Coronary and Pulmonary Circulations - Essay Example

Coronary and Pulmonary Circulations Coronary circulation Introduction The heart is responsible for pumping the blood to every cell in the body and also to the lungs, where the blood gives up carbon dioxide and takes on oxygen (Phillips, “The Heart and Circulatory System). Due to its continuous activity, heart requires a circulation that provides constant oxygen and removes metabolites quickly. Also, the oxygen demand is not constant and the circulation has to get adjusted to sudden increases in demand. The basal oxygen consumption of the heart is 8-10 ml of oxygen per minute per 100grams. Anatomy of coronary circulation: The various regions of the heart are supplied by coronary circulation. The major blood vessels which constitute this circulation are the left main coronary artery and the right main coronary artery, both of which originate at the base of the aorta from openings called the coronary ostia located behind the aortic valve leaflets (Klabunde, “Cardiovascular Physiology Concepts”). The left main coronary artery is usually larger than the right one and divides into left anterior descending and circumflex branches (Figure-1). The right coronary artery gives out branches like marginal artery and posterior interventricular artery. The coronary arteries and their branches lie on the surface of the heart and hence are also known as epicardial coronary vessels. The branches of the main arteries further branch out into arterioles and finally form a microvascular network of capillaries. These capillaries lie adjacent to cardiac myocytes. There will be many capillaries adjacent to each cardiomyocyte so that the capillary-to-cardiomyocyte ratio is high and this enables adequate oxygen supply to the heart cells. The arteries and arterioles have much less vascular resistance than the microvacular bed in healthy persons so that there is free flow of blood to the capillary bed. The capillary bed enters venules which join together and form cardiac veins. Removal of waste product from the heart cells is done through this circuit. Most of the cardiac veins drain into the coronary sinus located on the posterior side of the heart. The coronary sinus drains into the right atrium. This completes the coronary circulation. The anterior cardiac veins and the thesbian veins drain directly into the cardiac chambers (Klabunde, “Cardiovascular Physiology Concepts”). Most of the coronary flow occurs during diastole because, during systole, there is marked extravascular compression which affects coronary flow (Levick, “Introduction to Cardiovascular Physiology”). Figure-1: Coronary circulation (Klabunde, “Cardiovascular Physiology Concepts”). Autoregulation of coronary circulation: There is good autoregulation between 60 and 200 mmHg perfusion pressure that maintains normal coronary blood flow whenever coronary perfusion pressure changes due to changes in aortic pressure (Klabunde, “Cardiovascular Physiology Concepts”). Important mediators of autoregulation are adenosine and nitric oxide, which serve as a metabolic coupler between oxygen consumption and coronary blood flow. Response to nervous system: The coronary vasculature is innervated by both the sympathetic and parasympathetic nervous system. The sympathetic fibers act through alpha-1 receptors and activation causes transient vasoconstriction. Following this response, there is increased blood supply to the heart due to activation of beta-1 receptors. Hence, the final result of sympathetic stimulation is coronary vasodilatation and this is known as "functional sympatholysis”. Stimulation of parasympathetic nervous system results in vasodilatation only to an extent that the heart can function well. Beyond a certain limit, intrinsic coronary mechanisms increase the coronary vascular resistance by constricting the vessels (Klabunde, “Cardiovascular Physiology Concepts”). Collaterization: In progressive ischaemic coronary artery disease, there is growth of new vessels to surpass the block and continue adequate blood supply to the heart. The alternate circulation is known as collateral circulation (Figure-2) (Levick, “Introduction to Cardiovascular Physiology”). This circulation reduces the vascular resistance within the myocardium. This phenomenon is known as angiogenesis. Figure-2: Collaterization (“Preview of understanding angina and heart attacks”) Conclusion The coronary circulation is adapted to supply continuous blood supply to the heart so as to provide appropriate nutrition and remove metabolic waste quickly. Its anatomy is such that it allows free blood flow into the vessels and quick diffusion of oxygen to the cells of the heart. Due to the small size of the vessels, chances of block are very high. But, by way of formation of collaterals, the circulation ensures that the blood supply to the most important organ of the body is unaffected. Pulmonary circulation Introduction The purpose of the lungs is to remove carbondioxide from the body and drive in oxygen into the body. They are designed in such a way so as to optimize exposure of blood to oxygen. The transport of oxygen and carbondioxide to and from other parts of the body is by pulmonary circulation which may be defined as that part of the circulation which carries oxygen depleted blood away from the heart to the lungs and returns oxygenated blood to the heart so that this blood is supplied to all parts of the body (Widmaier, “Human Physiology”). Other than gas exchange, the pulmonary