Summary
Chapter 15 of the Class 11 Biology NCERT textbook, "Body Fluids and Circulation", covers this topic. Blood is a connective tissue of plasma and formed elements (red cells, white cells, platelets), lymph is a fluid derived from tissue fluid that transports nutrients and fats, and the circulatory system continuously delivers oxygen and nutrients to cells while removing waste.
- Blood and lymph as transport fluids — The chapter presents blood as plasma plus formed elements each with a role—RBCs carry gases, WBCs defend, platelets clot—while lymph, filtered from tissue fluid, ferries nutrients, fats, and immune cells.
- A self-regulating pump — It explains the four-chambered heart driven by nodal tissue, where the SA node acts as pacemaker and the cardiac cycle of systole and diastole, guarded by valves, keeps blood moving in one direction.
- Double circulation — The chapter teaches why humans have two separate loops: pulmonary circulation sends deoxygenated blood to the lungs and systemic circulation delivers oxygenated blood to the body, keeping the two blood streams from mixing.
Key points & formulas
- 01Blood comprises plasma (55%) and formed elements—RBCs (gas transport), WBCs (immunity), platelets (clotting)—with ABO and Rh antigen grouping for transfusions
- 02The sino-atrial node (SAN) acts as the heart's pacemaker, generating 70-75 action potentials per minute and maintaining normal heart rate of 72 beats/minute
- 03Cardiac cycle consists of atrial and ventricular systole (contraction) and diastole (relaxation), with heart valves preventing backward blood flow
- 04Double circulation involves two pathways: pulmonary circulation (right ventricle → lungs → left atrium) and systemic circulation (left ventricle → body tissues → right atrium)
- 05Lymph is a colorless tissue fluid containing lymphocytes for immunity, serving as transport medium for nutrients and fats absorbed in the intestines
- 06Blood clotting occurs through cascade enzymatic reactions converting fibrinogen to fibrin, forming networks that trap blood cells at wound sites
Frequently asked questions
01What are the main components of blood and their functions?
Blood comprises plasma (55%) and formed elements (45%). Plasma contains 90-92% water, 6-8% proteins (fibrinogen for clotting, globulins for defense, albumins for osmotic balance), and dissolved minerals and nutrients. Formed elements include: RBCs (~5-5.5 million/mm³) containing hemoglobin for oxygen transport; WBCs (6,000-8,000/mm³) including neutrophils, lymphocytes, eosinophils, and monocytes for immune defense; and platelets (1.5-3.5 million/mm³) for blood clotting.
02What is the difference between systole and diastole in the cardiac cycle?
Systole is the contraction phase of the heart chambers, which begins with atrial systole (both atria contract simultaneously) increasing blood flow into ventricles by 30%, followed by ventricular systole (ventricles contract) that forces blood into pulmonary arteries and aorta. Diastole is the relaxation phase where chambers fill with blood. During a complete cardiac cycle lasting 0.8 seconds, the heart beats once (72 beats/minute in healthy adults).
03How does double circulation work in humans?
Double circulation consists of two separate pathways: pulmonary circulation (right ventricle pumps deoxygenated blood to lungs via pulmonary artery; oxygenated blood returns to left atrium via pulmonary veins) and systemic circulation (left ventricle pumps oxygenated blood via aorta to all body tissues; deoxygenated blood returns to right atrium via vena cava). This separation ensures efficient oxygen delivery and prevents mixing of oxygenated and deoxygenated blood.
04Is the NCERT Class 11 Biology Chapter 15 PDF free to download?
Yes, the NCERT Class 11 Biology Chapter 15 PDF is free to download.
More chapters in Biology
Read Chapter 15 of Biology — the Class 11 Biology NCERT textbook (2026-27 edition) — online for free: the complete chapter as published by NCERT with every diagram, solved example and exercise, with step-by-step solutions, answers and revision notes. Open the NCERT PDF above, or browse all NCERT Class 11 textbooks.
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