GUYTON CHAPTER ON PRESSURE FLOW RESISTANCE, Assignments of Cardiology

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Guyton and Hall Textbook of Medical Physiology 13Ed (Chap 14) Overview of the Circulation; Biophysics of Pressure, Flow, and Resistance ¢ rate of blood flow through many tissues is controlled mainly in response to their need for nutrients e heart and blood vessels, in turn, are controlled to provide the necessary cardiac output and arterial pressure to cause the needed tissue blood flow PHYSICAL CHARACTERISTICS OF THE CIRCULATION e = The circulation, shown in Figure 14-1, is divided into the systemic circulation and the pulmonary circulation Putmon: ose Superior ea" Figure 1 e systemic circulation is also called the greater circulation or peripheral circulation Functional Parts of the Circulation e function of the arteries is to transport blood under high pressure to the tissues e the arteries have strong vascular walls, and blood flows at a high velocity ¢ arterioles are the last small branches of arterial system o they act as control conduits through which blood is released into the capillaries e Arterioles have strong muscular walls that can close the arterioles completely or can dilate the vessels several fold, > capable of vastly altering blood flow in each tissue in response to its needs ¢ capillaries exchange fluid, nutrients, electrolytes, hormones, and other substances between the blood and the interstitial fluid ¢ capillary walls are thin and minute capillary pores permeable to water and other small molecular substances e venules collect blood from the capillaries and gradually coalesce into progressively larger veins ¢ veins function as conduits for transport of blood from the venules back to the heart © they serve as a major resenvoirof extra blood - due to its low pressure and thin wall o they are muscular enough to contract or expand and thereby serve as a controllable reservoir for the extra blood Volumes of Blood in the Different Parts of the Circulation ° about in the systemic circulation and 16% isin the heart ~ 5 of the blood, and the + 34% an ee ea AE arterioles and capillaries e In capillaries, the most important function of the circulation occurs—diffusion of substances back and forth between the blood and the tissues Cross-Sectional Areas and Velocities of Blood Flow e If all the systemic vessels of each type were put side by side, approximate total cross-sectional areas are as follows Vessel Cross-Sectional Area (cm?) Aorta 25 Small arteries 20 Arterioles 40 Capillaries 2500 Venues 250 Small veins 80 Venae cavae 8 e Note particularly that the GFoss*sectional areas of the e There is large blood storage capacity of the venous system in comparison with the arterial system Because the same volume of blood flow (F) must pass through each segment of the circulation each minute, the velocity of blood flow (v) is inversely proportional to vascular cross-sectional area (A) veF/A] ® under resting conditions, the velocity averages about 33 cm/sec in the aorta but is only 0.3 mm/sec in the capillaries ein capillaries, however, blood remains in the capillaries for only 1 to 3 seconds ~> diffusion of Pressures in the Various Portions of the Circulation the mean pressure in the aorta is high, averaging about 100mmHg e heart pumping is pulsatile e arterial pressure alternates between a_ systolic pressure (120mmHg) and diastolic pressure (80mmHg) shown on Figure 14-2 i Puree caster onsitetes = a Mur ° ‘Syeme Paimanaey Figure 142, Nes bed preeter in te tnt gortors cf the ccuster ten hen stone hing m tre nena peste, e As the blood flows through the systemic circulation, its mean pressure falls progressively to about OmmnHg by the time it reaches SVC, IVC ® pressure in the systemic capillaries varies from as high as 35 mm Hg near the arteriolar ends to as low as 10 mm Hg near the venous ends ° e Note at the far right side of Figure 14-2 the respective pressures in the different parts of the pulmonary circulation e In pulmonary arteries, the pressure is pulsatile, like aorta but the pressure is far less: pulmonary artery diastolic pressure averages about 8 mm Hg, with a © mean pulmonary CAPILLARY pressure averages only 7mmHg > low pressures of the pulmonary system are in accord with the needs of the lungs BASIC PRINCIPLES OF CIRCULATORY FUNCTION © 3 basic principles underlie all functions of the system 1. Blood flow to most tissues is controlled according to the tissue need > when tissues are active, more blood flow 20-30x the resting level BUT itis not possible for the <3 simply to increase blood flow everywhere INSTEAD microvessels of each tissue monitor needs and act by dilating or constricting them, to contol local blod flow + with the help of CNS and hormones 2. Cardiac output is the sum of all the local tissue flows > when blood flows through a tissue, it immediately returns by way of the veins to the heart > heart responds automatically to this increased inflow of blood by pumping it immediately back into the arteries > heart acts as an automaton, responding