George R. Saade, MD

• Division Chief, Maternal- Fetal Medicine
• Professor, Obstetrics and Gynecology
• Department of Obstetrics and Gynecology
• University of Texas Medical Branch
• Galveston, Texas

Furthermore diabetes medications side effects weight loss precose 50 mg fast delivery, almost all aspects of this turn-on/turn-off patterning by the cerebellum can be learned with experience diabetes insipidus dogs symptoms buy discount precose on-line. The cerebellum functions with the cerebral cortex at still another level of motor control; it helps to program in advance muscle contractions that are required for smooth progression from a present rapid movement in one direction to the next rapid movement in another direction diabetes symptoms dry cough order cheapest precose and precose, with all this occurring in a fraction of a second blood sugar reading chart cheap precose american express. The neural circuit for this passes from the cerebral cortex to the large lateral zones of the cerebellar hemispheres and then back to the cerebral cortex managing diabetes juicing cheap precose 50 mg. What is it that arouses us from inactivity and sets into play our trains of movement Basically, the brain has an older core located beneath, anterior, and lateral to the thalamus-including the hypothalamus, amygdala, hippocampus, septal region anterior to the hypothalamus and thalamus, and even old regions of the thalamus and cerebral cortex. All of these function together to initiate most motor and other functional activities of the brain. Ullsperger M, Danielmeier C, Jocham G: Neurophysiology of performance monitoring and adaptive behavior. However, we do know the effects of damage or stimulation of various portions of the cerebral cortex. In the first part of this chapter, the known cortical functions are discussed, and then basic theories of neuronal mechanisms involved in thought processes, memory, analysis of sensory information, and so forth are presented briefly. Note particularly the large number of horizontal fibers that extend between adjacent areas of the cortex, but note also the vertical fibers that extend to and from the cortex to lower areas of the brain and some all the way to the spinal cord or to distant regions of the cerebral cortex through long association bundles. The functions of the specific layers of the cerebral cortex are discussed in Chapters 48 and 52. This layer is only 2 to 5 millimeters thick, with a total area of about 25% of a square meter. Most of the neurons are of three types: (1) granular (also called stellate); (2) fusiform; and (3) pyramidal, the latter named for their characteristic pyramidal shape. The granular neurons generally have short axons and, therefore, function mainly as interneurons that transmit neural signals only short distances in the cortex. The sensory areas of the cortex, as well as the association areas between sensory and motor areas, have large concentrations of these granule cells, suggesting a high degree of intracortical processing of incoming sensory signals within the sensory areas and association areas. The pyramidal and fusiform cells give rise to almost all the output fibers from the cortex. The pyramidal cells, which are larger and more numerous than the fusiform cells, are the source of the long, large nerve fibers that go all the way to the spinal cord. Functional areas of the human cerebral cortex as determined by electrical stimulation of the cortex during neurosurgical operations and by neurological examinations of patients with destroyed cortical regions. It is important to emphasize the relation between the cerebral cortex and the thalamus. When the thalamus is damaged along with the cortex, the loss of cerebral function is far greater than when the cortex alone is damaged, because thalamic excitation of the cortex is necessary for almost all cortical activity. These connections act in two directions, both from the thalamus to the cortex and then from the cortex back to essentially the same area of the thalamus. Furthermore, when the thalamic connections are cut, the functions of the corresponding cortical area become almost entirely lost. Therefore, the cortex operates in close association with the thalamus and can almost be considered both anatomically and functionally a unit with the thalamus; for this reason, the thalamus and the cortex together are sometimes called the thalamocortical system. Almost all pathways from the sensory receptors and sensory organs to the cortex pass through the thalamus, with the principal exception of some sensory pathways of olfaction. The electrically stimulated patients told their thoughts evoked by the stimulation, and sometimes they experienced movements. Occasionally they spontaneously emitted a sound or even a word or gave some other evidence of the stimulation. This figure shows the major primary and secondary premotor and supplementary motor areas of the cortex, as well as the major primary and secondary sensory areas for somatic sensation, vision, and hearing, all of which are discussed in earlier chapters. The primary motor areas have direct connections with specific muscles for causing discrete muscle movements. The primary sensory areas detect specific sensations-visual, auditory, or somatic- transmitted to the brain from peripheral sensory organs. For example, the supplementary and premotor areas function along with the primary motor cortex and basal ganglia to provide "patterns" of motor activity. Important association areas include (1) the parieto-occipitotemporal association area, (2) the prefrontal association area, and (3) the limbic association area. These areas are called association areas because they receive and analyze signals simultaneously from multiple regions of both the motor and sensory cortices, as well as from subcortical Supplemental and premotor Primary motor Primary somatic Secondary somatic Parieto-Occipitotemporal Association Area the parieto-occipitotemporal association area lies in the large parietal and occipital cortical space bounded by the somatosensory cortex anteriorly, the visual cortex posteriorly, and the auditory cortex laterally. As would be expected, it provides a high level of interpretative meaning for signals from all the surrounding sensory areas. An Prefrontal association area Parietooccipitotemporal association area Limbic association area Primary auditory Secondary auditory Primary visual Secondary visual area beginning in the posterior parietal cortex and extending into the superior occipital cortex provides continuous analysis of the spatial coordinates of all parts of the body, as well as of the surroundings of the body. This area receives visual sensory information from the posterior occipital cortex and simultaneous somatosensory information from the anterior parietal cortex. From all this information, it computes the coordinates of the visual, auditory, and body surroundings. Locations of major association areas of the cerebral cortex, as well as primary and secondary motor and sensory areas. We discuss this area more fully later; it is one of the most important regions of the entire brain for higher intellectual function because most of these intellectual functions are language based. Motor and Integrative Neurophysiology the Angular Gyrus Area Is Needed for Initial Processing of Visual Language (Reading). This angular gyrus area is needed to make meaning out of the visually perceived words. In its absence, a person can still have excellent language comprehension through hearing but not through reading; injury to the angular gyrus can result in agraphia (inability to write) with alexia (inability to read), a condition in which a person cannot read, write, or spell words. In the most lateral portions of terns for expressing individual words or even short phrases are initiated and executed. This area also works in close association with the Wernicke language comprehension center in the temporal association cortex, as we discuss more fully later in the chapter. An especially interesting discovery is the following: When a person has already learned one language and then learns a new language, the area in the brain where the new language is stored is slightly removed from the storage area for the first language. If both languages are learned simultaneously, they are stored together in the same area of the brain. This area is found in the anterior pole of the temporal lobe, in the ventral portion of the frontal lobe, and in the cingulate gyrus lying deep in the longitudinal fissure on the midsurface of each cerebral hemisphere. We discuss in Chapter 59 that the limbic cortex is part of a much more extensive system, the limbic system, that includes a complex set of neuronal structures in the midbasal regions of the brain. This limbic system provides most of the emotional drives for activating other areas of the brain and even provides motivational drive for the process of learning itself. The names are learned mainly through auditory input, whereas the physical natures of the objects are learned mainly through visual input. Prefrontal Association Area