Blood Vessels and Circulation

Blood Vessels and Circulation

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Blood Vessels and Circulation...

Blood Vessels and Circulation:

The role of blood vessels in the circulation of blood has been known since 1628, when William Harvey, an English anatomist, demonstrated that blood in veins always flowed toward the heart. Before that time blood was considered stationary, some of it within the vessels, but the rest of it in puddles throughout the body.

Blood vessels are the channels or conduits through which blood is distributed to body tissues. The vessels make up two closed systems of tubes that begin and end at the heart.

The vascular system consists of the arteries, capillaries, and veins through which the heart pumps. As you will see, the major business of the vascular system (which is the exchange of materials between the blood and tissues) takes place in the capillaries. The arteries and veins are just as important for their transportation function between the capillaries and the heart.

ARTERIES

Arteries carry blood away from the heart to the capillaries; smaller arteries are called arterioles. Systemic arteries transport oxygenated blood from the left ventricle to the body tissues. Pulmonary arteries transport blood that has a low oxygen content from the right ventricle to the lungs.

Blood is pumped from the ventricles into large elastic arteries that branch repeatedly into smaller and smaller arteries until the branching results in microscopic arteries called arterioles. The arterioles play a key role in regulating the blood flow into the tissue capillaries. About 10% of the total blood volume is in the systemic arterial system at any given time.

The wall of the artery consists of three layers. The innermost layer is called the tunica intima and it is lined with a simple squamous epithelium called the endothelium. This is the same type of tissue that forms in the endocardium. It's function is the same; it's extreme smoothness prevents abnormal clotting.

The tunica media or the middle layer, is made up of smooth muscle and elastic connective tissue. Both of these tissues are involved in the maintenance of normal blood pressure, especially the diastolic pressure or when the heart is relaxed.

Fibrous connective tissue forms the outer layer or the tunica externa or adventia. This tissue is very strong, which is important to prevent the rupture or bursting of larger arteries that carry blood under high pressure. The outer and middle layers of the large arteries are quite thick. The smooth muscle layer enables arteries to constrict or dilate. This is regulated by the medulla and the ANS.

An aneurysm is a bulge or bubble that develops at a weakened region in the wall of the artery. This is especially dangerous if it is in the aorta or arteries of the brain. If diagnosed soon enough, the aneurysm may be removed and the vessel surgically repaired. Because the wall is weakened, an aneurysm is subject to rupture. Little can be done when this happens because the massive bleeding usually leads to death before medical care can be obtained.

VEINS

Veins carry blood from the capillaries back to the heart; the smaller veins are called venules. The same three tissue layers are present in veins as in the walls of arteries; but there are some differences when compared to arterial layers. The inner layer of the veins is smooth endothelium but at intervals this lining is folded to form valves. Valves prevent backflow of blood and are most numerous in veins of the legs, where blood most often returns to the heart against the flow of gravity.

The middle layer of veins is a thin layer of smooth muscle. It is thin because veins do not regulate blood pressure and blood flow into capillaries as arteries do. Veins, can constrict extensively and this function becomes very important in certain situations such as severe hemorrhaging. The outer layer of veins is also thin; not as much fibrous connective tissue is necessary because blood pressure in veins is very low.

Varicose veins are veins that are twisted and dilated with accumulated blood. These frequently occur in legs. Conditions that hinder venous return, such as pregnancy, obesity, and standing for long periods of time, allow blood to accumulate in the veins of the extremities. This stretches the veins so the valve flaps no longer overlap and they permit the backflow of blood. Superficial veins are more susceptible because they receive less support from surrounding tissue.

Phlebitis is an inflammation of a vein. This condition is most common in the veins of the legs because they are subjected to a great amount of pressure as the blood is returned to the heart against the flow of gravity. Often no specific cause can be determined but advancing age and blood disorders have been known to be predisposing factors

About 70% of the total blood volume is in the veins at any given time. Venous valves are especially important in the arms and legs, where they prevent the backflow of blood in response to the pull of gravity.

CAPILLARIES

Capillaries carry blood from arterioles to venules. Their walls are only one cell in thickness; capillaries are actually the extension of just the lining of arteries and veins. These vessels are the smallest and most numerous of the blood vessels. When the arterioles branch into the capillaries, the middle and outer layers disappear so that the capillary wall is only a thin endothelium lining. This thin wall permits the exchange of materials between the blood in the capillary and the adjacent tissue cells. This exchange is the primary function of capillaries.

