By T. Karmok. Southwest Bible College and Seminary.
Certain areas of the body cheap himcolin 30 gm visa, such as the palms himcolin 30gm overnight delivery, soles, lips, and external genitalia, have a Layers of the Dermis greater concentration of sensory receptors and are therefore more The dermis is composed of two layers. Chapter 15 includes a detailed discussion of stratum papillarosum (papillary layer), is in contact with the epi- the structure and function of the various sensory receptors. Papillae Blood vessels within the dermis supply nutrients to the mitoti- form the base for the friction ridges on the fingers and toes. Fibers within this layer blood vessels play an important role in regulating body tempera- are more dense and regularly arranged to form a tough, flexible ture and blood pressure. It is quite distensible, as is evident in pregnant tion responses can either shunt the blood away from the women or obese individuals, but it can be stretched too far, caus- superficial dermal arterioles or permit it to flow freely throughout ing “tearing” of the dermis. Fever or shock can be detected by the color and leaves a white streak called a stretch mark, or linea albicans temperature of the skin. Lineae albicantes are frequently found on the sodilation of dermal blood vessels. It is important to maintain good blood circulation in people It is the strong, resilient reticular layer of domestic mammals who are bedridden to prevent bedsores, or decubitus (de- that is used in making leather and suede. When a person lies in one position for an ex- process, the hide of an animal is treated with various chemicals that tended period, the dermal blood flow is restricted where the body cause the epidermis with its hair and the papillary layer of the dermis presses against the bed. As a consequence, cells die and open to separate from the underlying reticular layer. Changing the position of the patient then softened and treated with protective chemicals before being cut frequently and periodically massaging the skin to stimulate blood and assembled into consumer goods. Integumentary System © The McGraw−Hill Anatomy, Sixth Edition Companies, 2001 112 Unit 4 Support and Movement 6. FUNCTIONS OF THE SKIN The skin not only protects the body from pathogens and external injury, it is a highly dynamic organ that plays a key role in main- taining body homeostasis. Objective 5 Discuss the role of the skin in the protection of the body from disease and external injury, the regulation of body fluids and temperature, absorption, synthesis, sensory reception, and communication. Physical Protection The skin is a barrier to microorganisms, water, and excessive sun- FIGURE 5. Bedsores are most common on skin overlying a bony pro- form an acidic protective film (pH 4. The protein ker- atin in the epidermis also waterproofs the skin, and the cornified outer layer (stratum corneum) resists scraping and keeps out mi- Hypodermis croorganisms. As mentioned previously, exposure to UV light The hypodermis, or subcutaneous tissue, is not actually a part of stimulates the melanocytes in the stratum basale to synthesize the skin, but it binds the dermis to underlying organs. In addition, sur- dermis is composed primarily of loose connective tissue and adi- face friction causes the epidermis to thicken by increasing the pose cells interlaced with blood vessels (see fig. Collagenous rate of mitosis in the cells of the stratum basale and stratum spin- and elastic fibers reinforce the hypodermis—particularly on the osum, resulting in the formation of a protective callus. The amount of adipose tissue in the hypodermis varies Regardless of skin pigmentation, everyone is susceptible to skin cancer if his or her exposure to sunlight is sufficiently intense. Females generally have about an 8% United States, and approximately 9,300 of these are diagnosed as the thicker hypodermis than males. Sunscreens are advised for people The hypodermis is the site for subcutaneous injections. Subcutaneous devices to administer slow-release, low-dosage medications are now available. For exam- Hydroregulation ple, insulin may be administered in this way to treat some forms of di- The thickened, keratinized, and cornified epidermis of the skin is abetes. Even a subcutaneous birth-control device (Norplant) is currently being marketed (see fig. In addition, the outer layers are dead and scalelike, and a protein-polysaccharide base- ment membrane adheres the stratum basale to the dermis. Knowledge Check Human skin is virtually waterproof, protecting the body from desiccation (dehydration) on dry land, and even from water ab- 4. List the layers of the epidermis and dermis and explain how sorption when immersed in water. Describe the sequence of cellular replacement within the epi- dermis and the processes of keratinization and cornification. Thermoregulation The skin plays a crucial role in the regulation of body tempera- ture. Body heat comes from cellular metabolism, particularly in hypodermis: Gk. Integumentary System © The McGraw−Hill Anatomy, Sixth Edition Companies, 2001 Chapter 5 Integumentary System 113 2. Responding to a lowering of the triggers various physiologic responses temperature, cutaneous sensory to produce and conserve heat. Responding to a rise in the temperature, (c) Perspiration stops as sweat cutaneous sensory receptors send a glands shut down. This triggers a response that can quickly generate up to 5 times the normal rate of body heat production. The volume of perspiration produced is largely a function of how much the body is overheated. This volume increases ap- proximately 100 to 150 ml/day for each 1° C elevation in body temperature. For each hour of hard physical work out-of-doors in the summertime, a person may produce 1 to 10 L of perspiration. A serious danger of continued exposure to heat and excessive water and salt loss is heat exhaustion, characterized by nau- sea, weakness, dizziness, headache, and a decreased blood pres- sure.