to the demands of the tissues with the help of nerve signals 3. Arterial pressure regulation is generally independent of either local blood flow control or cardiac output control > if pressure falls significantly below the normal level of about 100 mm Hg, nervous reflexes elicits a series of circulatory changes to raise the pressure back toward normal > nervous signals especially (a) increase the force of heart pumping, (b) cause contraction of the large venous to provide more blood to the heart, and (c) cause GSRRFSIEEBIEBRSHTEEDH of the BARBHBIES in many arteries to increase the arterial pressure > in more prolonged periods, kidneys secrete ADH INTERRELATIONSHIPS OF PRESSURE, FLOW, AND RESISTANCE ¢ Blood flow through a blood vessel is determined by two factors: © (2) pressure difference ofthe blood between the two ends of the vessel or the “pressure gradient” which pushes the blood through the vessel o (2) the impediment to blood flow through the vessel called vascular resistance e Figure 14-3 demonstrates these relationships, showing a blood vessel segment located anywhere in the circulatory system ry _—— Pressure gracient — Ps Blood flow | ‘sistant Figure 14- clationships of p: : Flow. P,, at the ongin of the end of t sel e¢ P1 represents the pressure at the origin of the vessel and P2 at the other hand e the rate of blood flow is directly proportional to the » which demonstrates once again that the diameter of a blood vessel plays by far the greatest role Importance of the Vessel Diameter “Fourth Power Law” in Determining Arteriolar Resistance e about 2/3 of the total systemic resistance to blood flow is arteriolar resistance in the small arterioles e it possible for the arterioles to turn off almost completely the blood flow to the tissue or at the other extreme to cause a vast increase in flow Resistance to Blood Flow in Series and Parallel Vascular Circuits ¢ When blood vessels are arranged in series, flow through each blood vessel is the same and the total resistance to blood flow (Rtotal) is equal to the sum Figure 14-9. Vascular resistances (R): A, in series and B, in parallel ¢ total peripheral vascular resistance is therefore equal to the sum of resistances of the arteries, arterioles, capillaries, venules, and veins © parallel arrangement permits each tissue to regulate its own blood flow, to a great extent * parallel blood vessels > easier for blood to flow > because each parallel vessel provides another pathway, or conductance Effect of Blood Hematocrit and Blood Viscosity on Vascular Resistance and Blood Flow another important factor in Poiseuille’s equation is the viscosity of the blood ¢ the viscosity of normal blood is about three times as great as the viscosity of water e large numbers of suspended red cells in the blood, each of which exerts frictional drag against adjacent cells and against the wall of the blood vessel making it viscous ¢ Hct of 40 = means that 40 percent of the blood volume is cells and the remainder is plasma e Hematocrit is determined by centrifuging blood in a calibrated tube, as shown in Figure 14-10 Increasing Hematoc Viscosity e viscosity of blood increases drastically as the hematocrit increases Markedly Increases Blood ¢ normal hematocrit is about 3 to 4, and it is the pressure is required to force whole blood EFFECTS OF PRESSURE ON VASCULAR RESISTANCE AND TISSUE BLOOD FLOW e increase in arterial pressure not only increases the force that pushes blood through the vessels but also initiates compensatory increases in vascular resistance e reductions in arterial pressure, vascular resistance is promptly reduced in most tissues and blood flow is maintained at a relatively constant rate e flow autoregulation > ability of each tissue to adjust flow during changes in arterial pressure . hormonal vasoconstrictors, such as norepinephrine, angiotensin lI vasopressin, or endothelin, can also reduce blood flow, at least transiently e local autoregulatory mechanisms _ eventually override most of the effects of vasoconstrictors to provide appropriate blood flow Pressure-Flow Relationship in Passive Vascular Beds e increased arterial pressure not only increases the force but also distends the elastic vessels, actually decreasing vascular resistance e decreased arterial pressure in passive blood vessels increases resistance as the elastic vessels gradually collapse due to reduced distending pressure e¢ Sympathetic stimulation and other vasoconstrictors can alter the passive pressure-flow relationship shown in Figure 14-13. ‘Sympathetic toa fou (niin Gympatnetic stimulation 0 20 40 60 80 100 120 140 160 180 200 Arterial pressure (mm Ha) I pressure on blood flow through a netic sti inhibition of sympathetic activity greatly dilates the vessels and can increase the blood flow very strong sympathetic stimulation can constrict the vessels Even in tissues that do not effectively autoregulate blood flow during acute changes in arterial pressure, blood flow is regulated according to the needs of the tissue when the pressure changes are sustained