Chris Ghaemmaghami

• Associate Professor, Vice Chair of Academics, and Program Director,
• Department of Emergency Medicine, University of Virginia,
• Charlottesville, VA, USA

Calcitonin is important 16 mainly in children weight loss programs that work buy orlistat online pills, who have 10 times as many C cells as adults; it has little effect in adults (see section 7 weight loss 4 doctors select nutraceuticals orlistat 60 mg buy visa. Each is about 3 to 8 mm long and 2 to 5 mm wide weight loss games orlistat 120 mg buy low cost, and is separated from the thyroid follicles by a thin fibrous capsule and adipose tissue (fig weight loss yahoo discount orlistat 60 mg mastercard. Often weight loss in a month cheap 120 mg orlistat with amex, they occur in other locations ranging from as high as the hyoid bone to as low as the aortic arch, and about 5% of people have more than four parathyroids. Calcium homeostasis is so crucial to neuromuscular and cardiovascular function that a person can die within just a few days if the parathyroids are removed without instituting hormone replacement therapy. This often happened to patients shortly following thyroid surgery before surgeons realized the existence and function of the tiny, nearly hidden parathyroids. The adult adrenal gland measures about 5 cm vertically, 3 cm wide, and 1 cm from anterior to posterior. It weighs about 8 to 10 g in the newborn, but loses half of this weight by the age of 2 years, mainly because of involution of its outer layer, the adrenal cortex. Like the pituitary, the adrenal gland forms by the merger of two fetal glands with different origins and functions. Surrounding it is a much thicker adrenal cortex, constituting 80% to 90% of the gland and having a yellowish color due to its high concentration of cholesterol and other lipids. The name glomerulosa ("full of little balls") refers to the arrangement of its cells in round clusters. Here the cells are arranged in parallel cords (fascicles), separated by blood capillaries, perpendicular to the gland surface. The cells are called spongiocytes because of a foamy appearance imparted by an abundance of cytoplasmic lipid droplets. Like the preceding layer, the zona reticularis also secretes glucocorticoids and androgens. Aldosterone is the most significant mineralocorticoid, and is produced only by the zona glomerulosa. In brief, blood pressure sensors (baroreceptors) in major arteries near the heart detect falling blood pressure and activate a sympathetic reflex. Water is retained with it by osmosis, so aldosterone helps to maintain blood volume and pressure. Cortisol (also known clinically as hydrocortisone) is the most potent glucocorticoid, but the adrenals also secrete a weaker one called corticosterone. They stimulate fat and protein catabolism, gluconeogenesis, and the release of fatty acids and glucose into the blood. Glucocorticoids also have an anti-inflammatory effect; hydrocortisone is widely used in ointments to relieve swelling and other signs of inflammation. Excessive glucocorticoid secretion or medical use, however, suppresses the immune system for reasons we will see in the discussion of stress physiology later in this chapter. Androgens are the primary adrenal sex steroids, but the adrenals also produce small amounts of estrogen. It has little biological activity in 18 19 20 the Adrenal Medulla the adrenal medulla has a dual nature, acting as both an endocrine gland and a ganglion of the sympathetic nervous system (see section 15. Named for their tendency to stain brown with certain dyes, these cells are essentially sympathetic postganglionic neurons, but they have no dendrites or axon and they release their products into the bloodstream like any other endocrine gland. Upon stimulation by the nerve fibers-as in a situation of fear, pain, or other stress-the chromaffin cells release a mixture of catecholamines that we previously encountered as neurotransmitters (see fig. They increase alertness and prepare the body in several ways for physical activity. The liver boosts glucose levels by glycogenolysis (hydrolysis of glycogen to glucose) and gluconeogenesis (conversion of fats, amino acids, and other noncarbohydrates to glucose). It inhibits the secretion of insulin, so the muscles and other insulindependent organs absorb and consume less glucose. They fall back on alternative fuels such as fatty acids, while the blood glucose is left for use by the brain, which is more glucose-dependent but not insulin-dependent. Adrenal catecholamines also raise the heart rate and blood pressure, stimulate circulation to the muscles, increase pulmonary airflow, and raise the metabolic rate. At the same time, they inhibit such temporarily inessential functions as digestion and urine production so that they do not compete for blood flow and energy. It produces more than 25 steroid hormones, known collectively as the corticosteroids or corticoids. All of them are synthesized from cholesterol; this and other lipids impart a yellow color to the cortex. Only five corticosteroids are secreted in physiologically significant amounts; the others are either negligible in quantity or, if more abundant, are in chemically less active forms. At puberty in both sexes, androgens induce the growth of pubic and axillary hair and their associated apocrine sweat glands, and they stimulate the libido (sex drive) throughout adolescent and adult life. In men, the large amount of androgen secreted by the testes overshadows that produced by the adrenals. It is normally of minor importance to women of reproductive age because its quantity is small compared with estrogen from the ovaries. After menopause, however, the ovaries no longer function and only the adrenals secrete estrogen. However, several other tissues, such as fat, convert androgens into additional estrogen. Both androgens and estrogens promote adolescent skeletal growth and help to sustain adult bone mass. The medulla and cortex are not as functionally independent as once thought; each of them stimulates the other. When stress activates the sympathetic nervous system, these cells stimulate the cortex to secrete corticosterone and perhaps other corticosteroids. It is primarily an exocrine digestive gland, and its gross anatomy is described in section 25. Scattered throughout the exocrine tissue, however, are 1 to 2 million endocrine cell clusters called pancreatic islets (islets of Langerhans21). Although they are less than 2% of the pancreatic tissue, the islets secrete hormones of vital importance, especially in the regulation of glycemia, the blood glucose concentration. Islet cells respond directly to blood nutrient levels associated with the cycle of eating and fasting. Their functions are as follows: · Alpha cells, or glucagon cells, secrete glucagon between meals when the blood glucose concentration falls below 100 mg/dL. Pancreatic islets are most concentrated in the tail of c: Al Telser/McGraw-Hill Education the pancreas. These effects lead to the release of glucose into circulation, thus raising the blood glucose level. In adipose tissue, glucagon stimulates fat catabolism and the release of free fatty acids. Glucagon is also secreted in response to rising amino acid levels in the blood after a high-protein meal. It promotes amino acid absorption and thereby provides cells with the raw material for gluconeogenesis. Insulin, "the hormone of nutrient abundance," is secreted during and immediately following a meal when blood nutrient levels are rising; even the appetizing aroma of food stimulates insulin release in anticipation of eating. Osteocalcin and lipocalin 2, two hormones from the osteoblasts of bone, stimulate multiplication of beta cells, insulin secretion, and insulin sensitivity of other body tissues. The principal targets of insulin are the liver, skeletal muscles, and adipose tissue. In times of plenty, insulin stimulates cells to absorb glucose, fatty acids, and amino acids and to store or metabolize them; therefore, it lowers the level of blood glucose and other nutrients. It promotes the synthesis of glycogen, fat, and protein, thereby promoting the storage of excess nutrients for later use and enhancing cellular growth and differentiation. The brain, liver, kidneys, and red blood cells absorb and use glucose without need of insulin, but insulin promotes glycogen synthesis in the liver. Insulin insufficiency or inaction is well known as the cause of diabetes mellitus, detailed later in this chapter. Amylin helps to reduce spikes in blood glucose by slowing the emptying of the stomach; modulating the secretion of gastric enzymes, acid, and bile; inhibiting glucagon secretion; and stimulating the sense of satiety (having had enough to eat). Delta cells, or somatostatin cells, secrete somatostatin (growth hormone­inhibiting hormone) concurrently with the release of insulin by the beta cells. Somatostatin helps to regulate the speed of digestion and nutrient absorption, and perhaps modulates the activity of other pancreatic islet cells. Each egg develops in its own follicle, which is lined by a wall of granulosa cells and surrounded by a capsule, the theca (fig. Theca cells synthesize the androgen androstenedione, and granulosa cells convert this to estradiol and lesser amounts of