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.

The structure of heart of fishes, reptiles, birds and mammals show many structures of evolutionary significance. Thicker walls of ventricles is one of them.

Ventricles have thick walls because these are mainly involved in pumping the blood to the body parts with certain pressure.

The walls of the left ventricle is about 3 times thicker from the right ventricle, while the atria acts as a collecting chambers for the blood which is starting to the heart thus have thin walled. Also they have to force the blood into the ventricles that lies just below there thus atria have thin walls.

(a) Difference between blood and lymph are as follows

Blood	Lymph
Blood is a connective tissue which have erythrocytes leucocytes and platelet present in fluid called plasma. It flows in all blood vessels.	Lymph is also a connective tissue which, is devoid of RBC but has large number of WBC (leucocytes) in plasma. It flows only in lymphatic system and is also found extracelluarly inside the tissue.

(b) Difference between basophils and eosinophils are as follows

Basophils	Eosinophils
These possess 3 lobed nucleus, with less number of coarse granules. These take basic stain. These are normally $0-1 \%$ is the blood.	These possess bilobed nucleus and coarse granules in cytoplasm These take acidic stain These are $1-6 \%$ is the blood.

(c) Difference between tricuspid valve and bicuspid valve are as follows

Tricuspid Value	Bicuspid Value
This valve separate the right atria from right ventricle. It is made of 3 cusps or flaps. This is also known as right atrio ventricular valve.	This valve separates the left atria from left ventricle. It has 2 cusps or flaps. This is also called mitral valve or left atrio ventricular valve.