Body Fluids and Circulation — NCERT Class 11 Biology

How to Use This Page

This page provides complete NCERT Solutions for CBSE Class 11 Biology Chapter 18 — Body Fluids and Circulation. All intext questions (organised by page number) and chapter-end exercise questions are answered in detail. Use the section headings to jump directly to the question you need help with, or read through the full page for a comprehensive revision of the chapter.

Key Concepts (Quick Recap)

• Blood Composition: Plasma (55%) + Formed elements (45%: RBCs, WBCs, platelets)
• Plasma Proteins: Fibrinogen (clotting), Albumins (osmotic pressure), Globulins (immunity)
• RBCs (Erythrocytes): Biconcave discs, no nucleus, contain haemoglobin, lifespan ~120 days, produced in red bone marrow
• WBCs (Leucocytes): Granulocytes (neutrophils, eosinophils, basophils) and Agranulocytes (lymphocytes, monocytes)
• ABO Blood Groups: A (A antigen, anti-B antibody), B (B antigen, anti-A antibody), AB (A and B antigens, no antibodies — universal recipient), O (no antigens, anti-A and anti-B antibodies — universal donor)
• Rh Factor: Rh-positive (Rh antigen present), Rh-negative (Rh antigen absent); important in erythroblastosis foetalis
• Coagulation: Fibrinogen →(Thrombin)→ Fibrin; Prothrombin →(Thromboplastin + Ca²⁺)→ Thrombin; cascade of clotting factors
• Lymph: Tissue fluid that enters lymphatic vessels; similar to plasma but has fewer proteins; helps in immune response and fat absorption
• Heart Anatomy: 4 chambers (2 atria, 2 ventricles); bicuspid (mitral) valve on left, tricuspid valve on right; semilunar valves at aortic and pulmonary openings
• Cardiac Cycle: 0.8 sec per cycle; atrial systole (0.1 sec) → ventricular systole (0.3 sec) → cardiac diastole (0.4 sec)
• Cardiac Output: Stroke volume (70 mL) × Heart rate (72/min) ≈ 5040 mL/min ≈ 5 L/min
• ECG Waves: P wave (atrial depolarisation), QRS complex (ventricular depolarisation), T wave (ventricular repolarisation)
• Double Circulation: Pulmonary circulation (right side → lungs → left side) + Systemic circulation (left side → body → right side)
• Hepatic Portal System: Vein from digestive tract → liver → inferior vena cava

Intext Questions

Page 284

Lymph is not a component of blood. Blood consists of plasma and formed elements (RBCs, WBCs, and platelets). Lymph is a separate fluid — it is the tissue fluid that has entered the lymphatic vessels. While lymph is derived from blood plasma and has a similar composition (but with fewer proteins and no RBCs), it circulates through the lymphatic system, not through blood vessels.

Individuals with blood group AB have both A and B antigens on the surface of their RBCs, but they have neither anti-A nor anti-B antibodies in their plasma. This is why they can receive blood from any blood group (universal recipients) — there are no antibodies to attack the donor RBCs. However, they can only donate to other AB individuals because their RBCs carry both antigens.

Page 291

The pericardium is a double-layered membrane that surrounds and protects the heart. It consists of an outer fibrous pericardium and an inner serous pericardium. The serous pericardium has two layers — the outer parietal layer and the inner visceral layer (epicardium). The space between these two layers is called the pericardial cavity, which contains pericardial fluid that reduces friction during heart contractions.

During ventricular systole (contraction of ventricles), the pressure in the ventricles increases. When ventricular pressure exceeds atrial pressure, the bicuspid (mitral) valve on the left side and the tricuspid valve on the right side close to prevent the backflow of blood into the atria. This closure produces the first heart sound ("lub"). Simultaneously, when ventricular pressure exceeds the pressure in the aorta and pulmonary artery, the semilunar valves open, allowing blood to be pumped out.

Page 297

In double circulation, blood passes through the heart twice during one complete circuit of the body: once through the pulmonary circulation (where blood is oxygenated in the lungs) and once through the systemic circulation (where oxygenated blood is distributed to body tissues). This ensures complete separation of oxygenated and deoxygenated blood, which is a characteristic feature of birds and mammals.

The P wave in an ECG represents the electrical impulse spreading through the atria, causing atrial depolarisation (which leads to atrial contraction/systole). The QRS complex represents ventricular depolarisation, and the T wave represents ventricular repolarisation. Atrial repolarisation occurs during the QRS complex but is hidden by the much larger ventricular depolarisation signal.