two other estrogens, estriol and estrone. In the middle of the monthly ovarian cycle, a mature follicle ovulates (ruptures and releases the egg). The remains of the follicle become the corpus luteum, which secretes progesterone for the next 12 days or so in a typical cycle (several weeks in the event of pregnancy). In brief, they contribute to the development of the reproductive system and feminine physique, promote adolescent bone growth, regulate the menstrual cycle, sustain pregnancy, and prepare the mammary glands for lactation. Its endocrine secretions are testosterone, lesser amounts of weaker androgens and estrogens, and inhibin. Nestled between the tubules are clusters of interstitial endocrine cells, the source of testosterone and the other sex steroids (fig. Testosterone stimulates development of the male reproductive system in the fetus and adolescent, the development of the masculine physique in adolescence, and the sex drive. The liver and kidneys further convert cholecalciferol to a calcium-regulating hormone, calcitriol (see the following paragraphs). The liver is involved in the production of at least five hormones: (1) It converts the cholecalciferol from the skin into calcidiol, the next step in calcitriol synthesis. The liver is therefore important in regulating the oxygen-carrying capacity of the blood. You may have noticed that glucagon is not the only hormone that does so; so do growth hormone, epinephrine, norepinephrine, cortisol, and corticosterone. Their exocrine products are eggs and sperm, and their endocrine products are the gonadal hormones, most of which are steroids. The granulosa cells of the ovary and interstitial cells of the testis are endocrine cells. In anemia, hepcidin levels fall and more iron is mobilized to support hemoglobin synthesis. In infections, hepcidin levels rise, reducing the level of free iron available to infectious microorganisms that need it for their reproduction. Calcitriol raises the blood concentration of calcium by promoting its intestinal absorption and slightly inhibiting its loss in the urine. This is a very potent hormone that constricts blood vessels throughout the body and thereby raises blood pressure. Rising blood pressure stretches the heart wall and stimulates cardiac muscle in the atria to secrete two similar natriuretic22 peptides. They coordinate different regions and glands of the digestive system with each other and affect feeding, digestion, gastrointestinal motility and secretion, and maintenance of the mucosa. Some of them are called gut­brain peptides because they originate in the digestive tract but stimulate the brain. Gastrin26 is secreted by the stomach upon the arrival of food and stimulates other cells of the stomach to secrete hydrochloric acid. Gut­brain peptides and the endocrine regulation of hunger are further discussed in section 26. Fat cells secrete at least three hormones that regulate carbohydrate and fat metabolism. The best-known one is leptin, which has long-term effects on appetiteregulating centers of the hypothalamus. A low level of leptin, signifying a deficiency of body fat, increases appetite and food intake, whereas a high level of leptin tends to blunt the appetite. Leptin also serves as a signal for the onset of puberty, which is delayed in persons with abnormally low · · body fat. Leptin is treated with other enteric hormones in the aforementioned discussion of appetite. Both of these hormones stimulate pancreatic beta cells and promote insulin secretion and action. Osseous tissue thus plays an important role in glucose metabolism and the regulation of blood glucose levels. This organ performs many functions in pregnancy, including fetal nutrition, oxygenation, and waste removal. It includes numerous discrete glands as well as individual cells in the tissues of other organs. The endocrine organs and tissues other than the hypothalamus and pituitary are surveyed in table 17. Identify three endocrine glands that are larger or more functional in infants or children than in adults. Name a glucocorticoid, a mineralocorticoid, and a catecholamine secreted by the adrenal gland. Does the action of glucocorticoids more closely resemble that of glucagon or insulin Define hypoglycemic hormone and hyperglycemic hormone and give an example of each. The monoamine hormones include dopamine, epinephrine, norepinephrine, melatonin, and thyroid hormone.

Starting at the central sulcus weight loss green tea generic orlistat 120 mg with mastercard, it extends caudally to the parieto­occipital sulcus weight loss pills vegetarian 120 mg orlistat visa, visible on the medial surface of each hemisphere (see fig weight loss pills hydroxycut 60 mg orlistat order amex. It is concerned with taste weight loss xyngular orlistat 60 mg purchase line, somatic sensation (such as touch weight loss juice recipes purchase generic orlistat from india, heat, and pain), and visual processing; multisensory integration such as correlating sights and sounds to holistically compre hend our sensory world; spatial perception and awareness of body orientation; language processing; and numerical aware ness (a sense of the quantity of things we see before us). The occipital lobe is at the rear of the head, caudal to the parieto­occipital sulcus and underlying the occipital bone. It is the principal visual center of the brain, where we first become aware of visual stimuli and process them to identify what we see. The temporal lobe is a lateral, horizontal lobe deep to the temporal bone, separated from the frontal and parietal lobes above it by a deep lateral sulcus. It is concerned with hearing; smell; emotion; learning; language comprehension and memory of the grammar and vocabulary of the languages we speak; memory consolidation (formation of new longterm memories); and storage of verbal, visual, and auditory memories. The insula31 is a small mass of cortex deep to the lateral sulcus, made visible only by retracting or cutting away some of the overlying cerebrum (see fig. It plays roles in taste; pain; visceral sensation; consciousness; emo tional responses and empathy (sympathetic awareness of the feelings of others); and cardiovascular homeostasis (such as heart rate and blood pressure responses to exercise). What route can commissural tracts take between the right and left cerebral hemispheres other than the one shown here This is com posed of glia and myelinated nerve fibers that transmit signals from one region of the cerebrum to another and between the cere brum and lower brain centers. Projection tracts extend vertically between higher and lower brain and spinal cord centers. The corticospinal tracts, for example, carry motor signals from the cerebrum to the brainstem and spinal cord. Such tracts form a broad, dense sheet called the internal capsule between the thalamus and basal nuclei (described shortly), then radiate in a diverging, fanlike array (the corona radiata32) to specific areas of the cortex. The great majority of commissural tracts pass through the large corpus callosum (see fig. Commissural tracts enable the two sides of the cerebrum to communicate with each other. Long association fibers connect dif ferent lobes of a hemisphere to each other, whereas short association fibers connect different gyri within a single lobe. The Cerebral Cortex Neural integration is carried out in the gray matter of the cerebrum, which is found in three places: the cerebral cortex, basal nuclei, and limbic system. We begin with the cerebral cortex,33 a layer cover ing the surface of the hemispheres (see fig. Even though it is only 2 to 3 mm thick, the cortex constitutes about 40% of the mass of the brain and contains 14 to 16 billion neurons. Stellate cells have spheroidal neurosomas with short axons and dendrites projecting in all directions. They are con cerned largely with receiving sensory input and processing infor mation on a local level. Their apex points toward the brain surface and has a thick dendrite with many branches and small, knobby dendritic spines (see fig. The base gives rise to horizontally oriented dendrites and an axon that passes into the white matter below. Py ramidal cell axons have collaterals that synapse with other neurons in the cortex or in deeper regions of the brain. About 90% of the human cerebral cortex is a sixlayered tissue called neocortex34 because of its relatively recent evolutionary ori gin. The parallel paths of nerve fibers and the insulating effect of their myelin sheaths, however, give a directionality to diffusion, since more water diffuses along the nerve fibers than across them. The vectors of diffusion can be color-coded so tracts passing between the left and right brain are colored red, anterior­posterior tracts are green, and superior­inferior tracts are blue. The most often mapped tract has been the corticospinal tract, the main output pathway for cerebral control of the muscles; the tract of second greatest interest is called the arcuate fasciculus, which connects the Wernicke and Broca speech areas (see section 14. Its greatest use to date is the assessment of acute ischemic stroke, and the second greatest is diagnosis of brain tumors and their effects on movement, language, and vision. This enables the surgeon to precisely target a problem