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:

The tunica interna forms the inner lining of the vessel. It contains endothelial cells and collagen.
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.
The tunica externa forms the outer layer and is made up of mostly collagen with some elastin fibres. Vessel walls need to be elastic as they have to cope with large fluctuations in blood volume.

Arteries and arterioles – Arteries always carry blood away from the heart. As the arteries branch and become smaller they eventually form arterioles. The largesrt arteries contain a lot of elastin fibres but as they get smaller the muscular middle layer becomes much thicker and the amount of elastin relatively less. The smaller, more muscular, arteries and the arterioles are used to control blood flow. Contraction of the smooth muscle is these vessels narrows their lumen and restricts blood flow.

Capillaries – The arterioles transport blood to the capillaries. The capillaries are the smallest of the blood vessels and their walls are very thin to enable the exchange of nutrients and gases to take place. Capillaries form an extensive network, especially around skeletal muscle, the heart and the lungs. Capillaries are so small that blood cells can only pass through them one at a time. The flow of blood through the capillaries is controlled by precapillary sphincters. Capillaries ensure a constant supply of blood to all cells.

Veins and venules – Blood flows from the capillaries into the venules. As the venules decrease in number they increase in size and eventually form veins. Veins have much thinner inner and middle layers than arteries and the larger veins contain valves. These valves allow blood to flow only in one direction back to the heart, helping venous return.

Venous return

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Venous return

Stroke volume depends on venous return. If the venous return decreases the stroke volume will decrease, reducing the overall cardiac output. A vein has a quite a large lumen and offers very little resistance to blood flow. However, by the time blood has entered the veins the blood pressure is low and active mechanisms are needed to ensure venous return.

The skeletal muscle pump – This is the most important mechanism in venous return. When we move our muscles contract, squeezing and compressing nearby veins. This action pushes the blood back towards the heart as the valves in the vein allow the blood to flow in one direction only, preventing backflow and pooling.

The respiratory pump – When air is breathed into and out of the lungs the volume of the thoracic cavity changes, creating pressure changes. During inspiration the pressure around the abdomen increases as the diaphragm lowers to increase the volume of the thoracic cavity. This pressure squeezes the blood in the abdominal veins back towards the heart. During expiration the pressure in the thoracic region increases as the diaphragm and ribs move back to reduce the volume of the thoracic cavity. This has a similar squeezing effect on the veins.

The valves – The valves play an important role in venous return as they direct the flow of blood to the heart.

Vasomotor control

The flow and pressure of blood are controlled by the vasomotor centre in the medulla of the brain. The vasomotor centre is stimulated by baroreceptors (which respond to changes in blood pressure) in the aorta and carotid arteries. Most blood vessels are stimulated by sympathetic nerves of the autonomic nervous system. Blood vessels receive a continual low frequency impulse that is known as the vasomotor tone. The vasomotor centre controls this stimulus by:

Increasing vasomotor tone, causing vasoconstriction (the lumen decreases in size, resulting in an increase in blood pressure and a reduction in blood flow).
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. This is called the vascular shunt. The vascular shunt involves two mechanisms:

Vasodilation of the arterioles supplying the skeletal muscles increases the blood flow to them. Vasoconstriction of the arterioles supplying the other organs reduces blood flow to these organs.
Opening of the precapillary sphincters in the capillary network supplying skeletal muscle and closure of the precapillary sphincters in the capillary networks supplying the other organs increases the flow of blood to the skeletal muscles and decreases flow to the other organs.

The net effect is to greatly increase the percentage of cardiac output going to the muscles. The vascular shunt mechanism does not only increase blood flow to working muscles. If you are involved in strenuous or prolonged periods of exercise you begin to get hot. The body’s response to overheating is to dilate the blood vessels near the skin, increasing the blood flow to the skin and allowing heat to escape from the body.

Blood flow and blood pressure

Blood flows from areas of high pressure to areas of low pressure. The area of high pressure is the pressure created by the contraction of the ventricles, which forces blood out of the heart into the aorta. Blood pressure is equal to blood flow x resistance. The resistance is caused by friction between the blood and the vessel walls.

During systole the blood pressure at rest for a young adult is roughly 120mmHg and during diastole the pressure drops to about 80mmHg. Blood pressure is usually measured using a sphygmomanometer. Various factors, including exercise, pregnancy, stress, etc., can affect blood pressure and blood pressure varies between individuals.

As blood flows into the large arteries the blood pressure is quite high because they have relatively large lumens and offer little resistance to blood flow. When the blood reaches the arterioles the pressure drops suddenly because the resistance exerted by the arteriole walls is much greater. By the time blood reaches the capillaries the blood pressure has dropped to about 35mmHg. As the blood passes back through the venous system the pressure continues to fall, and is almost zero by the time the blood enters the right atrium.

The arterioles play a significant role in regulating blood pressure. Through vasoconstriction or vasodilation of the arterioles their resistance can be increased or decreased, which in turn increases or decreases the blood pressure.

Velocity of blood flow

At rest a blood cell will take roughly a minute to be carried around the circulatory system, but the velocity of the blood flow is far from constant as it passes from one blood vessel to another. The velocity of the blood flow is affected by the cross-sectional area of the blood vessels. Blood travels through the aorta at about 40cm/s. As it travels through the smaller arteries and arterioles the total cross-sectional area increase (although the cross-sectional area of individual vessels decreases, there are many more of them), decreasing the velocity of the blood. The greatest total cross-sectional area is to be found in the capillary network as there are many capillaries. In the capillary network the velocity of the blood is only about 0.1cm/s, slow enough to allow the exchange of gases, waste products and nutrients. The blood then flows back through the venules and veins, where the total cross-sectional area decreases, resulting in an increase in velocity.

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.