RBCs, WBCs and Blood Platelets : Types and Functions

Formed (cellular) elements Formed (cellular) elements (RBCs, WBCs and Platelets) are shaped structures visible under the microscope as cells and cell-like structures present in the blood. They form 42 – 45% of blood. These are of three types i.e.,

Red Blood Cells (RBCs) or Erythrocytes.
White Blood Cells (WBCs) or Leucocytes.
Blood platelets or Thrombocytes.

(1) RBCs or Red Blood Corpuscles (Erythrocytes)

Red Blood Corpuscles are very small, measuring about 7.7 μm in diameter, biconcave, disc-like, denucleated cells in the mammalian blood. RBCs are the most abundant of all the three types of corpuscles.

They are about 5 – 5.4 million/mm³ in the blood of a man and 4 – 4.5 million/mm³ in the blood of a woman. Young immature RBCs contains nucleus, mitochondria, endoplasmic reticulum and other organelles. But, during maturation, these organelles disappear so haemoglobin for the transportation of oxygen.

Moreover, small size and concavity on either side of these cells provide more surface area for the efficient absorption of O2. The small size of these cells also enables them to pass through very narrow capillaries. more as to accommodate Erythrocytes are red in colour because of the presence of a respiratory pigment called haemoglobin (Hb).

It is present in the colourless spongy body or stroma of the RBCs. It is composed of haematin pigment and globin-a protein. Haemoglobin has great affinity for oxygen. It forms an unstable compound oxyhaemoglobin with 02. This oxyhaemoglobin haemoglobin and O, in the body tissues and O2 is supplied to the cells.

It can carry very small amount of CO2 in the form of carbamino haemoglobin (HBCO2). an iron containing dissociates into haemoglobin and O2 in the body tissues and O2 is supplied to the cells. It can carry very small amount of CO2 in the form of carbamino haemoglobin (HBCO2).

Functions of Erythrocytes (RBCs) :

The Erythrocytes (RBCs) perform the following functions :

Transportation of O2 : Haemoglobin in the cytoplasm of RBCs help in the transportation of O2 from the respiratory organsto the body cells.
Transportation of CO2 : About 23 % of CO2 produced in the cells is carried from tissue to respiratory organsfor its elimination by the RBCs.

Formation, life and destruction of RBCs.

About 1% RBCs are produced daily in our body and the same number is worn out. In adults the erythrocytes are produced in the red bone marrow of long bones, especially in ribs, breast bone and ilium of hip bones. In an embryo, these are produced in the liver and spleen.

During maturation, the erythrocytes in mammals lose their nuclei, mitochondria, endoplasmic reticulum, etc. to accommodate more haemoglobin. Due to the loss of nucleus the life of erythrocytes is reduced to about 120 days. The worn out erythrocytes are destroyed in the liver, spleen and bone marrow. Their iron part is stored in the liver cells. Whereas, the rest is excreted as bile pigments (bilirubin and biliverdin).

(2) WBCs or Leucocytes (White Blood Corpuscles)

The leucocytes (WBCs) in general are amoeboid (i.e., without any definite shape) nucleated corpuscles. They are known as phagocytes – as they engulf the foreign and worn out structures; scavengers m out tissues of the body; soldiers of the as they engulf the body – as they can accumulate at the site of a injury and fight with the invading micro-organisms, that may cause infection.

The leucocytes (WBCs) vary from 5000 – 9000/mm³ of human blood. They are produced either in the bone marrow or in lymphoid tissues, such as, lymph nodes, spleen, etc. Extraordinary increase in the number of leucocytes is called leukaemia (blood cancer) and decrease in their number is called leucopenia.

Types of leucocytes (WBCs) :

Based upon the structure, the leucocytes are classified into the following two types :
(a) Agranular and
(b) Granular

(a) leucocytes or Agranular Agranulocytes :

Agranulocytes are the leucocytes (WBCs) whose cytoplasm is not granular in appearance. They are produced in the lymph nodes, spleen and other lymphoid tissues. They have single large nucleus. These are further of two types:
(i) Monocytes
(ii) Lymphocytes

(b) Granular Leucocytes (Granulocytes):

Granulocytes have granular cytoplasm. All the granulocytes are produced in the red bone marrow. Their nucleus are usually constricted into lobes. Depending upon the chemical nature of their cytoplasm they are classified into three types:
(i) Acidophils
(ii) Basophils and
(iii) Neutrophils.

Functions of Leucocytes( WBCs) :

Defence of the body against diseases and invaders is the most essential function of leucocytes (WBCs) . They perform the following functions:

Phagocytosis : The engulfing of the particles, like solid substances, such as, bacteria, out tissues, etc. is called phagocytosis. Neutrophils and monocytes engulf pathogens which may enter the body and protect us against diseases. Leucocytes have the property to squeeze through the walls of capillaries (diapedesis) at the site of injury to fight with worn the pathogens. The killed leucocytes (dead WBCs) pass out of wounds together with tissue cells as pus. An abnormal increase in WBC count indicates some infection in the body.
Formation of antibodies and antitoxins : Lymphocytes produce antibodies. They combat with the antigens or pathogens and protect the body against specific diseases. Acidophils and basophils produce antitoxins that neutralize the toxins and protect the body. When disease causing germs gain entry into the bloodstream, they produce poisonous substances called toxins (poisons). Antibodies act as antitoxins to neutralize (detoxify) the poisonous effect of the toxins.
Immunity : Resistance of the body against diseases is called immunity. Lymphocytes are responsible to provide immunity against specific diseases.
Inflammation : Increased local heat, redness, swelling and pain, due to reaction of tissues to injury and to localized invasion of germs is called inflammation. Here, WBCs (especially the monocytes and neutrophils) show diapedesis and fight against the disease causing germs.

(3) Blood platelets (Thrombocytes)

They are the smallest sized corpuscles of all the three types. Their number varies from 2,00,000 – 3,00,000 per mm3 of blood. Their size is about 3 – 4 μm in diameter. They are denucleated. They are also produced in the red bone marrow from some giant cells called megakaryocytes. Its lifespan is about 3-5 days and are mainly destroyed in the spleen.

Functions of platelets :

The blood platelets are responsible for the clotting of blood at injuries. Blood platelets on disintegration produce a chemical called thromboplastin or thrombokinese. It initiates the chain of reactions involved in blood clotting.