As discussed in Chapter 57, the prefrontal association area functions in close association with the motor cortex to plan complex patterns and sequences of motor movements. To aid in this function, it receives strong input through a massive subcortical bundle of nerve fibers connecting the parieto-occipitotemporal association area with the prefrontal association area. Through this bundle, the prefrontal cortex receives much preanalyzed sensory information, especially information on the spatial coordinates of the body that is necessary for planning effective movements. Much of the output from the prefrontal area into the motor control system passes through the caudate portion of the basal ganglia­thalamic feedback circuit for motor planning, which provides many of the sequential and parallel components of movement stimulation. The prefrontal association area is also essential to carrying out "thought" processes. This characteristic presumably results from some of the same capabilities of the prefrontal cortex that allow it to plan motor activities. It seems to be capable of processing nonmotor and motor information from widespread areas of the brain and therefore to achieve nonmotor types of thinking, as well as motor types. In fact, the prefrontal association area is frequently described simply as important for elaboration of thoughts, and it is said to store on a short-term basis "working memories" that are used to combine new thoughts while they are entering the brain. Loss of these face recognition areas, strangely enough, results in little other abnormality of brain function. One may wonder why so much of the cerebral cortex should be reserved for the simple task of face recognition. However, most of our daily tasks involve associations with other people, and thus one can see the importance of this intellectual function. Facial recognition areas located on the underside of the brain in the medial occipital and temporal lobes. Organization of the somatic auditory and visual association areas into a general mechanism for interpretation of sensory experience. The types of thoughts that might be experienced include complicated visual scenes that one might remember from childhood, auditory hallucinations such as a specific musical piece, or even a statement made by a specific person. This area of confluence of the different sensory interpretative areas is especially highly developed in the dominant side of the brain-the left side in almost all right-handed people-and it plays the greatest single role of any part of the cerebral cortex for the higher comprehension levels of brain function that we call intelligence. Therefore, this region has been called by different names suggestive of an area that has almost global importance: the general interpretative area, the gnostic area, the knowing area, the tertiary association area, and so forth. Likewise, the person may be able to read words from the printed page but be unable to recognize the thought that is conveyed. Therefore, the person may be able to see words and even know that they are words but may not be able to interpret their meanings. The term "dyslexia" is used to describe difficulty in learning about written language, not complete word blindness. Therefore, it is easy to understand why the left side of the brain might become dominant over the right side. However, if for some reason this left side area is damaged or removed in very early childhood, the opposite side of the brain will usually develop dominant characteristics. The following theory can explain the capability of one hemisphere to dominate the other hemisphere. The attention of the "mind" seems to be directed to one principal thought at a time. Presumably, because the left posterior temporal lobe at birth is usually slightly larger than the right lobe, the left side normally begins to be used to a greater extent than is the right side. In about 95% of all people, the left temporal lobe and angular gyrus become dominant, and in the remaining 5%, either both sides develop simultaneously to have dual function or, more rarely, the right side alone becomes highly developed, with full dominance. This speech area is responsible for formation of words by exciting simultaneously the laryngeal muscles, respiratory muscles, and muscles of the mouth. The motor areas for controlling hands are also dominant in the left side of the brain in about 90% of persons, thus causing right-handedness in most people. Although the interpretative areas of the temporal lobe and angular gyrus, as well as many of the motor areas, are usually highly developed in only the left hemisphere, these areas receive sensory information from both hemispheres and are also capable of controlling motor activities in both hemispheres. For this purpose, they use mainly fiber pathways in the corpus callosum for communication between the two hemispheres. This unitary, cross-feeding organization prevents interference between the two sides of the brain; such interference could create havoc with both mental thoughts and motor responses. This close relation probably results from the fact that the first introduction to language is by way of hearing. Many other types of interpretative capabilities, some of which use the temporal lobe and angular gyrus regions of the opposite hemisphere, are retained. Thus, even though we speak of the "dominant" hemisphere, this dominance is primarily for languagebased intellectual functions; the so-called nondominant hemisphere might actually be dominant for some other types of intelligence. Yet efforts to show that the prefrontal cortex is more important in higher intellectual functions than other portions of the brain have not been successful. For example, when we read a book, we do not store the visual images of the printed words but instead store the words themselves or their conveyed thoughts, often in language form. Patients with damage to the prefrontal cortex may have normal motor functions and may even perform normally on some intelligence tests. These functions can be explained by describing what happens to patients in whom the prefrontal areas have become damaged, as follows. Several decades ago, before the advent of modern drugs for treating psychiatric conditions, it was discovered that some patients could receive significant relief from severe psychotic depression by severing the neuronal connections between the prefrontal areas of the brain and the remainder of the brain by a procedure called prefrontal lobotomy. This procedure was performed by inserting a blunt, thin-bladed knife through a small opening in the lateral frontal skull on each side of the head and slicing the brain at the back edge of the prefrontal lobes from top to bottom. Their level of aggressiveness decreased, sometimes markedly, and they often lost ambition. Their social responses were often inappropriate for the occasion, often including loss of morals and little reticence in relation to sexual activity and excretion. The patients could still talk and comprehend language, but they were unable to carry through any long trains of thought, and their moods changed rapidly from sweetness to rage to exhilaration to madness. The patients could also still perform most of the usual patterns of motor function that they had performed throughout life, but often without purpose. From this information, let us try to piece together a coherent understanding of the function of the prefrontal association areas. Decreased aggressiveness and inappropriate the capability of calling forth information from widespread areas of the brain and using this information to achieve deeper thought patterns for attaining goals. Although people without prefrontal cortices can still think, they show little concerted thinking in logical sequence for longer than a few seconds or a minute or so at most. Thus, people without prefrontal cortices are easily distracted from their central theme of thought, whereas people with functioning prefrontal cortices can drive themselves to completion of their thought goals, irrespective of distractions. Elaboration of Thought, Prognostication, and Performance of Higher Intellectual Functions by the Prefrontal Areas-Concept of a "Working Memory. This limbic area helps to control behavior, which is discussed in detail in Chapter 59. We learned earlier in Another function that has been ascribed to the prefrontal areas is elaboration of thought, which means simply an increase in depth and abstractness of the different thoughts put together from multiple sources of information. Psychological tests have shown that prefrontal lobectomized lower animals presented with successive bits of sensory information fail to keep track of these bits even in temporary memory, probably because they are distracted so easily that they cannot hold thoughts long enough for memory storage to take place. In fact, studies have shown that the prefrontal areas are divided into separate segments for storing different types of temporary memory, such as one area for storing shape and form of an object or a part of the body and another for storing movement.

