Blood Vessels and Blood Pressure-Notes.pdf

Donal S. O'Leary, Ph.D. Cardiovascular Physiology BLOOD VESSELS AND BLOOD PRESSURE Learning Objectives: 1. Describe the parallel arrangement of circulatory architecture. 2. Describe the relationship between pressure, flow and resistance. 3. Compare and contrast the anatomy of blood vessels at the different levels of the vascular tree. 4. Compare and contrast the changes in pressure, velocity, area, and blood volume at different levels of the vascular tree. 5. Understand the concept of compliance. 6. Discuss the effects of posture on transmural and perfusion pressures. 7. Understand the coupling between the vasculature and the ventricle. Lecture Outline 1. Parallel arrangement of the circulation 2. “The Equation” 3. Effects of changes in length and radius on resistance 4. Series vs. parallel resistors 5. Pressure, velocity, flow and cross sectional surface area at different levels 6. Compliance 7. Effects of posture on pressure 8. Ventricular – Vascular Coupling 1 Donal S. O'Leary, Ph.D. Cardiovascular Physiology Head & neck Upper limbs Lungs Bronchial Left atrium Right atrium Right vent. Coronary Left vent. Thorax Hepatic Portal Liver Spleen Hepatic Peritubular Glomerular Large & small intest. Kidneys Pelvic Lower limbs Parallel Blood Flow In the mammalian circulatory system blood flow to the systemic organs occurs in parallel. Blood is ejected by the left ventricle into the aorta. The aorta subdivides into the major arteries. These major arteries further subdivide into small arteries -> arterioles->capillaries. At the level of the capillaries diffusion of O2, CO2, and nutrients occurs between the cells and the blood. The capillaries come together into the venules, the venules combine into small veins which in turn combine into the major veins which carry blood back to the right heart. Blood is pumped by the right ventricle into the lungs and returns to the left heart. 2 Donal S. O'Leary, Ph.D. Cardiovascular Physiology ---------------------------------------------------NOTES--------------------------------------------------- When a pump (the heart) ejects a fluid (blood) through tubes (blood vessels), pressure is generated within the tubes. The magnitude of the pressure is dependent on the amount of blood flow and the resistance to flow the tubes impart according to THE EQUATION: P1 - P2 = Flow X Resistance where P1 is the pressure in the tube attached to the outflow of the pump and P2 is the pressure at the inflow to the pump. For the entire systemic circulation P1 is the pressure in the aorta and P2 is central venous pressure, flow is cardiac output, and resistance would be Total Peripheral Resistance (TPR, also termed Total Systemic Vascular Resistance [TSVR]). For the pulmonary circulation, P1 is the pressure in the pulmonary artery, P2 is the pressure in the left atrium. Question - what is the flow???? Resistance is then equal to the (P1 - P2)/Flow and has the units of mmHg/l/min. Many years ago a scientist using small tubes and ideal fluids discovered that resistance can be quantified based on several physical properties. 3 Donal S. O'Leary, Ph.D. Cardiovascular Physiology Poiseuille's Law R = (8  l) / ( r4)  - viscosity l - length r - radius of the tube - by far the most important component. Question - why?? Given the above equation, we can substitute Poiseuille's Law into THE EQUATION and FLOW = (P1 - P2) ( r4) (8  l) Effect of changes in length and radius on flow are shown below. 1 2 1 Q L r1 r 2 = rL L1 L2 = 2L1 Q1 = 10 ml/sec Q2 = 5 ml/sec 3 4 Qr r3 = 2rL L3 = L1 Q = flow Q3 = 160 ml/sec L = length r = radius Resistors in Series (artery - arteriole - capillary - venule - vein) add directly Rt = R1 + R2 + R3 +......Rn Resistors in Parallel (resistance of the skin vessels + muscle + kidney....) add inversely 1/Rt = 1/R1 + 1/R2 + 1/R3 +.....1/Rn 4 Donal S. O'Leary, Ph.D. Cardiovascular Physiology The Figure below shows resistors in series and parallel. Series Resistance Network Parallel Resistance Network R3 R1 Q1 = P/R1 R1 R2 Pi Q P0 Pi R2 Q2 = P/R2 P0 a b c d R3 Q3 = P/R3 Rs = R1 + R2 + R3 P = Pi – P0 1 1 1 1 Rp = R1 + R2 + R3 Q = P/Rs P = Pi – P0 Q flow Qtotal = Q1 + Q2 + Q3 a b c d position along the network Pi Qtotal = P/Rp P1 = Q x R1 pressure P2 = Q x R2 P P3 = Q x R3 P0 Rs = resistance series a b c d Rp = resistance parallel position along the network 5 Donal S. O'Leary, Ph.D. Cardiovascular Physiology Anatomy The Figures below schematically compare the anatomy of the different blood vessels in the vascular tree. Most notable is the variation in the ratio of the diameter to the wall thickness. NOTE that at the