Body Fluids & Circulation

The circulatory system in humans is a closed, double-circuit system driven by a four-chambered heart, transporting oxygen, nutrients, hormones, and waste products to and from every tissue. Understanding its components — blood, the heart, and the vascular network — is essential for NEET, where this topic consistently yields 4-5 questions annually.

Blood constitutes roughly 6-8% of body weight and separates into plasma (55%) and formed elements (45%). Plasma is a straw-coloured fluid containing water (90-92%), proteins (albumin for osmotic balance, globulins for immunity, fibrinogen for clotting), glucose, amino acids, lipids, electrolytes, and dissolved gases. Among formed elements, erythrocytes (RBCs) are biconcave, enucleated discs with a 120-day lifespan, produced in red bone marrow and destroyed in the spleen ("graveyard of RBCs"). Their primary function is oxygen transport via haemoglobin. Leucocytes (WBCs) are nucleated and classified as granulocytes — neutrophils (most abundant, phagocytosis), eosinophils (anti-parasitic, allergic response), basophils (secrete histamine and heparin) — and agranulocytes — lymphocytes (B-cells for antibody production, T-cells for cell-mediated immunity) and monocytes (differentiate into macrophages). Platelets (thrombocytes) are cell fragments derived from megakaryocytes, critical for blood clotting.

The ABO blood group system depends on the presence of antigens on RBC surfaces: type A has A-antigen (anti-B antibodies in plasma), type B has B-antigen (anti-A antibodies), type AB has both antigens (no antibodies — universal recipient), and type O has neither antigen (both antibodies — universal donor). The Rh factor adds another layer: Rh-positive individuals carry the D-antigen. If an Rh-negative mother carries an Rh-positive foetus, maternal anti-Rh antibodies can cross the placenta in subsequent pregnancies, destroying foetal RBCs — a condition called erythroblastosis fetalis, preventable by administering anti-Rh antibodies (RhoGAM) after the first delivery.

The human heart lies in the mediastinum, slightly tilted leftward. It has four chambers: two atria (receiving chambers) and two ventricles (pumping chambers), separated by the interatrial and interventricular septa. The tricuspid valve guards the right atrioventricular opening; the bicuspid (mitral) valve guards the left. Semilunar valves (pulmonary and aortic) prevent backflow from arteries into ventricles. The heart's conducting system ensures rhythmic, coordinated contraction: the sinoatrial node (SAN, the pacemaker, firing at 70-75 beats per minute) initiates impulses that spread through both atria, then reach the atrioventricular node (AVN, which delays the signal ~0.1 seconds to allow complete atrial emptying), continue through the Bundle of His, its left and right branches, and finally the Purkinje fibres that stimulate ventricular contraction from the apex upward.

The cardiac cycle lasts 0.8 seconds: atrial systole (0.1 s) forces remaining blood into ventricles; ventricular systole (0.3 s) generates high pressure to eject blood into pulmonary and aortic arteries; joint diastole (0.4 s) allows all chambers to relax and fill. Cardiac output equals stroke volume multiplied by heart rate (CO = SV x HR), approximately 5 litres per minute at rest.

The electrocardiogram (ECG) records electrical events: the P wave represents atrial depolarization, the QRS complex represents ventricular depolarization, and the T wave represents ventricular repolarization. Notably, atrial repolarization is masked by the QRS complex.

Double circulation comprises two circuits: pulmonary circulation (deoxygenated blood from right ventricle to lungs via pulmonary artery, returning oxygenated to left atrium via pulmonary veins) and systemic circulation (oxygenated blood from left ventricle to body via aorta, returning deoxygenated to right atrium via venae cavae). A critical NEET fact: pulmonary arteries carry deoxygenated blood, and pulmonary veins carry oxygenated blood — the exception to the general rule.

The key testable concept is the cardiac conducting system sequence (SAN to AVN to Bundle of His to Purkinje fibres), the ECG wave interpretations, and the blood group antigen-antibody relationships.