area and avoid injury to other critical tracts nearby. It has found clinical applications as well to Alzheimer disease, epilepsy, autism, and cocaine addiction. Clinicians hope that it will soon prove useful in the prognosis of diseases-predicting their developmental course to allow for earlier and more effective intervention. It is a ring of cortex on the medial side of each hemi sphere, encircling the corpus callosum and thalamus. There are still differences of opinion on what structures to con sider as parts of the limbic system, but these three are agreed upon. Other components include the mammillary bodies and other hypotha lamic nuclei, some thalamic nuclei, parts of the basal nuclei, and parts of the frontal lobe called prefrontal and orbitofrontal cortex. This ring of structures (shown in violet) includes important centers of learning and emotion. In the frontal lobe, there is no sharp rostral boundary to limbic system components. All of these structures are bi laterally paired; there is a limbic system in each cerebral hemisphere. The limbic system was long thought to be associated with smell because of its close association with olfactory pathways, but beginning in the early 1900s and continuing even now, experiments have abundantly demonstrated more significant roles in emotion and memory. Stimulation of a gratification center produces a sense of pleasure or reward; stimulation of an aversion center produces unpleasant sensations such as fear or sorrow. Gratification centers dominate some limbic structures, such as the nucleus accumbens (not illustrated), while aversion centers dominate others such as the amygdala. The roles of the amygdala in emotion and the hippocampus in memory are described in section 14. The Basal Nuclei the basal nuclei are masses of cerebral gray matter buried deep in the white matter, lateral to the thalamus (fig. The putamen and globus pallidus together are also called the lentiform43 nucleus, because they form a lensshaped body. They are involved in motor control and are further discussed in a later section on that topic. Name the five lobes of the cerebrum and describe their locations, boundaries, and principal functions. Distinguish between commissural, association, and projection tracts of the cerebrum. It is impossible in many cases to assign these functions to one specific brain region; as we have already seen (see fig. Some functions overlap anatomically and some cross anatomical boundaries from one region to another, often remote region. Thus, we will consider these as integrative functions of the brain, focusing especially on the cerebrum but in many cases in volving the combined action of multiple levels of the brain. Some of these present the most difficult challenges for neurobiology, but they are the most intriguing functions of the brain and involve its largest areas. The complete and persistent absence of brain waves is a common clinical and legal criterion of brain death. Alpha waves have a frequency of 8 to 13 Hz and are recorded especially in the parietooccipital area. They are suppressed when a person opens the eyes, receives specific sensory stimulation, or engages in a mental task such as performing mathematical calculations. Beta waves have a frequency of 14 to 30 Hz and occur in the frontal to parietal region. They are normal in children and in drowsy or sleeping adults, but a predominance of theta waves in awake adults suggests emotional stress or brain disorders. This section concerns such "higher" brain functions as sleep, memory, cognition, emotion, sensation, motor control, and lan guage. Sleep superficially resembles other states of prolonged unconsciousness such as coma and animal hibernation, except that individuals cannot be aroused from those states by sensory stimulation. The muscles relax, and the vital signs (body temperature, blood pressure, and heart and respiratory rates) fall. The muscles are now very relaxed, vital signs are at their lowest levels, and one becomes difficult to awaken. They occasion ally show 1 or 2 seconds of sleep spindles, high spikes resulting from interactions between neurons of the thalamus and cerebral cortex. This is moderate to deep sleep, typically beginning about 20 minutes after stage 1. This is so named because the eyes oscillate back and forth as if watching a movie. Humans are a diurnal (daytimeactive) species in whom fall ing light intensity induces sleepiness. It regulates not only sleep but also circadian rhythms of body temperature, urine production, hormone secre tion, and other functions. These stimulate the reticular activating system, which then stimulates the thalamus. As a person begins to wake, electrical activity increases in the thalamus and then spreads through the cerebral cortex. Thus, the cerebral awareness of environmental stimuli increases and the sleeper wakes up. Narcolepsy seems to be an autoimmune disease caused by antibodymediated destruction of the orexinproducing neurons. A few examples of the cognitive effects of cerebral lesions reveal some functions of the association areas: · Parietal lobe lesions can cause people to become unaware We spend about onethird of our lives asleep, and we might well regret such a terrible "waste of time. Sleep is associated with restorative anabolic processes in the immune, nervous, endocrine, and other systems. Theta wave activity and the sleep spindles of stage 2 sleep are also associated with memory consolidation. Sleep research subjects who have just learned a new task show a higher density of sleep spindles than controls who have not. In typical cases, men shave only half of the face, women apply makeup to only one side, patients dress only half of the body, and some people deny that one arm or leg belongs to them. Such patients are unable to find their way around- say, to describe the route from home to work or navigate within a familiar building. In prosopagnosia,48 a person cannot remember familiar faces, even his or her own reflection in a mirror. Frontal lobe lesions are especially devastating to the quali ties we think of as personality. The frontal lobe integrates information from sensory and motor regions of the cortex and from other association areas. It gives us a sense of our relationship to the rest of the world, enabling us to think about it and to plan and execute appropriate behavior. Lesions here may produce profound personality disorders and socially inappropriate behaviors. We studied its forms and its neural and molecular mech anisms earlier (see section 12. Now that you have been intro duced to the gross anatomy of the brain, we can consider where those processes occur anatomically. Infor mation management by the brain entails learning (acquiring new information), memory proper (information storage and retrieval), and forgetting (eliminating trivial information). Braininjured peo ple are sometimes unable to recall things they once knew (retrograde amnesia) or unable to store new information (anterograde amnesia). The hippocampus of the limbic system is an important memoryforming center (see fig. The hippocampus learns from sensory input while an experience is happening, but it has a short memory. Later, especially during sleep, it plays this memory repeatedly to the cerebral cortex, which is a "slow learner" but forms longer lasting memories. This process of "teaching the cerebral cor tex" until a longterm memory is established is called memory consolidation. Such functions are widely distributed over regions of cerebral cor tex called association areas, which constitute about 75% of all brain tissue. This is the most difficult area of brain research and the most incompletely understood aspect of cerebral function. Much of what we know about it has come from studies of pa tients with brain lesions-areas of tissue destruction resulting from cancer, stroke, and trauma. The operation had no adverse effect on his intelligence or explicit memory for things that had happened early in his life, but it left him with an inability to establish new explicit memories. He could hold a conversation with his psychologist, but a few minutes later deny that it had taken place. He was nevertheless able to learn new motor skills, thus showing explicit and implicit memory to involve sep arate brain regions. Other parts of the brain involved in memory include the cer ebellum, with a role in learning motor skills, and the amygdala, with a role in emotional memory. Emotional control centers of the brain have been identified by studying people with brain lesions and by such techniques as surgical removal, ablation (destruction) of small regions with electrodes, and stimulation with electrodes and chemical implants, especially in experimental animals. Changes in behavior following such procedures give clues to the functions that a region performs. However, interpretation of the results is diffi cult and controversial because of the complex connections between the emotional brain and other regions. The prefrontal cortex (frontal association area) is the most rostral part of the frontal lobe, just behind the forehead. It is the seat of judgment, intent, and control over the expression of our emotions. However we may feel, it is here that we decide the appropriate way to show those feelings.

