(a) Fibrinogens are inactive components of blood plasma. Under the action of enzyme thrombin they form a clot or coagulum of a network of threads called fibrin in which dead and damaged elements of blood are trapped.

(b) Globulins are primarily involved in immunity, i.e., defence mechanisms of the body.

(c) Neutrophils are phagocytic cells which destroy foreign organisms entering the body.

(d) Lymphocytes are specialised cells which are responsible for the immune responses in the body. Two major types of lymphocytes, that are involved in this process are B and T-lymphocytes.

A protein named as rhesus antigen, is present on the surface of red blood corpuscles. The population having this protein are called $\mathbf{R h}$ positive $\left(\mathrm{Rh}^{+}\right)$while others who do not have this protein are known as Rh negative ( $\mathrm{Rh}^{-}$) (dominant). Erythroblastosis Foetalis occurs due to Rh incompatibility between the foetus and mother during pregnancy. Rh antigen of the foetus do not get exposed to the $\mathrm{Rh}^{-}$blood of the mother in the first pregnancy. However, during the delivery of the first child, there is a possibility of exposure of the maternal blood to small amounts of the $\mathrm{Rh}^{+}$blood from the foetus. In such cases, the mother starts preparing antibodies against Rh antigen in her blood and during her subsequent pregnancies the Rh antibodies from the mother ( $R h$ - ve) leak into the blood of the foetus ( $\mathrm{Rh}+\mathrm{ve}$ ) resulting in the destruction of foetal RBCs. This could be fatal to the foetus or could cause severe anaemia and jaundice, even may lead to the death of the foetus.

Blood exhibits coagulation or clotting in response to an injury or trauma. Coagulation prevents excessive loss of blood from the body. In its absence their could be huge loss of blood, which can be fatal.

The time gap in the passage of action potential from sino-atrial node to the ventricles allows ventricles to relax. Thus, ventricular pressure falls, causing the closing of semilunar valves, and prevents the back flow of blood into ventricles.

Electrocardiograph (ECG) is a graphical representation of the electrical activity of the heart during a cardiac cycle. A patient is connected to the machine with three electrical leads (one to each wrist and one to the left ankle) that continuously monitor the heart activity. For a detailed evaluation of the heart functions multiple leads are attached to the chest region.

The QRS complex represents the depolarisation of the ventricles, which initiates the ventricular contraction. The contraction starts shortly after $Q$ and marks the beginning of the systole. The time taken in QRS complex is 0.12 second in normal ECG.

The larger Q and R wave indicate a myocardial infarction (heart attack). The S-T segment is elevated in acute myocardial infarction and depressed when the heart muscle receives insufficient oxygen.