Overview of the Cardiovascular System

Transcription

1 Overview of the Cardiovascular System 2 vascular (blood vessel) loops: Pulmonary circulation: from heart to lungs and back) Systemic circulation: from heart to other organs and back Flow through systemic and pulmonary circuits are in series Flow within systemic (and pulmonary) circuit is in parallel Parallel flow allows independent regulation of blood flow to organs Parallel Flow in the Cardiovascular System 1

2 Blood Vessels Heart Arteries Arterioles Capillaries Venules Veins Arteries relatively large, branching vessels that conduct blood away from the heart. Major artery is aorta Arterioles small branching vessels with high resistance Capillaries site of exchange between blood and tissues Venules small converging vessels - drain blood to veins Veins relatively large converging vessels that conduct blood to the heart. Major vein is vena cava (superior and inferior) Closed system Blood Vessel anatomy 2

3 Physical Laws Governing Blood Flow Blood flow is calculated as flow per unit time Dictated by Pressure Gradients in the vasculature Resistance in the vasculature Flow Rule Circulatory system is a closed system Pressure = force exerted by blood Flow occurs from high pressure to low pressure Heart creates pressure gradient for bulk flow of blood A gradient must exist throughout circulatory system to maintain blood flow Flow = pressure gradient/resistance Pressure gradient: Pressures throughout the vasculature are not constant 3

4 Effect of Resistance on Flow The lower the resistance, the greater the flow Total peripheral resistance (TPR) is the sum of the resistance of all peripheral vasculature in the systemic circulation. It decreases during exercise to allow blood flow to increase (next lecture) Flow =pressure gradient/resistance Poiseuille s Law Resistance = 8 x length x viscosity π x radius 4 LENGTH: The longer the vessel, the greater the resistance to flow Capillaries tend to be short compared with larger vessels Describes the behaviour of a perfect fluid in a rigid tube BUT Blood vessels are not rigid Blood is a 2-phase system (cells and plasma) 4

5 Regulate Blood Flow by Regulating Radius Regulation of radius of arterioles (and small arteries) Vasoconstriction decrease radius (by contracting smooth muscle) increase resistance decrease blood flow Vasodilation increase radius (by relaxing smooth muscle) decrease resistance increase blood flow VISCOSITY Mostly, when considering flow rate, viscosity remains constant Plasma almost twice as viscous as water Whole blood 3-4 times as viscous as water Viscosity depends on haematocrit Red blood cells tend to flow in centre stream (concept of plasma skimming) 5

6 Viscosity Branches leaving large vessels at right angles tend to receive rbc-poor blood (plasma skimming) Haematocrit (therefore viscosity) of capillary blood is 25% lower than whole-body haematocrit Polycythaemia: increased haematocrit Large vessels (arteries and veins) increased haematocrit increases viscosity Smaller vessels (arterioles, venules, veins) effect of increased haematocrit has less impact on viscosity because flow is different in these vessels. cross sectional area is increased rbc in capillaries single file rbc tend to flow in centre stream Blood vessels are not rigid tubes Laplace s Law: Wall tension = Pressure x radius wall thickness Wall tension on the aorta is high (pressure is high relative to radius) In chronic hypertension, aortic wall thickness increases in compensation Large arteries must have strong walls - an artery of twice the radius needs to withstand twice the tension at any given blood pressure Aneurysms tend to develop in larger arteries 6