This response—called Cushing’s re- flex—raises the arterial blood pressure generic 30gm himcolin, often dramatically purchase himcolin 30 gm with visa. While blood flow may improve, microvascular pres- sures are elevated, which worsens cerebral edema. The intestinal vasculature is unusual because three very different tissues—the muscle layers, submucosa, and mucosal layer—are served by branches from a common vasculature lo- SMALL INTESTINE CIRCULATION cated in the submucosa. Most of the intestinal vascular resistance is regulated by small arteries and arterioles preceding the separate The small intestine completes the digestion of food and then muscle and submucosal and mucosal vasculatures. MA, muscular absorbs the nutrients to sustain the remainder of the body. At arteriole; 1A to 5A, successive branches of the arterioles; 1V to rest, the intestine receives about 20% of the cardiac output 4V, successive branches of the venules; MV, muscular venule. Quantification of the architectural of these numbers nearly double after a large meal. Unless the changes observed in intestinal arterioles from diabetic rats. Blood flows of 70 to 100 mL/min high blood flow both at rest and during food absorption, per 100 g in this specialized tissue are probable and much the capillary blood pressure is usually 13 to 18 mm Hg higher than the average blood flow for the total intestinal and seldom higher than 20 mm Hg during food absorp- wall (see Table 17. Therefore, plasma colloid osmotic pressure is ing blood flow in the heart and brain. The interstitial space of the villi is mildly hy- sorption, the plasma protein reflection coefficient for the perosmotic ( 400 mOsm/kg H2O) at rest as a result of NaCl. It is assumed that to 600 to 800 mOsm/kg H2O near the villus tip, compared most of the decrease in reflection coefficient occurs in with 400 mOsm/kg H2O near the villus base. This lowers the ability of plasma cause of high osmolalities in the villi appears to be greater ab- proteins to counteract capillary filtration, with the net re- sorption than removal of NaCl and nutrient molecules. Eventu- is also a possible countercurrent exchange process in which ally, this fluid must be removed. Not surprisingly, the materials absorbed into the capillary blood diffuse from the highest rates of intestinal lymph formation normally oc- venules into the incoming blood in the arterioles. Food Absorption Requires a High Blood Flow Sympathetic Nerve Activity Can Greatly Decrease to Support the Metabolism of the Mucosal Intestinal Blood Flow and Venous Volume Epithelium The intestinal vasculature is richly innervated by sympa- Lipid absorption causes a greater increase in intestinal thetic nerve fibers. Major reductions in gastrointestinal blood flow, a condition known as absorptive hyperemia, blood flow and venous volume occur whenever sympa- and oxygen consumption than either carbohydrate or thetic nerve activity is increased, such as during strenuous amino acid absorption. During absorption of all three exercise or periods of pathologically low arterial blood classes of nutrients, the mucosa releases adenosine and pressure. Venoconstriction in the intestine during hemor- CO and oxygen is depleted. The hyperosmotic lymph and rhage helps to mobilize blood and compensates for the 2 venous blood that leave the villus to enter the submucosal blood loss. Gastrointestinal blood flow is about 25% of the tissues around the major resistance vessels are also major cardiac output at rest; a reduction in this blood flow, by contributors to absorptive hyperemia. By an unknown heightened sympathetic activity, allows more vital func- mechanism, hyperosmolality resulting from NaCl induces tions to be supported with the available cardiac output. Hyperosmolality result- decreased by a combination of low arterial blood pressure ing from large organic molecules that do not enter en- (hypotension) and sympathetically mediated vasoconstric- dothelial cells does not cause appreciable increases in NO tion that mucosal tissue damage can result. These observations suggest that NaCl entering the en- HEPATIC CIRCULATION dothelial cells is essential to induce NO formation. The hepatic circulation perfuses one of the largest organs in The active absorption of amino acids and carbohydrates the body, the liver. The liver is primarily an organ that and the metabolic processing of lipids into chylomicrons maintains the organic chemical