Vasodilation and Vasoconstriction - Essay Example

? VASODILATION AND VASOCONSTRICTION Vasodilation and Vasoconstriction One of the important aspects of the maintenance of a normal physiological circulatory mechanism is the adequate blood flow to the body tissues. The tissues are also capable of controlling the amount of blood flow they receive under a variety of circumstances. The control of blood flow is imperative to carry out a vast quantity of fundamental and essential functions. Delivery of oxygen and nutrients to the tissues, removal of unwanted substances such as carbon dioxide and hydrogen ions and the transport of various important hormones and ions across the tissues and to the other parts of the body are some of the chief functions performed by an adequate and regular blood flow. Hence, it becomes necessary for the tissues to control the blood they receive according to their necessities under a range of situations. Various organ tissues have different requirements. For instance, blood flow to skin is important for heat loss from the body. This is a significant step in the thermoregulatory functions of the body (Guyton & Hall, 2006). Vasoconstriction and vasodilation are two very fundamental and significant processes that are involved in controlling the amount of blood flow through a rapid and quick response under different body conditions and are brought about by a sequence of events and various mediators. Vasodilation and vasoconstriction is explained by various theories formulated in response to the body needs and metabolic conditions. Humoral control of the circulatory function is also an important regulator of vasodilation and vasoconstriction. The arterioles consist of small amount of elastic tissue which is lacking in the arteries. But, like arteries, they do consist of a thick layer of smooth muscle which is supplied by a large amount of sympathetic nerve fibres regulating the constriction and dilation. The smooth muscles also respond to other stimuli such as hormones, mechanical stretch and other local chemical changes. When smooth muscles of the arterioles are stimulated by the nerve fibers or by the other local chemical changes, the vessel circumference reduces and narrows because of the contraction of the smooth muscles surrounding the vessel circularly. This narrowing of the vessels is described as vasoconstriction and results in increased resistance and decreased blood flow to the target organs and tissues. Vasodilation, on the other hand, is the increase in the circumference of the blood vessel and is brought about by the relaxation of the smooth muscles of the arterioles. The enlargement of the vascular circumference causes reduced resistance to blood flow and thus increasing the flow to the target organs (Sherwood 2012). Apart from the vasodilation and vasoconstriction process which is achieved as a response to various stimuli, the arteriolar smooth muscles maintain a continuous state of contractility which is termed as “vascular tone”. Vascular tone is maintained by the myogenic capability of the smooth muscles which is self-induced and is not aided by any hormonal activity or sympathetic innervations. Second important factor that maintains vascular tone is the sympathetic nerve fibre innervations of the smooth muscles. These fibres are responsible for releasing nor-epinephrine, an important vasoconstrictor, for increasing the vascular tone (Sherwood 2012). The maintenance of vascular tone as an ongoing process is significant as the increase or decrease in vascular tone helps in achieving vasoconstriction and vasodilation, respectively. However, the factors that lead to the vasodilation and vasoconstriction are grouped into two categories- intrinsic controls and extrinsic controls. The former factors determine the distribution of cardiac output through vascular tone regulation while the extrinsic factors regulate the blood pressure (Sherwood 2012). Therefore, it is evident from the discussion that both vasodilation and vasoconstriction are imperative for blood flow control and are regulated by the smooth muscle contraction, sympathetic nerve fibres, circulating hormones and mechanical stress. These vascular processes ultimately regulate the cardiac output and blood pressure. The intrinsic factors are the local changes within an organ that determine the amount of blood it requires, hence, affecting the radius of vessels. This helps in adjusting the blood flow requirements of organs. The cardiac output is thus not distributed equally to all organs but is adjusted by each organ through altering the arteriole radius according to specific tissue demand. The local factors affecting the arteriolar radius are divided broadly into chemical influences and physical influences. The chemical factors influencing the vascular tone include histamine release and local metabolic changes. The local physical factors influencing the vascular tone are the smooth muscle response to the mechanical stretch, local heat or cold application (Sherwood 2012). Local metabolic needs are one of the most important influential factors for changes in the arteriolar radius. When the tissues are under an increased metabolic activity for instance skeletal muscles during exercise the tissues undergo various chemical changes. The oxygen concentration decreases because of active oxygen uptake and carbon dioxide concentration is increased. These factors trigger local vasodilation leading to increasing blood flow to the tissues to meet their high oxygen demands and remove the excess carbon dioxide. This increased blood flow due to the enhanced tissue activity is described as “active hyperemia”. On the other hand, in a relaxed state, the muscle does not require an active supply of blood and is less active metabolically too. Thus, the local metabolic circumstances of increased oxygen result in vasoconstriction (Sherwood 2012). Therefore, the tissue adjusts the blood flow according to its requirements with the help of local metabolic changes. The local metabolic alterations are enough to causes changes in vascular tone and hormonal or neural aid is not required. Regulation of the circulation during the exercise is both a crucial and important process. During exercise the skeletal muscles receive a greater share of the cardiac output due to the increased metabolic activity. Vasodilation is brought about by the local metabolic activity as discussed earlier. The adrenaline which is released from the sympathetic nerve fibres binds to the a-adrenergic