Cardiovascular Physiology

Transcription

1 Cardiovascular Physiology Lecture 1 objectives Explain the basic anatomy of the heart and its arrangement into 4 chambers. Appreciate that blood flows in series through the systemic and pulmonary circulations. Explain what causes fluid to flow through a tube. Understand the relationship between blood flow, pressure and resistance Appreciate the three factors that influence the resistance to flow through a tube. Introduction The primary function of the cardiovascular system is to deliver blood to tissues, and remove waste from cells. The heart acts as a pump to carry blood containing oxygen to the tissues, and the vessels consist of arteries and arterioles, carry a small percentage of this blood. The veins carry the majority of blood at any one time, and the thin capillaries are located between arteries and veins, and here is where nutrients, wastes and fluid are exchange. Homeostatic functions of the cardiovascular system includes regulation of arterial blood pressure, delivery of hormones, and various adjustments required to respond to haemorrhage, exercise, and changes in posture. Circuitry of the cardiovascular system Each side of the heart has two chambers, a ventricle and an atrium, which are connected by atrioventricular valves. AV valves ensure a one-way flow of blood from the atrium to the ventricle.

2 Left heart is connected to the systematic circulation, which encompasses the systematic arteries, capillaries and veins. System circulation involves the pumping of blood from the left ventricle of the heart to all organs of the body except the lungs. Right heart is connected to the pulmonary circulation, where the right ventricle pumps deoxygenated blood to the lungs. Both the left and right heart function in series so the blood is pumped sequentially from the left heart to the systemic circulation, to the right heart, to the pulmonary circulation, to the left heart again. Cardiac output the rate at which blood is pumped from either ventricle. Because the left and right side of the heart work in series, the cardiac output of the left ventricle is equal to the cardiac output of the right ventricle, in the steady state. Venous return the rate at which blood is returned from the veins to the atria. Because the left and right heart operate in series, the venous return to the left heart is equal to the venous return to the left atrium, in the steady state. CO = VR the cardiac output from the heart equals the venous return to the heart, in the steady state. Blood vessels: 1. Closed system of passive conduits 2. Deliver blood to and from tissues where nutrients and wastes are exchanged 3. Regulate blood flow to organs: when resistance alters, blood flow to organ alters

3 Circuitry of the heart: The steps below show the complete circuit through the cardiovascular system. o Oxygenated blood fills the left ventricle. Blood that has been oxygenated in the lungs returns the left atrium via the pulmonary vein. This blood then flows from the left atrium to the left ventricle via the mitral valve. o Blood is ejected from the left ventricle into the aorta. Blood leaves the left ventricle through the aortic valve (the semilunar valve of the left side of the heart) located between the left ventricle and aorta. When the left ventricle contracts, the pressure increases, opening the aortic valve and blood is ejected into the aorta. This amount per unit time is the cardiac output. Blood flows through the arterial system, driven by the pressure of the contraction of the left ventricle. o Cardiac output is distributed among various organs via sets of parallel arteries. 15% goes to the brain via cerebral arteries 5% goes to the heart via coronary arteries 25% goes to the kidneys via renal arteries o The proportion of blood to various organs is not fixed. During exercise, a higher proportion goes to skeletal muscle. The following mechanisms for achieving such a change are as follows: In the first mechanism, cardiac output remains constant, but the flow is redistributed among organ system by selective changes in arteriolar resistance. Blood flow increases in one organ at the expense of another. In the second mechanism, the cardiac output increases or decreases, but the percentage distribution remains constant. In the third mechanism, both cardiac output and percentage distribution change. This is the response used in exercise. Blood flow to the skeletal muscle increases to meet the increased demand for oxygen by a combination of increased cardiac output and increased percentage distribution to skeletal muscle. o Blood flow from the organs is collected in the veins and contains waste such as CO2. Mixed venous blood is collected in increasingly larger venous until it reaches the vena cava, which carries deoxygenated blood to the right heart. o Venous return to the right atrium occurs because pressure in the vena cava is higher than in the right atrium and the right atrium fills. In the steady state, venous return to the right atrium equals cardiac output from the left ventricle. o Mixed venous blood fills the right ventricle, by flowing through the tricuspid valve. o Blood is ejected from the right ventricle into the pulmonary artery, which carries the blood to the lungs. The cardiac output of the right ventricle is also equal to the cardiac output from the left ventricle. In the capillary beds of the lungs, oxygen is added to blood from alveolar gas and CO2 is removed from the blood and added to alveolar gas. o Blood flow from the lungs is returned to the heart via the pulmonary vein to begin a new cycle.

4 Haemodynamics Haemodynamics refers to the principles which govern blood flow. These are principles based in physics and include flow, pressure, resistance and capacitance both within the heart and blood vessels. Types and characteristics of blood vessels The aorta is the largest artery in the body, with medium and small arteries branching off of it. Their function is deliver oxygenated blood to organs. The arteries are thick-walled and have extensive elastic tissue, smooth muscle and connective tissue. Blood is under the highest pressure when it is in the arteries and the volume contained is called the stressed volume, because of this high pressure. The arterioles are the smallest branches of the arteries and their walls have extensive smooth muscle. They are the site of highest resistance to blood flow. Smooth muscle in

5 arterioles is tonically active (always contracted) and extensively innervated by sympathetic adrenergic nerve fibres. Alpha1-adrenergic are found on arterioles of several vascular beds including the skin and splanchnic vasculature. When activated, alpha1-adrenergic receptors cause constriction of smooth muscle, decreasing the diameter of the arteriole, which increases its resistance to blood flow. o Less common beta2-adrenergic receptors are found in arterioles in skeletal muscle and when activated, cause relaxation of the vascular smooth muscle, which increase sthe diameter and decreases resistance of these arterioles to blood flow. o Thus, arterioles are not only the sites of highest resistance but also the site where resistance can be changed by alterations in sympathetic nerve activity, by catecholamines and other vasoactive substances. Capillaries are thin-walled and lined with a single layer of endothelial cells, surrounded by a basal lamina. They are the site of nutrient, waste and gas exchange. Capillaries are selectively perfused, and this perfusion is determined by the degree of dilation of arterioles and precapillary sphincters. The degree of dilation is determined by the sympathetic innervation of smooth muscle and by vasoactive metabolites produced in the tissues. Venules and veins are thin-walled structures, the walls of which are composed of an endothelial cell layer and a modest amount of elastic tissue, smooth muscle, and connective tissue. Because of their thin wall and small amount of elastic tissue, veins have a large capacitance (capacity to hold blood). The volume of blood in the veins is the unstressed volume. The veins are innervated by the same alpha1 and beta2-adrenergic nerve fibres as arterioles are, and constriction of veins reduces their capacitance, and therefore reduces the unstressed volume. Velocity of blood flow Velocity is the rate of volume displaced per unit time and depends on the diameter and cross-sectional area of vessels. The radius can be a single radii of a blood vessel or the total radius of a group of blood vessels (all the capillaries). The figure below shows how changes in area alter the velocity of flow through a vessel. The flow rate is the same in all examples, but since the relationship between velocity and area is inverse, as area increases, velocity decreases.