The causes of minimal change nephropathy are unclear but may be at least partly related to an immunological response and abnormal T-cell secretion of cytokines that injure the podocytes and increase their permeability to some of the lower molecular weight proteins diabetes mellitus type 2 blood glucose levels discount 50 mg precose mastercard, especially albumin diabetes symptoms young male buy precose in united states online. This increased permeability permits the proteins to be filtered by the glomerular capillaries and excreted in the urine diabetes symptoms for male order precose american express, a condition known as proteinuria or albuminuria diabetes type 1 is it genetic order precose us. Minimal change nephropathy is most common in young children but can also occur in adults diabetic diet dr oz discount 25 mg precose mastercard, especially in those who have autoimmune disorders. Expressed mathematically, Thus, the net filtration pressure = 60 - 18 - 32 = +10 mm Hg. This high Kf for the glomerular capillaries contributes to their rapid rate of fluid filtration. Some diseases, however, lower Kf by reducing the number of functional glomerular capillaries (thereby reducing the surface area for filtration) or by increasing the thickness of the glomerular capillary membrane and reducing its hydraulic conductivity. For example, chronic uncontrolled hypertension may gradually reduce Kf by increasing the thickness of the glomerular capillary basement membrane and, eventually, by damaging the capillaries so severely that there is loss of capillary function. Increase in colloid osmotic pressure in plasma flowing through the glomerular capillary. Increases in the filtration fraction (glomerular filtration rate/renal plasma flow) increase the rate at which the plasma colloid osmotic pressure rises along the glomerular capillary; decreases in the filtration fraction have the opposite effect. Assuming that the normal colloid osmotic pressure of plasma entering the glomerular capillaries is 28 mm Hg, this value usually rises to about 36 mm Hg by the time the blood reaches the efferent end of the capillaries. Therefore, the average colloid osmotic pressure of the glomerular capillary plasma proteins is midway between 28 and 36 mm Hg, or about 32 mm Hg. Two factors that influence the glomerular capillary colloid osmotic pressure are the following: (1) the arterial plasma colloid osmotic pressure; and (2) the fraction of plasma filtered by the glomerular capillaries (filtration fraction). For example, precipitation of calcium or of uric acid may lead 334 Chapter 27 Glomerular Filtration, Renal Blood Flow, and Their Control by reducing renal plasma flow. Glomerular hydrostatic pressure is determined by three variables, each of which is under physiological control: (1) arterial pressure; (2) afferent arteriolar resistance; and (3) efferent arteriolar resistance. However, as discussed later, this effect is buffered by autoregulatory mechanisms that maintain a relatively constant glomerular pressure as arterial pressure fluctuates. Constriction of the efferent arterioles increases the resistance to outflow from the glomerular capillaries. However, because efferent arteriolar constriction also reduces renal blood flow, filtration fraction and glomerular colloid osmotic pressure increase as efferent arteriolar resistance increases. Therefore, if constriction of efferent arterioles is severe (more than about a threefold increase in efferent arteriolar resistance), the rise in colloid osmotic pressure exceeds the increase in glomerular capillary hydrostatic pressure caused by efferent arteriolar constriction. Effect of change in afferent arteriolar resistance or efferent arteriolar resistance on glomerular filtration rate and renal blood flow. Thus, the oxygen delivered to the kidneys far exceeds their metabolic needs, and the arterial-venous extraction of oxygen is relatively low compared with that of most other tissues. A large fraction of the oxygen consumed by the kidneys is related to the high rate of active sodium reabsorption by the renal tubules. If glomerular filtration ceases completely, renal sodium reabsorption also ceases and oxygen consumption decreases to about one-fourth normal. This residual oxygen consumption reflects the basic metabolic needs of the renal cells. As with other tissues, blood flow supplies the kidneys with nutrients and removes waste products. The purpose of this additional flow is to supply enough plasma for the high rates of glomerular filtration that are necessary for precise regulation of body fluid volumes and solute concentrations. This variable, in turn, is influenced by the sympathetic nervous system, hormones, autacoids (vasoactive substances that are released in the kidneys and act locally), and other feedback controls that are intrinsic to the kidneys. Most of the renal vascular resistance resides in three major segments-interlobular arteries, afferent arterioles, and efferent arterioles. Resistance of these vessels is controlled by the sympathetic nervous system, various hormones, and local internal renal control mechanisms, as discussed later. An increase in the resistance of any of the vascular segments of the kidneys tends to reduce the renal blood flow, whereas a decrease in vascular resistance increases renal blood flow if renal artery and renal vein pressures remain constant. This capacity for autoregulation occurs through mechanisms that are intrinsic to the kidneys, as discussed later in this chapter. However, as discussed in Chapter 28, even mild increases in renal sympathetic activity can stimulate renin release and increase renal tubular reabsorption, causing decreased sodium and water excretion. Blood flow in the renal medulla accounts for only 1% to 2% of the total renal blood flow. Flow to the renal medulla is supplied by a specialized portion of the peritubular capillary system called the vasa recta. These vessels descend into the medulla in parallel with the loops of Henle and then loop back along with the loops of Henle and return to the cortex before emptying into the venous system. In general, blood levels of these hormones parallel the activity of the sympathetic nervous system; thus, norepinephrine and epinephrine have little influence on renal hemodynamics except under conditions associated with strong activation of the sympathetic nervous system, such as severe hemorrhage. Another vasoconstrictor, endothelin, is a peptide that can be released by damaged vascular endothelial cells of the kidneys, as well as by other tissues. However, endothelin may contribute to hemostasis (minimizing blood loss) when a blood vessel is severed, which damages the endothelium and releases this powerful vasoconstrictor. Endothelial-Derived Nitric Oxide Decreases Renal Vascular Resistance and Increases Glomerular Filtration Rate. At the same time, though, the reduction in renal blood flow caused by efferent arteriolar constriction contributes to decreased flow through the peritubular capillaries, which in turn increases the 338 lar resistance and is released by the vascular endothelium throughout the body is endothelial-derived nitric oxide. A basal level of nitric oxide production appears to be important for maintaining vasodilation of the kidneys and normal excretion of sodium and water. In some hypertensive patients or in patients with atherosclerosis, damage of the vascular endothelium and impaired nitric oxide production may contribute to increased renal vasoconstriction and elevated blood pressure. Prostaglandins and Bradykinin Decrease Renal Vascular Resistance and Tend to Increase Glomerular Filtration Rate. Autoregulation of renal blood flow and glomerular filtration rate but lack of autoregulation of urine flow during changes in renal arterial pressure. These mechanisms still function in blood- perfused kidneys that have been removed from the body, independent of systemic influences. The primary function of blood flow autoregulation in most tissues, other than the kidneys, is to maintain the delivery of oxygen and nutrients at a normal level and remove the waste products of metabolism, despite changes in the arterial pressure. In the kidneys, the normal blood flow is much higher than that required for these functions. Because the total plasma volume is only about 3 liters, such a change would quickly deplete the blood volume. Even with these special control mechanisms, changes in arterial pressure still have significant effects on renal excretion of water and sodium; this effect is referred to as pressure diuresis or pressure natriuresis, and it is crucial in the regulation of body fluid volumes and arterial pressure, as discussed in Chapters 19 and 30. This feedback helps ensure a relatively constant delivery of sodium chloride to the distal tubule and helps prevent spurious fluctuations in renal excretion that would otherwise occur. The juxtaglomerular complex consists of macula densa cells in the initial portion of the distal tubule and juxtaglomerular cells in the walls of the afferent and efferent arterioles. Structure of the juxtaglomerular apparatus, demonstrating its possible feedback role in