Body fluids and circulation constitute one of the most consistently tested topics in NEET biology, yielding 4-5 questions annually. The chapter integrates blood composition, blood group genetics, cardiac anatomy, the cardiac conducting system, electrocardiography, and the pathophysiology of circulatory disorders — all of which demand precise factual recall rather than mere conceptual understanding.

Blood is a specialized fluid connective tissue comprising approximately 6-8% of total body weight. It separates into two fractions: plasma (55% by volume) and formed elements (45%). Plasma is a straw-coloured liquid consisting of approximately 90-92% water, with the remaining fraction occupied by dissolved plasma proteins, electrolytes, nutrients, hormones, and waste gases. The three principal plasma proteins are albumin — responsible for maintaining the colloid osmotic (oncotic) pressure of blood and preventing fluid from leaking into tissues — globulins, which serve immune functions as antibodies (immunoglobulins), and fibrinogen, the soluble precursor of fibrin in the blood clotting cascade. Serum is distinguished from plasma by the absence of fibrinogen and other clotting factors, which are consumed during clot formation.

The formed elements include three types of cells and cell fragments. Erythrocytes (red blood cells) are biconcave, anucleate discs with a lifespan of approximately 120 days. They are produced by erythropoiesis in red bone marrow (stimulated by the hormone erythropoietin from the kidneys) and are destroyed in the spleen — the "graveyard of RBCs." Each erythrocyte is packed with haemoglobin, a metalloprotein with four iron-containing haem groups that reversibly bind oxygen. Leucocytes (white blood cells) are nucleated and classified as granulocytes (neutrophils, eosinophils, basophils) or agranulocytes (lymphocytes, monocytes). Neutrophils, constituting 60-65% of all WBCs, are the most abundant and serve as the primary cellular defenders against bacterial infection through phagocytosis. Eosinophils (2-3%) combat parasitic infections and mediate allergic responses. Basophils (0.5-1%), the least abundant WBC, release histamine (promoting inflammation and vasodilation) and heparin (an anticoagulant). Among agranulocytes, lymphocytes (20-25%) include B-cells (responsible for humoral immunity through antibody production) and T-cells (responsible for cell-mediated immunity, maturing in the thymus). Monocytes (6-8%) differentiate into macrophages in tissues, performing phagocytosis and antigen presentation. Platelets (thrombocytes) are cell fragments derived from megakaryocytes in bone marrow, with a count of 1.5-3.5 lakh per $mm^{3}$ and a lifespan of 5-9 days; they initiate haemostasis.

The ABO blood group system classifies individuals based on the presence or absence of specific antigens on erythrocyte surfaces. Group A carries the A-antigen and anti-B antibodies in plasma; Group B carries the B-antigen and anti-A antibodies; Group AB carries both antigens and no ABO antibodies, making AB+ individuals universal recipients; Group O carries neither antigen but both anti-A and anti-B antibodies, making O- individuals true universal donors. The Rh system adds the D-antigen dimension: Rh-positive individuals express the D-antigen. When an Rh-negative mother carries an Rh-positive foetus, delivery causes foetal erythrocytes to enter maternal circulation, sensitizing the immune system. In subsequent Rh-positive pregnancies, maternal IgG anti-Rh antibodies cross the placenta and destroy foetal RBCs — a potentially fatal condition called erythroblastosis fetalis. Prevention is achieved by administering anti-D immunoglobulin (RhoGAM) within 72 hours after the first delivery, neutralizing foetal RBCs before maternal immunological memory is established.

The human heart, situated in the mediastinum and slightly tilted leftward, is a four-chambered organ enclosed in a pericardial sac. Two thin-walled atria (receiving chambers) communicate with two thick-walled ventricles (pumping chambers) via atrioventricular valves: the tricuspid valve on the right and the bicuspid (mitral) valve on the left. Semilunar valves — the pulmonary and aortic — guard the exits of the right and left ventricles, respectively, preventing arterial backflow. The left ventricle has the thickest wall, generating pressures of approximately 120 mmHg to drive systemic circulation.

The cardiac conducting system ensures coordinated, rhythmic contraction. The sinoatrial node (SAN), located in the upper right atrium near the superior vena cava opening, is the primary pacemaker, firing spontaneously at 70-75 bpm — the highest intrinsic rate in the system. Impulses spread through both atria, converge at the atrioventricular node (AVN), which introduces a critical 0.1-second delay allowing atria to complete systole before ventricular contraction. The impulse then travels through the Bundle of His and its left and right branches, terminating in the Purkinje fibres, which stimulate ventricular myocardium from the apex upward for efficient blood ejection.

A complete cardiac cycle lasts 0.8 seconds at 75 bpm: atrial systole (0.1 s, AV valves open), ventricular systole (0.3 s, beginning with isovolumetric contraction with all valves closed, followed by ejection when semilunar valves open), and joint diastole (0.4 s, the longest phase, when all chambers relax and passive filling occurs through open AV valves). Cardiac output equals stroke volume multiplied by heart rate, approximately 5 litres per minute at rest.

The electrocardiogram records these electrical events: the P wave represents atrial depolarization; the QRS complex represents ventricular depolarization (not contraction, which is the mechanical event triggered by depolarization); and the T wave represents ventricular repolarization. Atrial repolarization is not visible as a distinct wave because it is masked by the simultaneous, much larger QRS complex.

Double circulation separates oxygenated and deoxygenated blood completely. Pulmonary circulation carries deoxygenated blood from the right ventricle through the pulmonary artery to the lungs, returning oxygenated blood via pulmonary veins to the left atrium. Systemic circulation carries oxygenated blood from the left ventricle through the aorta to body tissues, returning deoxygenated blood via the venae cavae to the right atrium. The critical NEET exception: pulmonary arteries carry deoxygenated blood; pulmonary veins carry oxygenated blood — the only arteries and veins to deviate from the conventional oxygenation pattern.