capillaries also filter emboli and thrombi from entering systemic circulation and act as blood reservoir for left ventricle. Figure-3: Pulmonary and systemic circulations (Parker, “Cardiovascular mechanics”) Anatomy The main artery of pulmonary circulation is the main pulmonary artery. It extends from the right ventricle and branches into left and right pulmonary arteries. These extend into the left and right lungs respectively. The pulmonary artery carries deoxygenated blood from the heart. From the lungs, blood is brought back to the heart by 4 pulmonary veins, 2 from each lung, into the left atrium. From the left atrium blood flows into the left ventricle from where blood enters the aorta and is supplied to all parts of the body (Figure-3). The branches of the right and left pulmonary artery further branch out and finally end up in a capillary network in the lungs. These branches run parallel to the airways to the level of terminal bronchioles. The pulmonary alveoli which are the functional units of the lungs are adjacent to the capillaries so that gas exchange is easy. The pulmonary capillaries are dense within the alveolar walls. Oxygen diffuses into the blood and carbondioxide diffuses into the alveoli which is then removed from the body during expiration (Figure-4). Figure-4: Pulmonary and systemic circulations (“Blood Circulation”). Autoregulation of pulmonary circulation The main mediator of pulmonary autoregulation is oxygen (Effros, “GI Motolity online”). Pulmonary vasodilatation occurs in regions where partial pressure of oxygen is high so that more blood enters that region of lungs from where it gets more oxygen. Similarly, in regions where the partial pressure of oxygen is low, pulmonary vasoconstriction occurs. Thus, there is direct linear relationship between blood flow and partial pressure of oxygen in the region (Effros, “GI Motolity online”). The purpose of this is to optimize oxygenation of blood. Another regulator of pulmonary circulation is acidemia. In contrast to systemic circulation, acidemia causes vasoconstriction of the pulmonary circulation. Pulmonary vasculature The pulmonary circulation contains about 450ml of blood in an adult which is about 9% in volume. The pulmonary artery systolic pressures average about 15% those in systemic arteries. Also, the pulmonary vascular resistance is about one tenth of systemic vascular resistance. This is because the smooth muscle fibers are attenuated in the pulmonary arterioles unlike in systemic arterioles. Hence there is very less pressure gradient for the blood to traverse through the pulmonary vascular blood. The low pressures in the pulmonary circulation serve another important purpose i.e., they make the pulmonary circulation act as a filter that prevents emboli from the systemic veins from reaching the brain, heart, and other critical arterial beds (Effros, “GI Motolity online”). Determinants of blood flow The main determinants of blood flow in the pulmonary vasculature are active resistance, alveolar pressure and hydrostatic pressure. The normal capillary hydrostatic pressure is 8-10 mmHg (Effros, “GI Motolity online”). During inspiration, the alveoli expand and compress upon the capillaries, thus increasing the resistance and decreasing the flow of blood. The reverse occurs during expiration. The exchange of gases in dependent upon the arteriolar-alveolar pressure difference. Also, blood flow depends on the position of the individual. When a person in supine, the blood flow is uniform over all the parts of the lungs. When the person is supine, blood flow through the apex in minimal while that at the base is maximum. Blood supply to the lungs The lungs are supplied both by the pulmonary circulation and systemic circulation. This is known as dual supply (“Physiology of pulmonary circulation”). The purpose of pulmonary circulation has been discussed above. The systemic circulation supplies nutrition to the bronchial tree and to the pulmonary tissue. This is through bronchial arteries. The right side of the lung is supplied by one bronchial artery which arises from the 3rd posterior intercostal artery or from the upper left bronchial artery. The left lung is supplied by 2 bronchial arteries which arise from descending thoracic aorta. Venous blood is removed from the lung tissue by 4 bronchial veins, 2 from each side. The right bronchial veins drain into the azygous vein while the left bronchial veins drain either into the left superior intercostal vein or into the hemizygous vein. Due to the presence of 2 circulations supplying blood to the lungs, anatomic shunts can occur between these two circulations. An anatomic shunt can deliver unsaturated venous blood to the arterial circulation. Under normal conditions, the effect is minimal because, increasing ventilation has little effect on oxygen saturation in blood issuing from the ventilated vessels, which is already nearly saturated (Effros, “GI Motolity online”). However in pathological and congenital shunts, decreased oxygen can enter the body tissues causing hypoxemia. Conclusion Thus the characteristics of pulmonary circulation are low pressure, low compliance and low resistance. This is to allow free flow of blood into the vasculature and enable effective gas exchange. Also, the anatomy of lungs and its circulation are such that oxygen intake is optimized. References Blood circulation. 14 Oct. 2007 Effros, Richard. Anatomy, development, and physiology of the lungs. GI Motility online 2006. 14 Oct. 2007 Read More