Chapter-End Exercise

1. Plasma (55% of blood volume):

• Water: ~90% of plasma
• Plasma proteins: ~6–8%
• Fibrinogen: essential for blood clotting
• Albumins: maintain osmotic pressure (oncotic pressure) and transport substances
• Globulins: primarily involved in immune responses (antibodies/immunoglobulins)
• Minerals: Na⁺, K⁺, Ca²⁺, Cl⁻, HCO₃⁻, etc.
• Nutrients: glucose, amino acids, fatty acids, vitamins
• Hormones: various hormones transported in blood
• Waste products: urea, uric acid, creatinine, CO₂
• Dissolved gases: O₂ and CO₂

2. Formed Elements (45% of blood volume):

• Red Blood Cells (Erythrocytes): Most abundant blood cells (~5–5.5 million/mm³). Biconcave disc shape, no nucleus in mature state, contain haemoglobin (iron-containing pigment that carries oxygen). Lifespan: ~120 days. Produced in red bone marrow. Destroyed in the spleen (graveyard of RBCs).
• White Blood Cells (Leucocytes): ~6000–8000/mm³. Colourless, nucleated, provide immunity.
• Granulocytes (granules in cytoplasm): Neutrophils (most abundant, ~60–65%), Eosinophils (~2–3%, resist infections), Basophils (~0.5–1%, release histamine, heparin)
• Agranulocytes (no granules): Lymphocytes (~20–25%, produce antibodies), Monocytes (~2–6%, phagocytic)
• Platelets (Thrombocytes): ~1.5–3.5 lakh/mm³. Cell fragments (not true cells) derived from megakaryocytes. Essential for blood clotting. Lifespan: ~7–10 days.

1. Fibrinogen:

• Essential for blood coagulation (clotting)
• Converted to fibrin by thrombin during the clotting process
• Forms a mesh of fibrin threads that traps RBCs to form a blood clot

2. Albumins:

• Maintain osmotic pressure (oncotic pressure) of blood, which prevents fluid from leaking out of blood vessels
• Transport various substances including hormones, fatty acids, and drugs
• Act as buffer to maintain blood pH
• Contribute to blood viscosity

3. Globulins:

• Alpha and beta globulins: transport metals (iron, copper), hormones (thyroid hormones), and lipids (as lipoproteins)
• Gamma globulins (immunoglobulins/antibodies): provide immunity by recognising and neutralising foreign substances (antigens)

Key points:

• If RBCs with a specific antigen are mixed with plasma containing the corresponding antibody, agglutination (clumping) occurs.
• For example, if blood group A RBCs (with A antigen) are mixed with blood group B plasma (with anti-A antibody), agglutination occurs.
• Blood group O is called the universal donor because its RBCs have no antigens, so they will not be attacked by any antibodies.
• Blood group AB is called the universal recipient because its plasma has no antibodies, so it will not attack any donor RBCs.

Importance of Rh factor:

1. Blood transfusion: An Rh-negative person receiving Rh-positive blood will develop anti-Rh antibodies. A subsequent transfusion of Rh-positive blood can cause a severe transfusion reaction (agglutination and haemolysis of donor RBCs).

2. Erythroblastosis foetalis (Haemolytic disease of the newborn): This occurs when an Rh-negative mother carries an Rh-positive foetus (inherited from the father). During the first pregnancy, the mother's immune system is sensitised to the Rh antigen when foetal RBCs enter the maternal circulation (usually during delivery). In subsequent Rh-positive pregnancies, the mother's anti-Rh antibodies can cross the placenta and attack the foetal RBCs, causing severe haemolysis. This can lead to anaemia, jaundice, or even death of the foetus.

3. Prevention: Anti-Rh antibodies (RhoGAM) are administered to Rh-negative mothers after the first delivery to destroy any Rh-positive foetal cells in the maternal circulation, preventing sensitisation.

Stage 1: Formation of prothrombin activator (thromboplastin)

• When a blood vessel is injured, thromboplastin is released from damaged tissues and platelets.
• Thromboplastin, along with calcium ions (Ca²⁺) and other factors, converts inactive prothrombin into active thrombin.
• This can occur through two pathways:
• Extrinsic pathway: Initiated by tissue thromboplastin released from damaged cells (faster, a few seconds)
• Intrinsic pathway: Initiated by contact with damaged vessel wall (slower, several minutes)

Stage 2: Conversion of prothrombin to thrombin

• Prothrombin (inactive, produced by the liver, requires vitamin K) →(Prothrombin activator + Ca²⁺)→ Thrombin (active enzyme)

Stage 3: Conversion of fibrinogen to fibrin

• Fibrinogen (soluble plasma protein) →(Thrombin)→ Fibrin (insoluble, forms threads)
• Fibrin threads form a mesh that traps RBCs, WBCs, and platelets, forming a blood clot.
• The clot contracts (clot retraction) and seals the wound.

