Potassium Regulation of the Heart and Cardiovascular System - Term Paper Example

Potassium regulation of the heart and cardiovascular system. The cardiovascular system of the human body consists of the heart, blood vessels e.g. arteries, veins, arterioles, capillaries, and blood which flows in these vessels. The human heart is located in the middle of the thoracic cavity and is oriented in a diagonal plane such that its apex leans slightly towards the left side. It is covered in a fibrous sac known as the pericardium, except in areas where the great vessels attach to it. Within the pericardium is a small quantity of fluid which lubricates the heart surface and decreases friction between the pericardium and heart during contraction and relaxation. The heart has a pivotal function in maintaining blood circulation in the entire body. It acts as a pump which generates the energy needed to force out blood from it and circulate within blood vessels of the cardiovascular system to perfuse all the tissues of the body. Blood circulation in the body is a passive process and can only occur if the heart pumps blood with sufficient force so that the mean arterial pressure (i.e. the average pressure within the arteries which carry blood away from the heart) is greater than the venous pressure (i.e. the pressure within veins which carry blood towards the heart). If this pressure imbalance is not maintained there will be backflow of deoxygenated blood towards the heart and eventually inadequate tissue perfusion along with other complications. The right side of the heart maintains flow through the pulmonary circulation whereas the left pump sustains blood flow in systemic circulation to the rest of the body.As indicated above the cardiovascular system maintains two different but simultaneous circulation of blood in the body and each circulation is pumped by separate chambers of the heart. Deoxygenated blood from the systemic circulation reaches the right ventricle of the heart from two great veins; the inferior and superior vena cavae. From the right atrium the blood flows through the tricuspid valve into the right ventricle which subsequently pumps the blood, via the pulmonary valve, into the pulmonary veins which are part of the pulmonary circulation. Oxygenated blood returning from pulmonary circulation is contained within the pulmonary arteries which drain into the left atrium. From the left atrium blood flows through the mitral valve into the left ventricle and this ventricle in turn pumps the blood with a lot of force into the aorta thru the aortic valve. The aorta then carries blood to the systemic organs via different vessels of the systemic circulation. (David E.Moharman) Although an action potential is required by all striated muscle to trigger contraction cardiac muscle differ from the rest because they are capable of generating and propagating it themselves. As we already know all cells have an electrical potential present across their membrane the only way that this potential can change (to trigger an action potential) is by making electrolytes move across. In cardiac muscle this membrane potential is maintained by three major electrolytes; potassium which is concentrated within the cell cytoplasm, calcium and sodium which are concentrated outside the cell. (Nevins) In order to understand how such a membrane potential is generated it is important to see the role of potassium ion in isolation. As the concentration of potassium is greater inside the cell than outside there will be a movement of potassium ions down the chemical gradient to the outside of a cell. This outward movement can be stopped by applying a negative charge inside the cell and thus creating an electrical gradient to counter the chemical gradient present. This charge needed to create no movement of potassium ions across the membrane is known as the equilibrium potential for potassium. The resting membrane potential in a cardiac muscle cell or myocyte is around -90 mV whereas the equilibrium potential for potassium is -96 mV, thus there is a net leakage of potassium ions out of the cell. If this continued to happen there would be an eventual loss of the chemical gradient present but the action of sodium potassium ATPase which brings potassium ions back into the cell counters this leakage. (Richard E. Klabunde) The permeability of an ion across the myocyte membrane is directly proportional to the number of ion channels present on the membrane. When these channels open they allow their specific ions to traverse the membrane to the opposite side and this phenomenon is how membrane potentials are manipulated. When an action potential arrives to a myocyte specific ion channels are depolarized which cause an influx of sodium ions to the cytoplasm along with calcium. During this time the potassium channels cause the efflux of potassium to the outside of the cell. This influx eventually leads to an altered potential across the membrane which is translated into contraction through a complex process of excitation contraction coupling. There are a variety of potassium ion channels present on a myocyte, each serving a specific function during a contraction cycle of the heart. The inward rectifier potassium channels are responsible for maintain membrane potential at equilibrium or rest i.e. before the arrival of an action potential. When a cell is depolarized these channels are suppressed and thus an altered potential is achieved. The delayed rectifier potassium channels get open towards the repolarizing phase of an action potential and allow potassium ions to diffuse back into the cell bringing it potential back towards equilibrium. There is another voltage gated channel (i.e. channels dependent on voltage) on myocytes called the transient outward potassium channel and this channel, unlike the ones mentioned previously causes efflux of potassium ions outside during the depolarization phase of an action potential. Then there are two ligand gated (i.e. dependent on different molecules) channels of potassium. One of these is an ATP-sensitive channel and functions when ATP levels are low, whereas the other one is activated by acetylcholine and thus functions upon parasympathetic stimulation. (David E.Moharman) Apart from maintaining the normal physiology of the heart potassium also has many protective effects on cardiovascular system. There is a strong inverse co relation between potassium intake and the development of hypertension. The long term regulation of blood pressure is achieved by the renin angiotensin and aldosterone system. In the scenario of increased blood pressure kidenys release renin which activate angiotensin II which in turn causes the release of aldoterone. Aldosterone is a hormone which causes increased sodium and water excretion thereby bringing down the blood pressure in vessels. Elevated potassium levels cause a decrease within the renal vasculature and thus increases glomerular filtration. Hence it can be seen that elevated potassium levels can cause a shift in the connection between blood pressure and sodium excretion that will eventually lead to a decrease in arterial blood pressure. Another proposed mechanism whereby potassium protects the cardiovascular system is by inhibiting free redical formation by macrophages and vascular endothelial cells. At the same time when potassium levels are on the higher limit they hinder vascular smooth muscle proliferation and thus inhibit narrowing of the vessle lumen. Narrowing of vessel lumen is also reduced by elevated potassium levels as it inhibits platelet aggregation and arterial thrombosis within the vessels; all these effects thereby reduce the likelihood of the formation of atherosclerotic lesions and thrombosis within vessels, formation of which can have precarious effects on the cardiovascular system. Thus by way of these protective mechanisms of potassium it can be appreciated the importance of this electrolyte in our daily diet for healthier functioning of our cardiovascular system.(Young DB) References: David E.Moharman, Lois Jane Heller. Cardiovascular Physiology. McGraw Hill, 1997. Nevins, Patricia. Events that occur during resting phase of a cardia cycle. 3 May 2011. November 2011 . Richard E. Klabunde, PhD. Cardiovascular Physiology Concepts. 2011. November 2011 . Top of Form Young, DB, H Lin, and RD McCabe. "Potassiums Cardiovascular Protective Mechanisms." The American Journal of Physiology. 268.4 (1995): 825-37. Print. Bottom of Form Read More