the control of nephron function. The macula densa cells contain the Golgi apparatus, which consists of intracellular secretory organelles directed toward the arterioles, suggesting that these cells may be secreting a substance toward the arterioles. Decreased Macula Densa Sodium Chloride Causes Dilation of Afferent Arterioles and Increased Renin Release. Studies of individual blood vessels (especially small arterioles) throughout the body have shown that they respond to increased wall tension or wall stretch by contraction of the vascular smooth muscle. Stretch of the vascular wall allows increased movement of calcium ions from the extracellular fluid into the cells, causing them to contract through the mechanisms discussed in Chapter 8. On the other hand, this mechanism may be more important in protecting the kidney from hypertension-induced injury. In response to sudden increases in blood pressure, the myogenic constrictor response in afferent arterioles occurs within seconds and therefore attenuates transmission of increased arterial pressure to the glomerular capillaries. High Protein Intake and Hyperglycemia Increase Renal Blood Flow and Glomerular Filtration Rate. A high-protein meal releases into the blood amino acids, which are reabsorbed in the proximal tubules. Because amino acids and sodium are reabsorbed together by cotransport in the proximal tubules, increased amino acid reabsorption also stimulates sodium reabsorption. Because glucose, like some of the amino acids, is also reabsorbed along with sodium in the proximal tubule, increased glucose delivery to the tubules causes them to reabsorb excess sodium along with glucose. The main purpose of this feedback is to ensure a constant delivery of sodium chloride to the distal tubule, where final processing of the urine takes place. An opposite sequence of events occurs when proximal tubular reabsorption is reduced. For example, when the proximal tubules are damaged (which can occur as a result of poisoning by heavy metals, such as mercury, or large doses of drugs, such as tetracyclines), their ability to reabsorb sodium chloride is decreased. As a consequence, large amounts of sodium chloride are delivered to the distal tubule and, without appropriate compensation, would quickly cause excessive volume depletion. These examples again demonstrate the importance of this feedback mechanism for ensuring that the distal tubules receive the proper rate of delivery of sodium chloride, other tubular fluid solutes, and tubular fluid volume so that appropriate amounts of these substances are excreted in the urine. Although the mechanisms responsible for these sex differences are not fully understood, beneficial effects of estrogens and damaging effects of androgens on the kidneys have been suggested as a partial explanation. Along this course, some substances are selectively reabsorbed from the tubules back into the blood, whereas others are secreted from the blood into the tubular lumen. Eventually, the urine that is formed and all the substances in the urine represent the sum of three basic renal processes-glomerular filtration, tubular reabsorption, and tubular secretion: Urinary excretion = Glomerular ltration - Tubular reabsorption + Tubular secretion For many substances, tubular reabsorption plays a much more important role than secretion in determining the final urinary excretion rate. However, tubular secretion accounts for significant amounts of potassium ions, hydrogen ions, and a few other substances that appear in the urine. The rate at which each of these substances is filtered is calculated as follows: Filtration = Glomerular filtration rate × Plasma concentration excretion for many substances. Thus, a small change in glomerular filtration or tubular reabsorption can potentially cause a relatively large change in urinary excretion. In reality, changes in tubular reabsorption and glomerular filtration are closely coordinated so that large fluctuations in urinary excretion are avoided. Second, unlike glomerular filtration, which is relatively nonselective (essentially all solutes in the plasma are filtered except the plasma proteins or substances bound to them), tubular reabsorption is highly selective. Some substances, such as glucose and amino acids, are almost completely reabsorbed from the tubules, so the urinary excretion rate is essentially zero. Many ions in the plasma, such as sodium, chloride, and bicarbonate, are also highly reabsorbed, but their rates of reabsorption and urinary excretion are variable, depending on the needs of the body. Waste products, such as urea and creatinine, conversely, are poorly reabsorbed from the tubules and are excreted in relatively large amounts. Therefore, by controlling their reabsorption of different substances, the kidneys regulate excretion of solutes independently of one another, a capability that is essential for precise control of the body fluid composition. In this articler, we discuss the mechanisms that allow the kidneys to selectively reabsorb or secrete different substances at variable rates. This calculation assumes that the substance is freely filtered and not bound to plasma proteins. For example, if plasma glucose concentration is 1 g/L, the amount of glucose filtered each day is about 180 L/day × 1 g/L, or 180 g/day. Because virtually none of the filtered glucose is normally excreted, the rate of glucose reabsorption is also 180 g/day. Transport that is coupled indirectly to an energy source, such as that due to an ion gradient, is referred to as secondary active transport. Reabsorption of glucose by the renal tubule is an example of secondary active transport. Although solutes can be reabsorbed by active and/or passive mechanisms by the tubule, water is always reabsorbed passively across the tubular epithelial membrane by the process of osmosis. Solutes are transported through the cells (transcellular path) by passive diffusion or active transport, or between the cells (paracellular path) by diffusion. Transport of water and solutes from the interstitial fluid into the peritubular capillaries occurs by ultrafiltration (bulk flow). For example, water and solutes can be transported through the cell membranes (transcellular route) or through the spaces between the cell junctions (paracellular route). Then, after absorption across the tubular epithelial cells into the interstitial fluid, water and solutes are transported through the peritubular capillary walls into the blood by ultrafiltration (bulk flow) that is mediated by hydrostatic and colloid osmotic forces. The peritubular capillaries behave like the venous ends of most other capillaries because there is a net reabsorptive force that moves the fluid and solutes from the interstitium into the blood. Lateral intercellular spaces lie behind the tight junctions and separate the epithelial cells of the tubule. Solutes can be reabsorbed or secreted across the cells through the transcellular pathway or between the cells by moving across the tight junctions and intercellular spaces via the paracellular pathway. Sodium is a substance that moves through both routes, although most of the sodium is transported through the transcellular pathway. In some nephron segments, especially the proximal tubule, water is also reabsorbed across the paracellular pathway, and substances dissolved in the water, especially potassium, magnesium, and chloride ions, are carried with the reabsorbed fluid between the cells. Transport that is coupled directly to an energy source, such as the hydrolysis of adenosine 344 port is that it can move solutes against an electrochemical gradient. On the Primary Active Transport Through the Tubular Membrane Linked to Hydrolysis of Adenosine Triphosphatase. Sodium, water, and other substances are reabsorbed from the interstitial fluid into the peritubular capillaries by ultrafiltration, a passive process driven by the hydrostatic and colloid osmotic pressure gradients. The sodium-potassium pump transports sodium from the interior of the cell across the basolateral membrane, creating a low intracellular sodium concentration and a negative intracellular electrical potential. The low intracellular sodium concentration and negative electrical potential cause sodium ions to diffuse from the tubular lumen into the cell through the brush border. At the same time, potassium is transported from the interstitium to the inside of the cell. The operation of this ion pump maintains low intracellular sodium and high intracellular potassium concentrations and creates a net negative charge of about -70 millivolts within the cell. This active pumping of sodium out of the cell across the basolateral membrane of the cell favors passive diffusion of sodium across the luminal membrane of the cell, from the tubular lumen into the cell, for two reasons: (1) there is a concentration gradient favoring sodium diffusion into the cell because the intracellular sodium concentration is low (12 mEq/L) and tubular fluid sodium concentration is high (140 mEq/L); and (2) the negative, -70-millivolt, intracellular potential attracts the positive sodium ions from the tubular lumen into the cell.

