Cardiovascular Physiology-Hemodynamics PDF

Week 4: Cardiovascular Physiology-Hemodynamics Enumerate the vascular circuitry from the heart to the pulmonary and systemic circulation. Circuitry of the Cardiovascular System Left and Right Sides of the Heart Figure 4.1 is a schematic diagram of the circuitry of the cardiovascular system. The lef...

Week 4: Cardiovascular Physiology-Hemodynamics Enumerate the vascular circuitry from the heart to the pulmonary and systemic circulation. Circuitry of the Cardiovascular System Left and Right Sides of the Heart Figure 4.1 is a schematic diagram of the circuitry of the cardiovascular system. The left and right sides of the heart and the blood vessels are shown in relation to each other. Each side of the heart has two chambers: an atrium and a ventricle, connected by one-way valves, called atrioventricular (AV) valves. The AV valves are designed so that blood can flow only in one direction, from the atrium to the ventricle. The left heart and right heart have different functions. The left heart and the systemic arteries, capillaries, and veins are collectively called the systemic circulation. The left ventricle pumps blood to all organs of the body except the lungs. The right heart and the pulmonary arteries, capillaries, and veins are collectively called the pulmonary circulation. The right ventricle pumps blood to the lungs. The left heart and right heart function in series so that blood is pumped sequentially from the left heart to the systemic circulation, to the right heart, to the pulmonary circulation, and then back to the left heart. The rate at which blood is pumped from either ventricle is called the cardiac output. Because the two sides of the heart function in series, the cardiac output of the left ventricle equals the cardiac output of the right ventricle in the steady state. The rate at which blood is returned to the atrium from the veins is called the venous return. Again, because the left and right sides of the heart operate in series, venous return to the left heart equals venous return to the right heart in the steady state. Finally, in the steady state, cardiac output from the left heart equals venous return to the right heart. Explain the function of each major class of vessels within the vasculature. Blood Vessels The blood vessels have several functions. They serve as a closed system of passive conduits, delivering blood to and from the tissues where nutrients and wastes are exchanged. The blood vessels also participate actively in the regulation of blood flow to organs. When resistance of the blood vessels, particularly of the arterioles, is altered, blood flow to an organ is adjusted. Circuitry The steps in one complete circuit through the cardiovascular system are shown in Figure 4.1. The circled numbers in the figure correspond with the steps described here: 1. Oxygenated blood fills the left ventricle. Blood that has been oxygenated in the lungs returns to the left atrium via the pulmonary vein. This blood then flows from the left atrium to the left ventricle through the mitral valve (the AV valve of the left heart). 2. 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), which is located between the left ventricle and the aorta. When the left ventricle contracts, the pressure in the ventricle increases, causing the aortic valve to open and blood to be ejected forcefully into the aorta. (As noted previously, the volume of blood ejected from the left ventricle per unit time is called the cardiac output.) Blood then flows through the arterial system, driven by the pressure created by contraction of the left ventricle. 3. Cardiac output is distributed among various organs. The total cardiac output of the left heart is distributed among the organ systems via sets of parallel arteries. Thus simultaneously, approximately 15% of the cardiac output is delivered to the brain via the cerebral arteries, 5% is delivered to the heart via the coronary arteries, 25% is delivered Week 4: Cardiovascular Physiology-Hemodynamics to the kidneys via the renal arteries, and so forth. Given this parallel arrangement of the organ systems, it follows that the total systemic blood flow must equal the cardiac output. The percentage distribution of cardiac output among the various organ systems is not fixed, however. For example, during strenuous exercise, the percentage of the cardiac output going to skeletal and cardiac muscle increases, compared with the percentages at rest. There are three major mechanisms for achieving such changes in blood flow to an organ system. In the first mechanism, the cardiac output remains constant, but the blood flow is redistributed among the organ systems by the selective alteration of arteriolar resistance. In this scenario, blood flow to one organ can be increased at the expense of blood flow to other organs. In the second mechanism, the cardiac output increases or decreases, but the percentage distribution of blood flow among the organ systems is kept constant. Finally, in a third mechanism, a combination of the first two mechanisms occurs in which both cardiac output and the percentage distribution of blood flow are altered. This third mechanism is used, for example, in the response to strenuous exercise: Blood flow to skeletal and cardiac muscle increases to meet the increased metabolic demand by a combination of increased cardiac output and increased percentage distribution to skeletal and cardiac muscle. 4. Blood flow from the organs is collected in the veins. The blood leaving the organs is venous blood and contains waste products from metabolism, such as carbon dioxide (CO₂). This mixed venous blood is collected in veins of increasing size and finally in the largest vein, the vena cava. The vena cava carries blood to the right heart. 5. Venous return to the right atrium. Because the pressure in the vena cava is higher than in the right atrium, the right atrium fills with blood, called the venous return. In the steady state, venous return to the right atrium equals cardiac output from the left ventricle. 6. Mixed venous blood fills the right ventricle. Mixed venous blood flows from the right atrium to the right ventricle through the AV valve in the right heart, the tricuspid valve. 7. 7. Blood is ejected from the right ventricle into the pulmonary artery. When the right ventricle contracts, blood is ejected through the pulmonary valve (the semilunar valve of the right side of the heart) into the pulmonary artery, which carries blood to the lungs. Note that the cardiac output ejected from the right ventricle is identical to the cardiac output that was ejected from the left ventricle. In the capillary beds of the lungs, oxygen (O₂) is added to the blood from alveolar gas, and carbon dioxide (CO₂) is removed from the blood and added to the alveolar gas. Thus, the blood leaving the lungs has more O₂ and less CO₂ than the blood that entered the lungs. 