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Body Fluids and Circulation – Class 11 Biology Chapter 15 Notes & NCERT PDF

Class 11BiologyChapter 15NCERT book: Biology - Class 11

Notes and a simple summary of Chapter 15 of the NCERT Class 11 Biology book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 15: Body Fluids and Circulation

Introduction

All living cells need nutrients, O₂ and other essential substances, and their wastes must be removed continuously. Simple organisms such as sponges and coelenterates circulate water from their surroundings through body cavities. Complex organisms use special fluids — blood (the most common) and lymph. This chapter covers the composition of blood and lymph, circulatory pathways, the human heart and its working, and circulatory disorders.

Key Concepts

15.1 Blood

Blood is a special connective tissue: a fluid matrix (plasma) with formed elements.

15.1.1 Plasma — straw-coloured, viscous fluid, about 55% of blood.

  • 90–92% water, 6–8% proteins.
  • Major proteins: fibrinogen (clotting), globulins (defence) and albumins (osmotic balance).
  • Small amounts of minerals (Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻, Cl⁻), glucose, amino acids and lipids in transit, and inactive clotting factors.
  • Plasma without clotting factors is serum.

    • 15.1.2 Formed elements — about 45% of blood: erythrocytes, leucocytes and platelets.

  • RBCs (erythrocytes): most abundant; 5–5.5 million per mm³ in a healthy adult man; formed in red bone marrow; no nucleus in most mammals; biconcave; contain haemoglobin (12–16 g per 100 mL of blood); life span 120 days; destroyed in the spleen ("graveyard of RBCs").
  • WBCs (leucocytes): colourless, nucleated, 6000–8000 per mm³, generally short-lived.

    • - Granulocytes: neutrophils (most abundant, 60–65%, phagocytic), eosinophils (2–3%, resist infection, allergy), basophils (least, 0.5–1%, secrete histamine, serotonin, heparin; inflammation).

    - Agranulocytes: lymphocytes (20–25%, B and T types, immune responses) and monocytes (6–8%, phagocytic).

  • Platelets (thrombocytes): cell fragments from megakaryocytes in bone marrow; 1,50,000–3,50,000 per mm³; release substances for clotting; fewer platelets cause clotting disorders.

    • 15.1.3 Blood groups

  • ABO grouping: based on antigens A and B on RBCs and natural antibodies in plasma.

    • - Group A: antigen A, antibody anti-B; can receive from A, O.

    - Group B: antigen B, antibody anti-A; can receive from B, O.

    - Group AB: antigens A and B, no antibodies; can receive from AB, A, B, O — universal recipient.

    - Group O: no antigens, antibodies anti-A and anti-B; can receive only from O; donates to all — universal donor.

  • Rh grouping: Rh antigen (similar to that of Rhesus monkeys) is present in nearly 80% of humans (Rh+ve); absent in Rh–ve. An Rh–ve person exposed to Rh+ve blood forms anti-Rh antibodies, so Rh must also be matched.
  • Erythroblastosis foetalis: Rh–ve mother, Rh+ve foetus. In the first pregnancy the placenta separates the bloods; during the first delivery the mother may be exposed to foetal Rh+ve blood and make antibodies. In later pregnancies these antibodies can enter the foetus and destroy its RBCs — fatal, or causing severe anaemia and jaundice. Prevented by giving anti-Rh antibodies to the mother immediately after the first delivery.

    • 15.1.4 Coagulation of blood — prevents excessive blood loss. The clot (coagulum) is a network of fibrin threads trapping dead and damaged formed elements.

  • Injury stimulates platelets (and tissues at the site) to release factors that start a cascade of linked enzymic reactions forming thrombokinase.
  • Thrombokinase: prothrombin → thrombin.
  • Thrombin: fibrinogen → fibrin.
  • Calcium ions play a very important role.

    • 15.2 Lymph (Tissue Fluid)

    As blood passes through capillaries, water and small water-soluble substances move into spaces between cells, leaving larger proteins and most formed elements behind: this is interstitial (tissue) fluid, with the same mineral distribution as plasma. The lymphatic system collects it and drains it back into the major veins; the fluid in it is lymph — colourless, with specialised lymphocytes for immunity; it also carries nutrients and hormones. Fats are absorbed through lymph in the lacteals of intestinal villi.

    15.3 Circulatory Pathways

  • Open circulation (arthropods, molluscs): blood pumped into large vessels and then into open spaces (sinuses).
  • Closed circulation (annelids, chordates): blood always flows in a closed network — more advantageous, flow can be regulated precisely.
  • Vertebrate hearts: fishes — 2 chambers (1 atrium, 1 ventricle), single circulation; amphibians and reptiles (except crocodiles) — 3 chambers (2 atria, 1 ventricle), mixed blood, incomplete double circulation; crocodiles, birds and mammals — 4 chambers, double circulation.

