Body Fluids and Circulation Class 11 Notes

Blood — Composition and Functions

Plasma

Plasma is a straw-coloured, slightly alkaline (pH 7.35–7.45) fluid that constitutes about 55% of blood volume. It is ~90% water and contains dissolved proteins (albumin, globulins, fibrinogen, prothrombin), electrolytes, nutrients, hormones, waste products and gases.

Formed Elements

• Red blood cells (erythrocytes) — biconcave, enucleated in mammals; ~5–5.5 million/mm³ of blood. Contain haemoglobin (Hb ≈ 12–16 g/100 mL blood) for O₂ transport. Lifespan ≈ 120 days; destroyed in the spleen ('graveyard of RBCs').
• White blood cells (leucocytes) — nucleated, fewer in number (6000–8000/mm³). Types: granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes). They defend against infection and foreign material.
• Platelets (thrombocytes) — cell fragments derived from megakaryocytes in bone marrow; ~150,000–350,000/mm³. Essential for blood clotting (haemostasis). Lifespan ≈ 7–11 days.

Lymph

• Transports absorbed fats from the intestine (chylomicrons in lacteals) to the blood.
• Returns excess interstitial fluid and proteins back to the blood.
• Lymph nodes filter lymph and house immune cells (lymphocytes and macrophages) that destroy pathogens.
• Lymphatic vessels carry lymph through lymph nodes and ultimately drain into the subclavian veins.

Blood Groups — ABO and Rh Systems

• Blood group A — A antigen on RBCs, anti-B antibody in plasma.
• Blood group B — B antigen on RBCs, anti-A antibody in plasma.
• Blood group AB — both A and B antigens on RBCs; no anti-A or anti-B antibodies. Universal recipient for RBC transfusion.
• Blood group O — no A or B antigens on RBCs; both anti-A and anti-B antibodies in plasma. Universal donor for RBC transfusion.

Coagulation of Blood

Haemostasis (stopping of bleeding) involves vascular spasm, platelet plug formation and coagulation (clotting). Coagulation is a cascade of enzymatic reactions involving clotting factors.

• The key reaction: prothrombin (inactive, produced by liver, requires vitamin K) is converted to thrombin (active enzyme) by prothrombinase (activated by tissue thromboplastin and Ca²⁺).
• Thrombin then converts soluble fibrinogen (produced by liver) into insoluble fibrin threads that form the mesh of the clot, trapping blood cells.
• The clot retracts and is eventually dissolved by plasmin (fibrinolysis).
• Anticoagulants: heparin (produced by basophils, prevents thrombin formation) and EDTA or sodium citrate (chelate Ca²⁺).

The Human Heart — Structure and Cardiac Cycle

• Right atrium — receives deoxygenated blood from the body via superior and inferior venae cavae; pumps to right ventricle through the tricuspid (atrioventricular) valve.
• Right ventricle — pumps deoxygenated blood to the lungs via the pulmonary artery through the pulmonary (semilunar) valve.
• Left atrium — receives oxygenated blood from the lungs via the four pulmonary veins; pumps to left ventricle through the bicuspid (mitral) valve.
• Left ventricle — the thickest chamber; pumps oxygenated blood to the entire body via the aorta through the aortic (semilunar) valve.

• Atrial systole (~0.1 s) — both atria contract, pushing remaining blood into ventricles (atrial kick).
• Ventricular systole (~0.3 s) — both ventricles contract; AV valves close (first heart sound, 'lub'), semilunar valves open; blood is ejected.
• Joint diastole (~0.4 s) — all chambers relax; semilunar valves close (second heart sound, 'dub'); blood fills the atria.

Double Circulation and ECG

Double circulation consists of two circuits working simultaneously.

• Pulmonary circulation — right ventricle → pulmonary artery → lungs (gas exchange) → pulmonary veins → left atrium. This circuit operates at lower pressure.
• Systemic circulation — left ventricle → aorta → body tissues (O₂ delivery, CO₂ pickup) → venae cavae → right atrium. This circuit operates at higher pressure due to the thick-walled left ventricle.

Disorders of the Circulatory System

• Hypertension (high blood pressure) — sustained BP above 140/90 mm Hg. Damages arteries, heart, kidneys and brain over time. Risk factors: obesity, salt excess, stress, genetic predisposition.
• Coronary artery disease (atherosclerosis) — deposition of cholesterol plaques in coronary arteries, reducing blood flow to the heart muscle (myocardium). Can lead to angina and myocardial infarction (heart attack).
• Angina — temporary chest pain (angina pectoris) due to reduced blood supply to the heart muscle during exertion or stress; a warning sign of coronary artery disease.
• Heart failure — the heart cannot pump enough blood to meet the body's needs. Not the same as cardiac arrest (sudden stop of heartbeat). Managed with diuretics, ACE inhibitors and lifestyle changes.

Solved Examples

Example: A patient has blood group A. Can they safely receive blood from a donor with blood group AB? Explain.

Solution: No. A person with blood group A has anti-B antibodies in their plasma. AB blood contains both A and B antigens on the RBCs. If AB blood is transfused into an A recipient, the anti-B antibodies will attack the B antigens, causing agglutination (clumping). Therefore, AB blood cannot be safely given to a group A patient. A group A patient should receive group A or group O blood.

Example: Calculate the cardiac output of a person whose stroke volume is 70 mL and heart rate is 80 beats per minute.

Solution: Cardiac output = Stroke volume × Heart rate = 70 mL × 80 = 5600 mL/min = 5.6 L/min. This is slightly above the average resting value of ~5 L/min (70 mL × 72 bpm), which is within the normal range and may reflect mild physical activity or a larger body size.