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Therefore metabolic disease crossword clue generic precose 25 mg without a prescription, if the hippocampus signals that a neuronal input is important diabete 5g order precose line, the information is likely to be committed to memory diabetes medications that cause hypoglycemia purchase precose with visa. Thus blood sugar 4 hours after eating buy precose with mastercard, a person rapidly becomes habituated to indifferent stimuli but learns assiduously any sensory experience that causes either pleasure or pain blood sugar watch monitor precose 25 mg purchase online. It has been suggested that the hippocampus provides the drive that causes translation of short- term memory into longterm memory-that is, the hippocampus transmits signals that seem to make the mind rehearse over and over the new information until permanent storage takes place. Whatever the mechanism, without the hippocampi, consolidation of long- term memories of the verbal or symbolic thinking type is poor or does not take place. Functions of the Amygdala the amygdala is a complex of multiple small nuclei located immediately beneath the cerebral cortex of the medial anterior pole of each temporal lobe. It has abundant bidirectional connections with the hypothalamus, as well as with other areas of the limbic system. In lower animals, the amygdala is concerned to a great extent with olfactory stimuli and their interrelations with the limbic brain. It was noted in Chapter 54 that one of the major divisions of the olfactory tract terminates in a portion of the amygdala called the corticomedial nuclei, which lies immediately beneath the cerebral cortex in the olfactory pyriform area of the temporal lobe. In the human being, another portion of the amygdala, the basolateral nuclei, has become much more highly developed than the olfactory portion and plays important roles in many behavioral activities not generally associated with olfactory stimuli. The amygdala receives neuronal signals from all portions of the limbic cortex, as well as from the neocortex of the temporal, parietal, and occipital lobes-especially from the auditory and visual association areas. Because of these multiple connections, the amygdala has been called the "window" through which the limbic system sees the place of the person in the world. In turn, the amygdala transmits signals (1) back into these same cortical areas, (2) into the hippocampus, (3) into the septum, (4) into the thalamus, and (5) especially into the hypothalamus. In general, stimulation in the amygdala can cause almost all the same effects as those elicited by direct stimulation of the hypothalamus, Role of the Hippocampus in Learning Anterograde Amnesia After Bilateral Removal of the Hippocampi. Portions of the hippocampi have been surgically removed bilaterally in a few human beings for treatment of epilepsy. However, they often can learn essentially no new information that is based on verbal symbolism-they often cannot even learn the names of people with whom they come in contact every day. Yet they can remember for a moment or so what transpires during the course of their activities. Thus, they are capable of short-term memory for seconds up to a minute or two, although their ability to establish memories lasting longer than a few minutes is either completely or almost completely abolished. In many lower animals, this cortex plays 750 Chapter 59 the Limbic System and the Hypothalamus-Behavioral and Motivational Mechanisms of the Brain plus other effects. Effects initiated from the amygdala and then sent through the hypothalamus include the following: (1) increases or decreases in arterial pressure and heart rate; (2) increases or decreases in gastrointestinal motility and secretion; (3) defecation or micturition; (4) pupillary dilation or, rarely, constriction; (5) piloerection; and (6) secretion of various anterior pituitary hormones, especially the gonadotropins and adrenocorticotropic hormone. Aside from these effects mediated through the hypothalamus, amygdala stimulation can also cause several types of involuntary movement. These types include the following: (1) tonic movements, such as raising the head or bending the body; (2) circling movements; (3) occasionally clonic, rhythmical movements; and (4) different types of movements associated with olfaction and eating, such as licking, chewing, and swallowing. Stimulation of certain amygdaloid nuclei can also cause a pattern of rage, escape, punishment, severe pain, and fear similar to the rage pattern elicited from the hypothalamus, as described earlier. Finally, excitation of still other portions of the amygdala can cause sexual activities that include erection, copulatory movements, ejaculation, ovulation, uterine activity, and premature labor. When the anterior parts of both many behavioral patterns can be elicited by stimulation of specific portions of the limbic cortex. When the anterior temporal cortex is ablated bilaterally, the amygdalas are almost invariably damaged as well and, as discussed earlier, the Klüver-Bucy syndrome occurs. The animal especially develops consummatory behavior: it investigates any and all objects, has intense sex drives toward inappropriate animals or even inanimate objects, and loses all fear-and thus develops tameness as well. Bilateral removal of the posterior portion of the orbital frontal cortex often causes an animal to develop insomnia associated with intense motor restlessness; the animal becomes unable to sit still and moves about continuously. This removal causes changes in behavior called the KlüverBucy syndrome, which is demonstrated by an animal that (1) is not afraid of anything, (2) has extreme curiosity about everything, (3) forgets rapidly, (4) has a tendency to place everything in its mouth and sometimes even tries to eat solid objects, and (5) often has a sex drive so strong that it attempts to copulate with immature animals, animals of the wrong sex, or even animals of a different species. Although similar lesions in human beings are rare, afflicted people respond in a manner not too different from that of the monkey. The amygdalas seem to be behavioral awareness areas that operate at a semiconscious level. Destruction of these gyri bilaterally releases the rage centers of the septum and hypothalamus from prefrontal inhibitory influence. Therefore, the animal can become vicious and much more subject to fits of rage than normally. Until further information is available, it is perhaps best to state that the cortical regions of the limbic system occupy intermediate associative positions between the functions of the specific areas of the cerebral cortex and functions of the subcortical limbic structures for control of behavioral patterns. Thus, in the anterior temporal cortex, one especially finds gustatory and olfactory behavioral associations. In the middle and posterior cingulate cortex, there is reason to believe that sensorimotor behavioral associations occur. Bibliography Anacker C, Hen R: Adult hippocampal neurogenesis and cognitive flexibility - linking memory and mood. This cortex functions as a transitional zone through which signals are transmitted from the remainder of the brain cortex into the limbic system and also in the opposite direction. Therefore, the limbic cortex in effect functions as a cerebral association area for control of behavior. Stimulation of the different regions of the limbic cortex has failed to give any clear idea of their functions. Motor and Integrative Neurophysiology stalt: mechanisms of fear, threat, and trauma memory encoding. All these states result from different activating or inhibiting forces generated usually within the brain. In Chapter 59, we began a partial discussion of this subject when we described different systems that are capable of activating large portions of the brain. In this articler, we present brief surveys of specific states of brain activity, beginning with sleep. It is an active form of sleep usually associated with dreaming and active bodily muscle movements. Muscle tone throughout the body is exceedingly depressed, indicating strong inhibition of the spinal muscle control areas. Heart rate and respiratory rate usually become irregular, which is characteristic of the dream state. Despite the extreme inhibition of the peripheral muscles, irregular muscle movements do occur in addition to the rapid movements of the eyes. This type of sleep is also called paradoxical sleep because