Q.Complete the missing word in the statement given below:
a. Plasma without _________ factors is called serum.
b. ___________ and monocytes are phagocytic cells.
c. Eosinophils are associated with _____ reactions.
d. _______ ions play a significant role in clotting.
e. One can determine the heart beat rate by counting the number of _________ in an ECG.

The 'formed elements' of blood are its cellular/particulate components suspended in the fluid plasma: red blood cells (gas transport), white blood cells (immune defence, of several distinct types), and platelets (clotting).

1. Red Blood Cells (RBCs / erythrocytes):

• Small, biconcave, disc-shaped cells; in mammals they are anucleate at maturity, which increases the internal space available for haemoglobin.
• Packed with the respiratory pigment haemoglobin, which reversibly binds oxygen, making RBCs the primary vehicle for oxygen transport from the lungs/gills to body tissues (and assisting CO2 transport back).
• Produced continuously in bone marrow (erythropoiesis) and are the most numerous formed element in blood.

2. White Blood Cells (WBCs / leukocytes):

• Nucleated cells, far less numerous than RBCs, whose role is defence against infection and foreign matter. They are broadly divided into two groups:
• Granulocytes (cytoplasm shows visible granules):
  • Neutrophils — the most abundant WBC type; phagocytose bacteria and are first responders to infection.
  • Eosinophils — combat parasitic infections and are involved in allergic responses.
  • Basophils — release histamine and heparin, involved in inflammatory and allergic reactions.
• Agranulocytes (no visible granules):
  • Lymphocytes — central to specific/adaptive immunity; B-lymphocytes produce antibodies, T-lymphocytes mediate cell-based immune responses.
  • Monocytes — the largest WBCs; circulate and then migrate into tissues to become macrophages, which phagocytose pathogens and debris.

3. Platelets (thrombocytes):

• Small, disc-shaped cell fragments (not complete cells in mammals) derived from large bone-marrow cells called megakaryocytes. …