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It takes one type of cell and neural circuit to provide sensitive night vision and a different type to provide highresolution daytime vision weight loss on whole30 purchase orlistat 120 mg online. The high sensitivity of rods in dim light stems partly from the extensive neural convergence that occurs between the rods and ganglion cells weight loss pills backed by science discount orlistat 60 mg free shipping. Up to 600 rods converge on each bipolar cell weight loss pills or powder orlistat 120 mg order with mastercard, and many bipolar cells converge on each ganglion cell weight loss kidney disease 120 mg orlistat purchase. Weak stimulation of many rods can produce an additive effect on one bipolar cell rapid 60 weight loss pills orlistat 120 mg buy on-line, and several bipolar cells can collaborate to excite one ganglion cell. Thus, a ganglion cell can respond in dim light that only weakly stimulates any individual rod. Scotopic vision is functional even at a light intensity less than starlight reflected from a sheet of white paper. One ganglion cell receives input from all the rods in about 1 mm2 of retina-its receptive field. What the brain perceives is therefore a coarse, grainy image similar to an overenlarged photograph or low-resolution digital image. Around the edges of the retina, receptor cells are especially large and widely spaced. Our peripheral vision is a low-resolution system that serves mainly to alert us to motion in the periphery and to stimulate us to look that way to identify what is there. When you look directly at something, its image falls on the fovea, which is occupied by about 4,000 tiny cones and no rods. Each cone synapses with only one bipolar cell and each bipolar cell with only one ganglion cell. This gives each foveal cone a "private line to the brain," and each ganglion cell of the fovea reports to the brain on a receptive field of just 2 µm2 of retinal area (fig. Cones distant from the fovea exhibit some neural convergence but not nearly as much as rods do. The price of this lack of convergence at the fovea, however, is that cone cells are incapable of spatial summation, and the cone system is therefore less sensitive to light. The threshold of photopic (cone) vision lies between the intensity of starlight and moonlight reflected from white paper. Color vision is especially well developed in primates for evolutionary reasons discussed in chapter 1 (see section 1. These were formerly called blue, green, and red cones-a less accurate terminology. Our perception of colors is based on a mixture of nerve signals representing cones with different absorption peaks. At 500 nm, all three types of cones are stimulated; the L cones respond at 60% of their maximum capacity, M cones at 82% of their maximum, and S cones at 20%. The table in this figure shows how some other color sensations are generated by other response ratios. Some individuals have a hereditary alteration or lack of one photopsin or another and thus exhibit color blindness. The most common form is red­green color blindness, which results from a lack of either L or M cones and causes difficulty distinguishing these and related shades from each other. It depends on having two eyes with overlapping visual fields, which allows each eye to look at the same object from a different angle. Stereoscopic vision contrasts with the panoramic vision of mammals such as rodents and horses, in which the eyes are on opposite sides of the head. Although stereoscopic vision covers a smaller visual field than panoramic vision and provides less alertness to sneaky predators, it has the advantage of depth perception. The evolutionary basis of our stereoscopic vision was also explained along with color vision in section 1. When you fixate on something within 30 m (100 ft), each eye views it from a slightly different angle and focuses its image on the fovea centralis. Objects farther away than the fixation point cast an image somewhat medial to the foveas, and closer objects cast their images more laterally (fig. The distance of an image from the two foveas provides the brain with information used to judge the position of other points relative to the fixation point. Persons Wavelength (nm) 400 450 500 550 625 675 Percentage of maximum cone response (S:M:L) Perceived hue 50: 0: 0 72: 30: 0 20: 82: 60 0: 85: 97 0: 3: 35 0: 0: 5 Violet Blue Blue-green Green Orange Red with normal vision see the number 74. In the middle column of the table, each number indicates how strongly the respective cone cells respond as a percentage of their maximum capability for a given light intensity. At 550 nm, for example, L cones respond at 97% of their maximum, M cones at 85%, and S cones not at all. If you were to add another row to this table for 600 nm, what would you enter in the middle and right-hand columns Within the chiasm, half of the fibers from each optic nerve cross over to the opposite side of the brain (fig. As a result, the right cerebral hemisphere sees objects in the left visual field, because their images fall on the right half of each retina (the medial half of the left eye and lateral half of the right eye). You can trace the nerve fibers from each half-retina in the figure to see that they lead to the right hemisphere. Since the right brain controls motor responses on the left side of the body and vice versa, each side of the brain sees what is on the side of the body where it exerts motor control. In horses and other animals with panoramic vision, nearly 100% of the optic nerve fibers of the right eye decussate to the left brain and vice versa. When the eyes converge on the fixation point (F), more distant objects (D) are focused on the retinas medial to the fovea and the brain interprets them as being farther away than the fixation point. Nearby objects (N) are focused lateral to the fovea and interpreted as being closer. Blue and yellow indicate the visual fields of the left and right eyes; green indicates the area of overlap and stereoscopic vision. Nerve fibers from the medial side of the right eye (red) descussate to the left side of the brain, while fibers from the lateral side remain on the right side of the brain. The right occipital lobe thus monitors the left side of the visual field and the left occipital lobe monitors the right side. Lesions along these tracts or in the occipital lobe can cause blindness even if the eyes are fully functional, with various effects on one of both eyes, and part or all of the visual field of either eye, depending on where the lesion occurs. A few optic nerve fibers come from the photosensitive, melanopsin-containing ganglion cells and take a different route, ending in the superior colliculi and pretectal nuclei of the midbrain. The superior colliculi control the visual reflexes of the extrinsic eye muscles, and the pretectal nuclei are involved in the photopupillary and accommodation reflexes. Some processing, such as contrast, brightness, motion, and stereopsis, begins