level of the arterioles (and precapillary sphincters) this ratio is 3/2, thus for its size the arteriole has a very thick wall. A large fraction of the arteriole wall is composed of vascular smooth muscle. Through activation and relaxation of the vascular Structure of Vessels Figure 19-6 from Boron and Boulpaep’s Medical Physiology 2nd ed smooth muscle the arteriole diameter can change. You will recall from Poiseuille's equation that resistance is inversely proportional to the radius of the vessel to the fourth power (r4). Thus as radius increases resistance decreases dramatically and as radius decreases resistance increases 6 Donal S. O'Leary, Ph.D. Cardiovascular Physiology dramatically. With this ability to alter diameter (radius), the arterioles are chiefly responsible for changes in resistance. The Figure below shows the changes in pressure, velocity of blood flow, cross sectional area and the percentage of total blood volume at each level of the vascular tree. Percent of Total Blood Volume Cross-Sectional Area- cm2 Velocity- cm/sec Mean Pressure- mmHg 5000 50 100 Pressure % Blood 30 45 Velocity Volume 4000 80 35 20 3000 60 25 2000 40 Cross- Sectional 10 15 Area 1000 20 5 0 0 0 0 Aorta Large Small Arter- Capill- Venules Small Large Venae arteries Arteries ioles aries Veins Veins Cavae PRESSURE: Blood pressure continually decreases as we progress from the aorta to the vena cava. NOTE that the largest decrease in pressure occurs across the arterioles. This is because of the high resistance imparted by these vessels. VELOCITY: The velocity of blood flow is highest in the aorta as blood is ejected by the left ventricle. Velocity decreases markedly across the arterioles and capillaries as cross sectional area increases markedly. Velocity increases on the venous side as cross sectional area decreases. CROSS SECTIONAL AREA: Cross sectional area refers to the area of all the blood vessels at each level. Thus, while the aorta is large, there is only one aorta and thus cross sectional area is very small compared to that at the level of the capillaries. While each capillary is very small, there are millions (billions) of capillary thus, total cross sectional area is greatest at this level. 7 Donal S. O'Leary, Ph.D. Cardiovascular Physiology FLOW = VELOCITY X CROSS SECTIONAL AREA cm3/min = cm/min X cm2 PERCENTAGE TOTAL BLOOD VOLUME: The amount of blood at each level is dependent on the pressure and compliance of the vessels. The below shows the relationship between volume and pressure for arteries and veins. Compliance is defined as the ratio of the change in volume produced by a change in pressure. Thus, while pressure is high in the aorta, total volume is low because compliance is low. While pressure is low in the veins, compliance is high and thus total volume is high. Approximately 70% of the total blood volume resides on the venous side of the circulation. 8 Donal S. O'Leary, Ph.D. Cardiovascular Physiology EFFECT OF POSTURE: In the upright posture a hydrostatic pressure gradient exists due to the fluid column. The pressure generated by the column = height of the column x density of fluid x gravity. Given a constant density and gravity, the hydrostatic pressure generated with changes in posture are proportional to the distance above or below the heart. NOTE that the effect of posture on pressure occurs equally in the arteries and veins and thus, in general, has little effect on tissue perfusion pressure unless pressure falls so low that the veins collapse. Note that the absolute TRANSMURAL pressure in the legs is high, a major contributor to varicose veins seen in some patients. TRANSMURAL PRESSURE: Pressure across the wall of the blood vessels. PERFUSION PRESSURE: Pressure gradient across the tissue (arterial pressure - venous pressure). Note that the effects of posture equally affect the arteries and the veins so that the pressure difference (energy for flow) is not normally affected. 9 Donal S. O'Leary, Ph.D. Cardiovascular Physiology Arterial Blood Pressure Arterial blood pressure is not static but oscillates with each ventricular contraction. With ventricular systole, blood is ejected into the aorta causing arterial blood pressure to increase because blood is flowing through tubes which have resistance. Pressure rises to its highest level - termed systolic pressure. With ventricular relaxation and closure of the aortic valve, ejection ceases and pressure falls. Pressure then reaches its lowest level just prior to the next ventricular ejection period - termed diastolic pressure. The mean arterial pressure (MAP) can be determined by integrating the arterial pressure wave form over time however, a "rule of thumb" is that: MAP = diastolic pressure + 1/3 pulse pressure Unlike the ventricle, during diastole, arterial pressure does not fall to near zero. This is because of the capacitance properties of the major arterial vessels - so called windkessel vessels. During ejection, some of the fluid energy is absorbed bey the elastic properties of the arteries. During diastole, this energy is released back via elastic recoil which thereby acts to maintain arterial pressure above ventricular pressure during diastole. 