CARDIOVASCULAR PHYSIOLOGY With a few exceptions (e. g. portal vessels), blood flow through the various organs if arranged in parallel rather than in series. This allows blood to be SHUNTED from one area to another (depending on need). With the closed 4-chambered circulation of mammals (and birds), the VOLUME FLOW through the SYSTEMIC CIRCUIT must necessarily equal the volume flow through the PULMONARY CIRCUIT. However, systemic blood PRESSURE can be (and is) much greater than pulmonary blood pressure. The reptilian 3-chambered circulation is advantageous in that blood can be SHUNTED (e.g. from the pulmonary to the systemic) because the volume flows need not be equal. However, the pressures in the two circuits can not differ greatly. This can be a limitation in that excessive pulmonary pressure can force excess fluid into the lungs. The Principle of Laplace This principle states that the TENSION on the walls of blood vessels and of the heart ventricles is proportional to the product of internal blood pressure and vessel radius: T is proportional to aPr, where a = 1.0 for a cylinder and 0.5 for a sphere, P = pressure, and r is midwall radius. For a detailed account, see: Seymous & Blaylock, 2000 Although the effect of body mass is modest (with a scaling exponent of 1.06), large mammals tend to have, relatively, somewhat larger hearts than small mammals and also to have somewhat higher blood pressures. Gravity also has a pronounced influence for terrestrial animals. If the head is elevated aboe the heart, additionall pressure is needed to pump the blood against gravity. Long-necked animals with elevated heads must have high blood pressure and, according to the principle of Laplace, thick ventricular walls in order to withstand the tension or stress. Giraffes have measure blood pressures around 235 mm Hg and ventricular walls about 5.6 cm (over two inches) thick. What about the extremely long-necked sauropod dinosaurs? Argentinosaurus may have been the largest terrestrial animal that ever lived (possibly weighing as much as 100 metric tons). IF this animal fully extended its very long neck, it would have required a blood pressure around 700 mm Hg! If it was also ENDOTHERMIC, it would have needed a 6 metric ton heart (approximately 2.25 m in diameter) with ventricular walls over 50 cm thick - a very unlikely scenario*. *Calculations based on Seymour & Lillywhite, 2000. [see Figure] [see Animation] [Fig. 12-31] SYSTOLIC PRESSURE results from ventricular systole, whereas DIASTOLIC PRESSURE represents the passive decline in pressure during ventricular diastole. PULSE PRESSURE = Systolic - Diastolic MEAN PRESSURE = (approx.) Diastolic + 1/3 Pulse Pressure Ventricular pressure drops to near zero during ventricular diastole; however, diastolic arterial pressure is maintained by ELASTIC RECOIL of the aorta. [see Figure] [Fig. 12-30] [Fig. 12-29] Note the progressive drop in pressure from arteries to veins and the large difference between the systemic and the pulmonary circuit. HEMODYNAMICS For the entire systemic circuit: Mean Arterial Pressure (MAP) = Cardiac Output (CO) x total peripheral resistance This actually is a special case of a more general relationship. The pressure gradient (delta P) = flow (F) x resistance to flow (R). i.e. (delta P) = F x R We tend to focus on blood pressure. However, from the perspective of the individual body cell, it is flow that is really important. We can thus rearrange this equation as: F = (delta P)/R It is thus clear that we can increase flow by increasing the pressure gradient AND/OR by decreasing resistance to flow. Resistance to flow (R) = (8•eta•L/pi•r^4), where eta = viscosity, L = length, and r = radius Resistance is regulated primarily by altering the diameter of a vessel. Arteries and (especially) arterioles are capable of considerable CONSTRICTION and DILATION and are referred to as RESISTANCE VESSELS. [Fig. 12-50] In this example, the total flow remains the same; however, the distribution is greatly altered by dilating # 1 and constricting # 2, 3, and 4. [see Figure] As this diagram illustrates, we can increase local flow (e.g. through capillary bed A) by decreasing resistance across capillary bed A and/or by increasing blood pressure across A (which can result from an increase in overall resistance. [Fig. 12-39(a)] Total flow through an area (here, the number of balls/min) will be the same; however, the velocity of flow (here, distance moved/min) will vary inversely