Body Fluids and CirculationNEET Zoology · Class 11 · NCERT Chapter 15

Introduction

Every cell in your body needs oxygen and nutrients and produces waste. The circulatory system is the transport network that moves these molecules around. Blood is the carrier, the heart is the pump, and lymph helps drain fluid that escapes from capillaries.

Expect 2 to 3 NEET questions every year from this chapter. The most reliable scoring areas are: WBC types and their percentages, blood groups and Rh factor, the clotting cascade, the four heart chambers, the cardiac cycle, and the ECG waves.

Blood

Blood is a red, opaque, slightly alkaline (pH 7.4) connective tissue. A healthy adult has about 5 to 6 litres of blood. Blood has two parts:

• Plasma: about 55% by volume. The liquid part.
• Formed elements: about 45% by volume. RBCs, WBCs and platelets.

Plasma

Plasma is straw-coloured, mostly water (about 90 to 92%). Important plasma proteins:

• Albumin (about 60% of plasma proteins): maintains osmotic pressure of blood.
• Globulins: include the antibodies (gamma-globulins) made by B cells.
• Fibrinogen: involved in blood clotting; converted to fibrin when needed.

Plasma also carries glucose, amino acids, lipids, urea, mineral ions (sodium, potassium, chloride, calcium, bicarbonate), hormones, dissolved gases (mainly CO2 as bicarbonate), and waste products. Plasma without the clotting factors is called serum.

Red Blood Cells (Erythrocytes)

• Most abundant blood cell. Count: 4 to 6 million per cubic mm of blood.
• Biconcave disc shape, no nucleus (mature mammalian RBCs lose their nucleus and most organelles). The biconcave shape gives more surface area and lets RBCs squeeze through capillaries.
• Made in bone marrow (red bone marrow of long bones in adults).
• Carry oxygen (and some CO2) using the iron-containing protein hemoglobin (about 12 to 16 g per 100 mL of blood).
• Lifespan: about 120 days. Destroyed in the spleen ("graveyard of RBCs") and liver.

White Blood Cells (Leukocytes)

WBCs have a nucleus, are colourless (no haemoglobin) and fight infection. Count: 5,000 to 11,000 per cubic mm. Two groups:

Granulocytes (have visible granules)

• Neutrophils (60 to 65%, the most abundant WBC): phagocytose bacteria. First responders to a bacterial infection.
• Eosinophils (2 to 3%): fight parasitic worm infections and allergic reactions.
• Basophils (0.5 to 1%, the least common WBC): release histamine and heparin during inflammation.

Agranulocytes (no visible granules)

• Lymphocytes (20 to 25%): two types. B lymphocytes produce antibodies (humoral immunity). T lymphocytes carry out cell-mediated immunity. Memory cells live for years.
• Monocytes (6 to 8%): the largest WBC. In tissue they become macrophages, which phagocytose pathogens and dead cells.

Quick mnemonic for order (most to least common): "Never Let Monkeys Eat Bananas" = Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils.

Platelets (Thrombocytes)

• Small cell fragments without a nucleus.
• Count: 1.5 to 3.5 lakh per cubic mm (150,000 to 350,000).
• Made by fragmentation of megakaryocytes in the bone marrow.
• Release thromboplastin when a blood vessel is damaged, triggering the clotting cascade. Low platelet count causes excessive bleeding.

Blood Groups

ABO System

Discovered by Karl Landsteiner. Based on two antigens (A and B) on the surface of RBCs and two antibodies (anti-A, anti-B) in plasma.

• Group A: antigen A on RBCs; anti-B in plasma.
• Group B: antigen B on RBCs; anti-A in plasma.
• Group AB: both antigens on RBCs; NO antibody in plasma. Universal recipient.
• Group O: no antigens on RBCs; both antibodies in plasma. Universal donor.

For a safe transfusion, the donor's RBC antigens must not meet matching antibodies in the recipient's plasma.

Rh System

Rh antigen (so named because first found in rhesus monkey blood) is another important RBC antigen. About 80% of Indians are Rh-positive (Rh+).

• An Rh- person who receives Rh+ blood develops anti-Rh antibodies after the first transfusion. A second Rh+ transfusion can cause a severe transfusion reaction.
• Erythroblastosis foetalis: happens in the second pregnancy when an Rh- mother carries an Rh+ fetus. At the first delivery some Rh+ fetal blood crosses into the mother. The mother makes anti-Rh antibodies. In the second Rh+ pregnancy these antibodies cross the placenta and destroy fetal RBCs.

