ii. White blood corpuscles (WBCs) or leucocytes

iii. Platelets.

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1. Red blood corpuscles (RBCs):

Red blood corpuscles or erythrocytes (erythros means red) are biconcave and enucleated (i.e. without a nucleus, young RBCs have a nucleus, but as they mature, the nucleus degenerates) disc-like biconcave shaped, flat in the centre, and thick and rounded at the periphery. Such a shape of RBCs as they mature, is of great advantage to these cells as it increases their surface area.

Due to lack of organelles (such as mitochondria and endoplasmic reticulum), their oxygen demand is negligible. The RBCs are very small in size. This small size helps in absorbing oxygen and enables them to travel in very fine blood capillaries throughout the body of a person.

RBCs are produced in the bone marrow of long bones, such as ribs, vertebrae and skull bones. The average life span of erythrocytes is about 120 days. Their number ranges from 5-5.5 millions in an average human male and 4.5-5 millions in an average human female.

The RBCs have a colourless spongy body which contains haemoglobin. The haemoglobin is the respiratory pigment which is formed of an iron- containing part known as haeme and a protein part known as globin. The oxygen after diffusing into blood capillaries combines with iron-containing part of RBCs and forms oxyhaemoglobin.

2. White blood cells (WBCs):

White blood corpuscles or leucocytes (leucos means white) are rounded to irregular-shaped cells and are capable of amoeboid movement. They are colourless (lack haemoglobin). They are produced in the bone marrow. Leucocytes are of two types granulocytes (contain granules in their cytoplasm and have lobed nuclei) and agranulocytes (lack granules in their cytoplasm and have unlobed nuclei).

A. Granulocytes:

These are spherical in shape and larger than red blood cells. They have lobed nucleus and their cytoplasm contains granules. There are three types of granulocytes neutrophils, basophils and eosinophils.

i. Neutrophils:

They account for 50-70% of the WBCs in our blood. Their cytoplasm contains very fine granules. The nucleus has 3-6 lobes. The granules stain with both acidic and basic dyes. Neutrophils are highly phagocytic in nature.

ii. Basophils:

They account for only 0.5% of the total WBCs. They are of the same size or slightly smaller than neutrophils. The nucleus is indistinctly lobed. The cytoplasmic granules stain with basic dyes. Basophils produce histamin which dilates the blood vessels making their walls leaky for other leucocytes to fight germs.

The leucocytes squeeze out of the blood capillaries to engulf the germs. This process is known as diapedesis. They also produce allergy to foreign materials and help in defence mechanism of the body.

iii. Eosinophils:

These account for 1-4% of total WBCs. The nucleus is two-lobed. The cytoplasmic granules are large and rough and stain dark red with acidic dyes (e.g. eosin). They produce antitoxins to detoxify poison secreted by germs. They also activate the chemicals to fight allergy.

B. Agranulocytes:

Their nucleus is single and typically spherical or kidney-shaped. The cytoplasm does not contain granules. The nucleus is single. There are two types of agranulocytes lymphocytes and monocytes.

i. Lymphocytes:

They account for about 25% of the total WBCs. The nucleus is large, spherical, with a dent-like depression on one side, which occupies most of the cell volume. They produce antibodies and play a crucial role in immunity.

ii. Monocytes (macrophages):

They are the largest leucocytes and account for 3-8% of the total WBCs. The nucleus is large and U- or kidney- shaped. They are phagocytic in nature and ingest germs and clean up damaged cell.

Functions:

i. WBCs help in destroying solid substances and germs, specially bacteria, by engulfing them. By doing so they protect the body from diseases, i.e. they are responsible for immunity.

ii. They fight against disease-causing germs and destroy the damaged cells by the process called phagocytosis.

iii. They also help in the formation of antibodies which neutralize or kill the germs that enter our body.

3. Blood platelets (Thrombocytes) These are colourless, oval or round, cytoplasmic fragments formed from giant cells of the bone marrow.

These are found floating in the blood. These are enucleated (without a Platelets nucleus), bounded by a membrane and contain a few organelles. Their life span is about 5-10 days. Thrombocytes helps in blood clotting. They release a chemical, thromboplastin, which initiates the process of clotting of blood. Their life span is about 5-10 days.