in the retina. The primary visual cortex in the occipital lobe is connected by association tracts to association areas in the temporal and parietal lobes. The primary visual cortex relays information forward to association areas in the temporal and parietal lobes for further processing. One pathway, called the ventral stream, runs forward through the lower temporal lobe. It is concerned with color vision, object recognition, and visual memory, including our ability to read, recognize faces, and identify other things we see. The other, called the dorsal stream, runs forward through the upper parietal lobe. It is concerned with recognizing the locations and spatial relationships of objects and with analysis of motion. What is yet to be learned about visual processing has important implications for biology, medicine, psychology, and even philosophy. The effects of aging on the senses-especially taste, smell, hearing, and vision-are described at "Sense Organs" in section 29. List as many structural and functional differences between rods and cones as you can. Explain how the absorption of a photon of light leads to excitation of an optic nerve fiber. Discuss the duplicity theory of vision, summarizing the advantage of having separate types of retinal photoreceptor cells and neural circuits for photopic and scotopic vision. People from the Stone Age to the preColumbian civilizations of the Americas practiced trephination-cutting a hole in the skull to let out "evil spirits" that were thought to cause headaches. The ancient Hindus were expert surgeons for their time, and the Greeks and Romans pioneered military surgery. Surgeons rarely attempted anything more complex than amputations or kidney stone removal. A surgeon had to be somewhat indifferent to the struggles and screams of his patient. Most operations had to be completed in 3 minutes or less, and a strong arm and stomach were more important qualifications for a surgeon than extensive anatomical knowledge. At least three things were needed for surgery to be more effective: better knowledge of anatomy, asepsis61 for the control of infection, and anesthesia62 for the control of pain. Early efforts to control surgical pain were crude and usually ineffective, such as choking a patient into unconsciousness and trying to complete the surgery before he or she awoke. Alcohol and opium were often used as anesthetics, but the dosage was poorly controlled; some patients were underanesthetized and suffered great pain anyway, and others died of overdoses. Often there was no alternative but for a few strong men to hold the struggling patient down as the surgeon worked. Charles Darwin originally intended to become a physician, but left medical school because he was sickened by observing "two very bad operations, one on a child," in the days before anesthesia. In 1799, the English chemist Sir Humphry Davy suggested using nitrous oxide to relieve pain. Nitrous oxide ("laughing gas") was a popular amusement in the 1800s, when traveling showmen went from town to town demonstrating its effects on volunteers from the audience. Ether was commonly used in small oral doses for toothaches and "nervous ailments," but its main claim to popularity was its use as a party drug for so-called ether frolics. Long himself was a bit of a bon vivant who put on demonstrations for some of the young ladies, with the disclaimer that he could not be held responsible for whatever he might do under the influence of ether (such as stealing a kiss). At these parties, Long noted that people sometimes fell and suffered considerable cuts and bruises without feeling pain. In 1842, he had a patient who was terrified of pain but needed a tumor removed from his neck. Long excised the tumor without difficulty as his patient sniffed ether from a towel. The operation created a sensation in town, but other physicians ridiculed Long and pronounced anesthesia dangerous. His medical practice declined as people grew afraid of him, but over the next 4 years he performed eight more minor surgeries on patients under ether. Struggling to overcome criticisms that the effects he saw were due merely to hypnotic suggestion or individual variation in pain sensitivity, Long even compared surgeries done on the same person with and without ether. Long failed to publish his results quickly enough, and in 1844 he was scooped by a Connecticut dentist, Horace Wells, who had tried nitrous oxide as a dental anesthetic. Another dentist, William Morton of Boston, had tried everything from champagne to opium to kill pain in his patients. He too became interested in ether and gave a demonstration at Massachusetts General Hospital, where he etherized a patient and removed a tumor before a medical audience. Within a month of this successful and sensational demonstration, ether was being used in other cities of the United States and England. Wells, who had engaged in a bitter feud to establish himself as the inventor of ether anesthesia, committed suicide at the age of 33. Crawford Long ran a successful medical practice in Athens, Georgia, but to his death he remained disappointed that he had not received credit as the first to perform surgery on etherized patients. General anesthetics such as isoflurane render a patient unconscious by crossing the blood­brain barrier and blocking nervous transmission through the brainstem. Local anesthetics such as procaine (Novocaine) and tetracaine selectively deaden specific nerves. They decrease the permeability of membranes to Na+, thereby reducing their ability to produce action potentials. A sound knowledge of anatomy, control of infection and pain, and development of better tools converged to allow surgeons time to operate more carefully. It attracted a more educated class of practitioner, which put it on the road to becoming the remarkable lifesaving approach that it is today. Names of some analgesic neuropeptides and how they affect the sensation of pain 7. The bony and membranous labyrinths of the inner ear; the names and distribution of the two inner-ear fluids in relation to the labyrinths 8. Structure of the spiral organ, especially the hair cells and tectorial membrane; differences between inner and outer hair cells 11. How vibrations of the tympanic membrane lead to stimulation of the cochlear nerve 13. How the outer hair cells tune the cochlea to improve its sensitivity to differences in pitch 15. The pathway from cochlear nerve to auditory centers of the brain; the feedback pathway from the pons back to the cochlea, and its purpose 16. Differences between static and dynamic equilibrium and between linear and angular acceleration 17. Structure of the saccule and utricle and the relevance of the spatial orientation of the macula in each one 18. How linear acceleration stimulates the hair cells of the saccule and utricle during linear acceleration; how the body senses the difference between vertical and horizontal acceleration 19. Structure of the semicircular ducts, especially the crista ampullaris and cupula 20.

