Blood circulatory system (Blood vascular system )

Blood circulatory system    (Blood vascular system )

Blood circulatory system is regarded as a kind of vascular system and therefore, it is called blood vascular system. A vascular system is that which has tubes full of fluid to be transported from one place to another.  The blood vascular system comprises of heart, the organ which pumps and receives the blood, and blood vessels, which are tubes through which the blood flows. 

   Blood vessels-

The blood vessels are elastic muscular tubes which carry blood. There are three kinds of blood vessels in human body.

    They are of three types-(1)Arteries 

(2)veins and 

(3)capillaries.

(1)Arteries-

Arteries are the thick walled blood vessels which carry blood away from the heart for distribution to the body. The walls of arteries particularly those near the heart are thick that enables them to dilate but not rupture when the heart contracts and forces blood into them. Thus the blood passing through narrow lumen of arteries is aerated and has a considerable pressure.

(2) Veins -

veins are thin walled blood vessels which bring blood from the body back to the heart. They are larger and hold more blood than the arteries. The blood passing through wide lumen of veins is nonaerated and has low pressure. It flows slow and steady. The veins have valves that allowed the blood to flow only towards the heart and prevent the backflow.

(3) Capillaries-

Capillaries are thin walled and extremely narrow blood vessels which occur at the terminals of artery and vein. They join the arteries and veins together. They form a vast network of vessels. The wall of capillaries are permeable to water and dissolved substances so that the exchange of materials between the blood and body cells takes place. The blood also flows very slow which facilitates the exchange of food material, gases and waste products.

The Human Heart-

The heart is a hollow, muscular organ roughly of the size of our clenched fist (12 × 9 CM). Its average weight is about 300 gram in males and about 250 gram in females. It is reddish- brown in colour and somewhat conical in form. The heart is situated between the two lungs in the middle of the thoracic cavity.  It is surrounded by a tough, two layer Sac, the pericardium. The pericardial fluid is secreted in the pericardial cavity between the pericardium and heart which reduces the friction between the heart wall and surrounding tissues when the heart is beating.

Structure of heart-

The heart is a double pump. It is divided by septa into two halves: the right and the left. Each half consists of two communicating Chambers: upper smaller auricle or atrium and lower larger ventricle. Thus the heart has four Chambers: The two upper Chambers called atria and two lower Chambers called left and right ventricles. There are valves between left atrium and left ventricle and between right atrium and right ventricle. These valves provide one way passage and prevent the return of blood. That means these two valves permit the flow of blood from atrium to ventricle and not in the reverse order . The walls of heart are composed of special muscle cells, called cardiac muscle fibres.

Function of heart-

The most vital function of heart is heartbeats which takes place all the time throughout one's life

. The sequence of events which takes place during the completion of one heartbeat is called the cardiac cycle. It involves repeated rhythmic contraction and relaxation of heart muscles. Contraction is called systole and relaxation is called diastole. The cardiac cycle involves the  followings:

(1) During the time when the muscles of all four Chambers of the heart and relaxed the blood returning to the heart under low pressure in the veins enters the two atria. Blood from large veins called vena cava pour into right atrium. This blood comes from head, upper body parts and lower body parts where oxygen has been used up and the blood is free from oxygen (deoxygenated). Thus the deoxygenated blood enters the right atrium. At the same time the pulmonary vein from lungs pours oxygenated blood into the left atrium. Thus the oxygenated blood enters the left atrium.

(2) As the right and left Atria filled with blood pressure in them rises so that the walls between left atrium and left ventricle (bicuspid valve) and between right atrium and right ventricle (tricuspid valve) open and the Atria contract. Atrial contraction (atrial diastole) faces pumping of deoxygenated blood from right atrium into the right ventricle through tricuspid valve and oxygenated blood from left atrium into left ventricle through bicuspid valve.

(3) Almost immediately the ventricles contract. This is called ventricular systole. During contraction of ventricles the deoxygenated blood from right ventricle flows to the lungs through pulmonary artery and the oxygenated blood from left ventricle is distributed to all the parts of the body through the largest artery called aorta.

Double circulation-
The blood circulation in human heart is double circulation. This means that blood passes through the heart twice for each circuit of the body. One circulation involves the entry of blood from all body parts into the heart. This blood is deoxygenated which goes to the lungs for oxygenation. The second circulation involves entry of oxygenated blood from lungs into the heart and then its distribution to all parts of the body. Double circulation is made possible because the human heart is divided into two. One-half pumps de- oxygenated blood to the lungs and the other half pumps oxygenated blood to the rest of the body.