    • 15.3.1 Human circulatory system (blood vascular system): heart, closed branching vessels and blood.

  • Heart: mesodermal; in the thoracic cavity between the lungs, slightly tilted to the left; size of a clenched fist; enclosed in a double-walled pericardium with pericardial fluid.
  • Chambers: two small upper atria, two larger lower ventricles. Inter-atrial septum and inter-ventricular septum separate left and right; the thick fibrous atrio-ventricular septum separates atrium and ventricle of the same side (with openings).
  • Valves: tricuspid (right atrium–right ventricle, three cusps); bicuspid or mitral (left atrium–left ventricle); semilunar valves at openings of the pulmonary artery and aorta. Valves allow one-way flow.
  • Heart is made of cardiac muscle; ventricular walls are much thicker than atrial walls.
  • Nodal tissue: SAN (sino-atrial node) in the right upper corner of the right atrium; AVN (atrio-ventricular node) in the lower left corner of the right atrium near the AV septum; AV bundle (bundle of His) passes through the septa, divides into right and left bundles, giving Purkinje fibres to the ventricular musculature. Nodal tissue is autoexcitable. The SAN generates the most action potentials (70–75 per minute) and is the pacemaker; the heart normally beats 70–75 times a minute (average 72).

    • 15.3.2 Cardiac cycle:

  • Joint diastole: all chambers relaxed; tricuspid and bicuspid valves open; blood from pulmonary veins and vena cava flows into ventricles via atria; semilunar valves closed.
  • Atrial systole: SAN fires; both atria contract, increasing ventricular filling by about 30%.
  • Impulse passes via AVN and AV bundle; bundle of His carries it through the ventricles → ventricular systole (atria relax). Rising pressure closes tricuspid and bicuspid valves; further rise opens semilunar valves; blood enters pulmonary artery and aorta.
  • Ventricular diastole: pressure falls, semilunar valves close; then tricuspid and bicuspid valves open as atrial pressure pushes them; joint diastole again.
  • Heart rate 72/min → cardiac cycle = 0.8 s.
  • Stroke volume ≈ 70 mL per ventricle per beat.
  • Cardiac output = stroke volume × heart rate ≈ 5000 mL (5 L) per minute; higher in athletes.
  • Heart sounds: first sound 'lub' — closure of tricuspid and bicuspid valves; second sound 'dub' — closure of semilunar valves.

    • 15.3.3 Electrocardiogram (ECG): graphical record of electrical activity of the heart during a cardiac cycle. A standard ECG uses three leads (both wrists and left ankle).

  • P-wave: depolarisation of the atria → atrial contraction.
  • QRS complex: depolarisation of the ventricles → ventricular contraction begins shortly after Q (start of systole).
  • T-wave: repolarisation of the ventricles; its end marks the end of systole.
  • Counting QRS complexes gives the heart rate; deviations from the normal shape indicate abnormality.

    • 15.4 Double Circulation

    Arteries and veins have three layers: tunica intima (squamous endothelium), tunica media (smooth muscle and elastic fibres; thinner in veins) and tunica externa (fibrous connective tissue with collagen).

  • Pulmonary circulation: right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium.
  • Systemic circulation: left ventricle → aorta → arteries, arterioles, capillaries → tissues → venules, veins, vena cava → right atrium.
  • Hepatic portal system: the hepatic portal vein carries blood from the intestine to the liver before it enters systemic circulation.
  • Coronary system: vessels supplying the cardiac muscle itself.

    • 15.5 Regulation of Cardiac Activity

    The heart is myogenic — activity is auto-regulated by nodal tissue. A neural centre in the medulla oblongata moderates it through the autonomic nervous system: sympathetic signals increase heart rate, strength of contraction and cardiac output; parasympathetic signals decrease heart rate, conduction speed and cardiac output. Adrenal medullary hormones also increase cardiac output.

    15.6 Disorders of the Circulatory System

  • Hypertension: blood pressure higher than normal (120/80 mm Hg — 120 systolic, 80 diastolic). Repeated readings of 140/90 or higher indicate hypertension; it leads to heart disease and affects brain and kidney.
  • Coronary Artery Disease (CAD) / atherosclerosis: deposits of calcium, fat, cholesterol and fibrous tissue narrow the lumen of arteries supplying heart muscle.
  • Angina (angina pectoris): acute chest pain when not enough oxygen reaches heart muscle; more common in middle-aged and elderly.
  • Heart failure (congestive heart failure): heart does not pump effectively enough; congestion of lungs is a main symptom. It differs from cardiac arrest (heart stops beating) and heart attack (heart muscle suddenly damaged by inadequate blood supply).

    • Diagrams to Know

  • Formed elements of blood (Figure 15.1).
  • Section of the human heart with chambers, septa, valves and nodal tissue (Figure 15.2).
  • Standard ECG with P, QRS and T (Figure 15.3).
  • Schematic plan of blood circulation — pulmonary and systemic (Figure 15.4).

