Composition of blood formed element

The formed elements of blood are erythrocytes, leucocytes, and platelets these elements have some peculiar features.

1. Erythrocytes lack nuclei and platelets are simply cell fragment.

2. Their life span varies from a few days to 4 months.

3. Most of them don't divide. instead, they are continuously renewed by the division of cells in the bone     marrow, where they originate.

Erythrocytes or red blood cells (RBCs)

Erythrocytes are the most numerous formed elements of the blood. An adult human male has a total of about 5 million RBCs per cubic mm of blood. while an adult female has 4.5 million RBCs per cubic mm of blood. these are small- sized 7um in diameter having the shape of a biconcave disk a disk thicker at the edge than in the center shape is maintained by a net of fibrous proteins, especially Spectris their shape and small size bestow them a high surface-to-volume ratio, the goal that respiratory gases can quickly diffuse to and from the cell's interior.

Mature RBCs can effortlessly squeeze from narrow blood capillaries due to the presence of a flexible and robust plasma membrane which allows them to deform without rupturing.

RBCs are produced in bone marrow under the influence of erythropoietin, a hormone secreted by juxtaglomerular cells in the kidney and the process is known explicitly as erythropoiesis. mature erythrocytes lack a nucleus (anucleate) and other organelles.

You know that erythrocytes contain the protein hemoglobin, which binds to the oxygen. each hemoglobin molecules is composed of four subunit bound together. each subunit comprises a molecular group known as heme and a polypeptide appended to the heme. the four peptides of hemoglobin molecules are collectively called globin. each of the heme group contains one atom of iron (Fe). which can bind one oxygen molecules. the average concentration of hemoglobin in blood is 14g/dl in women and 16g/dl in men. 

The average life span of an erythrocyte is roughly 120 days.

Leucocytes or white blood cells (WBCs)

Leucocytes are the only formed elements that are considered as complete cells with nuclei and typical organelles. they form a portable army that safeguards the body from harm by microbes, infections, toxins and tumor cells.

WBCs can move out through the intact blood vessel walls-a phenomenon called diapedesis (also known as leucocyte extravasation or leucocyte adhesion cascade), during which WBCs moves towards the site of infection or damage and provide inflammatory or immune responses.

Based on their structural and chemical attributes, leucocytes are categorized into two classes: granulocytes with specialized membrane- bound cytoplasmic granules and agranulocytes without granules. their size, concentration in blood, the percentage in the WBCs populace and a few other characteristics are tabulated.

granulocytes:- 

Neutrophil  -   phagocytize bacteria

Eosinophils -   Engulf bacteria and parasitic worms, inactivate some inflammatory chemicals of allergy.

Basophils    -   Release histamine and mediators of inflammation,

agranulocytes:-

Lymphocytes - mount immune response by direct cell attack or via antibodies.

Monocytes     - phagocytosis, develop into macrophage in tissue.

platelets

In mammals, platelets are the cytoplasmic fragments of huge enucleated cell called megakaryocytes. however, in other vertebrates, platelets are nucleated and are named thrombocytes. each cubic millimeter of blood contains somewhere in the range of 2,50,000 to 5,00,000 platelets. every platelet has a couple of granules comprising of different chemicals viz. serotonin, ADP, calcium ,platelet-derived growth factor (PDGF) and an assortment of enzyme that act in the clotting mechanism. the life expectancy of platelets is around ten days and fresh platelets are formed from their ancestral hemocytoblasts under the regulation of a hormone called thrombopoietin.

As specified previously, platelets play a key role in the process of clotting when blood vessels are ruptured. by sticking to the damaged site, platelets forma temporary plug that helps seal the break. the detailed mechanism of this process will be discussed in the following sections.