Capillaries are microscopic vessels that form a connection between arteries and veins. Their primary function is the exchange of materials between the blood and tissue cells. The diameter of a capillary is so small that RBC's must pass through them in single file. It is estimated that if all the capillaries in the body were place end to end, they would encircle the earth at the equator 2.5 times!!!!!!!

PHYSIOLOGY OF CIRCULATION

ROLE OF THE CAPILLARIES

Capillaries have a vital role in the exchange of gases, nutrients, and metabolic waste products between the blood and the tissue cells. Substances pass through the capillary wall by diffusion, filtration, and osmosis.

Gases move by diffusion, that is, from their area of greater concentration to their area of lesser concentration. O2, therefore, diffuses from the blood in systemic capillaries to the tissue fluid, and carbon dioxide diffuses from tissue fluid to the blood to be brought to the lungs and exhaled.

Fluid movement across a capillary wall is determined by a combination of hydrostatic and osmotic pressures. The end result of the capillary circulation created by these two pressures is that substances leave the blood at one end of the capillary and return at the other end. Normally more fluid leaves the capillaries than is returned to them. About 90% of the fluid is returned at the venule end. Because of the capillary circulation, the interstitial fluid is continually changing and nutrients, gases, and waste products are moved between the tissue cells and the blood.

Edema is an abnormal accumulation of interstitial fluid, or swelling. This may be caused by a disruption of normal capillary circulation. Factors that may lead to edema include an increase in capillary blood pressure, a decrease in the quantity of plasma proteins, and an increase in the permeability of the capillary wall so that proteins leak out.

RELATIONSHIP OF BLOOD FLOW TO BLOOD PRESSURE

Blood flow refers to the movement of blood through the vessels from arteries to the capillaries and then into the veins. Pressure is a measure of the force that the blood exerts against the vessel walls. Like all fluids, blood flows from a high pressure area to a region with a low pressure. Because the contraction of the ventricles provides this force, or pressure, it is greatest during ventricular systole when the blood is pumped from the left ventricle into the aorta.

The velocity or speed with which blood flows differs in the various poarts of the vascular system. Velocity is inversely related (meaning as one value goes up, the other goes down) to different segments of the vascular system.

The aorta receives all the blood from the left ventricle, and the blood moves very rapidly. Each time the aorta branches, the area in which the blood is traveling becomes spread out and the blood velocity slows or the blood flow decreases. Think of a river that begins in a narrow bed and is flowing rapidly. If the river bed widens, the water spreads out to fill it and flows more slowly. If the river were to narrow again, the water would move faster. This is what happens with the vascular system.

The capillaries cover the largest amount of area, and blood velocity there is the slowest. When capillaries unite to form venules, and then veins, the area decreases and the blood flow speeds up.

If you remember, it is in the capillary network where there will be an exchange of gases, nutrients, and wastes. The slow rate of blood flow in the capillaries permits sufficient time for these essential exchanges. Think of a train slowing down at a station but not actually stopping to allow people to jump off and on, then speeding up to get to the next station. The capillaries are the stations of the vascular system.

The more rapid blood velocity in other vessels makes circulation time quite short.

Blood pressure is the force the blood exerts against the wall of the blood vessels. The pumping of the ventricles creates blood pressure, which is measured in mmHg. When a systemic blood pressure is taken, two numbers are obtained: systolic and diastolic. Systolic is always the higher of the two and represents when the left ventricle is contracting. The lower number is the diastolic pressure, when the left ventricle is relaxed and does not exert force. Diastolic pressure is maintained by the arteries and aterioles.

Systemic blood pressure is highest in the aorta which receives all the blood pumped by the left ventricle. As blood travels farther away from the heart, blood pressure decreases.

The brachial artery is most often used to take a blood pressure reading. In the arterioles blood pressure decreases further and then systolic and diastolic pressures merge into one pressure.

MAINTENANCE OF SYSTEMIC BLOOD PRESSURE

Because blood pressure is so important, there are many factors and physiological processes that interact to keep blood pressure within normal limits.

VENOUS RETURN- the amount of blood that returns to the heart by way of the veins. Venous return is important because the heart can pump only the blood it receives. If venous return decreases, the cardiac muscle fibers will not be stretched, the force of the ventricular systole will decrease and blood pressure will decrease. This is what happens following a severe hemorrhage. When the body is horizontal, venous return can be maintained fairly easily, but when the body is vertical, gravity must be overcome to return body from the lower part of the body to the heart. There are three mechanisms that help promote venous return; constriction of the veins, the skeletal muscle pump, and the respiratory pump. Veins contain smooth muscle which enables them to constrict and force blood toward the heart.