CBSE Class 9 Answered

Mammalian blood is composed of plasma and three types of cells, red blood cells, white blood cells, and platelets.

Plasma

• It is a light yellow-coloured alkaline liquid.
• It mainly consists of water (90–92%), proteins (7–8%), inorganic salts (1%) and other substances (trace amounts). 

Cellular Elements

Red Blood Cells (RBCs/Erythrocytes)

• RBCs are minute, disc-like structures.
• In adults, RBCs are produced in the marrow of long bones.
• In children, RBCs are produced in the bone marrow of all the cells until 5 years of age.
• Mature RBCs do not have nuclei.
• The average life of RBCs is 120 days.

White Blood Cells (WBCs/Leucocytes)

• WBCs are amoeboid.
• WBCs are produced in the bone marrow, lymph nodes and sometimes even in the liver and spleen.
• The average life of WBCs is about 2 weeks.
• WBCs are of different types-Neutrophils, Eosinophils, Basophils, Lymphocytes and Monocytes.

Functions of WBCs

• Phagocytosis:
• Inflammation:
• Formation of Antibodies

Blood Platelets (Thrombocytes)

• Blood platelets are minute, oval or round, non-nucleated structures floating in the blood.
• Platelets are derived from megakaryocytes in the red bone marrow.
• Their life span is 3 to 5 days.
• Blood platelets play an important role in blood clotting.