    • Key-Facts Sheet

  • Plasma 55%, formed elements 45%.
  • RBC 5–5.5 million/mm³; Hb 12–16 g/100 mL; life 120 days.
  • WBC 6000–8000/mm³; neutrophils 60–65%, lymphocytes 20–25%, monocytes 6–8%, eosinophils 2–3%, basophils 0.5–1%.
  • Platelets 1.5–3.5 lakh/mm³.
  • Heart rate 72/min; cycle 0.8 s; stroke volume 70 mL; cardiac output 5 L/min.
  • Normal BP 120/80; hypertension ≥ 140/90.

    • Exam Focus

    Exercises ask you to name the components of formed elements and give one function of each; explain the importance of plasma proteins; match columns (eosinophils–immune response/allergy, RBC–gas transport, platelets–coagulation, lymphocytes–immunity); explain why the heart is called myogenic and why the SAN is the pacemaker; state the significance of the AVN and AV bundle; define cardiac cycle and cardiac output; explain heart sounds and draw and explain a standard ECG; describe double circulation; distinguish blood and lymph, open and closed circulation, P-wave and T-wave; and describe the evolutionary change in vertebrate heart chambers.

    Key Terms

  • Plasma (प्लाज़्मा)
  • Serum (सीरम)
  • Erythrocyte (लाल रुधिर कणिका)
  • Leucocyte (श्वेत रुधिर कणिका)
  • Platelet (पट्टिकाणु)
  • Antigen (प्रतिजन)
  • Antibody (प्रतिरक्षी)
  • Coagulation (स्कंदन)
  • Lymph (लसीका)
  • Pacemaker (गति-प्रेरक)
  • Cardiac cycle (हृद चक्र)
  • Cardiac output (हृद निकास)
  • Double circulation (दोहरा परिसंचरण)
  • Hypertension (उच्च रक्तचाप)

    • Common Mistakes

  • Calling serum the same as plasma — serum is plasma without clotting factors.
  • Mixing up universal donor (O) and universal recipient (AB).
  • Assuming erythroblastosis foetalis affects the first child — it usually affects later Rh+ve pregnancies.
  • Attributing 'lub' to semilunar valves — 'lub' is closure of AV (tricuspid and bicuspid) valves; 'dub' is closure of semilunar valves.
  • Reading the T-wave as atrial activity — it is ventricular repolarisation.
  • Confusing heart failure with cardiac arrest or heart attack.

    • 💡 Key Learning Points

    • ✓Describe the composition of plasma and the formed elements with their counts and functions
    • ✓Explain ABO and Rh blood groups, transfusion matching and erythroblastosis foetalis
    • ✓Outline the clotting cascade and the formation and role of lymph
    • ✓Explain the structure of the human heart, its conducting system and the cardiac cycle
    • ✓Interpret a standard ECG and describe double circulation, its regulation and common disorders

    👨‍🏫 Teaching Tips

    • →Show a prepared blood smear under the microscope to identify RBCs and types of WBCs
    • →Use a blood-grouping kit demonstration (or a simulation) to show agglutination
    • →Dissect or show a model of a goat heart to locate chambers, valves and vessels
    • →Let students listen to heart sounds with a stethoscope and relate lub-dub to valve closure
    • →Calculate cardiac output from stroke volume and pulse rate measured in class
    • →Draw an ECG trace and have students label P, QRS and T with the heart event for each
    • →Discuss healthy lifestyle choices in relation to hypertension and coronary artery disease

    📋 Assessment Questions

    1. Name the plasma proteins and state the role of each.
    2. Why is a person with blood group O called a universal donor?
    3. Why is the SA node called the pacemaker of the heart?
    4. Calculate the cardiac output if stroke volume is 70 mL and heart rate is 72 per minute.
    5. What do the P-wave, QRS complex and T-wave of an ECG represent?

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    Keep learning

    All chapters in Biology - Class 11

    1. 1. The Living World
    2. 2. Biological Classification
    3. 3. Plant Kingdom
    4. 4. Animal Kingdom
    5. 5. Morphology of Flowering Plants
    6. 6. Anatomy of Flowering Plants
    7. 7. Structural Organisation in Animals
    8. 8. Cell: The Unit of Life
    9. 9. Biomolecules
    10. 10. Cell Cycle and Cell Division
    11. 11. Photosynthesis in Higher Plants
    12. 12. Respiration in Plants
    13. 13. Plant Growth and Development
    14. 14. Breathing and Exchange of Gases
    15. 15. Body Fluids and Circulation
    16. 16. Excretory Products and their Elimination
    17. 17. Locomotion and Movement
    18. 18. Neural Control and Coordination
    19. 19. Chemical Coordination and Integration