it is a paradox that a person can still be asleep, despite the presence of marked activity in the brain. However, the person is not fully aware of the surroundings and therefore is truly asleep. It is to be distinguished from coma, which is unconsciousness from which a person cannot be aroused. Sleep researchers also divide sleep into two entirely different types of sleep that have different qualities, as described in the following section. This type of sleep is not so restful, and it is often associated with vivid dreaming. Slow-Wave Sleep We can understand the characteristics of deep slow-wave sleep by remembering the last time we were kept awake for more than 24 hours and the deep sleep that occurred during the first hour after going to sleep. This sleep is exceedingly restful and is associated with decreases in peripheral vascular tone and many other vegetative functions of the body. For example, 10% to 30% decreases occur in blood pressure, respiratory rate, and basal metabolic rate. Although slow-wave sleep is frequently called "dreamless sleep," dreams and sometimes even nightmares do occur during slow-wave sleep. Also, the dreams of slow-wave sleep are usually not remembered because consolidation of the dreams in memory does not occur. An earlier theory of sleep was that the excitatory areas of the upper brain stem, the reticular activating system, simply became fatigued during the waking day and became inactive as a result. An important experiment changed this thinking to the current view that sleep is caused by an active inhibitory process, because it was discovered that transecting the brain stem at the level of the midpons creates a brain cortex that never goes to sleep. In other words, a center located below the midpontile level of the brain stem appears to be required to cause sleep by inhibiting other parts of the brain. Neuronal Centers, Neurohumoral Substances, and Mechanisms That Can Cause Sleep-Possible Role for Serotonin Stimulation of several specific areas of the brain can produce sleep with characteristics near those of natural sleep. The raphe nuclei in the lower half of the pons and in the medulla is the most conspicuous stimulation area for causing almost natural sleep. Nerve fibers from these nuclei spread locally in the brain stem reticular formation and also upward into the thalamus, hypothalamus, most areas of the limbic system, and even the neocortex of the cerebrum. In addition, fibers extend downward into the spinal cord, terminating in the posterior horns, where they can inhibit incoming sensory signals, including pain, as discussed in Chapter 49. When a drug that blocks the formation of serotonin is administered to an animal, the animal often cannot sleep for the next several days. Therefore, it has been assumed that serotonin is a transmitter substance associated with the production of sleep. Stimulation of some areas in the nucleus of the tractus solitarius can also cause sleep. This nucleus is the termination in the medulla and pons for visceral sensory signals entering by way of the vagus and glossopharyngeal nerves. Sleep can be promoted by stimulation of several regions in the diencephalon, including (1) the rostral part of the hypothalamus, mainly in the suprachiasmal area, and (2) an occasional area in the diffuse nuclei of the thalamus. This phenomenon is also true of bilateral lesions in the medial rostral suprachiasmal area in the anterior hypothalamus. In both cases, the excitatory reticular nuclei of the mesencephalon and Chapter 60 States of Brain Activity-Sleep, Brain Waves, Epilepsy, Psychoses, and Dementia upper pons seem to become released from inhibition, thus causing intense wakefulness. Indeed, sometimes lesions of the anterior hypothalamus can cause such intense wakefulness that the animal actually dies of exhaustion. Experiments have shown that the cerebrospinal fluid and the blood or urine of animals that have been kept awake for several days contain a substance or substances that will cause sleep when injected into the brain ventricular system of another animal. One likely substance has been identified as muramyl peptide, a low-molecularweight substance that accumulates in the cerebrospinal fluid and urine in animals kept awake for several days. When only micrograms of this sleep-producing substance are injected into the third ventricle, almost natural sleep occurs within a few minutes, and the animal may stay asleep for several hours. Another substance that has similar effects in causing sleep is delta sleep­inducing peptide, a nonapeptide found in the cerebrospinal fluid after electrical stimulation of the thalamus to induce sleep. Several other potential sleep factors, mostly peptides, have been isolated from the cerebrospinal fluid or neuronal tissues of the brain stem of animals kept awake for days. It is possible that prolonged wakefulness causes progressive accumulation of a sleep factor or factors in the brain stem or cerebrospinal fluid that lead(s) to sleep. It is not understood why Therefore, once wakefulness begins, it has a natural tendency to sustain itself because of all this positive feedback activity. Then, after the brain remains activated for many hours, even the neurons in the activating system presumably become fatigued. Consequently, the positive feedback cycle between the mesencephalic reticular nuclei and the cerebral cortex fades and the sleep-promoting effects of the sleep centers take over, leading to rapid transition from wakefulness back to sleep. This overall theory could explain the rapid transitions from sleep to wakefulness and from wakefulness to sleep. Therefore, it has been postulated that the large acetylcholine-secreting neurons in the upper brain stem reticular formation might, through their extensive efferent fibers, activate many portions of the brain. Orexin (also called hypocretin) is produced by neurons in the hypothalamus that provide excitatory input to many other areas of the brain where there are orexin receptors. Loss of orexin signaling as a result of defective orexin receptors or destruction of orexin-producing neurons causes narcolepsy, a sleep disorder characterized by overwhelming daytime drowsiness and sudden attacks of sleep that can occur, even when a person is talking or working. Patients with narcolepsy may also experience a sudden loss of muscle tone (cataplexy) that can be partial or even severe enough to cause paralysis during the attack. These observations point to an important role for orexin neurons in maintaining wakefulness, but their contribution to the normal daily cycle between sleep and wakefulness is unclear. Even mild sleep restriction over a few days may degrade cognitive and physical performance, overall productivity, and the health of a person. The essential role of sleep in homeostasis is perhaps most vividly demonstrated by the fact that rats deprived of sleep for 2 to 3 weeks may actually die. Despite the obvious importance of sleep, our understanding of why sleep is an essential part of life is still limited. Sleep causes two major types of physiological effects: first, effects on the nervous system, and second, effects on other functional systems of the body. Mammals, and even invertebrate animals, sleep more in the setting of infectious as well as non-infectious illnesses. Therefore, we might suggest the following possible mechanism for causing the sleepwakefulness cycle. When the sleep centers are not activated, the mesencephalic and upper pontile reticular activating nuclei are released from inhibition, which allows the reticular activating nuclei to become spontaneously active. This spontaneous activity in turn excites both the cerebral cortex and the peripheral nervous system, both of which send numerous positive feedback signals back to the same reticular activating nuclei to activate them still further. Motor and Integrative Neurophysiology Lack of sleep certainly affects the functions of the central nervous system. Prolonged wakefulness is often associated with progressive malfunction of the thought processes and sometimes even causes abnormal behavioral activities. We are all familiar with the increased sluggishness of thought that occurs toward the end of a prolonged wakeful period, but in addition, a person can become irritable or even psychotic after forced wakefulness. Therefore, we can assume that sleep in multiple ways restores both normal levels of brain activity and normal "balance" among the different functions of the central nervous system.