composition of the blood by mucosal epithelial cells place a major burden on the mi- plasma. For example, all plasma proteins are produced by crovasculature of the small intestine. There is an extensive the liver, and the liver adds glucose from stored glycogen network of capillaries just below the villus epithelial cells to the blood. The villus capillaries are unusual in and bacteria and detoxifies many man-made or natural or- that portions of the cytoplasm are missing, so that the two ganic chemicals that have entered the body. These areas of fusion, or closed fenestrae, are thought to facilitate the uptake of absorbed materials by The Hepatic Circulation Is Perfused by capillaries. In addition, intestinal capillaries have a higher Venous Blood From Gastrointestinal Organs filtration coefficient than other major organ systems, which and a Separate Arterial Supply probably enhances the uptake of water absorbed by the villi (see Chapter 16). However, large molecules, such as plasma The human liver has a large blood flow, about 1. It is perfused by both the reflection coefficient for the intestinal vasculature is arterial blood through the hepatic artery and venous greater than 0. CHAPTER 17 Special Circulations 283 The venous blood arrives via the hepatic portal vein and accounts for about 67 to 80% of the total liver blood flow (see Table 17. The remaining 20 to 33% of the total flow is through the hepatic artery. The majority of blood flow to the liver is determined by the flow through the stomach and small intestine. About half of the oxygen used by the liver is derived from venous blood, even though the splanchnic organs have removed one third to one half of the available oxygen. The liver has a high metabolic rate and is a large organ; consequently, it has the largest oxygen consumption of all organs in a resting person. The Liver Acinus Is a Complex Microvascular Unit With Mixed Arteriolar and Venular Blood Flow The liver vasculature is arranged into subunits that allow the arterial and portal blood to mix and provide nutrition for the liver cells. The core of each acinus is supplied by a single ter- minal portal venule; sinusoidal capillaries originate from this venule (Fig. A sin- gle liver acinus, the basic subunit of liver struc- ies have fenestrated regions with discrete openings that fa- ture, is supplied by a terminal portal venule and a terminal hepatic cilitate exchange between the plasma and interstitial spaces.
R 8 L/ r4 (2) where r is the radius of the tube himcolin 30 gm on-line, L is its length cheap 30gm himcolin mastercard, and is the though blood viscosity increases with hematocrit and with viscosity of the fluid; 8 and are geometrical constants. In contrast, radius changes have a much maintaining the arterial pressure relatively constant so greater influence because resistance is inversely propor- there is a steady force to drive blood through the cardio- tional to the fourth power of the radius (Fig. Small changes in arteriolar radius can tion 1 shows that if pressure and flow are expressed in units cause large changes in flow to a tissue or organ because flow of mm Hg and mL/min, respectively, R is in mm Hg is related to the fourth power of the radius. Poiseuille’s law incorporates all of the factors influencing Conditions in the Cardiovascular System Deviate flow, so that From the Assumptions of Poiseuille’s Law 4 Q P r /8 Despite the usefulness of Poiseuille’s law, it is worthwhile to In the body, changes in radius are usually responsible for examine the ways the cardiovascular system does not variations in blood flow. First, CHAPTER 12 An Overview of the Circulation and Hemodynamics 213 the cardiovascular system is composed of tapering, branch- ing, elastic tubes, rather than rigid tubes of constant diam- eter. These conditions, however, cause only small devia- tions from Poiseuille’s law. Application of Poiseuille’s law requires that flow be steady rather than pulsatile, yet the contractions of the heart cause cyclical alterations in both pressure and flow. Despite this, Poiseuille’s law gives a good estimate of the re- lationship between pressure and flow averaged over time. Another criterion for applying Poiseuille’s law is that flow be streamlined. Streamline (laminar) flow describes the movement of fluid through a tube in concentric layers that slip past each other. The layers at the center have the fastest velocity and those at the