NCERT Solutions for Class 11 Biology Chapter 15: Body Fluids and Circulation (NCERT 2026–27)
These Class 11 Biology Chapter 15 solutions cover Body Fluids and Circulation with complete, step-by-step answers to every NCERT exercise question. The chapter explains how blood and lymph transport nutrients, gases and wastes, the structure and working of the human heart, the cardiac cycle, ECG, double circulation and common disorders — all written in exam-ready prose for session 2026–27.
Class: 11Subject: BiologyChapter: 15Chapter Name: Body Fluids and CirculationExercises: 14 questionsSession: 2026–27
Chapter 15, Body Fluids and Circulation, studies the fluids that animals use to transport essential substances. Blood, a fluid connective tissue, has a fluid matrix (plasma) and formed elements (RBCs, WBCs, platelets); humans are grouped by the ABO and Rh systems. Lymph (tissue fluid) carries nutrients, fats and lymphocytes. The chapter then describes the four-chambered human heart, its valves and nodal tissue, the cardiac cycle, heart sounds and the ECG. It explains double circulation (pulmonary + systemic), the neural and hormonal regulation of cardiac activity (the heart is myogenic), and disorders such as hypertension, coronary artery disease, angina and heart failure.
Key Concepts & Definitions
Plasma: the straw-coloured fluid matrix of blood (~55%); ~90–92% water with proteins (fibrinogen, globulins, albumins), minerals, glucose and clotting factors. Plasma without clotting factors is serum.
Formed elements: erythrocytes (RBCs, ~5–5.5 million mm–3, carry haemoglobin), leucocytes (WBCs, 6000–8000 mm–3, defence) and platelets/thrombocytes (1.5–3.5 lakh mm–3, clotting).
Blood groups: ABO grouping is based on antigens A and B on RBCs; Rh grouping on the Rh antigen. ‘O’ is the universal donor, ‘AB’ the universal recipient.
Lymph (tissue fluid): colourless fluid similar to plasma but lacking large proteins and most formed elements; rich in lymphocytes; drains tissue fluid back to veins and absorbs fats via lacteals.
Double circulation: two pathways — pulmonary (heart→lungs→heart) and systemic (heart→body→heart) — that keep oxygenated and deoxygenated blood separate.
Cardiac cycle: the sequential systole and diastole of atria and ventricles; lasts ~0.8 s; produces lub (AV-valve closure) and dub (semilunar-valve closure) sounds.
Cardiac output: volume of blood pumped by each ventricle per minute = stroke volume (~70 mL) × heart rate (~72) ≈ 5 litres.
Myogenic heart: a heart whose contraction is initiated by its own nodal tissue (SAN, the pacemaker), not by nerves; modulated by the ANS and adrenal hormones.
NCERT Exercises — Full Solutions
All questions below are reproduced verbatim from the NCERT textbook (Reprint 2026–27). Answers are original and written in CBSE exam-ready style.
1. Name the components of the formed elements in the blood and mention one major function of each of them.
ANSWERThe formed elements are erythrocytes (RBCs), leucocytes (WBCs) and platelets (thrombocytes).Erythrocytes (RBCs): contain haemoglobin and transport respiratory gases — mainly oxygen (and some carbon dioxide) between the lungs and the tissues.Leucocytes (WBCs): protect the body against disease; granulocytes and monocytes are phagocytic and destroy pathogens, while lymphocytes (B and T) bring about immune responses.Platelets (thrombocytes): release factors that help in the coagulation (clotting) of blood, preventing excessive blood loss after an injury.
2. What is the importance of plasma proteins?
ANSWERPlasma contains 6–8% proteins, mainly fibrinogen, globulins and albumins, each with a key role:Fibrinogen is essential for the clotting or coagulation of blood, helping to seal wounds and prevent blood loss.Globulins are primarily involved in the defence mechanisms of the body (immunity).Albumins help maintain the osmotic balance of the blood, keeping water in the vessels and regulating fluid distribution. Plasma proteins also help carry various substances and maintain blood viscosity and pH.