The arterial blood pressure falls very low nephrogenic diabetes insipidus quizlet order precose amex, and renal blood flow and urine output decrease diabetes diet best fruits precose 25 mg order otc. If the total amount of free hemoglobin released into the circulating blood is greater than the quantity that can bind with haptoglobin (a plasma protein that binds small amounts of hemoglobin) diabetes prevention models generic 50 mg precose with amex, much of the excess leaks through the glomerular membranes into the kidney tubules diabetes type 2 reversal 50 mg precose buy with mastercard. If this amount is still slight type 2 diabetes definition dictionary com buy precose 25 mg without prescription, it can be reabsorbed through the tubular epithelium into the blood and will cause no harm; if large, then only a small percentage is reabsorbed. Yet, water continues to be reabsorbed, causing the tubular hemoglobin concentration to rise so high that the hemoglobin precipitates and blocks many of the kidney tubules. Thus, renal vasoconstriction, circulatory shock, and renal tubular blockage together cause acute renal shutdown. If the shutdown is complete and fails to resolve, the patient dies within 7to12days,asexplainedinChapter 32, unless treated with an artificial kidney. The following specific procedures have met with some degrees of clinical or experimental success. Consequently, foreign cells transplanted anywhere into the body of a recipient can produce an immune reaction. A transplant of a tissue or whole organ from one part of the same animal to another part is called an autograft; from one identical twin to another, an isograft; from one person to another or from an animal to another animal of the same species, an allograft; and from a nonhuman animal to a human or from an animal of one species to one of another species, a xenograft. In the case of autografts and isografts, cells in the transplant contain virtually the same types of antigens as in the tissues of the recipient and will almost always continue to live normally and indefinitely if an adequate blood supply is provided. Some of the different cellular tissues and organs that have been transplanted from one person to another as allografts, either experimentally or for therapeutic purposes, are skin, kidney, heart, liver, glandular tissue, bone marrow, and lung. With proper matching of tissues between persons, many kidney allografts have been successful for at least 5 to 15 years and allograft liver and hearttransplantsfor1to15years. Development of significant immunity against any of these antigens can cause graft rejection. The lymphocytes are mixed with appropriate antisera and complement; after incubation, the cells are tested for membrane damage, usually by determining the rate of transmembrane uptake by the lymphocytic cells of a special dye. Therefore, a precise match of some antigens between donor and recipient is not always essential to allow allograft acceptance. By using a more advanced method of genetic testing and obtaining the best possible match between donor and recipient, the grafting procedure has become far less hazardous. The best success has been with tissue type matches between siblings and between parent and child. The match in identical twins is exact, so transplants between identical twins are almost never rejected because of an immune reaction. Prevention of Graft Rejection by Suppressing the Immune System If the immune system were completely suppressed, graft rejection would not occur. In fact, in a person who has serious depression of the immune system, grafts can be successful without the use of significant therapy to prevent rejection. However, in the person with a healthy immune system, even with the best possible tissue typing, allografts seldom resist rejection for more than a few days or weeks without the use of specific therapy to suppress the immune system. Furthermore, because the T cells are mainly the portion of the immune system important for killing grafted cells, their suppression is much more important than suppression of plasma antibodies. Various drugs that have a toxic effect on the lymphoid system and therefore block formation of antibodies and T cells, especially the drug azathioprine. Cyclosporine and tacrolimus, which inhibit formation of T-helper cells and, therefore, are especially efficacious in blocking the T-cell rejection reaction. These agents have proven to be highly valuable drugs because they do not depress some other portions of the immune system. Use of these agents often leaves the person unprotected from infectious disease; therefore, sometimes bacterial and viral infections become rampant. In addition, the incidence of cancer is several times greater in an immunosuppressed person, presumably because the immune system is important in destroying many early cancer cells before they can begin to proliferate. Transplantation of living tissues in people has been successful mainly because of the development of drugs that suppress the responses of the immune system. With the introduction of improved immunosuppressive agents, successful organ transplantation has become much more common. The current approach to immunosuppressive therapy attempts to balance acceptable rates of rejection with moderation of the adverse effects of immunosuppressive drugs. Whenever a vessel is severed or ruptured, hemostasis is achieved by several mechanisms: (1) vascular constriction; (2) formation of a platelet plug; (3) formation of a blood clot as a result of blood coagulation; and (4) eventual growth of fibrous tissue into the blood clot to close the hole in the vessel permanently. The normal concentration of platelets in the blood is between 150,000 and 450,000/l. Platelets have many functional characteristics of whole cells, even though they do not have nuclei and cannot reproduce. On the platelet cell membrane surface is a coat of glycoproteins that repulses adherence to normal endothelium and yet causes adherence to injured areas of the vessel wall, especially to injured endothelial cells and even more so to any exposed collagen from deep within the vessel wall. In addition, the platelet membrane contains large amounts of phospholipids that activate multiple stages in the blood-clotting process, as discussed later. It has a half-life in the blood of only 8 to 12 days, so over several weeks its functional processes run out; it is then eliminated from the circulation mainly by the tissue macrophage system. More than half of the platelets are removed by macrophages in the spleen, where the blood passes through a latticework of tight trabeculae. The contraction results from the following: (1) local myogenic spasm; (2) local autacoid factors from the traumatized tissues, vascular endothelium, and blood platelets; and (3) nervous reflexes. The nervous reflexes are initiated by pain nerve impulses or other sensory impulses that originate from the traumatized vessel or nearby tissues. However, even more vasoconstriction probably results from local myogenic contraction of the blood vessels initiated by direct damage to the vascular wall. And, for the smaller vessels, the platelets are responsible for much of the vasoconstriction by releasing a vasoconstrictor substance, thromboxane A2. The more severely a vessel is traumatized, the greater the degree of vascular spasm. The spasm can last for many minutes or even hours, during which time the processes of platelet plugging and blood coagulation can take place. To understand this process, it is important that we first discuss the nature of platelets themselves. Physical and Chemical Characteristics Platelets (also called thrombocytes) are minute discs 1 to 4 micrometers in diameter. They are formed in the bone marrow from megakaryocytes, which are extremely large hematopoietic cells in the marrow; the megakaryocytes Mechanism of Platelet Plug Formation Platelet repair of vascular openings is based on several important functions of the platelet. These platelet-secreted factors recruit additional platelets (aggregation) to form a hemostatic plug. Therefore, at the site of a puncture in a blood vessel wall, the damaged vascular wall activates successively increasing numbers of platelets that attract more and more additional platelets, thus forming a platelet plug. This plug is loose at first but is usually successful in blocking blood loss if the vascular opening is small. These threads attach tightly to the platelets, thus constructing an unyielding plug. The clot begins to develop in 15 to 20 seconds if the trauma to the vascular wall is severe and in 1 to 2 minutes if the trauma is minor. Activator substances from the traumatized vascular wall, from platelets, and from blood proteins adhering to the traumatized vascular wall initiate the clotting process. Within 3 to 6 minutes after rupture of a vessel, the entire opening or broken end of the vessel is filled with clot if the vessel opening is not too large. Platelets also play an important role in this clot retraction, as discussed later. Indeed, multiple small holes through the endothelial cells themselves are often closed by platelets actually fusing with the endothelial cells to form additional endothelial