edge of the tube have the slowest. This is the most efficient pattern of flow velocities, in that the fluid exerts the least resistance to flow in this configuration. Turbulent flow has crosscurrents and ed- dies, and the fastest velocities are not necessarily in the middle of the stream. Several factors contribute to the ten- dency for turbulence: high flow velocity, large tube diame- ter, high fluid density, and low viscosity. All of these fac- tors can be combined to calculate Reynolds number (NR), Axial streaming and flow velocity. As flow velocity increases, red blood cells NR vd / (4) move toward the center of the blood vessel (axial streaming), where v is the mean velocity, d is the tube diameter, is the where velocity is highest. Axial streaming of red blood cells low- fluid density, and is the fluid viscosity. This value is hardly ever exceeded in a normal cardiovascular system, but high flow velocity is the most common cause of turbulence in pathological states. Once turbulence occurs, a given in- sure gradient along a tube and flow changes at the point crease in pressure gradient causes less increase in flow be- cause the turbulence dissipates energy that would other- wise drive flow. Under normal circumstances, turbulent flow is found only in the aorta (just beyond the aortic valve) and in certain localized areas of the peripheral sys- tem, such as the carotid sinus. Pathological changes in the cardiac valves or a narrowing of arteries that raise flow velocity often induce turbulent flow. Turbulent flow generates vibrations that are transmitted to the surface of Streamline flow Turbulent flow the body; these vibrations, known as murmurs and bruits, can be heard with a stethoscope. Finally, blood is not a strict newtonian fluid, a fluid that exhibits a constant viscosity regardless of flow velocity. When measured in vitro, the viscosity of blood decreases as the flow rate increases. This is because red cells tend to Critical velocity collect in the center of the lumen of a vessel as flow veloc- ity increases, an arrangement known as axial streaming (Fig. Axial streaming reduces the viscosity and, therefore, resistance to flow. Because this is a minor effect in the range of flow velocities in most blood vessels, we usually assume that the viscosity of blood (which is 3 to 4 Pressure gradient times that of water) is independent of velocity. When flow is streamlined, concentric layers of fluid PRESSURES IN THE CARDIOVASCULAR SYSTEM slip past each other with the slowest layers at the interface be- Pressures in several regions of the cardiovascular system are tween blood and vessel wall. When the critical velocity is reached, turbulent readily measured and provide useful information. In the presence of turbulent flow, flow does not in- pressure is too high, it is a risk factor for cardiovascular dis- crease as much for a given rise in pressure because energy is lost eases, including stroke and heart failure. A more compliant structure exhibits a greater change in volume for a given transmural pressure change. The lower The Contractions of the Heart Produce the compliance of a vessel, the greater the pressure that will Hemodynamic Pressure in the Aorta result when a given volume is introduced. For example, The left ventricle imparts energy to the blood it ejects into each time the left ventricle contracts and ejects blood into the aorta, and this energy is responsible for the blood’s cir- the aorta, the aorta expands; in doing so, it exerts an elastic cuit from the aorta back to the right side of the heart. This of this energy is in the form of potential energy, which is force is measured as the pressure in the aorta. This is hemody- the aorta becomes less compliant, and aortic pressure rises namic pressure, produced by contractions of the heart and more for a given increase in aortic volume. The next section de- of the legs, the volume of the veins expands much more scribes a third form of energy, hydrostatic pressure, derived than that of the arteries. Mean Arterial Pressure Depends on Cardiac A Column of Fluid Exerts Hydrostatic Pressure Output and Systemic Vascular Resistance Fluid standing in a container exerts pressure proportional A simple model is useful in seeing how the pressures, flows to the height of the fluid above it. The pressure at a given and volumes are established in the cardiovascular system.
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