The eyelids are separated from each other by the palpebral fissure and meet each other at the corners called the medial and lateral commissures weight loss 64055 cheap orlistat 60 mg buy online. The eyelid consists largely of the orbicularis oculi muscle covered with skin (fig weight loss pills garcinia cambogia dr oz purchase 60 mg orlistat with amex. It also contains a supportive fibrous tarsal plate that is thickened along the margin of the eyelid weight loss using apple cider vinegar cheap orlistat 60 mg with mastercard. Within the plate are 20 to 25 tarsal glands that open along the edge of the eyelid weight loss 21 day fix 60 mg orlistat purchase with amex. They also disrupt airflow across the eye surface weight loss pills like adderall buy orlistat without prescription, thus protecting the eyes from drying. It is also very vascular, which is especially evident when the vessels are dilated and the eyes are "bloodshot. The lacrimal gland, about the size and shape of an almond, is nestled in a shallow fossa of the frontal bone in the superolateral corner of the orbit. The arrows indicate the flow of tears from the lacrimal gland, across the front of the eye, into the lacrimal sac, and down the nasolacrimal duct. After washing across the eye, tears collect near the medial commissure and flow into a tiny pore, the lacrimal punctum,38 on the margin of each eyelid. The punctum opens into a short lacrimal canaliculus, which leads to the lacrimal sac in the medial wall of the orbit. From this sac, a nasolacrimal duct carries the tears to the inferior meatus of the nasal cavity; thus an abundance of tears from crying or watery eyes can result in a runny nose. Once the tears enter the nasal cavity, they normally flow back to the throat and are swallowed. When you have a cold, the nasolacrimal ducts become swollen and obstructed, the tears cannot drain, and they may overflow from the brim of the eye. Six extrinsic eye muscles attach to the walls of the orbit and the external surface of the eyeball. Extrinsic means "arising externally"; it distinguishes these from the intrinsic muscles inside the eye that control the lens and pupil. The superior, inferior, medial, and lateral rectus originate from a shared tendinous ring on the posterior wall of the orbit and insert on the anterior region of the eyeball, just beyond the visible "white of the eye. The inferior oblique extends from the · medial wall of the orbit to the inferolateral aspect of the eye. The oblique muscles slightly rotate the eyes when you tilt your head toward either shoulder, keeping the visual axes of the eyes aligned. People with weakness in the oblique muscles have to tilt their heads to fixate on objects; with the head erect, they have double vision. The oblique muscles also pull the eye forward, opposing tension in the rectus muscles that would otherwise pull the eye back more deeply into the orbit. It cushions the eye, allows it to move freely, and protects blood vessels and nerves in the rear of the orbit. The Tunics the three tunics of the eyeball are as follows: · the outer fibrous layer. It serves as a tough fibrous protective cover for the eye and provides for attachment of the extrinsic muscles that move it. The cornea is the anterior transparent region of modified sclera that admits light into the eye. Most of it is composed of very compact layers of collagen fibrils and thin flat fibroblasts. It is covered by a thin stratified squamous epithelium anteriorly and a simple squamous epithelium posteriorly. Water follows by osmosis, so this mechanism prevents the cornea from overhydrating, swelling, and losing transparency. The anterior epithelium also is a source of stem cells that give the cornea a great capacity for regeneration if it is injured. Its dense network of blood vessels provides all oxygenation, nourishment, and waste-removal services to the retina, which has no blood vessels of its own. The ciliary body, a thickened extension of the choroid, forms a muscular ring around the lens. The iris is an adjustable diaphragm that controls the diameter of the pupil, its central opening. If the melanin is scanty, light reflects from the posterior pigment epithelium and gives the iris a blue, green, or gray color. The Optical Components the optical components of the eye are transparent elements that admit light rays, bend (refract) them, and focus images on the retina. Normally the rate of reabsorption balances the rate of secretion (see Deeper Insight 16. Blue arrows indicate the flow of aqueous humor from the ciliary processes into the posterior chamber; through the pupil into the anterior chamber; and finally into the scleral venous sinus, the vein that reabsorbs the fluid. It is suspended behind the pupil by a ring of fibers called the suspensory ligament (fig. The vitreous43 body is a transparent jelly that fills a space called the vitreous chamber behind the lens. The vitreous body maintains intraocular pressure within the eye, which supports its spherical shape and enables the extrinsic muscles to pull the eye in different directions without compressing and distorting it. It also keeps the retina smoothly pressed against the wall of the eye, which keeps it in close contact with its blood supply and is essential for focusing images on the retina. An oblique channel through this body called the hyaloid canal is the remnant of an artery present in the embryo (see fig. The retina forms from a cup-shaped outgrowth of the diencephalon called the optic vesicle (see fig. It is a thin transparent membrane attached to the rest of the eye at only two points: the optic disc, where the optic nerve leaves the rear (fundus) of the eye, and its scalloped anterior margin, the ora serrata. They occur as the lens fibers darken with age, fluid-filled clefts appear between them, and the clefts accumulate debris from degenerating fibers. Cataracts are a common complication of diabetes mellitus, but can also be induced by heavy smoking, ultraviolet radiation, radiation therapy, certain viruses and drugs, and other causes. They cause the vision to appear milky or as if one was looking from behind a waterfall. The implanted lens improves vision almost immediately, but glasses still may be needed for near vision. Pressure in the anterior and posterior chambers drives the lens back and puts pressure on the vitreous body. The vitreous body presses the retina against the choroid and compresses the blood vessels that nourish the retina. Without a good blood supply, retinal cells die and the optic nerve may atrophy, producing blindness. Glaucoma can be detected early in the course of regular eye examinations by measurements of intraocular pressure and visual field. Macular degeneration is the death of receptor cells in the macula, the central part of the retina and location of the sharpest vision. It can develop so slowly that one fails to notice a change in vision, but eventually it results in loss of vision in the center of the visual field, often making it difficult or impossible to read, drive a car, or do fine daily tasks. It cannot yet be cured, but early detection can allow for treatments to slow its progression. It is a retinal degeneration caused by the effects of diabetes mellitus on the blood vessels that nourish the retina. With early detection and control of diabetes, blindness can be prevented in 90% of cases. Posterior view of the lens and the suspensory ligament that anchors it to the ciliary body. It leads to blindness if the retina remains separated for too long from the blood supply in the choroid. The retina is examined with an illuminating and magnifying instrument called an ophthalmoscope (fig. Directly posterior to the center of the lens, on the visual axis of the eye, is a patch of cells called the macula lutea47 about 3 mm in diameter. In the center of the macula is a tiny pit, the fovea48 centralis, which produces the most finely detailed images for reasons explained later. Nerve fibers from all regions of the retina converge on this point and leave here in a bundle that constitutes the optic nerve. Blood vessels travel through the core of the optic nerve and enter and leave the eye at the optic disc. Eye examinations serve for more than evaluating the visual system; they allow for a direct, noninvasive examination of blood vessels for signs of hypertension, diabetes mellitus, atherosclerosis, and other vascular diseases. The optic disc contains no receptor cells, so it produces a blind spot in the visual field of each eye. You can detect your blind spot and observe an interesting visual phenomenon with the help of figure 16. Close or cover your right eye and hold the image about 30 cm (1 ft) from your face. Without taking your gaze off the X, move your head or the page slightly forward and back, or right and left, until the red dot disappears. This occurs because the image of the dot is falling on the blind spot of your left eye. You should notice something else happen at the same time as the dot disappears-a phenomenon called visual filling. Note the blood vessels