Body Fluids and Circulation notes

This chapter explains how transport of gases, nutrients, hormones, wastes and immune cells occurs in the human body. NCERT focuses mainly on blood, lymph, the human heart, blood vessels, cardiac cycle, double circulation and common circulatory disorders. Blood is a connective tissue made of plasma and formed elements. Lymph returns tissue fluid to blood and helps immunity. The four-chambered human heart pumps blood through pulmonary and systemic circuits. The cardiac cycle explains rhythmic systole and diastole, while ECG records electrical activity of the heart. NEET frequently asks blood groups, coagulation, cardiac output, heart sounds, hypertension, CAD and double circulation.

Blood is a specialized fluid connective tissue that transports gases, nutrients, hormones, wastes and immune components. It consists of plasma, the straw-coloured fluid matrix, and formed elements: RBCs, WBCs and platelets. Plasma contains water, proteins, ions, nutrients, hormones and wastes. RBCs are biconcave, enucleated cells rich in haemoglobin and transport oxygen. WBCs are nucleated immune cells classified as granulocytes and agranulocytes. Platelets are cell fragments that help blood coagulation. Blood grouping depends on antigens present on RBCs, mainly ABO and Rh systems. In NEET, blood composition, cell counts, universal donor/recipient and clotting factors are frequently tested.

Lymph is a colourless fluid formed when tissue fluid enters lymphatic capillaries. As blood passes through capillaries, some plasma and small molecules filter into intercellular spaces, forming tissue fluid. Most returns directly to blood, while the remaining fluid enters lymph vessels and becomes lymph. Lymph is similar to plasma but has fewer proteins, no RBCs and many lymphocytes. It transports absorbed fats from intestinal lacteals, returns excess tissue fluid to blood and supports immunity. Lymphatic vessels ultimately drain into large veins near the heart. Lymphoid organs such as lymph nodes, spleen, thymus and tonsils help filter pathogens and produce immune responses.

Human circulation is a closed circulatory system driven by a four-chambered muscular heart. The right atrium receives deoxygenated blood from the venae cavae and sends it to the right ventricle, which pumps it to lungs through the pulmonary artery. The left atrium receives oxygenated blood from pulmonary veins and sends it to the left ventricle, which pumps it through the aorta to the body. Valves prevent backflow. The conducting system includes SA node, AV node, bundle of His and Purkinje fibres, ensuring rhythmic contraction. Arteries carry blood away from the heart, veins carry blood toward the heart and capillaries allow exchange.

The cardiac cycle is the sequence of events completed during one heartbeat. At an average heart rate of about 72 beats per minute, one cycle lasts about 0.8 seconds. It includes joint diastole, atrial systole and ventricular systole. During joint diastole, all chambers relax and blood flows passively into ventricles. Atrial systole completes ventricular filling. Ventricular systole closes AV valves, produces the first heart sound, and pumps blood into aorta and pulmonary artery. Ventricular relaxation closes semilunar valves, producing the second heart sound. ECG records electrical activity: P wave, QRS complex and T wave, which correspond to atrial and ventricular electrical events.

Double circulation means blood passes through the heart twice during one complete circulation: once through the pulmonary circuit and once through the systemic circuit. In pulmonary circulation, deoxygenated blood from the right ventricle goes to the lungs via pulmonary artery and returns oxygenated to the left atrium through pulmonary veins. In systemic circulation, oxygenated blood from the left ventricle goes to body tissues through the aorta and returns deoxygenated to the right atrium through venae cavae. This separation maintains high oxygen efficiency and prevents mixing. Blood pressure measures force exerted by blood on arterial walls, while cardiac output measures blood pumped per minute.

Circulatory disorders arise when normal heart or blood vessel function is disturbed. Hypertension is persistently elevated blood pressure, commonly above 140/90 mm Hg, and can damage heart, brain and kidneys. Coronary artery disease occurs when coronary arteries supplying the heart become narrowed, often due to atherosclerosis. Atherosclerosis is deposition of calcium, fat, cholesterol and fibrous tissue in arterial walls, reducing lumen size. Angina pectoris is chest pain due to insufficient oxygen supply to heart muscle, usually during exertion or stress. Heart failure means the heart cannot pump enough blood to meet body needs; it is not the same as cardiac arrest or heart attack.