10 Donal S. O'Leary, Ph.D. Cardiovascular Physiology Pulse Pressure: Pulse pressure can vary dependent on the stroke volume and the compliance of the arteries. When the heart ejects the stroke volume pressure rises in the arterial circulation due to the sudden addition of volume. Some of this energy is absorbed by the elastic properties of the arteries and this acts to limit the rise in systolic pressure. During diastole, this energy is released back to the blood and this limits the fall in pressure during diastole. 11 Donal S. O'Leary, Ph.D. Cardiovascular Physiology Effect of Stroke Volume: - As stroke volume rises, more blood is expelled into the arterial circulation and therefore pressure rises. Effect of Arterial Compliance: - If arterial compliance falls, the arteries become stiffer and for the same stroke volume pressure rises higher because the arteries cannot stretch as much during systole and therefore during diastole they cannot give back this energy as much and therefore pressure falls more in diastole. This is commonly seen in older individuals. Laminar vs. Turbulent Flow: Normally blood flow is laminar as shown in the figure below, with the velocity profile showing lowest velocity at the edges and the highest in the middle. However, at branch points or sudden partial occlusions turbulent flow may occur. Turbulent flow is wasteful as it takes more energy for a give level of total flow. A predictor of whether laminar or turbulent flow will occur is the Reynolds Number as shown in the equation 12 Donal S. O'Leary, Ph.D. Cardiovascular Physiology below. When Re > 2200 turbulence is predicted. The most impactful part of this relationship is the velocity. At constant flow, velocity is related to 1/Cross sectional surface area which =  r2, so as the diameter of the blood vessel decreases, the velocity increases disproportionately since it is a squared relationship and turbulent flow becomes more likely and often can be heard. Turbulent flow is often observed and heard with a partial blockage of a major artery, a cardiac valve that does not open properly, and is one explanation for the Korotkoff sounds heard while taking a patient’s blood pressure using a cuff described above. While both velocity and diameter are in the numerator, the effect of a narrowing would reduce D and therefore tend to decrease Reynolds Number, with a narrowing velocity increases. Flow = Velocity x Surface Area, thus Velocity = Flow/Surface Area and Surface Area = (xr2), therefore a narrowing has a greater effect on Velocity than it does on Diameter and Reynolds Number increases making turbulent flow more likely. Comparison of the systemic and pulmonary circulations: Note that in comparison, arterial pressures are lower on the pulmonary side vs. the systemic side of the circulation. This is because of the much lower pulmonary vascular resistance as compared to the systemic circulation. Question – which circulation has the higher cardiac output? 13 Donal S. O'Leary, Ph.D. Cardiovascular Physiology 14 Donal S. O'Leary, Ph.D. Cardiovascular Physiology Coupling Between the Vasculature and the Ventricle The effect of vasodilation or vasoconstriction of the arterioles on arterial pressure and central venous pressure with cardiac output being held constant by a pump. With vasodilation, there will be less volume in the arterial circulation and more in the venous side so MAP falls and RAP rises. The converse with vasoconstriction. The effect of changes in Cardiac Output on Mean Arterial Pressure and Right Atrial Pressure. As cardiac output falls, mean arterial pressure decreases and right atrial pressure rises as less volume is being translocated from the venous side of the circulation to the arterial side. Alternatively, if CO increases, the MAP increases and RAP falls due to more volume being taken from the venous side of the circulation and translocated into the arterial side. It is a dynamic process. When CO is zero, pressures eventually equalize and this is termed Mean Circulatory Filling Pressure. 15

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