diverging from the optic disc, where they enter the eye with the optic nerve. Some adjustment of the distance from your face may be needed depending on the print or electronic medium in which you view the image. When fully dilated, the pupil admits five times as much light as it does when fully constricted. Its diameter is controlled by two sets of contractile elements in the iris: (1) the pupillary constrictor consists of smooth muscle cells that encircle the pupil. When stimulated by the parasympathetic nervous system, it narrows the pupil and admits less light to the eye. When stimulated by the sympathetic nervous system, these cells contract, widen the pupil, and admit more light to the eye (see fig. Constriction in response to a shift in gaze is part of the near response described shortly. When light intensity rises, signals are transmitted from the eye to the pretectal region of the upper midbrain. Preganglionic parasympathetic fibers travel by way of the oculomotor nerve from here to the ciliary ganglion in the orbit. From the ganglion, postganglionic fibers continue into the eye, where they stimulate the pupillary constrictor. Sympathetic innervation to the pupil originates, like all other sympathetic efferent fibers, in the spinal cord. Preganglionic fibers ascend from the thoracic cord to the superior cervical ganglion. From there, postganglionic fibers follow the carotid arteries into the head and lead ultimately to the pupillary dilator. The greater the difference between the refractive indices of two media, the more strongly light rays are refracted when passing from one to the next. The lens merely fine-tunes the image, especially as you shift your focus between near and distant objects. If the gaze shifts to something closer, light rays from the source are too divergent to be focused without effort. In other words, the eye is automatically focused on things in the distance unless you make an effort to focus elsewhere. For a wild animal or our prehistoric ancestors, this arrangement would be adaptive because it allows for alertness to predators or prey at a distance. This convergence of the eyes orients the visual axis of each eye toward the object in order to focus its image on each fovea. If the eyes cannot converge accurately-for example, when the extrinsic muscles are weaker in one eye than in the other-double vision, or diplopia,50 results. The images 49 50 Refraction Image formation depends on refraction, the bending of light rays. Light travels at a speed of 300,000 km/s in a vacuum, but it slows down slightly in air, water, glass, and other media. The refractive index of a medium (n) is a measure of how much it retards light rays relative to air. If it strikes at any other angle, however, the light ray changes direction-it is refracted (fig. The greater the difference in refractive index between the two media, and the greater the angle of incidence, the stronger the refraction is. Light rays striking the very center of the cornea pass straight through, but because of the curvature of the cornea, rays striking off center are bent toward the center (fig. You can simulate this effect by pressing gently on the corner of one eyelid as you look at this text; the image of the text will fall on noncorresponding regions of the two eyes and cause you to see double. Lenses cannot refract light rays at their edges as well as they can closer to the center. The image produced by any lens is therefore somewhat blurry around the edges; this spherical aberration is quite evident in an inexpensive microscope. It can be minimized by screening out the peripheral light rays and looking only at the better-focused center. In the eye, the pupil serves this purpose by constricting as you focus on nearby objects. Like the diaphragm setting (f-stop) of a camera, the pupil thus has a dual purpose: to adjust the eye to variations in brightness and to reduce spherical aberration. Accommodation is a change in the curvature of the lens that enables you to focus on a nearby object.

But healthy conducting (elastic) arteries expand with each systole weight loss 6 weeks 120 mg orlistat order free shipping, absorb some of the force of the ejected blood weight-loss supplement zantrex-3 60 mg orlistat visa, and store potential energy weight loss in dogs order line orlistat. Then weight loss pills that are fda approved order orlistat online, when the heart is in diastole weight loss pills zoloft cheap 60 mg orlistat otc, their elastic recoil releases that as kinetic energy, exerts pressure on the blood, and maintains blood flow throughout the cardiac cycle. The elastic arteries thus smooth out pressure fluctuations and reduce stress on the smaller arteries. In the aorta, blood rushes forward at 120 cm/s during systole and has an average speed of 40 cm/s over the cardiac cycle. When measured farther away from the heart, systolic and diastolic pressures are lower and there is less difference between them (fig. In capillaries and veins, the blood flows at a steady speed with little if any pulsation because the pressure surges have been damped out by the distance traveled and the elasticity of the arteries. In the inferior vena cava near the heart, however, venous flow fluctuates with the respiratory cycle for reasons explained later, and there is some fluctuation in the jugular veins of the neck. As we get older, our arteries become less distensible and absorb less systolic force. This increasing stiffness of the arteries is called arteriosclerosis6 ("hardening of the arteries"). The primary cause of it is cumulative damage by free radicals, which cause gradual deterioration of the elastic and other tissues of the arterial walls- much like old rubber bands that become less stretchy. Another contributing factor is atherosclerosis, the growth of lipid deposits in the arterial walls (see Deeper Insight 19. These deposits can become calcified complicated plaques, giving the arteries a hard, crunchy or bonelike consistency. Common blood pressures at the age of 20 are about 123/76 for males and 116/72 for females. For healthy persons at age 70, typical blood pressures are around 145/82 and 159/85 for the two sexes, respectively. It may be a consequence of blood loss, dehydration, anemia, or other factors and is normal in people approaching death. Blood pressure is physiologically determined by three principal variables: cardiac output, blood volume, and resistance to flow. Blood volume is regulated mainly by the kidneys, which have a greater influence than any other organ on blood pressure (assuming there is a beating heart). Moving blood would exert no pressure against a vessel wall unless it encountered at least some downstream resistance. Thus, pressure and resistance are not independent variables in blood flow-rather, pressure is affected by resistance, and flow is affected by both. Resistance, in turn, hinges on three variables that we consider now: blood viscosity, vessel length, and vessel radius. Increasing distance from left ventricle Blood Viscosity the viscosity of blood stems mainly from its plasma proteins (albumin) and erythrocytes (see section 18. A deficiency of erythrocytes (anemia) or albumin (hypoproteinemia) reduces viscosity and speeds up blood flow. On the other hand, viscosity increases and flow declines in such conditions as polycythemia and dehydration. Because of arterial elasticity and the effect of friction against the vessel wall, all measures of blood pressure decline with distance-systolic pressure, diastolic pressure, pulse pressure, and mean arterial pressure. There is no pulse pressure beyond the arterioles, but there are slight pressure oscillations in the venae cavae caused by the respiratory pump described later in this chapter. Partly for this reason, if you were to measure mean arterial pressure in a reclining person, you would obtain a higher value in the arm, for example, than in the ankle. In a reclining person, a strong pulse in the dorsal artery of the foot is a good sign of adequate cardiac output. If perfusion is good at that distance from the heart, it is likely to be good elsewhere in the systemic circulation. In a healthy individual, the only significant ways of controlling peripheral resistance from moment to moment are vasoconstriction, the narrowing of a vessel, and vasodilation, the widening of a vessel. Vasodilation, however, is brought about not by any muscular effort to widen a vessel, but rather by muscular passivity-relaxation of the smooth muscle, allowing blood pressure to expand the vessel. Vasomotion is controlled in part by a nucleus in the medulla oblongata of the brain called the vasomotor center. The effect of vessel radius on blood flow stems from the friction of the moving blood against the vessel walls. That is, it flows in layers- faster near the center of a vessel, where it encounters less friction, and slower near the walls, where it drags against the vessel. The current may be very swift in the middle of a river but quite sluggish near shore, where the water encounters more friction against the