Sunday, 12 April 2015


13:09 - 6 comments

Blood is a fluid medium in animals which transports materials like gases, wastes, hormones, etc throughout the body. It transports necessary substances to the cells and delivers metabolic wastes away from the same cells. Exchange of gases takes place when blood reaches to pulmonary alveoli.

Blood mainly comprises of plasma and cell like bodies i.e. red blood cells, white blood cells and platelets.

Blood Cells and Cell like Bodies:

Cells or cell like bodies usually refers to red blood cells (erythrocytes), white blood cells (leucocytes) and platelets. They form about 45 percent by volume of blood.

                                   <<<Also Read: Functions of Blood>>>

Red blood cells (Erythrocytes):

Red Blood Cell
Erythrocytes, also called Red blood cells, are round flattish aligned centered cells which deal with delivering of oxygen to body tissues by blood flow via circulatory system. They take oxygen from the lungs or gills in fishes and release it into the cells or tissues. Lipids and proteins make up the cell membrane of red blood cells. Hemoglobin, an iron containing biomolecule, is the rich component of the cytoplasm of red blood cells mainly responsible for the oxygen binding and red color of the erythrocytes.


Red blood cells are the most numerous cells found in the blood. A cubic millimeter contains 5-5.5 million of them in males and 4-4.5 million in females.


These cells, when formed, have nucleus which is lost before they enter the circulatory fluid or blood. Only mammals (except camel and Llama) have non-nucleated red blood cells. All other vertebrates have nucleated red blood cells.


95% of the cytoplasm of red blood cells is the red pigment, called hemoglobin; the remaining 5% consists of enzymes, salts and other-proteins.

No Division after Maturation:

The Red blood cells once mature, do not divide.


These cells are biconcave in mammals and have an elastic cell membrane. They are not biconcave, but oval or spherical in shape in all the other vertebrates.


Red blood cells are formed principally in the red bone marrow of short bones, such as the sternum, ribs and vertebrae. In the embryonic life, they are formed in the liver and spleen. The formation of red blood cells is called hemopoiesis.

Life Span:

The average life span of red blood cells is about four months after which it breaks down and disintegrated in the liver and spleen partly by phagocytes. During phagocytosis, the iron from the hemoglobin is retained in the liver and spleen cells and is again used in the formation of red blood cells in the body. About 2-10 million red blood cells are formed and destroyed each second in a normal person.


Their main function is to transport oxygen to oxygen deficient cells and tissues in the body and take away carbon dioxide from the cells and tissues.

White Blood Cells (Leucocytes):

Leucocytes, also called as white blood cells are the chief cells of a body involved in protecting the body against foreign invaders and infectious diseases. Hematopoietic stem cell in the bone marrow is a multipotent cell where all white blood cells are produced. There are five different types of leucocytes as mentioned below under the heading types.


These blood cells are colorless, as they do not contain any pigments while erythrocytes have red pigment in them.


One cubic millimeter of blood contains 7000 to 8000 cells of leucocytes.


They are much larger than that of red blood cells.


There are at least five different types of leucocytes which can be distinguished on the basis of the shape of the nucleus and density of granules in the cytoplasm. They can be grouped into two main types, granulocytes and agranulocytes.


Granulocytes have granules in their cytoplasm and their nucleus is characteristically 2-4 lobed. They constitute about 71 to 72% of total white blood cells count in normal person and include neutrophils, eosinophils and basophils.


As the word Agranulocyte show that they have clear cytoplasm (no granules), and the nucleus does not have lobed appearance, but is-rounded. Agranulocytes include monocytes and lymphocytes (B and T).


Granulocytes are formed in the red bone marrow, while the agranulocytes are formed in lymphoid tissue, such as those of the lymph nodes, spleen, tonsils, adenoids and the thymus. (Leucocytes are not restricted to blood being more abundant in lymphatic system. They are also found wandering free in loose connective tissue, and occasionally in other tissues).

Life Span:

Granulocytes (neutrophils, eosinophils and basophils) survive for one week. Monocytes stay 10-20 hours in the blood, then enter tissues and become tissue macrophages. In this form they can live for months or even for years, unless destroyed by performing phagocytic function. Lymphocytes have life spans of months or even years; but this depends on the body's need for these cells.


Leucocytes protect the body against foreign invaders, and use circulatory system to travel to the site of invasion.

Monocytes and Neutrophils travel through capillaries and reach the site of wound where bacteria have gained entry and destroy them.

Macrophages and Neutrophils feed on invaded bacterial cells or other foreign cells, including cancer cells. They typically die in the process, and their dead bodies accumulated and contribute to the white substance called pus seen at infection sites. So, pus is actually our dead white blood cells.

Eosinophils are phagocytic and ingest foreign proteins and immune complexes rather than bacteria.

Basophils produce heparin a substance that inhibits blood clotting. They also produce other chemicals, such as histamine, that participate in allergic reactions in response to tissue damage and microbial invasion.

Lymphocytes help to provide immunity against the disease.


Thrombocytes, also called platelets are the chief cells in the body that are involved in blood clotting i.e. to stop bleeding. Thrombocytes are only found in mammals, while in amphibians, birds, etc they circulate as intact mononuclear cells.


These are not cells, but are fragments of large cells called megakaryocytes, about 2-31.micrometer in diameter.

Number and Nucleus:

There are about 250,000 platelets per cubic millimeter of blood and have no nucleus.

No Pigment:

Like leucocytes, there is no pigment in platelets.


Their important function is blood clotting. In this process they cause conversion of fibrinogen, a solid plasma protein, into its insoluble form called fibrin. The fibrin threads enmesh (catch) red blood cells and other platelets in the area of damaged tissue, ultimately forming a blood clot. The clot serves as .a temporary seal to prevent bleeding until the damaged tissue can be repaired.

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