cell membranes. Literally thousands of small hemorrhagic areas develop each day under the skin (petechiae, which appear as purple or red dots on the skin) and throughout the internal tissues of a person who has few blood platelets. The usual course for a clot that forms in a small hole of a vessel wall is invasion by fibroblasts, beginning within a few hours after the clot is formed, which is promoted at least partially by growth factor secreted by platelets. This process continues to complete organization of the clot into fibrous tissue within about 1 to 2 weeks. Conversely, when excess blood has leaked into the tissues, and tissue clots have formed where they are not needed, special substances in the clot usually become activated. These substances function as enzymes to dissolve the clot, as discussed later in the chapter. However, when a vessel is ruptured, procoagulants from the area of tissue damage become activated and override the anticoagulants, and then a clot does develop. In response to rupture of the vessel or damage to the blood itself, a complex cascade of chemical reactions occurs in the blood involving more than 12 blood coagulation factors. The net result is the formation of a complex of activated substances collectively called prothrombin activator. The thrombin acts as an enzyme to convert fibrinogen into fibrin fibers that enmesh platelets, blood cells, and plasma to form the clot. We will first discuss the mechanism whereby the blood clot is formed, beginning with conversion of prothrombin to thrombin, and then come back to the initiating stages in the clotting process whereby prothrombin activator is formed. Prothrombin activator is formed as a result of rupture of a blood vessel or as a result of damage to special substances in the blood. Thrombin causes polymerization of fibrinogen molecules into fibrin fibers within another 10 to 15 seconds. Thus, the rate-limiting factor in causing blood coagulation is usually the formation of prothrombin activator and not the subsequent reactions beyond that point because these terminal steps normally occur rapidly to form the clot. Whether blood will coagulate depends on the balance between these two groups of substances. Platelet Release of phospholipid tissue factor complex Thrombin Fibrin Cross-linked fibrin Fibrinogen Endothelium Fibrin clot Platelets also play an important role in the conversion of prothrombin to thrombin because much of the prothrombin first attaches to prothrombin receptors on the platelets that are already bound to the damaged tissue. It is an unstable protein that can split easily into smaller compounds, one of which is thrombin, which has a molecular weight of 33,700, almost half that of prothrombin. Prothrombin is formed continually by the liver, and it is continually being used throughout the body for blood clotting. If the liver fails to produce prothrombin, in a day or so prothrombin concentration in the plasma falls too low to provide normal blood coagulation. Vitamin K is required by the liver for normal activation of prothrombin, as well as a few other clotting factors. Therefore, lack of vitamin K or the presence of liver disease that prevents normal prothrombin formation can decrease the prothrombin to such a low level that a bleeding tendency results. Fibrinogen is a high-molecular-weight protein (molecular weight 340,000) that occurs in the plasma in quantities of 100 to 700 mg/dl. Fibrinogen is formed in the liver, and liver disease can decrease the concentration of circulating fibrinogen, as it does the concentration of prothrombin, noted earlier. Because of its large molecular size, little fibrinogen normally leaks from the blood vessels into the interstitial fluids, and because fibrinogen is one of the essential factors in the coagulation process, interstitial fluids ordinarily do not coagulate. Yet, when the permeability of the capillaries becomes pathologically increased, fibrinogen does leak into the tissue fluids in sufficient quantities to allow clotting of these fluids in much the same way that plasma and whole blood can clot. It acts on fibrinogen to remove four lowmolecular-weight peptides from each molecule of fibrinogen, forming one molecule of fibrin monomer that has the automatic capability to polymerize with other fibrin monomer molecules to form fibrin fibers. Therefore, many fibrin monomer molecules polymerize within seconds into long fibrin fibers that constitute the reticulum of the blood clot. In the early stages of polymerization, the fibrin monomer molecules are held together by weak noncovalent hydrogen bonding, and the newly forming fibers are not cross-linked with one another. However, another process occurs during the next few minutes that greatly strengthens the fibrin reticulum. This process involves a substance called fibrin stabilizing factor that is present in small amounts in normal plasma globulins but is also released from platelets entrapped in the clot. Before fibrin stabilizing factor can have an effect on the fibrin fibers, it must be activated. The same thrombin that causes fibrin formation also activates the fibrin stabilizing factor. This activated substance then operates as an enzyme to form covalent bonds between more and more of the fibrin monomer molecules, as well as multiple cross-linkages between adjacent fibrin fibers, thus adding tremendously to the three-dimensional strength of the fibrin meshwork. The fibrin fibers also adhere to damaged surfaces of blood vessels; therefore, the blood clot becomes adherent to any vascular opening and thereby prevents further blood loss. Within a few minutes after a clot is formed, it begins to contract and usually expresses most of the fluid from the clot within 20 to 60 minutes. The fluid expressed is called serum because all its fibrinogen and most of the other clotting factors have been removed; in this way, serum differs from plasma and cannot clot because it lacks these factors. Chapter 37 Hemostasis and Blood Coagulation Platelets are necessary for clot retraction to occur. Therefore, failure of clot retraction is an indication that the number of platelets in the circulating blood might be low. Electron micrographs of platelets in blood clots show that they become attached to the fibrin fibers in such a way that they actually bond different fibers together. Furthermore, platelets entrapped in the clot continue to release procoagulant substances, one of the most important of which is fibrin stabilizing factor, which causes more and more cross-linking bonds between adjacent fibrin fibers. In addition, the platelets contribute directly to clot contraction by activating platelet thrombosthenin, actin, and myosin molecules, which are all contractile proteins in the platelets; they cause strong contraction of the platelet spicules attached to the fibrin. The contraction is activated and accelerated by thrombin and by calcium ions released from calcium stores in the mitochondria, endoplasmic reticulum, and Golgi apparatus of the platelets. As the clot retracts, the edges of the broken blood vessel are pulled together, thus contributing still further to hemostasis. One of the most important causes of this clot promotion is that the proteolytic action of thrombin allows it to act on many of the other blood-clotting factors in addition to fibrinogen. For example, thrombin has a direct proteolytic effect on prothrombin, tending to convert it into still more thrombin, and it acts on some of the bloodclotting factors responsible for formation of prothrombin activator. Prothrombin activator is generally considered to be formed in two ways, although, in reality, the two ways interact constantly with each other: (1) by the extrinsic pathway that begins with trauma to the vascular wall and surrounding tissues; and (2) by the intrinsic pathway that begins in the blood.

References

• Assoian RK, Klein EA: Growth control by intracellular tension and extracellular stiffness, Trends Cell Biol 18:347n352, 2008.
• Merkus D, Houweling B, de Beer JV, Everon Z, Duncker DJ. Alterations in endothelial control of the pulmonary circulation in exercising swine with secondary pulmonary hypertension after myocardial infarction. J Physiol 2007; 580(Pt.3):907-23.
• Singh SP, Besner GE, Schauer GM. Pulmonary endodermal tumor resembling fetal lung: report of a case in a 14-year-old girl. Pediatr Pathol Lab Med 1997;17:951-8.
• Kidwell CS, Chalela JA, Saver JL, et al. Comparison of MRI and CT for detection of acute intracerebral hemorrhage. JAMA 2004;292:1823-30.
• Chang RR, Allada V. Electrocardiographic and echocardiographic features that distinguish anomalous origin of the left coronary artery from pulmonary artery from idiopathic dilated cardiomyopathy. Pediatr Cardiol. 2001;22(1):3-10.
• Juliano N, Wong SC, Naidu SS. Alcohol septal ablation for failed surgical myectomy. J Invasive Cardiol. 2005;17(10):569-571.
• Cresswell, J., Asterling, S., Chaudhary, M., Sheikh, N., Greene, D. Third-generation cryotherapy for prostate cancer in the UK: a prospective study of the early outcomes in primary and recurrent disease. BJU Int 2006;97: 969-974.
• Brown CD, Zitelli JA. Choice of wound dressings and ointments. Otolaryngol Clin North Am 1995;28:1081-1091.