receptors leading to vasoconstriction. This vasoconstriction is necessary to reduce the blood flow to the other areas of body such as kidneys, skin and splanchnic area. This increases the oxygen uptake of the skeletal muscles by 0.5 L (Astrand 2003). Hence vasodilation and vasoconstriction go side by side during the process of strenuous exercise. The local metabolic changes stimulate vasodilation of the arterioles while vasoconstriction of the precapillary vessels causes reduced flow to other organs hence providing more blood and oxygen to the active skeletal muscles. During acute inflammation, the local tissue demand for blood and nutrients is increased due to tissue injury or infection. Histamine is an important vasoactive amine and is released in response to physical injury or trauma. In cases of inflammation, histamine is released which causes arteriolar dilation causing increased blood flow to the injured area. Serotonin, prostaglandins and platelet-activating factor are other important cell-derived factors which help in vasodilation. The local response to injury or trauma is followed by a release of series of cell-derived and plasma-derived mediators that regulate the arteriolar radius according to tissue demand and requirement. The vasodilation mediated primarily by histamine release causes the characteristic erythema and warmth over the inflamed area. As a consequence, the vascular permeability is also increased leading to increased exchange of nutrients and proteins (Kumar et al 2007). Hence, the local physical factor influencing vascular tone majorly involves physical injury which can either be trauma from the outside or intrinsic infection and inflammation. The important endothelial derived vasoactive paracrines are the locally acting chemicals that alter the arteriolar radius under different circumstances. Nitric oxide brings about local arteriolar dilation by acting on the smooth muscles. The endothelial cells also release other paracrines such as endothelin which causes the arteriolar constriction. It acts on the smooth muscles causing their contraction. Apart from the local and acute changes brought about by the vasoactive paracrines some chemicals also bring long-term changes that alter the blood flow to an organ chronically. These include vascular endothelial growth factor which causes the growth of new blood vessels described by the term angiogenesis (Sherwood 2012). As mentioned earlier, blood vessels also respond to the hormonal action in response to which they constrict or dilate. The most important vasoconstrictor agents include norepinephrine and epinephrine. These are secreted by the sympathetic nerve fibres in release to stress or exercise and results in contraction of vessel smooth muscles. Angiotensin II is another powerful vasoconstrictor and acts by increasing the total peripheral resistance thus increasing the arterial blood pressure. Important vasodilator agents are bradykinin and histamine. Bradykinin not only causes arteriolar dilation but also increases vascular permeability (Guyton & Hall 2006). Vascular function can be evaluated through various laboratory techniques. Laboratory data and research trials have put forward that non-invasive techniques of measuring hemodynamic variables are very important in analyzing the hypertensive and cardiovascular conditions alongside the clinical practices (Smith & Levy 2008). Endothelial cell function assessment includes the testing of the amount of response of the endothelial cells to different types of stimulations such as vasoactive substance. Endothelial function can also be assessed by platelet function and inflammation activation like C-reactive protein. It can be assessed by both non-invasive and invasive techniques in the coronary and peripheral circulations of the humans. In normal individuals with healthy endothelium, vasodilation is produced after induction of stimuli however; individuals who are at risk will produce a reduced response. The various experiments and research data has made it evident and clear that endothelial function is an important biomarker of vascular function and the risk of atherosclerotic vascular diseases can be measured through these techniques (Verma et al 2003). Skin microcirculation is considered as a very easily accessible and authentic source of representing the functions and defects of the micro vascular system. With using minimally invasive techniques such as laser methods the skin microcirculation can be easily used for evaluating any dysfunctions in the endothelium. Among non-invasive methods the most recent laser development is the laser speckle contrast imaging technique that helps in evaluating any dysfunctions by monitoring the skin microcirculation (Mahe et al 2012). Therefore, skin serves as a very important and helpful circulation model for the assessment of the whole body’s microcirculatory system. Vasoconstriction and vasodilation are two fundamental processes that are required by the organs to meet their various metabolic requirements under a variety of circumstances. As it was discussed, both actions are stimulated by many intrinsic and extrinsic factors and bring about increased or decreased blood flow to the target organs. Both processes are controlled by their smooth muscle tone, sympathetic innervations, local chemical changes and the hormonal mediators. Vasodilation and vasoconstriction are important for the normal haemostasis as they maintain the adequate supply of nutrients and oxygen to the organs in situations such as stress, exercise, injury, heat, cold and infections. Bibliography A?STRAND, P.-O. Textbook of work physiology: physiological bases of exercise. Champaign, IL, Human Kinetics. GUYTON, A C AND HALL, J E. (2006). Textbook of medical physiology. Saunders, Elsevier Inc. KUMAR, V, ABBAS, A K, FAUSTO, N AND MITCHELL, R N. (2007). Robbins Basic pathology. Saunders, Elsevier Inc. MAHE G., LEFTHERIOTIS G., ABRAHAM P., HUMEAU-HEURTIER A., & DURAND S. (2012). Assessment of skin microvascular function and dysfunction with laser speckle contrast imaging. Circulation: Cardiovascular Imaging. 5, 155-163. SMITH, R., & LEVY, P. (2008). Review: New techniques for assessment of vascular function. Therapeutic Advances in Cardiovascular Disease. 2, 373-385. SHERWOOD, L. (2012).Fundamentals of Human Physiology. Belmont, CA : Brooks/Cole Cengage Learning. VERMA S, BUCHANAN MR, & ANDERSON TJ. (2003). Endothelial function testing as a biomarker of vascular disease. Circulation. 108, 2054-9. Read More