8. 8. Blood flow from the lungs is returned to the heart via the pulmonary vein. Oxygenated blood is returned to the left atrium via the pulmonary vein to begin a new cycle. Hemodynamics The term hemodynamics refers to the principles that govern blood flow in the cardiovascular system. These basic principles of physics are the same as those applied to the movement of fluids in general. The concepts of flow, pressure, resistance, and capacitance are applied to blood flow to and from the heart and within the blood vessels. Week 4: Cardiovascular Physiology-Hemodynamics Distribution of Blood Volume The total blood volume in a 70-kg male is approximately 5 L. Of this, normally, 85% is present in the systemic circulation, 10% is present in the pulmonary circulation, and 5% is present in the cardiac chambers at the end of diastole. Of the blood volume in the systemic circulation, most (three-fourths) resides in the veins, and the remaining one-fourth resides in the arteries and capillaries; thus, the systemic veins constitute a significant reservoir for the blood volume. Characteristics of Blood Vessels Blood vessel walls have the following components. The relative amount of each component varies between arteries, arterioles, capillaries, and veins, thus conveying their different functional properties. Endothelial cells comprise a single layer that lines all blood vessels. Endothelial cells are connected by junctional complexes in arteries and, to a lesser extent, in veins. In capillaries, the “leakiness” of these junctional complexes varies, depending on the organ. Brain capillaries have narrow (“tight”) junctions that comprise the blood-brain barrier; small intestinal and glomerular capillaries have “fenestrated” capillaries, where the endothelial layer is perforated to allow passage of large volumes of fluid and solutes; liver capillaries have large gaps between endothelial cells. Elastic fibers, comprised of an elastin core covered by microfibrils, convey the elastic properties of arteries, arterioles, and veins; they are not present in capillaries. Collagen fibers are much stiffer than elastic fibers and are present in arteries, arterioles, and veins; they are not present in capillaries. Together with elastic fibers, collagen fibers are responsible for the passive tension of blood vessel walls. Vascular smooth muscle cells are present in all blood vessels except capillaries. Contraction of vascular smooth muscle is responsible for active tension in blood vessels. Types of Blood Vessels Blood vessels are the conduits through which blood is carried from the heart to the tissues and from the tissues back to the heart. In addition, some blood vessels (capillaries) are so thin walled that substances can exchange across them. The size of the various types of blood vessels and the histologic characteristics of their walls vary, as described above. These variations have profound effects on their resistance and capacitance properties. Arteries The aorta is the largest artery of the systemic circulation. Medium- and small-sized arteries branch off the aorta. The function of the arteries is to deliver oxygenated blood to the organs. The arteries are thick-walled structures with extensive development of elastic tissue, vascular smooth muscle, and connective tissue. The thickness of the arterial wall is a significant feature: The arteries receive blood directly from the heart and are under the highest pressure in the vasculature. The volume of blood contained in the arteries is called the stressed volume (meaning the blood volume under high pressure). Week 4: Cardiovascular Physiology-Hemodynamics Arterioles The arterioles are the smallest branches of the arteries. Their walls have an extensive development of vascular smooth muscle, and they are the site of highest resistance to blood flow. The smooth muscle in the walls of the arterioles is tonically active (i.e., always contracted). It is extensively innervated by sympathetic adrenergic nerve fibers. α1-Adrenergic receptors are found on the arterioles of several vascular beds (e.g., skin and splanchnic vasculature). When activated, these receptors cause contraction, or constriction, of the vascular smooth muscle. Constriction produces a decrease in the diameter of the arteriole, which increases its resistance to blood flow. Less common, β2-adrenergic receptors are found in arterioles of skeletal muscle. When activated, these receptors cause dilation, or relaxation, of the vascular smooth muscle, which increases the diameter and decreases the resistance of these arterioles to blood flow. Thus, arterioles are not only the site of highest resistance in the vasculature, but they also are the site where resistance can be changed by alterations in sympathetic nerve activity, by circulating catecholamines, and by other vasoactive substances. Capillaries The capillaries are thin-walled structures lined with a single layer of endothelial cells, which is surrounded by a basal lamina. Capillaries are the site where nutrients, gases, water, and solutes are exchanged between the blood and the tissues and, in the lungs, between the blood and the alveolar gas. Lipid-soluble substances (e.g., O₂ and CO₂) cross the capillary wall by dissolving in and diffusing across the endothelial cell membranes. In contrast, water-soluble substances (e.g., ions) cross the capillary wall either through water-filled clefts (spaces) between the endothelial cells or through large pores in the walls of some capillaries (e.g., fenestrated capillaries). Not all capillaries are always perfused with blood. Rather, there is selective perfusion of capillary beds, depending on the metabolic needs of the tissues. This selective perfusion is determined by the degree of dilation or constriction of the arterioles and precapillary sphincters (smooth muscle bands that lie “before” the capillaries). The degree of dilation or constriction is, in turn, controlled by the sympathetic innervation of vascular smooth muscle and by vasoactive metabolites produced in the tissues. Venules and Veins In the vasculature, the veins are thin-walled structures. The walls of the veins are composed of a small amount of elastic tissue, smooth muscle, and connective tissue. The veins have a large capacitance (capacity to hold blood) and contain the largest percentage of blood in the cardiovascular system. The volume of blood contained in the veins is called the unstressed volume (meaning the blood volume under low pressure). The smooth muscle in the walls of the veins is, like that in the walls of the arterioles, innervated by sympathetic nerve fibers. Increases in sympathetic nerve activity, via α1-adrenergic receptors, cause contraction of the veins, which reduces their capacitance and therefore reduces the unstressed volume.