Blood Coagulation (Clotting)

The classical cascade in four steps:

1. Damage to a blood vessel exposes collagen and causes platelets to release thromboplastin (Factor III).
2. Thromboplastin and Ca²⁺ convert plasma prothrombin (inactive) to thrombin (active).
3. Thrombin converts soluble fibrinogen to insoluble fibrin.
4. Fibrin threads form a mesh and trap blood cells, making the clot.

Vitamin K is needed by the liver to synthesise prothrombin. Calcium ions are needed at every step. Anticoagulants like heparin (made by liver and basophils) and EDTA prevent clotting.

Lymph (Interstitial Fluid)

When blood reaches a capillary, water and small molecules leak out into the spaces between cells. This fluid is called tissue fluid or interstitial fluid. Most of it returns to venous capillaries, but some enters lymphatic vessels and becomes lymph.

• Colourless and transparent (no RBCs).
• Contains some white blood cells, mostly lymphocytes.
• Less protein and less oxygen than blood.
• Flows through lymph nodes (which filter out pathogens) and finally drains into the venous system at the subclavian vein.
• Lymph from the small intestine (lacteals) carries absorbed fats and is called chyle.

Open vs Closed Circulation

• Open circulation: blood is pumped into open spaces called sinuses where it bathes the organs directly. Slow, low pressure. Found in arthropods and most molluscs.
• Closed circulation: blood always flows inside vessels (arteries, capillaries, veins). Fast, high pressure, efficient. Found in annelids and all chordates.

The Human Heart

The human heart is a four-chambered, fist-sized, muscular organ inside a double-walled sac called the pericardium. It lies in the thoracic cavity between the two lungs.

Chambers, Valves and Vessels

• Two atria (right and left) above two ventricles (right and left).
• Right atrium: receives deoxygenated blood from the body via superior and inferior venae cavae.
• Right ventricle: pumps deoxygenated blood to the lungs via the pulmonary artery (the only artery with deoxygenated blood).
• Left atrium: receives oxygenated blood from the lungs via the four pulmonary veins (the only veins with oxygenated blood).
• Left ventricle: pumps oxygenated blood to the whole body via the aorta. Wall is much thicker than the right ventricle because of the higher pressure needed.
• Atrioventricular (AV) valves: between atria and ventricles. Right side is tricuspid (3 cusps). Left side is bicuspid (mitral) (2 cusps).
• Semilunar valves: guard the exit of each ventricle. Pulmonary valve at right ventricle exit, aortic valve at left ventricle exit.

Conducting System of the Heart

The heart is myogenic: it generates its own beat without nerves. The signal travels through specialised cardiac muscle fibres in this order:

1. Sino-atrial node (SAN): the pacemaker, in the wall of the right atrium. Fires 70 to 75 times per minute.
2. Signal spreads across both atria, causing atrial systole.
3. Atrio-ventricular node (AVN): at the junction of atria and ventricles. Briefly delays the signal so atria can finish emptying.
4. Bundle of His: conducts the signal down the interventricular septum.
5. Purkinje fibres: spread the signal across the ventricular walls; ventricles contract from the apex upwards.

Heart rate is modulated by nerves: vagus nerve (parasympathetic) slows the heart; sympathetic nerves speed it up. Adrenaline from the adrenal medulla also speeds the heart.

Cardiac Cycle and Heart Sounds

One cardiac cycle is the sequence of events from the start of one beat to the start of the next. A normal cycle is about 0.8 seconds. At 72 beats per minute, the heart pumps about 5 litres a minute (cardiac output) and 7,200 litres a day.

1. Atrial systole (0.1 sec): atria contract, ventricles relaxed. About 30% of ventricular filling happens here.
2. Ventricular systole (0.3 sec): ventricles contract, atria relax. AV valves close (first heart sound, "lubb"). Pressure rises; semilunar valves open; blood ejects into aorta and pulmonary artery.
3. Joint diastole (0.4 sec): all chambers relaxed. Semilunar valves close (second heart sound, "dubb"). AV valves open. Ventricles fill passively from atria (about 70% of filling).

