BLOOD AND ITS CIRCULATION

BLOOD

Composition of blood: Blood  is made up of a fluid called plasma (60%) and a great number of blood cells called corpuscles (40%). The plasma consists of 90% water , proteins and inorganic salts. Organic substances such as glucose, amino acids ,fats , urea, hormones and enzymes occur in plasma. The corpuscles are of two types, red and white . 

Red corpuscles are produced in the spleen. They form the majority of blood corpuscles. They contain a protein pigment called haemoglobin which gives the colour to the corpuscles. It  also has iron.

White corpuscles are much less in number then red corpuscles. These are of different types , some of which eat up disease germs which may enter the blood.

Haemoglobin

It is a protein pigment contained by RCB`s, giving them red colour. Haemoglobin carries out a respiratory function. It combines loosely with oxygen to form oxyhaemoglobin which transfers to the tissues where it breaks up into the haemoglobin and oxygen.

Blood group

 It is the grouping of the people whose blood may be mixed without clumping of blood corpuscles. A,B, AB  AND O are the four main blood group.

Blood bank

It is a reservoir of blood maintained in hospitals for transfusing it into the body of patients.

Blood circulation

The human heart is responsible for circulation of blood. Auricles and ventricles of the heart contract and relax alternately. The right auricle receives impure blood from a large vein and the left auricles receives the pure blood from the lungs. Both kinds of blood are forced into two ventricles by the contracting of two auricles. Now, the two ventricles contract  (systole) , valves close the opening between the auricles and ventricles, hence no blood can go back into the auricles. Thus the pure blood from left ventricle goes into a large aorta and the impure blood from the right ventricles goes into the pulmonary artery .

The aorta  takes blood to various part of the body and pulmonary artery to the lungs. The contractions of ventricles are called heart beat.

Following are the chief functions of the circulatory system:

1.    Maintenance of osmotic balance.Water being a major component of body mass, great solvent, tissues must maintain optimum level of water to carry out metabolic processes. Blood absorbs water from the gut and distributes it to all organs. Loss of water from blood as happens in the case of vomiting and diarrhoea, results in serious consequences, sometimes leading to death.

2.    Transport of respiratory gases.Micro-organisms have more surface area as compared to the bulk and hence gases simply diffuse in and out of the body, requiring no specific respiratory organs. However, in larger animals oxygen must be absorbed in specialized respiratory organs and then transported via an oxygen-carrying pigment in blood, such as haemoglobin or hemocyanin and delivered to the tissues. Carbon dioxide takes a reverse route to the outside.

3.    Distribution of nutrients.Food is digested and absorbed in the intestine and then transported via the blood and lymphatic circulation to the liver, where it is assimilated and then supplied to all parts of body.

4.    Temperature regulation.If excess metabolic heat is generated in the body, blood circulation carries it to the body surface where it is dissipated via the skin. In endotherms metabolic heat is evenly distributed throughout the body by blood, while in ectothermic animals heat is absorbed from atmosphere by skin and then transported to all parts of body evenly so that metabolic activities can take place.

5.    Transport of hormones and other chemicals.Hormones are secreted in endocrine glands but the target organs of these hormones are located in different parts of body. Blood carries these hormones from endocrine glands to target organs. Pituitary located on hypothalamus of brain secretes hormones such as TSH, GH, Gonadotropin, prolactin, ADH etc. which are transported by blood to other endocrine glands, which are stimulated to produce their own hormones.

6.    Defence against invaders.Blood contains granular and agranular leucocytes which attack and kill pathogens that manage to enter the body. T-cells kill the invaders chemically while B-lymphocytes produce antibodies to inactivate pathogens. Monocytes or macrophages ingest foreign materials and tissue debris by endophagy and digest it.

 7.    Disposal of wastes.Carbon dioxide and nitrogenous wastes are constantly produces all over the body, and being toxic must be removed from body quickly. Blood carries them to the excretory organs such as kidneys, nephridia, malpighian tubules, lungs etc. from where they are released outside into the atmosphere. Toxins and excess salts are also carried via blood to the skin, where they are excreted via the sweat glands.

 8.    Diagnostic material. Many kinds of chemical transformations and production of antibodies take place in blood, which is also a carrier of a variety of chemicals and hence can be uses as an ideal material to analyse and know the physiological condition of the body. Therefore, blood test for leucocyte count, haemoglobin, liver, kidney and thyroid function tests and a variety of other tests can be carried out with ease using patient’s blood.   

Lungs

In lungs the blood is purified. The blood coming into the lungs is impure and contains too much of carbon dioxide. Some of the oxygen in the air passes into the blood and some of the carbon dioxide is given to the blood in the bag. The oxygen is taken by the blood all over the body.

Veins

Veins are provided with valves and contain blood flowing towards the heart. The backward flow of the blood by the pulsation of the heart is checked by these valves. 


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