Body Fluids and CirculationNEET Zoology · Class 11 · NCERT Chapter 15

You can expect 2 to 3 questions from Body Fluids and Circulation in NEET 2027. The most reliable scoring topics are WBC types and their percentages, ABO and Rh blood group inheritance, the coagulation cascade, the four heart chambers and valves, the cardiac cycle phases with the SAN pacemaker, and the meaning of ECG waves (P, QRS, T).

WBCs are grouped as granulocytes and agranulocytes. Granulocytes: neutrophils (60 to 65 percent, the most abundant, phagocytose bacteria), eosinophils (2 to 3 percent, fight allergy and parasites), basophils (0.5 to 1 percent, release histamine in inflammation). Agranulocytes: lymphocytes (20 to 25 percent, the B and T cells of immunity), monocytes (6 to 8 percent, become macrophages in tissue).

Blood is a red, opaque connective tissue made of plasma plus RBCs, WBCs and platelets. It flows in arteries, veins and capillaries. Lymph is a colourless, transparent fluid formed by tissue fluid (interstitial fluid). It lacks RBCs and platelets but has white blood cells (mostly lymphocytes). Lymph also contains less protein and oxygen than blood. It flows in lymphatic vessels and is filtered through lymph nodes back into the bloodstream.

When a blood vessel is damaged: (1) Platelets release thromboplastin at the wound site. (2) Thromboplastin and calcium ions convert prothrombin (an inactive plasma protein) into the active enzyme thrombin. (3) Thrombin converts soluble fibrinogen into insoluble fibrin. (4) Fibrin threads trap blood cells to form the clot. Vitamin K is needed for prothrombin synthesis in the liver; calcium is needed at every step.

The pacemaker is the sino-atrial node (SAN), located on the wall of the right atrium. It fires about 70 to 75 times a minute, on its own. The signal spreads across both atria (causing atrial systole), reaches the atrio-ventricular node (AVN), then travels down the bundle of His into the ventricles, and finally into the Purkinje fibres, which trigger ventricular contraction. The heart is myogenic, meaning it generates its own beat without nerves.

One full cardiac cycle is the sequence of events in the heart from the start of one beat to the start of the next. It has three main phases: (1) Atrial systole (atria contract, push blood into ventricles), (2) Ventricular systole (ventricles contract, push blood into the aorta and pulmonary artery), (3) Joint diastole (all chambers relax, ventricles fill passively from atria). A normal cycle takes about 0.8 seconds. The first heart sound (lubb) is closure of the AV valves at the start of ventricular systole; the second sound (dubb) is closure of the semilunar valves at the start of joint diastole.

P wave: depolarisation (electrical activation) of the atria, leading to atrial contraction. QRS complex: depolarisation of the ventricles, leading to ventricular contraction. The atrial repolarisation is hidden inside QRS. T wave: repolarisation of the ventricles, leading to ventricular relaxation. The time between two successive R peaks is used to calculate heart rate.