Body Fluids and Circulation

Blood is a special fluid connective tissue with a liquid matrix, and it is the chief transport medium of the body. By volume it is roughly 55% plasma and 45% formed elements. Plasma is a straw-coloured fluid, about 90–92% water, carrying dissolved proteins (mainly fibrinogen, globulins and albumins), ions, glucose, amino acids, hormones and wastes; fibrinogen is needed for clotting, globulins for defence and albumins for osmotic balance.

The formed elements are three cell types. Red blood cells (erythrocytes) are the most abundant (about 5–5.5 million per mm³), biconcave, and in mammals lack a nucleus; they are packed with haemoglobin to carry oxygen, are formed in the red bone marrow, live about 120 days, and are destroyed in the spleen (the 'graveyard of RBCs'). White blood cells (leucocytes) are nucleated and colourless (6000–8000 per mm³); they are granulocytes (neutrophils — the most abundant, ~60–65%; eosinophils; basophils) or agranulocytes (lymphocytes, monocytes), and they provide immunity. Platelets (thrombocytes) (1.5–3.5 lakh per mm³) are cell fragments that help in clotting.

Blood groups are defined by antigens on the red cells. In the ABO system, group A has antigen A (and anti-B antibody), B has antigen B (anti-A), AB has both antigens and no antibodies, and O has neither antigen but both antibodies. Hence AB is the universal recipient and O the universal donor. The Rh factor is another antigen; an Rh-negative mother carrying a second Rh-positive child can suffer erythroblastosis foetalis, in which maternal anti-Rh antibodies attack the foetal red cells.

Coagulation (clotting) seals an injury. Damaged tissue and platelets release factors that form an enzyme (thromboplastin/thrombokinase); this converts inactive prothrombin to the active enzyme thrombin (a step requiring calcium ions, Ca²⁺), and thrombin then converts soluble fibrinogen into insoluble fibrin threads that trap blood cells to form the clot. The exam-critical chain to remember is: prothrombin → thrombin (needs Ca²⁺) → fibrinogen → fibrin.

Besides blood, the body has a second fluid, lymph (tissue fluid). As blood flows through fine capillaries, some plasma along with small dissolved substances leaks out into the spaces between cells; this colourless fluid is the lymph. It is essentially plasma minus the large proteins and red cells, but it does contain white blood cells (lymphocytes). The lymphatic system collects this fluid and returns it to the blood, and its lacteals in the intestine also absorb digested fats. Lymph thus carries nutrients, hormones and immune cells.

Animals show two broad patterns of circulation. In an open circulatory system, blood (called haemolymph) is pumped out into open spaces (sinuses) and bathes the tissues directly before returning — seen in arthropods (cockroach) and most molluscs. In a closed circulatory system, blood always flows within a continuous network of vessels and never leaves them, allowing precise, regulated distribution — seen in annelids (earthworm) and all vertebrates.

Within closed systems, vertebrates differ in how many times the blood passes through the heart per circuit. In single circulation the blood passes through the heart only once per complete circuit — typical of fishes, whose two-chambered heart pumps deoxygenated blood to the gills, from where oxygenated blood goes directly to the body. In double circulation, the blood passes through the heart twice: a pulmonary circuit carries blood to the lungs and back, and a systemic circuit carries it to the body and back.

Double circulation is the pattern in birds and mammals, whose four-chambered hearts keep oxygenated and deoxygenated blood completely separate, making oxygen delivery highly efficient. Amphibians and most reptiles have an incomplete double circulation (their hearts allow some mixing because the ventricle is not fully divided). For NEET, fix the pairs: open = arthropods/molluscs; closed = annelids/vertebrates; single = fish; complete double = birds and mammals.

The human heart is a muscular, four-chambered pump lying in the thoracic cavity, enclosed in a double-walled pericardium with lubricating fluid. The upper chambers are the two atria (auricles) and the lower are the two ventricles. The right side carries deoxygenated blood and the left side carries oxygenated blood, and the two sides are completely separate (double circulation).

The flow follows a fixed path. The right atrium receives deoxygenated blood from the body via the venae cavae and passes it to the right ventricle, which pumps it through the pulmonary artery to the lungs. Oxygenated blood returns via the pulmonary veins to the left atrium, then to the left ventricle, which pumps it through the aorta to the whole body. To prevent backflow, valves guard the openings: the tricuspid valve (right atrioventricular), the bicuspid or mitral valve (left atrioventricular), and the semilunar valves at the bases of the pulmonary artery and aorta. Because it must push blood around the entire body, the left ventricle has the thickest wall.

One heartbeat is the cardiac cycle, lasting about 0.8 second at a resting rate of ~72 beats per minute. It has three phases: atrial systole (atria contract, topping up the ventricles), ventricular systole (ventricles contract, ejecting blood into the arteries) and joint diastole (all chambers relax and fill). The volume pumped by a ventricle per beat is the stroke volume (~70 mL), and the cardiac output = stroke volume × heart rate (about 5 litres per minute at rest).

The heartbeat produces two characteristic heart sounds. The first, 'lubb', is the closure of the atrioventricular (tricuspid and bicuspid) valves at the start of ventricular systole; the second, 'dup', is the closure of the semilunar valves at the end of ventricular systole. Matching each sound to the valves that cause it, and remembering the chambers, valves and the cardiac-output formula, covers most heart questions.

The heart beats on its own because it is myogenic — the impulse arises from heart muscle, not from a nerve. The signal starts at the sino-atrial (SA) node in the wall of the right atrium, which generates impulses about 70–75 times a minute and is therefore called the pacemaker. From the SA node the impulse spreads over the atria to the atrioventricular (AV) node, then down the bundle of His and its branches, and finally through the Purkinje fibres into the ventricle walls, causing them to contract. This conducting pathway ensures the chambers contract in the correct order.

The electrical activity of the heart can be recorded as an electrocardiogram (ECG), a graph of voltage against time with three characteristic deflections. The P wave represents the depolarisation of the atria (which leads to atrial contraction); the QRS complex represents the depolarisation of the ventricles (the start of ventricular contraction); and the T wave represents the repolarisation (relaxation) of the ventricles. The shape, timing and number of these waves are used clinically to diagnose heart conditions, so knowing which wave reflects which event is a high-yield NEET fact.

Although the heart sets its own basic rhythm, the rate and force of the beat are regulated by the nervous system and hormones to match the body's needs. A cardiac centre in the medulla oblongata sends two opposing nerve supplies: sympathetic nerves speed up the heart and increase its force, while parasympathetic (vagus) nerves slow it down.

Hormonal control reinforces this: adrenaline (and noradrenaline) from the adrenal medulla increases heart rate and output during stress or exercise. Finally, several disorders are part of the syllabus. Hypertension is persistently high blood pressure (above about 140/90 mm Hg; normal is ~120/80). Coronary artery disease (atherosclerosis) narrows the vessels supplying the heart muscle; angina pectoris is the chest pain felt when the heart muscle gets too little oxygen; and heart failure is the state in which the heart cannot pump enough blood for the body's needs. For NEET, remember the conduction order (SA → AV → His → Purkinje), the ECG wave meanings, the sympathetic/vagus and adrenaline controls, and the definitions of these disorders.