riverbank and bottom. When a blood vessel dilates, a greater portion of the blood is in the middle of the stream and the average flow may be quite swift. When the vessel constricts, more of the blood is close to the wall and the average flow is slower (fig. Indeed, flow (F) is proportional not merely to vessel radius (r) but to the fourth power of radius-that is, F r4. For the sake of simplicity, consider a hypothetical blood vessel with a 1 mm radius when maximally constricted and a 3 mm radius when completely dilated. By the formula F r4, consider how the flow would change as radius changed: if r = 1 mm, then r4 = 14 = 1, and F = 1 mL/min. Blood flows more slowly near the vessel wall, as indicated by shorter arrows, than it does near the center of the vessel. Each arrow can be construed as the distance that a hypothetical blood cell would travel in a given amount of time, varying with its distance from the vessel wall. Flow is fastest in the aorta because it is a large vessel close to the pressure source, the left ventricle. From aorta to capillaries, velocity diminishes for three reasons: (1) the blood has traveled a greater distance, so friction has slowed it down. The aorta has a cross-sectional area of 3 to 5 cm2, whereas the total cross-sectional area of all the capillaries is about 4,500 to 6,000 cm2. Thus, a given volume of aortic blood is distributed over a greater total area in the capillaries, which collectively form a wider path in the bloodstream. Just as water slows down when a narrow mountain stream flows into a lake, blood slows down as it enters pathways with a greater total area or volume. Note, however, that blood in the veins never regains the velocity it had in the large arteries. This is because the veins are farther from the pressure head (the heart) and because they are more compliant than arteries-they stretch to accommodate more blood, and this reduces pressure and flow. Arterioles are the most significant point of control over peripheral resistance and blood flow because (1) they are on the proximal sides of the capillary beds, so they are best positioned to regulate flow into the capillaries and thus regulate perfusion of the organs; (2) they greatly outnumber any other class of arteries and thus provide the most numerous control points; and (3) they are more muscular in proportion to their diameters than any other class of blood vessels and are highly capable of changing radius. Arterioles alone account for about half of the total peripheral resistance of the circulatory system. However, larger arteries and veins also influence peripheral resistance through their own constriction and dilation. There are three ways of controlling vasomotor activity: local, neural, and hormonal mechanisms. A single drop of epinephrine applied here caused the arteriole to constrict to about one-third of its dilated diameter. Local Control Autoregulation is the ability of tissues to regulate their own blood supply. As the bloodstream delivers oxygen and carries away the metabolites, the vessels reconstrict. In addition, platelets, endothelial cells, and the perivascular tissues secrete a variety of vasoactive chemicals that stimulate vasodilation under such conditions as trauma, inflammation, and exercise. The drag of blood flowing against the endothelial cells creates a shear stress (like rubbing your palms together) that stimulates them to secrete prostacyclin and nitric oxide, which are vasodilators. Reactive hyperemia is seen when the skin flushes (reddens) after a person comes in from the cold. It also occurs in the forearm if a blood pressure cuff is inflated for too long and then loosened. Over a longer time, a hypoxic tissue can increase its own perfusion by angiogenesis8-the growth of new blood vessels. One reason for this is that the veins are larger than the capillaries, so they create less resistance. There is great clinical importance in determining how growth factors and inhibitors control angiogenesis. Malignant tumors secrete growth factors that stimulate a dense network of vessels to grow into them and provide nourishment to the cancer cells. Elevated blood pressure Reduced blood pressure Vasodilation Arteries stretched Reduced heart rate Reduced vasomotor tone Baroreceptors increase firing rate Increased vagal tone Neural Control In addition to local control, the blood vessels are under remote control by the central and autonomic nervous systems. The vasomotor center of the medulla oblongata exerts sympathetic control over blood vessels throughout the body. The role of sympathetic and vasomotor tone in controlling vessel diameter is explained in section 15. The vasomotor center is an integrating center for three autonomic reflexes-baroreflexes, chemoreflexes, and the medullary ischemic reflex. A baroreflex9 is a negative feedback response to changes in blood pressure (see fig. Glossopharyngeal nerve fibers from these sinuses transmit signals continually to the brainstem. This inhibits the sympathetic cardiac and vasomotor neurons and reduces sympathetic tone, and it excites the vagal fibers to the heart. Thus, it reduces the heart rate and cardiac output, dilates the arteries and veins, and reduces blood pressure (fig. This occurs because gravity draws the blood into the large veins of the abdomen and lower limbs when you stand, which reduces venous return to the heart and cardiac output to the brain. Normally, the baroreceptors respond quickly to this drop in pressure and restore cerebral perfusion (see fig. It is initiated by the chemoreceptors called aortic bodies and carotid bodies described earlier. The primary role of chemoreflexes is to adjust respiration to changes in blood chemistry, but they have a secondary role in vasoreflexes. High blood pressure activates this cycle of reactions that ideally return blood pressure to normal. Chemoreceptors also stimulate breathing, so increased ventilation of the lungs matches their increased perfusion. Increasing one without the other (airflow without blood flow, or vice versa) would be of little use. The medulla oblongata monitors its own blood supply and activates corrective reflexes when it senses a state of ischemia (insufficient perfusion). Within seconds of a drop in perfusion, the cardiac and vasomotor centers of the medulla send sympathetic signals to the heart and blood vessels that accelerate the heart and constrict the vessels. These actions raise the blood pressure and ideally restore normal cerebral perfusion. The cardiac and vasomotor centers also receive input from other brain centers, so stress, anger, and arousal can raise the blood pressure. The hypothalamus acts through the vasomotor center to redirect blood flow in response to exercise or changes in body temperature. Since water follows sodium osmotically, Na+ retention promotes water retention, thereby supporting blood pressure. They increase Na+ excretion by the kidneys, thus reducing blood volume and pressure. They also have a generalized vasodilator effect that helps to lower blood pressure. These adrenal and sympathetic catecholamines bind to -adrenergic receptors on the smooth muscle of most blood vessels. Widespread vasoconstriction raises the overall blood pressure because the whole "container" (the blood vessels) squeezes on a fixed amount of blood, like water pressure rising if you squeeze a plastic water bottle. This can be important in supporting cerebral perfusion in situations such as hemorrhaging or dehydration, in which blood volume has significantly fallen. The rerouting of blood and changes in the perfusion of individual organs can be achieved by either central or local control. For example, during periods of exercise, the sympathetic nervous system can selectively reduce flow to the kidneys and digestive tract. Yet as we saw earlier, metabolite accumulation in a tissue can stimulate local vasodilation and increase perfusion of that tissue without affecting circulation elsewhere in the body. If a specific artery constricts, pressure downstream from the constriction drops and pressure upstream from it rises. If blood can travel by either of two routes and one route puts up more resistance than the other, most blood follows the path of least resistance. High resistance in the circulation of the limbs and low resistance in the superior mesenteric artery route blood to the small intestine, where it is needed to absorb the nutrients you are digesting. The vasomotion that redirects such blood flow occurs mainly at the level of the arterioles in the respective organs. To increase the circulation in these routes, vasoconstriction must occur elsewhere, such as the kidneys and digestive tract (figs. That reduces their perfusion for the time being, making more blood available to the organs important in sustaining exercise. Thus, local changes in peripheral resistance can shift blood flow from one organ system to another to meet the changing metabolic priorities of the body.

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References

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