Electrocardiogram (ECG)

ECG is a recording of the electrical activity of the heart. The standard wave has three named deflections:

• P wave: depolarisation (electrical activation) of the atria, leading to atrial contraction.
• QRS complex: depolarisation of the ventricles, leading to ventricular contraction. Atrial repolarisation is hidden inside it.
• T wave: repolarisation of the ventricles, leading to relaxation.

Heart rate is calculated from the time between successive R peaks (the R-R interval).

Double Circulation

In mammals and birds, blood passes through the heart twice in one full circuit. This is double circulation.

• Pulmonary circulation: right ventricle to lungs to left atrium. Carries deoxygenated blood to the lungs for gas exchange, then oxygenated blood back.
• Systemic circulation: left ventricle to body to right atrium. Carries oxygenated blood to all body tissues, then deoxygenated blood back.

Why it matters: double circulation completely separates oxygenated and deoxygenated blood, so the body always gets fully oxygenated blood. Fish have a single circulation (one circuit through gills and body), amphibians and most reptiles have incomplete double circulation, and only birds, mammals and crocodiles have full double circulation with a four-chambered heart.

Circulatory Disorders

• Hypertension (high blood pressure): blood pressure consistently above 140/90 mm Hg. Causes long-term damage to the heart, blood vessels, kidneys and retina.
• Coronary artery disease (CAD): narrowing of coronary arteries by deposition of cholesterol and calcium. Reduces blood supply to the heart muscle. Also called atherosclerosis.
• Angina pectoris: chest pain when the heart muscle does not get enough oxygen, often during exertion. A warning sign of CAD.
• Heart attack (myocardial infarction): a coronary artery is fully blocked, part of the heart muscle dies. Symptoms: severe chest pain, sweating, breathlessness.
• Heart failure: the heart fails to pump enough blood. May follow long-term CAD or hypertension.

Worked NEET Problems

Summary Cheat Sheet

• Blood: plasma 55%, formed elements 45%. Total volume 5 to 6 L.
• Plasma proteins: albumin (osmotic balance), globulins (immunity), fibrinogen (clotting). Plasma minus clotting factors = serum.
• RBCs: 4 to 6 million per mm³, biconcave, no nucleus, lifespan 120 days, haemoglobin carries O₂.
• WBCs (5,000 to 11,000 per mm³): Neutrophils 60-65%, Lymphocytes 20-25%, Monocytes 6-8%, Eosinophils 2-3%, Basophils 0.5-1%.
• Platelets: 1.5 to 3.5 lakh per mm³. Release thromboplastin.
• ABO groups: A (anti-B), B (anti-A), AB (universal recipient), O (universal donor).
• Rh: Rh- mother + Rh+ fetus, second pregnancy = erythroblastosis foetalis.
• Clotting: Thromboplastin → Prothrombin to Thrombin → Fibrinogen to Fibrin. Needs Ca²⁺ and vitamin K.
• Lymph: no RBCs, mostly lymphocytes; chyle = fatty lymph from intestine.
• Heart: 4 chambers; right side deoxygenated, left side oxygenated.
• Valves: tricuspid (right AV), bicuspid/mitral (left AV), semilunar (at exits of both ventricles).
• Conducting system: SAN (pacemaker, 70 to 75 bpm) → AVN → bundle of His → Purkinje fibres.
• Cardiac cycle (~0.8 s): atrial systole 0.1 s, ventricular systole 0.3 s, joint diastole 0.4 s.
• Heart sounds: "lubb" = AV valves close (start of ventricular systole); "dubb" = semilunar valves close (start of joint diastole).
• ECG waves: P = atrial depolarisation, QRS = ventricular depolarisation, T = ventricular repolarisation.
• Pulmonary artery: only artery with deoxygenated blood. Pulmonary veins: only veins with oxygenated blood.
• Double circulation: blood passes through heart TWICE per cycle. Birds + mammals + crocodile only.
• Disorders: hypertension (BP above 140/90); CAD = narrowed coronary arteries; angina = pain on exertion; MI = full block, muscle dies.

Next: use the interactive learning widgets to explore the labelled heart, drill blood composition and scrub through the cardiac cycle with ECG, or work through the 25+ NEET PYQs with full solutions. To time yourself, take the free 10-question mock test.