with pooled cross-section area. [see Figure] In spite of the large pressure differences, total flow through each region remains the same. [see Figure] The velocity of flow, however, varies inversely with pooled cross-section area. (Although an individual capillary is very tiny, the pooled cross-section area is very large) [Fig. 12-39(b)] CAPILLARY EXCHANGE The capillaries are the exchanging parts. The slow flow through these tiny vessels allows maximum time for exchange (which occurs largely via diffusion). [Fig. 12-38] Blood flow through a capillary bed is largely regulated by selective constriction of arterioles and precapillary sphincters. Fluid is constantly moving into or out of capillaries. This is a consequence of the relationships among four (Starling) forces: Two of which (i.e. capillary hydrostatic pressure and interstitial oncotic pressure) tend to move fluid out and two of which (i.e. plasma oncotic pressure and interstitial hydrostatic pressure) tend to return fluid to the capillary. NOTE: ONCOTIC pressure is also called COLLOID OSMOTIC pressure, and represents the osmotic pressure exerted by colloidal sized particles. [Fig. 12-42] The most important factors are: capillary hydrostatic pressure and plasma oncotic pressure. We tend to have net FILTRATION at the arteriole end and net REABSORPTION at the venule end of a capillary. [see Figure] Fluid efflux normally exceeds influx by a small amount. This excess is normally drained by the lymphatic vessels. EDEMA = excess accumulation of interstitial fluid. This can result from several factors, including blockage of lymphatic circulation. [Fig. 12-47] Lymph capillaries are highly permeable (to protein as well as small particles). The lymphatic system drains excess fluid and returns proteins (in the interstitial fluid) to the blood. One-way valves promote unidirectional flow. [Fig. 12-44] Veins and venules are CAPACITANCE VESSELS. They are of large diameter and offer low resistance to flow. At any given time, 60% or more of the total blood volume may be within the systemic veins. [Fig. 12-45] VENOUS RETURN is promoted by the blood pressure gradient (which is typically < 10 mm Hg), by changes in thoracic and abdominal pressure associated with breathing (RESPIRATORY PUMP), and by contractions of skeletal muscles (SKELETAL MUSCLE PUMP) coupled with one-way valves. REGULATION OF ARTERIAL PRESSURE Cells are most concerned with flow, and some local control is accomplished through local dilation (e.g. in response to hypoxia) or constriction (e.g. in response to chemicals released from injured blood vessels. However, flow is not easily directly monitored and is rather indirectly controlled by monitoring and regulating blood pressure. This, in turn, is accomplished by controlling cardiac output and by controlling vascular resistance. Arterial pressure receptors (BARORECEPTORS) are located in the aortic arch and carotid sinus. These influence the CARDIOVASCULAR CENTER in the medulla oblongata. These can adjust cardiac output by ANS influences on heart rate and stroke volume. There is also output (sympathetic) to the arterioles and veins, which can alter peripheral resistance and venous return. [see Figure] [see Figure] [see Animation] [see Animation] EXERCISE During exercise cardiac output markedly increases (primarily due to increased heart rate, with a modest increase in stroke volume). Systolic pressure increases and total peripheral resistance decreases. Blood flow is redistributed, with large increases to skin, skeletal and heart muscle, decreased flow to intestine and kidney, and little or no change in blood supply to the brain. Trained individuals tend to exhibit a lower heart rate and greater stroke volume during exercise. [Fig. 12-61] CORONARY CIRCULATION Even though the heart is filled with blood, a separate circulation to the myocardium is absolutely essential for mammals (though not for lower vertebrates). On average, the coronary vessels receive about 5% of the cardiac output. The coronary vessels fill during ventricular diastole and empty during systole. Tachycardia allows less time for diastolic filling. [see Figure] Oxygen and nutrients are provided from maternal blood (via the PLACENTA and UMBILICAL VEIN). This connects with the fetal circulation via the DUCTUS VENOSUS. The FORAMEN OVALE and DUCTUS ARTERIOSUS shunt much of the blood from the fetal pulmonary circuit to the systemic circuit.
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