Human Circulatory System

The Circulatory System is the system of organs in the human beings and other animals which is responsible for transport of materials inside the body. It consists of heart, arteries, veins, capillaries and blood. The heart acts as a pump to push out blood. The arteries, veins and capillaries act as pipes or tubes through which the blood flows. These tubes carries blood are called blood vessels. So, there are three types of blood vessels in the human body: arteries, veins and capillaries. Now in detail we will understand all the parts of the circulatory system.

Roughly the shape of the heart is triangular and is made up of special muscle called cardiac muscle. The heart has four compartments known as ‘chambers’ inside it. The upper two chambers of heart are known as atria (in singular atrium) and the lower two chambers of heart are called ventricles. From two main veins these two atria receives blood. And the two ventricles transport blood to the entire body and the lungs. The left atrium is connected to the left ventricle through a valve V₁. Similarly, the right atrium is connected to the right ventricle through another valve V₂. These valves prevent the back flow of blood in to atria when ventricles contract to pump blood out of the heart to the rest of the body. This is because when the ventricles contract, the valves V₁ and V₂ close automatically so that the blood may not go back to atria. So, heart pumps blood around our body. All the atria and veins contract and relax and make heart to pump blood. Since ventricles have to pump blood in to various organs with high pressure, they have thicker walls than atria.

A sheath of tissue called ‘pericardium’ protects the muscular heart and the chambers of the heart are separated by a partition called septum. Now, before describing the circulation of blood in the human body it is necessary to know more about arteries, veins and capillaries.

• Arteries are thick walled blood vessels which carry blood from the heart to all the parts of the body. They are thick because blood emerges from the heart under high pressure. Arteries are found in whole of our body. The main artery called aorta is connected to the left ventricle of the heart through a valve V₃. The function of main artery is to carry oxygenated blood from left ventricle to all the parts of the body except lungs. Another artery called pulmonary artery is connected to the right ventricle of the heart through another valve V₄. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs.
• The Capillaries are thin walled and extremely narrow tubes or blood vessels which connect arteries to veins. Thus, the exchange of various materials like oxygen, food, carbon dioxide etc between the blood and the body cells takes place through capillaries only. The other end of capillaries is joined to some wider tubes called veins. The deoxygenated blood or dirty blood coming from the capillaries enters in to veins.
• Veins are the thin walled blood vessels which carry blood from all parts of the body back to the heart. Veins do not need thick walls because the blood flowing through them is no longer under high pressure. Veins have valves in them which allow the blood to flow in only one direction towards the heart.

The main difference between artery and vein is that an artery carries blood from the heart to the body organs whereas a vein carries blood from the body organs back to the heart. The blood which carries oxygen is called oxygenated blood and the blood which does not carry oxygen is called deoxygenated blood, however carries carbon dioxide in it.

Also when blood circulates in our body it supplies oxygen, digested food and other chemicals like hormones to all the cells of the body. It also carries back waste products like carbon dioxide etc. from the body cells.

Process of blood circulation in humans

• The pulmonary vein brings the oxygenated blood from the lungs in to the left atrium of the heart, when the muscles of all the four chambers of the heart are relaxed.
• When the left atrium contracts, the oxygenated blood is pushed in to the left ventricle through the valve V₁.
• When the left ventricle contracts, the oxygenated blood is forced in to the main artery called ‘aorta’. These arteries are in branched form and goes in to the different body organs. The smaller arteries called arterioles further branch in to capillaries.
• The main artery carries blood to all the organs of the body like head, chest, arms, stomach, intestines etc. In this way the cells of the body gets oxygen from capillaries and then blood becomes deoxygenated by loosing oxygen. Now the deoxygenated blood from the body organs enters in to the main vein called vena cava. The main vein carries the deoxygenated blood to the right atrium of the heart.
• When the right atrium contracts, deoxygenated blood is pushed in to the right ventricle through the valve V₂.
• And when the right ventricle contract, the deoxygenated blood is pumped in to the lungs through the pulmonary artery. In the lungs, deoxygenated blood releases its carbon dioxide and absorbs fresh oxygen from air. So, the blood becomes oxygenated again. This oxygenated blood is again sent to the left atrium of heart by pulmonary vein for circulation in the body.

This whole process is repeated continuously.

So, now we can say that a circulatory system is that in which the blood travels twice through the heart in one complete cycle of the body called double circulation. In human circulatory system the pathway of blood from the heart to the lungs and back to the heart is called pulmonary circulation and the pathway of blood from the heart to rest of the body and back to the heart is called systemic circulation. And these two together make double circulation.

Heart Beats:

One complete contraction and relaxation of the heart is called a heart beat. The heart beat usually beats about 70 to 72 times in a minute when we are resting. A stethoscope is an apparatus due to which doctors listens our heart beat. The heart beat faster during and after exercise because the body needs more energy under these conditions.

Blood Pressure:

The pressure at which blood is pumped around the body by the heart is called blood pressure. The maximum pressure at which the blood leaves the heart through the main artery during contraction phase is called the systolic pressure and the minimum pressure in the arteries during the relaxation phase of heart is called diastolic pressure.

Systolic pressure: 120 mm Hg

Diastolic pressure: 80 mm Hg

So, the normal blood pressure is 120/80 and is measured by an instrument called sphygmomanometer.

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