3. Match Column I with Column II :
ANSWER
Column I
Column II
(a) Eosinophils
(iii) Resist Infections
(b) RBC
(v) Gas transport
(c) AB Group
(ii) Universal Recipient
(d) Platelets
(i) Coagulation
(e) Systole
(iv) Contraction of Heart
Thus: a–(iii), b–(v), c–(ii), d–(i), e–(iv).
4. Why do we consider blood as a connective tissue?
ANSWERBlood is considered a connective tissue because, like all connective tissues, it develops from the mesoderm and consists of cells (formed elements) suspended in an extensive non-living matrix — here the fluid matrix is plasma.It performs the chief role of connective tissue: it links different organs and tissues of the body by transporting nutrients, gases, hormones and wastes between them, integrating the body into a functional whole. Because its matrix is fluid rather than solid, it is called a fluid connective tissue.
5. What is the difference between lymph and blood?
ANSWER
Blood
Lymph
Red coloured (due to haemoglobin in RBCs).
Colourless fluid (no RBCs).
Contains RBCs, WBCs and platelets.
Contains only WBCs, mainly lymphocytes; lacks RBCs and platelets.
Plasma is rich in proteins (fibrinogen, globulins, albumins).
Has less protein (large proteins stay in the vessels).
Flows through the heart, arteries, veins and capillaries.
Flows through the lymphatic system, draining back to the major veins.
Transports nutrients, O2, CO2 and wastes; clots readily.
Carries nutrients, hormones and fats (absorbed via lacteals); chiefly involved in immunity.
6. What is meant by double circulation? What is its significance?
ANSWERDouble circulation means blood passes through the heart twice to complete one full round of the body, through two separate circulatory pathways:Pulmonary circulation: the right ventricle pumps deoxygenated blood through the pulmonary artery to the lungs, where it is oxygenated and returned by the pulmonary veins to the left atrium.Systemic circulation: the left ventricle pumps oxygenated blood through the aorta to all body tissues; deoxygenated blood is collected by veins and vena cava and emptied into the right atrium.Significance: the two pathways keep oxygenated and deoxygenated blood completely separate (no mixing), so highly oxygenated blood is delivered efficiently to the tissues. This maintains a high metabolic rate and meets the energy needs of warm-blooded animals.
7. Write the differences between :
(a) Blood and Lymph
ANSWER (a)
Blood
Lymph
Red, contains RBCs, WBCs and platelets.
Colourless, contains only WBCs (mainly lymphocytes).
Rich in plasma proteins; flows in heart and blood vessels.
Poor in large proteins; flows in lymphatic vessels.
Carries O2, CO2, nutrients and wastes.
Carries nutrients, hormones and fats; chiefly immunity.
(b) Open and Closed system of circulation
ANSWER (b)
Open circulatory system
Closed circulatory system
Blood pumped by the heart flows through open spaces / sinuses (body cavities).
Blood is always confined within a closed network of blood vessels.
Found in arthropods and molluscs.
Found in annelids and chordates (all vertebrates).
Flow of fluid is less precisely regulated.
Flow can be more precisely regulated — more advantageous.
(c) Systole and Diastole
ANSWER (c)
Systole
Diastole
Contraction phase of the heart chambers.
Relaxation phase of the heart chambers.
Forces blood out of the atria into ventricles and out of ventricles into arteries.
Allows the chambers to fill with blood.
Increases pressure within the chamber.
Pressure within the chamber falls.
(d) P-wave and T-wave
ANSWER (d)
P-wave
T-wave
Represents the electrical excitation (depolarisation) of the atria.
Represents the return of the ventricles from excited to normal state (repolarisation).
Leads to contraction of both atria.
Marks the end of systole (end of T-wave).
8. Describe the evolutionary change in the pattern of heart among the vertebrates.
ANSWERAll vertebrates have a muscular chambered heart, but the number of chambers and the efficiency of separating oxygenated from deoxygenated blood increase as we move up the vertebrate series:Fishes: a 2-chambered heart (one atrium, one ventricle). It pumps only deoxygenated blood, which is oxygenated in the gills and supplied to the body — single circulation.Amphibians and reptiles (except crocodiles): a 3-chambered heart (two atria, one ventricle). Oxygenated and deoxygenated blood mix in the single ventricle — incomplete double circulation.Crocodiles, birds and mammals: a 4-chambered heart (two atria, two ventricles). Oxygenated and deoxygenated blood are kept completely separate — complete double circulation, allowing efficient, high-metabolism functioning.
9. Why do we call our heart myogenic?
ANSWEROur heart is called myogenic because the impulse for its contraction (heartbeat) originates from the cardiac muscle itself — specifically the nodal tissue — and not from any nerve.The nodal musculature is autoexcitable: the sino-atrial node (SAN) generates action potentials on its own (70–75 per minute) and sets the rhythm. Since this self-generated impulse initiates and maintains the heartbeat, the heart is described as myogenic (in contrast to a neurogenic heart, where a nerve initiates the beat).
10. Sino-atrial node is called the pacemaker of our heart. Why?
ANSWERThe sino-atrial node (SAN) is a patch of nodal tissue in the right upper corner of the right atrium. Although all nodal tissue is autoexcitable, the SAN can generate the maximum number of action potentials (70–75 per minute).Because it generates impulses faster than any other part, the SAN initiates and maintains the rhythmic contractile activity of the heart and sets the pace at which the whole heart beats. For this reason it is called the pacemaker.
11. What is the significance of atrio-ventricular node and atrio-ventricular bundle in the functioning of heart?
ANSWERThe atrio-ventricular node (AVN) lies in the lower left corner of the right atrium, near the atrio-ventricular septum. It picks up the action potential generated by the SAN and relays it onward, also introducing a slight delay so that the atria finish contracting before the ventricles do.The atrio-ventricular bundle (AV bundle / bundle of His) continues from the AVN, passes through the atrio-ventricular septa and divides into right and left bundle branches on the inter-ventricular septum, giving rise to the Purkinje fibres throughout the ventricular walls.Together they conduct the impulse from the atria to the ventricles, ensuring the ventricles contract in a coordinated way (ventricular systole) at the correct time after the atria. This coordinated conduction is essential for the proper pumping action of the heart.
12. Define a cardiac cycle and the cardiac output.
ANSWERCardiac cycle: the sequential events that take place in the heart and are cyclically repeated in each heartbeat. It consists of the systole (contraction) and diastole (relaxation) of both the atria and the ventricles. In a healthy person it lasts about 0.8 seconds, giving about 72 cardiac cycles per minute.Cardiac output: the volume of blood pumped out by each ventricle per minute. It equals the stroke volume × heart rate. With a stroke volume of about 70 mL and a heart rate of about 72 beats/min, the cardiac output averages about 5000 mL (5 litres) per minute in a healthy individual.
13. Explain heart sounds.
ANSWERDuring each cardiac cycle two prominent sounds are produced that can be heard with a stethoscope:First heart sound (lub): a low-pitched, longer sound produced by the closure of the tricuspid and bicuspid (atrioventricular) valves at the start of ventricular systole.Second heart sound (dub): a sharper, shorter sound produced by the closure of the semilunar valves (at the openings of the pulmonary artery and aorta) at the start of ventricular diastole.These sounds are of clinical diagnostic significance, as abnormal heart sounds can indicate valve defects or other heart problems.
14. Draw a standard ECG and explain the different segments in it.
ANSWERAn electrocardiogram (ECG) is a graphical record of the electrical activity of the heart during a cardiac cycle, obtained using an electrocardiograph (standardly via leads at both wrists and the left ankle). A normal ECG shows a series of waves labelled P, Q, R, S and T: a small upward P-wave, followed by a sharp QRS complex (a small Q dip, a tall R peak and an S dip), and then a rounded T-wave.P-wave: represents the electrical excitation (depolarisation) of the atria, which leads to the contraction of both atria.QRS complex: represents the depolarisation of the ventricles, which initiates ventricular contraction. The contraction starts shortly after Q and marks the beginning of systole.T-wave: represents the return of the ventricles from the excited to the normal state (repolarisation). The end of the T-wave marks the end of systole.By counting the number of QRS complexes in a given time, the heart rate can be determined. Since ECGs from healthy people have roughly the same shape for a given lead, any deviation indicates a possible abnormality — making the ECG of great clinical importance.(In the exam, draw a horizontal baseline and label the small upward P, the sharp tall QRS complex, and the rounded T-wave in order.)
Extra Practice Questions
Short Answer Type Questions
Q1. Why is ‘O’ blood group called the universal donor?
ANSWERGroup ‘O’ RBCs carry neither antigen A nor antigen B on their surface. Hence, when O blood is transfused, there are no surface antigens for the recipient’s antibodies to react against, so it can be given to a person of any ABO group. This is why O is called the universal donor.
Q2. What is serum? How does it differ from plasma?
ANSWERSerum is plasma without the clotting factors (especially fibrinogen). Plasma is the straw-coloured fluid matrix of blood that still contains clotting proteins, whereas serum is the fluid left after the blood has clotted and these factors have been used up.
Q3. What is erythroblastosis foetalis, and how can it be avoided?
ANSWERIt is a condition in which an Rh-negative mother, sensitised to Rh antigens during her first delivery, produces anti-Rh antibodies that, in a later pregnancy, leak across the placenta and destroy the RBCs of an Rh-positive foetus, causing anaemia and jaundice. It can be avoided by administering anti-Rh antibodies to the mother immediately after the delivery of the first child.
Q4. State the role of calcium ions and thrombin in blood clotting.
ANSWERCalcium ions play a very important role in the cascade of clotting reactions. The enzyme thrombin (formed from inactive prothrombin via the enzyme complex thrombokinase) converts the soluble plasma protein fibrinogen into insoluble fibrin threads, which form the network of the clot.
Q5. What is the hepatic portal system and its function?
ANSWERThe hepatic portal system is a unique vascular connection between the digestive tract and the liver. The hepatic portal vein carries blood from the intestine to the liver first, before it is delivered to the general systemic circulation, so that absorbed nutrients can be processed and regulated by the liver.
Long Answer Type Questions
Q1. Describe the events of the cardiac cycle.
ANSWERThe cardiac cycle begins with joint diastole, when all four chambers are relaxed; the AV (tricuspid and bicuspid) valves are open and the semilunar valves are closed, so blood flows from the pulmonary veins and vena cava through the atria into the ventricles. The SAN then fires an action potential, causing atrial systole, which pushes about 30% more blood into the ventricles. The impulse passes via the AVN and AV bundle (bundle of His) to the ventricular muscle, producing ventricular systole while the atria relax (atrial diastole). Rising ventricular pressure closes the AV valves (first sound, ‘lub’) and forces open the semilunar valves, ejecting blood into the pulmonary artery and aorta. The ventricles then relax (ventricular diastole); falling pressure closes the semilunar valves (second sound, ‘dub’), and as pressure drops further the AV valves reopen, returning the heart to joint diastole. The whole cycle lasts about 0.8 s and is repeated about 72 times a minute, each ventricle ejecting a stroke volume of ~70 mL.
Q2. Explain how the activity of the heart is regulated.
ANSWERThe heart is myogenic and auto-regulated by its own nodal tissue, so the SAN normally sets the heart rate without any external stimulus. However, this intrinsic activity can be moderated by neural and hormonal mechanisms. A special neural centre in the medulla oblongata acts through the autonomic nervous system (ANS): sympathetic nerves increase the heart rate, the force of ventricular contraction and hence the cardiac output, while parasympathetic nerves decrease the heart rate, slow the conduction of action potentials and reduce cardiac output. In addition, adrenal medullary hormones (such as adrenaline) can increase the cardiac output. This combination of intrinsic and extrinsic control lets the heart adjust its output to the body’s changing needs, for example during exercise.
Q3. Give an account of the major disorders of the human circulatory system.
ANSWERHypertension (high blood pressure): blood pressure higher than normal (120/80 mm Hg); a repeated reading of 140/90 or higher indicates hypertension, which can damage the heart and vital organs like the brain and kidneys. Coronary artery disease (CAD / atherosclerosis): deposits of calcium, fat, cholesterol and fibrous tissue narrow the lumen of the arteries that supply the heart muscle, reducing its blood supply. Angina (angina pectoris): acute chest pain felt when not enough oxygen reaches the heart muscle; it is more common in middle-aged and elderly people and arises from conditions affecting blood flow. Heart failure: the state in which the heart is not pumping blood effectively enough to meet the body’s needs; it is often called congestive heart failure because congestion of the lungs is a main symptom. It is not the same as cardiac arrest (heart stops beating) or a heart attack (sudden damage to heart muscle from inadequate blood supply).
For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.
A-R 1. Assertion: Blood is classified as a connective tissue.
Reason: Blood has formed elements suspended in a fluid matrix called plasma and connects different organs of the body.
A-R 2. Assertion: The SA node is called the pacemaker of the heart.
Reason: The SA node generates the maximum number of action potentials and sets the heart’s rhythm.
A-R 3. Assertion: Group O persons are universal donors.
Reason: Group O RBCs lack both A and B surface antigens.
A-R 4. Assertion: Fishes show double circulation.
Reason: Fishes have a four-chambered heart that keeps oxygenated and deoxygenated blood separate.
A-R 5. Assertion: The second heart sound (dub) is associated with the closure of the semilunar valves.
Reason: The semilunar valves close at the beginning of ventricular diastole, preventing backflow into the ventricles.
Answer key: 1-(A), 2-(A), 3-(A), 4-(D), 5-(A).
Common Mistakes to Avoid
Watch out for these
Confusing plasma (with clotting factors) and serum (without them).
Mixing up valve positions — tricuspid is on the right, bicuspid/mitral on the left.
Saying fishes have double circulation — they have a 2-chambered heart and single circulation.
Swapping the universal donor (O) and universal recipient (AB).
Confusing ECG waves — P = atrial depolarisation, QRS = ventricular depolarisation, T = ventricular repolarisation.
Forgetting that the heart is myogenic (beat starts in nodal tissue), not neurogenic.
How to score full marks in this chapter
Learn the exact figures — RBC count (5–5.5 million mm–3), WBC count (6000–8000), platelet count (1.5–3.5 lakh), heart rate (~72), stroke volume (~70 mL), cardiac output (~5 L), cardiac cycle (0.8 s) — these fetch easy marks. For differences (blood/lymph, open/closed, systole/diastole, P/T wave), always answer in a neat two-column table. For the ECG question, draw and label P, QRS and T and state what each represents. Use precise terms: SAN (pacemaker), AVN, bundle of His, Purkinje fibres.
Frequently Asked Questions
What is Class 11 Biology Chapter 15 Body Fluids and Circulation about?
It deals with the body fluids blood and lymph and the human circulatory system — the composition of blood, ABO and Rh blood groups, coagulation, lymph, the structure and working of the four-chambered heart, the cardiac cycle, heart sounds, ECG, double circulation, regulation of cardiac activity and circulatory disorders.
How many exercise questions are in Class 11 Biology Chapter 15?
The NCERT “Exercises” section has 14 numbered questions, several with sub-parts (Q3 is a matching question and Q7 has four pairs of differences). All are solved step by step on this page.
Why is the human heart called myogenic?
Because the impulse for the heartbeat originates from the heart’s own nodal tissue (the autoexcitable SA node) rather than from a nerve, the heart is described as myogenic.
Are these Class 11 Biology Chapter 15 solutions free?
Yes. All ClearStudy NCERT Solutions for Class 11 Biology are free and follow the official NCERT textbook for session 2026–27.
