Circulation of Blood in Human Body – The Circulatory System

How circulation of blood in human body occurs? The oxygenated blood is pumped to the human body by the heart and then deoxygenated blood is pumped to the lungs. The system through which the blood is pumped by the heart is called Circulatory system. The heart uses one atrium and one ventricle for each circulation. In Systemic and pulmonary circulation four chambers are used by the heart: (1) – left atrium (2) – left ventricle (3) – right atrium (4) – right ventricle.

1. Systemic and Pulmonary Circulations

There are two separate circulatory systems. The systemic circulation consists of arteries, capillaries, and veins which circulate the blood to all parts of the body except the lungs. The function of the systemic circulation is to supply oxygenated blood to all tissues of the body and to take back the reduced or unoxygenated blood back to the heart. The second circulation is the pulmonary circulation which circulates blood to the lungs for the purpose of oxygenation.

Systemic and pulmonary blood circulation system

2. Blood Vessels

Blood vessels are of 3 types:

 Arteries:

Arteries are thick walled blood vessels that carry blood under high pressure from the heart to the tissues. The end of the artery is highly specialized; it can constrict or dilate, thus adjusting the diameter of the blood vessel and offering resistance to the flow of blood from the arteries to the capillaries. The highly specialized part at the end of the artery is called the arteriole.

Capillaries:

Capillaries are very thin walled blood vessels which permit the passage of water, gases, and small molecules from inside the capillary to the “outside” into the tissues. The pressure inside the capillary is high at the arterial end, thus forcing out liquid, nutrients, and oxygen into the tissues. At the venous end of the capillaries the pressure is low, so that tissue fluid is sucked back in the capillary by the osmotic tension of the proteins in the plasma. The function of the capillary is to supply fluid, gases (especially oxygen) and nutrients to the tissues and to take back tissue fluid, carbon dioxide, and the products of metabolism. The blood from the capillaries goes into
the veins.

Blood Vessels
Veins:

Veins are the collecting channels which take the blood from the capillaries and transport it to the heart. To prevent back flow of blood in the veins, there is a system of valves which allow the blood to travel towards the heart but not in the reverse direction.

Structure of artery and vein

Lymphatics:

Not all the fluid that goes out of the capillary (arterial end), returns to it at the venous end. A large volume of tissue fluid enters fine channels in the tissues, called lymphatics, which also serve to carry the fluid, called lymph, back to the heart. During its passage lymph has to pass through a system of filters called lymph which not only act as filters but also destroy foreign substances and invading micro-organisms, The lymph nodes form a part of the immune system. In infections, the lymph nodes may swell up due to inflammation. In cancer, deposits of cancer cells in the lymph nodes causes their enlargement. Lymph ultimately drains into the great veins which empty into the right auricle.

Interior structure of a vein

3. Peripheral Resistance

The resistance to the flow of blood is dependent on many factors such as the viscosity of the blood and the diameter of the blood vessels. The narrowest blood vessel encountered by the blood flow is at the arteriole. Arterioles have a coiled muscle which their diameter, and thus can increase or decrease the resistance to the blood flow, Peripheral resistance is the most important factor which determines resistance to the flow of blood.

peripheral resistance

4. Vascular Distensibility

Blood vessels are elastic and therefore as the pressure increases, they can distend. This is important as otherwise at every heart beat the pressure in the blood vessel would rise very steeply.

5. Vascular Compliance

vascular compliance is the property of the blood vessel to distend and accommodate extra blood. The compliance of the veins is the maximum. If all the veins were to dilate simultaneously it could accommodate 800 times the amount of blood in the body. Compliance of the arteries allows the extra blood pumped into the arterial system by systole of the heart, to be accommodated.

6. Cardiac Output

1. Cardiac output is the volume of blood pumped out by the heart with each beat. This is roughly equal to 70ml/beat. However, when the heart beats vigorously, e.g. after exercise, the cardiac output per beat may go up to 6 times the resting cardiac output. The cardiac output per minute depends on the cardiac output per beat, multiplied by the number of beats per minute.
2. The optimum cardiac output per beat depends on the amount of blood in the left ventricles just before it contracts. For this to be optimum, it is necessary that there is sufficient time for the blood from the auricles to fill the ventricles. If the heart rate is very fast, the cardiac output per beat falls, although because of the more rapid beating, the cardiac output per minute may rise. With a slow heart rate, the stroke output (per beat) is high but the minute output may be small. With a very fast rate, the stroke output is so low (the diastole is too short) that even the fast heart rate cannot compensate and the minute output falls. At rest, the optimum heart rate is 60-90 per minute. During exercise, this may go up to 120-150/min.
3. The cardiac output depends very greatly on the efficiency of the left ventricle. When the left ventricle is healthy, it pumps 60% of the blood that it Contains. When the left ventricle is diseased, i.e. there is heart failure, it can pump only a smaller fraction of the blood that it contains. In heart failure, at the end of the ventricular contraction, there is a large quantity residual blood in the left ventricle. The ratio of the volume of blood pumped out at each stroke, to the blood contained inside the left ventricle before contraction is called the ejection fraction. When the left ventricle fails, the ejection fraction falls to less than 30%.
4. There are many sophisticated investigations to determine the thickness of the muscle of the left ventricle, the efficiency of its contraction and the amount of blood that it pumps out per beat. The commonly employed investigation is an echocardiogram.
5.In addition to the force of cardiac contraction, some other factors may also influence the cardiac output. The total blood volume influences the cardiac output. A rise in blood volume will increase the venous return of blood to the heart. This will cause greater filling of the left ventricle and increase the cardiac output.
6. The left ventricle contracts and pushes the blood into the arterial system against the pressure inside the arterial system. The higher the pressure in the arterial system, the greater is the resistance offered to the pumping of blood by the left ventricle. Therefore, blood pressure is another factor which determines the cardiac output per beat.

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