Introduction

All living cells have to be provided with nutrients, O₂ and other essential substances. Also, the waste or harmful substances produced, have to be removed continuously for healthy functioning of tissues. It is therefore, essential to have efficient mechanisms for the movement of these substances to the cells and from the cells. Different groups of animals have evolved different methods for this transport. Simple organisms like sponges and coelenterates circulate water from their surroundings through their body cavities to facilitate the cells to exchange these substances. More complex organisms use special fluids within their bodies to transport such materials. Blood is the most commonly used body fluid by most of the higher organisms including humans for this purpose.Another body fluid, lymph, also helps in the transport of certain substances.The flow of fluid in the body is called circulation and the structures concerned constitute the circulatory system.

Functions of Circulatory System

1. Transport of respiratory gases

2. Transport of metabolic waste products

3. Transport of digested food substances

4. Transport of hormones

5. Temperature regulation

6. Defense

Blood

Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements.

A) Plasma

Plasma is a straw-colored, viscous fluid constituting nearly 55 percent of the blood. 90-92 percent of plasma is water and proteins contribute 6-8 percent of it. Fibrinogen, globulin and albumin are the major proteins. Fibrinogens are needed for clotting or coagulation of blood. Globulin are involved in defense mechanisms of the body and the albumin help in osmotic balance. Plasma also contains small amounts of minerals like Na+, Ca++, Mg++, HCO3–, Cl–, etc. Glucose, amino acids, lipids, etc., are also present in the plasma as they are always in transit in the body. Factors for coagulation or clotting of blood are also present in the plasma in an inactive form. Plasma without the clotting factors is called serum.

B) Formed elements

Erythrocytes, leukocytes and platelets are collectively called formed
elements and they constitute nearly 45 percent of the blood.

1) Erythrocytes
Erythrocytes or red blood cells (RBC) are the most abundant of all the cells in blood. A healthy adult man has, on an average, 5 millions to 5.5 millions of RBCs mm^–3 of blood. RBCs are formed in the red bone marrow in the adults. RBCs are devoid of nucleus in most of the mammals and are biconcave in shape. They have a red colored, iron containing complex protein called hemoglobin, hence the color and name of these cells. These molecules play a significant role in transport of respiratory gases. RBCs have an average life span of 120 days after which they are destroyed in the spleen (graveyard of RBCs).

2) Leukocytes
Leukocytes are also known as white blood cells (WBC) as they are colorless due to the lack of hemoglobin. They are nucleated and are relatively lesser in number which averages 6000-8000 mm^–3 of blood. Leukocytes are generally short lived.

There are two main categories of WBCs:
a) Granulocytes

- Neutrophils, eosinophils and basophils are different types of granulocytes.

b) Agranulocytes 

- Lymphocytes and monocytes are the agranulocytes.

Neutrophils are the most abundant cells (60-65 percent) of the total WBCs and basophils are the least (0.5-1 percent) among them. Neutrophils and monocytes (6-8 percent) are phagocytic cells which destroy foreign organisms entering the body. Basophils secrete histamine, serotonin, heparin, etc., and are involved in inflammatory reactions. Eosinophils (2-3 per cent) resist infections and are also associated with allergic reactions. Lymphocytes (20-25 per cent) are of two major types – ‘B’ and ‘T’ forms. Both B and T lymphocytes are
responsible for immune responses of the body.

3) Platelets
Platelets also called thrombocytes, are cell fragments produced from megakaryocytes (special cells in the bone marrow). Blood normally contains 1,500,00-3,500,00 platelets mm^–3. Platelets can release a variety of substances most of which are involved in the coagulation or clotting of blood. A reduction in their number can lead to clotting disorders which will lead to excessive loss of blood from the body.

Lymph (Tissue fluid)

As the blood passes through the capillaries in tissues, some water along with many small water soluble substances move out into the spaces between the cells of tissues leaving the larger proteins and most of the formed elements in the blood vessels. This fluid released out is called the interstitial fluid or tissue fluid. It has the same mineral distribution as that in plasma. Exchange of nutrients, gases, etc., between the blood and the cells, always occur through this fluid. An elaborate network of vessels called the lymphatic system collects this fluid and drains it back to the major veins. The fluid present in the lymphatic system is called the lymph.

Lymph is a colorless fluid containing specialized lymphocytes which are responsible for the immune responses of the body. Lymph is also an important carrier for nutrients, hormones, etc. Fats are absorbed through lymph in the lacteal present in the intestinal villi.