Summary: Tissue injury → Thromboplastin release → Prothrombin →(Ca²⁺)→ Thrombin → Fibrinogen → Thrombin → Fibrin → Blood clot

External structure:

• Protected by the pericardium (double-layered membrane with pericardial fluid)
• The outer layer is the fibrous pericardium; the inner serous pericardium has parietal and visceral layers

Internal structure — Four chambers:

1. Right Atrium: Receives deoxygenated blood from the superior vena cava (from upper body), inferior vena cava (from lower body), and coronary sinus (from heart muscle). It has the sino-atrial (SA) node, which is the pacemaker of the heart.

2. Right Ventricle: Receives deoxygenated blood from the right atrium through the tricuspid valve. Pumps blood to the lungs through the pulmonary artery.

3. Left Atrium: Receives oxygenated blood from the lungs through the four pulmonary veins.

4. Left Ventricle: Receives oxygenated blood from the left atrium through the bicuspid (mitral) valve. Pumps blood to the body through the aorta. Has the thickest muscular wall to generate high pressure for systemic circulation.

Valves:

• Tricuspid valve: Between right atrium and right ventricle (3 flaps)
• Bicuspid (Mitral) valve: Between left atrium and left ventricle (2 flaps)
• Pulmonary semilunar valve: At the opening of the pulmonary artery
• Aortic semilunar valve: At the opening of the aorta

Septum: The interatrial septum separates the two atria; the interventricular septum separates the two ventricles. These prevent mixing of oxygenated and deoxygenated blood.

Phase 1: Atrial Systole (0.1 sec)

• Both atria contract simultaneously.
• The SA node generates an electrical impulse that spreads through the atria.
• Blood is pushed from the atria into the ventricles through the bicuspid and tricuspid valves.
• At the end of atrial systole, the ventricles receive about 70% of their blood volume.
• The remaining 30% of ventricular filling occurs passively during diastole.

Phase 2: Ventricular Systole (0.3 sec)

• Both ventricles contract simultaneously.
• The AV node conducts the impulse to the ventricles via the bundle of His and Purkinje fibres.
• As ventricular pressure rises above atrial pressure, the bicuspid and tricuspid valves close (producing the first heart sound — "lub").
• As ventricular pressure exceeds the pressure in the aorta and pulmonary artery, the semilunar valves open.
• Blood is ejected from the right ventricle into the pulmonary artery and from the left ventricle into the aorta.

Phase 3: Cardiac Diastole (0.4 sec)

• All four chambers relax.
• The semilunar valves close as ventricular pressure drops below aortic and pulmonary pressure (producing the second heart sound — "dub").
• Blood flows passively from the atria into the ventricles.
• The coronary blood flow to the heart muscle is maximum during this phase.

Cardiac Output: The amount of blood pumped by each ventricle per minute. Cardiac Output = Stroke Volume × Heart Rate = 70 mL × 72/min = 5040 mL/min ≈ 5 L/min

The normal ECG consists of the following waves:

1. P Wave:

• Represents atrial depolarisation (electrical excitation of the atria)
• Indicates that the atria are about to contract
• Duration: ~0.08 seconds
• Abnormal P wave may indicate atrial enlargement or arrhythmia

2. QRS Complex:

• Represents ventricular depolarisation (electrical excitation of the ventricles)
• The Q wave is the first negative deflection, R wave is the first positive deflection, and S wave is the second negative deflection
• Indicates that the ventricles are about to contract
• Duration: ~0.08 seconds
• Atrial repolarisation also occurs during this period but is hidden by the large QRS complex
• An enlarged QRS complex may indicate ventricular hypertrophy

3. T Wave:

• Represents ventricular repolarisation (recovery/resting phase of the ventricles)
• Indicates that the ventricles are relaxing
• Duration: ~0.16 seconds
• An inverted or flattened T wave may indicate myocardial ischaemia

Significance of ECG: 1. Detects abnormal heart rhythms (arrhythmias) 2. Diagnoses heart attacks (myocardial infarction) 3. Identifies enlargement of heart chambers (cardiac hypertrophy) 4. Detects electrolyte imbalances (especially potassium and calcium) 5. Monitors the effect of medications on the heart

1. Pulmonary Circulation (lesser circulation):

• Deoxygenated blood from the body enters the right atrium via the superior and inferior vena cava.
• Blood flows from the right atrium to the right ventricle through the tricuspid valve.
• The right ventricle pumps blood into the pulmonary artery, which carries it to the lungs.
• In the lungs, CO₂ is released and O₂ is absorbed (gas exchange at the alveoli).
• Oxygenated blood returns to the left atrium via the pulmonary veins.

