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Structure and function of the vascular system

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Introduction

Structure and function of the vascular system The blood vessels are part of the cardiovascular system and form the body's transport network. It is essential that a sports performer has an efficient vascular system, to deliver oxygen and food supplies to the working muscles and to remove waste products such as carbon dioxide. The blood carries all the vital ingredients needed for the muscles to work and the blood vessels form a closed circulatory network, allowing distribution of blood to all cells. During exercise there is a dramatic change in the distribution of blood around the body, with up to 85% of cardiac output going to the working muscles. The heart, vascular and respiratory systems all work together to coordinate the increase in oxygen delivery needed to cope with the increased demand for energy. Blood vessels Five different types of blood vessels in the body link together to form the vascular system. All blood vessels are basically a muscular wall surrounding a central opening called a lumen. The walls of the blood vessels (except the capillaries) comprise three layers: 1. The tunica interna forms the inner lining of the vessel. It contains endothelial cells and collagen. 2. The tunica media forms the middle layer and is made up of smooth muscle and elastin fibres. The smooth muscle is stimulated by the sympathetic nerves of the autonomic nervous system. ...read more.

Middle

Blood vessels receive a continual low frequency impulse that is known as the vasomotor tone. The vasomotor centre controls this stimulus by: 1. Increasing vasomotor tone, causing vasoconstriction (the lumen decreases in size, resulting in an increase in blood pressure and a reduction in blood flow). 2. Decreasing vasomotor tone, causing vasodilation (the lumen increases in size, resulting in a decrease in blood pressure and an increase in blood flow). As the arteries have a relatively thick tunica media they are responsible for most of the changes in blood flow and blood pressure. There is also a degree of local control of blood distribution, called autoregulation. The arterioles in some areas of the body react directly to chemical changes in the tissues that they supply. An increased demand by the tissue for oxygen seems to trigger the response of vasodilation of the surrounding arterioles, so do increases in carbon dioxide and lactic acid. The vascular shunt - During exercise the demand for oxygen from the skeletal muscles increases dramatically and more oxygenated blood must flow to them to meet this demand. The increase in stroke volume and heart rate helps to increase the overall cardiac output and therefore increases oxygen supply, but this in itself is not enough. Blood must also be redistributed so that more goes to the skeletal muscles and less to the other organs. ...read more.

Conclusion

Effects of exercise on blood pressure and blood volume Systolic blood pressure tends to increase during exercise. The vasodilation that occurs in skeletal muscle causes a drop in blood pressure because of the decrease in resistance, but the cardiac output increases significantly and negates the effect of this vasodilation. During exercise there is very little change in diastolic pressure, which only increases during isometric work because of the resistance to blood flow caused by the contracting muscle. After a period of exercise it is much better to perform a series of cool-down activities than to stop abruptly. If you stop suddenly the blood 'pools' in the working muscles, and as during heavy exercise, up to 85% of the cardiac output is distributed to them, the venous return will instantly drop. The knock-on effect is that less blood enters the heart during diastole which means that the stroke volume will be much lower, leading to a drastic reduction in blood pressure causing sickness and dizziness. Blood volume can change during exercise, but whether it increases or decreases depends on the type of activity and the fitness of the individual. A decrease in volume is mostly caused by plasma moving out of the capillaries into the surrounding tissues. This increases the viscosity of the blood and therefore increases the peripheral resistance. After a period of aerobic training the usual trend is an increase in blood volume. This is of great benefit to performers as it increases their capacity to carry oxygen. ...read more.

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