Chapter 15: Body Fluids and Circulation | Summary

Erythrocytes (RBCs), leucocytes (WBCs), and platelets (thrombocytes) collectively constitute the formed elements, making up about 45 per cent of blood. RBCs are the most abundant, numbering 5–5.5 million per mm3 in a healthy adult male. They are biconcave, nucleus-free (in most mammals), and contain haemoglobin — an iron-containing protein that transports respiratory gases. RBCs are produced in red bone marrow and have a lifespan of 120 days, after which they are destroyed in the spleen.

Leucocytes are nucleated, colourless cells averaging 6,000–8,000 per mm3. They are divided into granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes). Neutrophils (60–65%) are the most abundant and, along with monocytes, are phagocytic. Basophils secrete histamine, serotonin, and heparin; eosinophils resist infections and mediate allergic reactions; and lymphocytes (B and T types) are responsible for immune responses. Platelets, fragments of megakaryocytes, number 1,50,000–3,50,000 per mm3 and are crucial for blood clotting.

Human blood is classified using the ABO and Rh systems. ABO grouping is based on the presence or absence of surface antigens A and B on RBCs and corresponding antibodies (anti-A and anti-B) in plasma. Blood group O individuals are universal donors as they have no antigens, while AB individuals are universal recipients as they have no antibodies in their plasma. Rh grouping depends on the presence of the Rh antigen, found in about 80 per cent of humans (Rh+ve). Rh incompatibility during pregnancy can lead to erythroblastosis foetalis, preventable by administering anti-Rh antibodies after the first delivery.

Blood coagulation is a protective cascade initiated by injury. Platelets and damaged tissues release factors that activate a cascade converting prothrombin to thrombin via thrombokinase. Thrombin then converts soluble fibrinogen to insoluble fibrin threads, which trap blood cells to form a clot. Calcium ions are essential at multiple steps of this process.

As blood passes through tissue capillaries, water and small soluble substances filter out into the intercellular spaces, forming interstitial or tissue fluid. This fluid has the same mineral composition as plasma but lacks most large proteins and formed elements. The lymphatic system collects this fluid and returns it to major veins — the fluid within this system is called lymph.

Lymph is a colourless fluid rich in lymphocytes that contribute to immune defence. It also transports nutrients and hormones. Notably, dietary fats are absorbed into the bloodstream via lymph through lacteals present in the intestinal villi, bypassing the portal circulation.

Circulatory systems are either open (blood flows into sinuses — arthropods and molluscs) or closed (blood remains within vessels — annelids and chordates). Closed systems allow more precise flow regulation. Among vertebrates, fishes have 2-chambered hearts and single circulation; amphibians and most reptiles have 3-chambered hearts with incomplete double circulation; and crocodiles, birds, and mammals have 4-chambered hearts with complete double circulation.

The human heart is a mesodermally derived, fist-sized muscular organ in the thoracic cavity, protected by the pericardium. It has two atria (upper) and two ventricles (lower). The tricuspid valve guards the right atrio-ventricular opening; the bicuspid (mitral) valve guards the left. Semilunar valves guard the exits into the pulmonary artery and aorta. Nodal tissue (SAN and AVN) regulates the heartbeat. The SAN, located in the right atrium, generates 70–75 action potentials per minute and is called the pacemaker. The AVN and Bundle of His propagate impulses to the ventricles via Purkinje fibres.

The cardiac cycle consists of sequential systole and diastole of the atria and ventricles, completing in 0.8 seconds (72 cycles per minute). Joint diastole is followed by atrial systole (SAN-initiated), then ventricular systole, and finally joint diastole again. Each ventricle ejects approximately 70 mL per beat (stroke volume). Cardiac output equals stroke volume multiplied by heart rate, averaging 5,000 mL (5 litres) per minute in a healthy individual. Two heart sounds — lub (closure of AV valves) and dub (closure of semilunar valves) — are produced per cycle.

An electrocardiogram (ECG) records the electrical activity of the heart using an electrocardiograph. The P-wave represents atrial depolarisation; the QRS complex represents ventricular depolarisation (onset of systole); and the T-wave represents ventricular repolarisation (end of systole). ECGs are of great clinical significance — any deviation from the normal pattern suggests cardiac abnormality.

In humans, two distinct circulatory pathways operate simultaneously. In pulmonary circulation, deoxygenated blood from the right ventricle travels via the pulmonary artery to the lungs, gets oxygenated, and returns to the left atrium through pulmonary veins. In systemic circulation, oxygenated blood from the left ventricle flows via the aorta to all body tissues and returns deoxygenated blood through venae cavae to the right atrium. The hepatic portal system carries blood from the intestines to the liver before it enters systemic circulation. A specialised coronary system serves the heart muscle exclusively.

Though the heart is intrinsically auto-regulated (myogenic) by nodal tissue, the medulla oblongata moderates cardiac function via the autonomic nervous system. Sympathetic nerves increase heart rate and contractile strength, while parasympathetic nerves decrease them. Adrenal medullary hormones also enhance cardiac output in response to physiological demands.

Several disorders can impair the circulatory system. Hypertension is defined as blood pressure consistently at or above 140/90 mmHg (normal: 120/80) and can damage the heart, brain, and kidneys. Coronary Artery Disease (CAD) or atherosclerosis results from deposits of calcium, fat, cholesterol, and fibrous tissue narrowing arterial lumens, reducing blood supply to the heart.

Angina (angina pectoris) is acute chest pain caused by insufficient oxygen reaching the heart muscle, often related to impaired coronary blood flow, and is more common in middle-aged and elderly individuals. Heart failure (congestive heart failure) occurs when the heart cannot pump blood effectively to meet body needs. It is distinct from cardiac arrest (complete cessation of beating) and heart attack (sudden ischaemic damage to heart muscle).

Body Fluids and Circulation is a foundational chapter that integrates the structure and function of blood, lymph, the heart, and blood vessels into a coherent understanding of how the human body maintains homeostasis. Blood — with its plasma and formed elements — serves as both a transport medium and a defence system. The cardiac cycle, regulated by the pacemaker activity of the SAN and modulated by the autonomic nervous system, ensures uninterrupted supply of oxygen and nutrients to every cell while removing metabolic wastes. Double circulation keeps oxygenated and deoxygenated blood completely separate, ensuring maximal efficiency — a key evolutionary advance in mammals.

From an examination perspective, this chapter is among the highest-weightage topics in CBSE Class 11 and frequently features in NEET. Questions regularly appear on blood composition, ABO and Rh grouping, erythroblastosis foetalis, the mechanism of coagulation, cardiac cycle events and timings, heart sounds, ECG wave interpretation, the distinction between pulmonary and systemic circulation, and circulatory disorders. A thorough command of the diagrams (heart section, ECG, schematic circulation plan) alongside the numerical values (RBC count, haemoglobin levels, normal blood pressure, cardiac output) is essential for scoring full marks.