2. Systemic Circulation (greater circulation):

• Oxygenated blood from the left atrium flows into the left ventricle through the bicuspid (mitral) valve.
• The left ventricle pumps blood into the aorta, which distributes it to all body tissues.
• In the tissues, O₂ is released and CO₂ is collected.
• Deoxygenated blood returns to the right atrium via the superior and inferior vena cava.

Importance of double circulation: 1. Complete separation of oxygenated and deoxygenated blood — ensures efficient oxygen supply to tissues. 2. Different pressures — the left ventricle generates high pressure for systemic circulation, while the right ventricle generates lower pressure for pulmonary circulation (protecting delicate lung capillaries). 3. Efficient oxygen delivery — all body tissues receive fully oxygenated blood.

Pathway: Blood from the digestive tract (stomach, intestines, spleen, pancreas) → Hepatic portal vein → Liver → Hepatic vein → Inferior vena cava → Right atrium

Components: 1. Hepatic portal vein: Carries nutrient-rich, deoxygenated blood from the gastrointestinal tract and spleen to the liver. It is formed by the union of the superior mesenteric vein and the splenic vein. 2. Hepatic artery: Supplies oxygenated blood to the liver. 3. Hepatic vein: Carries blood from the liver to the inferior vena cava.

Significance: 1. Detoxification: The liver removes toxins, drugs, and harmful substances absorbed from the digestive tract before they enter the systemic circulation. 2. Nutrient processing: The liver processes and stores nutrients (glucose is stored as glycogen, amino acids are deaminated, fats are metabolised). 3. Blood sugar regulation: The liver maintains blood glucose levels by converting glycogen to glucose (glycogenolysis) or glucose to glycogen (glycogenesis). 4. Bile production: The liver produces bile from the breakdown products of haemoglobin and other substances.

1. Hypertension (High Blood Pressure):

• Normal blood pressure: 120/80 mm Hg (systolic/diastolic)
• Hypertension is defined as sustained blood pressure above 140/90 mm Hg
• Causes: Obesity, excessive salt intake, smoking, stress, sedentary lifestyle, genetic factors, kidney disease
• Effects: Increases the workload on the heart, damages blood vessels, can lead to heart attack, stroke, kidney failure, and vision loss
• Prevention/Treatment: Regular exercise, reduced salt intake, weight management, stress reduction, medication (antihypertensives)

2. Coronary Artery Disease (CAD / Atherosclerosis):

• Characterised by the deposition of fatty substances (cholesterol, calcium) on the inner walls of coronary arteries, forming plaques (atheromas)
• This narrows the lumen of the arteries, reducing blood flow to the heart muscle
• Causes: High cholesterol, smoking, hypertension, diabetes, obesity, sedentary lifestyle, family history
• Effects: Reduced oxygen supply to heart muscle, chest pain (angina), risk of blood clot formation, heart attack (myocardial infarction)
• Prevention/Treatment: Healthy diet (low saturated fat), regular exercise, no smoking, medication (statins, aspirin), angioplasty, bypass surgery

3. Angina Pectoris:

• Angina means "strangling" — it is chest pain caused by reduced blood flow to the heart muscle
• Occurs when the coronary arteries cannot supply enough oxygen to the heart muscle during exertion or stress
• Symptoms: Pain or discomfort in the chest (may radiate to arms, neck, jaw, back), shortness of breath, sweating, nausea
• Types: Stable angina (predictable, occurs with exertion) and Unstable angina (unpredictable, can occur at rest, more dangerous)
• Treatment: Nitroglycerin (dilates coronary arteries), beta-blockers, calcium channel blockers, lifestyle modifications

Internal Links

• [Chapter 18 Guide — Body Fluids and Circulation](/school/cbse/class/11/biology/cbse-body-fluids-and-circulation)
• [All Class 11 Biology Solutions](/school/cbse/class/11/biology/solutions)
• [Class 11 Biology Subject Guide](/school/cbse/class/11/biology)