NCERT Solutions for Class 11 Biology Chapter 15: Body Fluids and Circulation

Concept. Blood is a special connective tissue consisting of a fluid matrix, plasma (about 55% of the blood), and the cellular part called the formed elements (about 45%). The formed elements are erythrocytes (red blood cells, RBCs), leucocytes (white blood cells, WBCs) and platelets (thrombocytes). They are produced from the bone marrow and each type is specialised for a particular function.

1. Erythrocytes (RBCs). They are the most abundant cells in blood: a healthy adult man has 5 to 5.5 million RBCs per mm³. RBCs are formed in the red bone marrow, are biconcave and, in most mammals, have no nucleus; they contain the red, iron-containing protein haemoglobin (12–16 g per 100 mL blood). Major function: transport of respiratory gases (O₂ and some CO₂). They live about 120 days and are destroyed in the spleen.
2. Leucocytes (WBCs). Colourless because they lack haemoglobin, nucleated and fewer in number (6000–8000 per mm³). They are of two kinds: granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes). Major function: defence of the body. Specifically: neutrophils (60–65%) and monocytes (6–8%) are phagocytic cells that destroy foreign organisms; basophils (0.5–1%) secrete histamine, serotonin and heparin and take part in inflammatory reactions; eosinophils (2–3%) resist infections and are associated with allergic reactions; lymphocytes (20–25%, B and T forms) are responsible for the immune responses.
3. Platelets (thrombocytes). Cell fragments produced from megakaryocytes (special cells of the bone marrow); 1,50,000–3,50,000 per mm³. Major function: coagulation (clotting) of blood — they release a variety of substances, most of which are involved in clotting. A reduction in their number leads to clotting disorders and excessive loss of blood.

Extra depth. RBCs are enucleate in most mammals, which leaves more space for haemoglobin, and their biconcave shape increases the surface area for gas exchange. The WBC types are easily remembered in order of abundance: Neutrophils (60–65%) > Lymphocytes (20–25%) > Monocytes (6–8%) > Eosinophils (2–3%) > Basophils (0.5–1%) — the mnemonic “Never Let Monkeys Eat Bananas”.

Conclusion: The formed elements are erythrocytes (gas transport), leucocytes (defence and immunity) and platelets (blood clotting), together forming about 45% of blood.

Concept. Plasma is a straw-coloured, viscous fluid forming nearly 55% of blood. About 90–92% of plasma is water, and proteins contribute 6–8%. The major plasma proteins are fibrinogen, globulins and albumins. Though present in small quantity, they perform vital functions.

1. Fibrinogen — blood clotting. Fibrinogen is needed for the coagulation of blood. When injury occurs, the inactive fibrinogen is converted by the enzyme thrombin into insoluble fibrin threads that form the clot and stop bleeding. Other clotting factors are also plasma proteins present in inactive form.
2. Globulins — defence. Globulins are primarily involved in the defence mechanisms of the body; the antibodies (immunoglobulins) that act against foreign agents are globulins, giving protection against infections.
3. Albumins — osmotic balance. Albumins help maintain the osmotic balance of blood: they keep water within the blood vessels and so help in maintaining blood volume and blood pressure. They also carry some substances.
4. Transport and other roles. Plasma proteins also help carry certain substances in the blood and act as a reserve of amino acids (general physiology).

Extra depth. Plasma without the clotting factors is called serum. Plasma also carries minerals (Na⁺, Ca⁺⁺, Mg⁺⁺, HCO₃⁻, Cl⁻) and glucose, amino acids and lipids that are always in transit. A low level of albumin disturbs osmotic balance and can cause tissue swelling (oedema).

Conclusion: Plasma proteins — fibrinogen for clotting, globulins for defence and albumins for osmotic balance — together support clotting, immunity and maintenance of blood volume.

Concept. Each item in Column I is a blood cell, blood group or phase of the cardiac cycle, and Column II lists its function or feature. The correct matches are explained below.

