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Download Lecture 7 and 8 Arterial Circulation Venous Circulation High
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Lecture 7 and 8 Arterial Circulation High Resistance High Pressure Low Compliance Venous Circulation Low Resistance Low Pressure High Compliance Body contains about 5 L of blood High pressure (15%) vs low pressure (60%) Thus, the venous system acts as a volume reservoir Changes in the diameter of veins has a major impact on the amount of blood they contain Abrupt increase in venous capacity causes pooling of blood in venous segments and may lead to syncope. The relative proportions of smooth muscle, elastic and collagen fibers that determines how distensible a vessel is and how that vessel will accommodate a given volume of blood. The most useful index of dispensability we have is compliance. ance Compliance = ΔV/ΔP (Physical property of the container/Stiff Balloon) ΔPressure = Flow x Resistance ΔPressure = Flow x Resistance Movement of Flow into the Arteries during the Cardiac Cycle When LV ejects blood into the aorta, it expands to accommodate the sudden increased volume. Remember, we talked about how the aorta and major arteries are both stiff and springy, in that it has large amounts of both elastic and collagen fibers. It takes a lot of energy to overcome the stiffness; however, stretch the wall of an artery outward. Only about 1/3 of the stroke volume leaves the arteries during systole. The rest remains in the arteries during systole, distending them and raising pressure. When ventricular contraction ends, the stretched walls recoil passively, like a stretched rubber band being released, and blood continues to be driven into the arterioles during diastole. Systolic blood pressure (SBP): peak pressure during contraction of the heart (systole) Diastolic blood pressure (DBP): minimum arterial pressure during relaxation of the o heart (diastole) Pulse pressure = SBP – DBP Mean arterial blood pressure = DBP + 1/3 (SBP – DBP) Since atmospheric pressure is 760 mmHg, mean arterial blood pressure is actually o 863 mmHg (i.e., 760 + 93). Zero reference point! Determinants of SBP: Left Ventricular Stroke Volume Diastolic Blood Pressure Vessel Wall Compliance Determinants of Diastolic Blood Pressure HR (Increase Heart Rate=increase inflow and decrease outflow time) Peripheral Vascular Resistance Random Blood pressure increases gradually between birth and late adolescence In normal healthy adults blood pressure changes little 20 and 50 years of age After 50 years of age systolic pressure increases at a greater rate than does diastolic pressure So called “hardening of the arteries” occurs with aging Phenylephrine acts on alpha receptors and increases both Diastolic and Systolic BP. In younger people, the reflected wave occurs during diastole In older people, the reflected wave occurs during systole Poiseuille’s Law Resistance to blood flow varies inversely to the 4th power of vessel radius Slide 18 if you wanna see all the math, but I doubt we need to know that… Factors that Alter Vessel Radius Vasoconstriction Vasodilation Sympathetic Nervous System Metabolites Adrenal NE Autoregulation Angiotensin II Nitric Oxide Vasopressin Adrenal Epinephrine More Random Equations Flow = D Pressure / Resistance Or Factors Affecting Laminar vs Turbulent Blood Flow: Velocity of Blood Flow Viscosity of Blood Increased Reynolds number predicts turbulent flow v = Q/A v = velocity of blood flow Q = blood flow rate A = cross-sectional area The effects of Blood Flow on Pathology: Shear stress velocity of flow near the vessel wall Shear stress stimulates release of paracrine substances from endothelial cells Atherosclerosis is enhanced in areas of low shear stress or turbulent flow Anemia= a decrease in Viscosity=Increase in Reynolds Number=More Turbulent Flow Thrombii= decrease vessel diameter=increased velocity of BF= increased Reynolds Number=Increased Turbulence Vessels connected in parallel: o E.g., brain, kidney, liver, lower limbs all connected in parallel RP R1 F1 R2 FT F2 R3 F3 o Vessels connected in series: o E.g., arteryarteriolecapillaryvenulevein F P1 R1 R2 R3 P2 F o Modifying regional vascular resistance directs blood flow to different regions Distribution of cardiac output at rest and during strenuous exercise:
