Blood is a connective tissue comprising plasma (55%) and formed elements (45%). Understanding its composition is critical for NEET as questions frequently test the identification of blood components and their specific functions. The exam typically dedicates 2-3 questions to blood composition and associated disorders.
Plasma consists of 90% water and 10% solutes. The solute fraction includes three major proteins: albumin (60%), globulins (35%), and fibrinogen (5%). Albumin maintains osmotic pressure and transports substances, while globulins function in immunity. Fibrinogen is essential for blood clotting. NEET frequently asks about serum versus plasma—serum is plasma without fibrinogen, obtained after blood clotting.
Red blood cells (RBCs) contain hemoglobin and transport oxygen. Normal RBC count is 4.5-5.5 million/mm³ in males and 4.0-5.0 million/mm³ in females. White blood cells (WBCs) are nucleated cells involved in immunity, with a normal count of 4,500-11,000/mm³. Platelets (150,000-400,000/mm³) are cell fragments responsible for blood clotting. The exam tests these normal ranges, clinical correlations of abnormal counts, and the distinction between different WBC types: neutrophils (50-70%), lymphocytes (20-30%), monocytes (3-8%), eosinophils (1-4%), and basophils (0.5-1%).
| Component | Normal Range | Primary Function |
|---|---|---|
| Hemoglobin | 12-16 g/dL (females), 13.5-17.5 g/dL (males) | Oxygen transport |
| RBC | 4.0-5.5 million/mm³ | Gas exchange |
| WBC | 4,500-11,000/mm³ | Immune response |
| Platelets | 150,000-400,000/mm³ | Hemostasis |
Blood groups are determined by antigens on RBC surfaces. The ABO system (groups A, B, AB, O) and Rh system (Rh+ or Rh-) are tested extensively in NEET. Group O is the universal donor, while AB+ is the universal recipient. Rh incompatibility is crucial in pregnancy, causing hemolytic disease of the newborn (HDN) when an Rh- mother carries an Rh+ fetus. The exam tests transfusion compatibility and hemolytic transfusion reactions.
The lymphatic system collects tissue fluid (lymph) from interstitial spaces and returns it to the bloodstream, maintaining fluid homeostasis. NEET questions often compare blood, tissue fluid, and lymph compositions, emphasizing differences in protein content and cellular composition.
Lymph forms when blood filtrate accumulates in tissue spaces. It lacks RBCs and contains fewer proteins than plasma but carries lymphocytes and antibodies. Unlike blood circulation powered by the heart, lymph circulation depends on muscle contractions and one-way valves in lymph vessels. The thoracic duct is the major collecting vessel, draining lymph into the left subclavian vein. NEET typically includes questions about factors affecting lymph formation: hydrostatic pressure (increases lymph formation) and plasma protein concentration (decreases lymph formation when decreased).
Lymphoid tissues include the thymus, spleen, lymph nodes, and Peyer's patches. The thymus produces T-lymphocytes, the spleen filters blood and produces antibodies, and lymph nodes filter lymph. The exam tests the anatomical locations and immune functions of these organs. Questions frequently assess why lymph nodes enlarge during infection (increased lymphocyte proliferation) and the role of lymphatic vessels in fat absorption from the intestine through lacteals.
The cardiac cycle comprises systole (contraction) and diastole (relaxation). NEET dedicates substantial marks to understanding cardiac cycle events, pressure changes, valve movements, and heart sounds. This section appears in 4-5 exam questions, making it a priority topic.
The cardiac cycle begins with atrial systole (0.1 seconds), when both atria contract, pushing blood into relaxed ventricles. Ventricular systole (0.3 seconds) follows, with ventricles contracting to eject blood into pulmonary artery and aorta. During ventricular systole, atrioventricular valves (tricuspid and mitral) close, producing the first heart sound (S1). Diastole (0.4 seconds) involves complete relaxation of both atria and ventricles. When aortic and pulmonary valves close, the second heart sound (S2) occurs. The normal heart rate is 60-100 beats/minute at rest, resulting in a complete cycle duration of approximately 0.8 seconds.
Understanding pressure dynamics during the cardiac cycle is essential for NEET. Left ventricular pressure increases from 5 mmHg (early diastole) to 120 mmHg during systole, exceeding aortic pressure (80 mmHg) and opening the aortic valve. When ventricular pressure falls below aortic pressure, the aortic valve closes. The mitral valve opens when atrial pressure exceeds ventricular pressure during diastole. Questions test students' ability to identify which valves are open during specific phases and predict pressure relationships.
The ECG records electrical activity of the heart through P-wave (atrial depolarization), QRS complex (ventricular depolarization), and T-wave (ventricular repolarization). Normal PR interval is 0.12-0.2 seconds, QRS duration is less than 0.12 seconds, and QT interval varies with heart rate. NEET questions test ECG interpretation, identification of arrhythmias, and correlation between mechanical and electrical events. Abnormalities like prolonged PR interval or ST elevation have clinical significance that exam questions emphasize.
Cardiac output (CO) = Heart rate (HR) × Stroke volume (SV). Normal cardiac output is approximately 5 L/minute at rest. The Frank-Starling mechanism states that increased venous return increases preload (end-diastolic volume), stretching myocardial fibers and increasing contractility, thereby increasing stroke volume. NEET questions assess how exercise, sympathetic stimulation, and increased venous