Detailed explanation of each match

1. (a) Eosinophils → (iii) Resist infections. Eosinophils are granulocytes making up 2–3% of the total WBCs. They resist infections and are also associated with allergic reactions.
2. (b) RBC → (v) Gas transport. The red colour of RBCs is due to haemoglobin, an iron-containing complex protein; haemoglobin binds oxygen to form oxyhaemoglobin and also carries a part of CO₂ as carbamino-haemoglobin.
3. (c) AB group → (ii) Universal recipient. According to Table 15.1, group AB has antigens A and B on RBCs and no antibodies in plasma, so there is no clumping when it receives A, B, AB or O blood; hence AB individuals are called universal recipients. (Group O, with no antigens, is the universal donor.)
4. (d) Platelets → (i) Coagulation. Platelets (thrombocytes) release a variety of substances, most of which are involved in the coagulation of blood; injury also stimulates platelets to release factors that activate the clotting mechanism.
5. (e) Systole → (iv) Contraction of heart. Systole is the contraction of the atria (atrial systole) or ventricles (ventricular systole) that forces blood onward; diastole is relaxation.

Common errors. Do not match AB with “universal donor” (that is O) and do not match eosinophils with coagulation (that is platelets). Eosinophils and basophils are both granulocytes but have different roles.

Conclusion: Eosinophils resist infections, RBCs transport gases, AB group individuals are universal recipients, platelets help coagulation and systole is the contraction of the heart: (a)–(iii), (b)–(v), (c)–(ii), (d)–(i), (e)–(iv).

Concept. Connective tissues are tissues that connect, support, transport and protect other tissues and organs. They have a characteristic structure: cells embedded in an extracellular matrix. The matrix may be solid (bone), semi-solid (cartilage), fibrous (tendon) or fluid. Blood fits into this framework and NCERT describes it as a special connective tissue consisting of a fluid matrix, plasma, and formed elements.

1. It has a matrix. Plasma, the fluid matrix (about 55% of blood), is the non-living extracellular substance in which the cells are suspended. It contains water (90–92%), proteins (6–8%), minerals, glucose, amino acids and lipids.
2. It has cells embedded in the matrix. The formed elements — RBCs, WBCs and platelets (about 45% of blood) — are the cells suspended in the plasma, as in all connective tissues.
3. It is mesodermal in origin, like other connective tissues (general knowledge), and formed from bone-marrow cells.
4. It connects different parts of the body. Blood links all organs and tissues by transporting nutrients, respiratory gases, hormones, and waste products between them — a connecting and distributing function, just as other connective tissues connect and support organs.
5. It protects. Like other connective tissues it has protective functions: WBCs defend against pathogens, and platelets and clotting factors prevent loss of blood.

Extra depth. Unlike other connective tissues the matrix of blood is fluid and has dissolved fibrinogen rather than fibres; the soluble fibrinogen is converted into insoluble fibrin threads during clotting. This is why blood is called a fluid connective tissue.

Conclusion: Blood is a connective tissue because it has living cells (formed elements) suspended in a non-living fluid matrix (plasma) and it connects, transports materials between and protects the other tissues of the body.

Concept. As blood passes through the capillaries, some water along with small water-soluble substances moves out into the spaces between cells, leaving the larger proteins and most formed elements behind. This fluid is the interstitial or tissue fluid, collected by the lymphatic system and drained into major veins; the fluid within the lymphatic system is lymph. Thus lymph is derived from blood and is similar to it, but differs in important respects.

Extra depth. Lymph is important for the immune system because it contains specialised lymphocytes. In the intestinal villi the lymphatic capillaries called lacteals absorb fats, which is why lymph after a fatty meal appears milky (general knowledge). Lymph returns the fluid that leaks from blood back to the circulation; without this drainage tissues would swell.

Conclusion: Lymph is a colourless fluid derived from blood plasma that lacks RBCs and platelets and has fewer proteins but contains lymphocytes, whereas blood is a red fluid with RBCs, WBCs, platelets and abundant plasma proteins.

Concept. In birds and mammals the oxygenated blood received by the left atrium and the deoxygenated blood received by the right atrium pass on to the ventricles of the same sides. The ventricles pump the blood out without any mixing, so two separate circulatory pathways are present. Such a pattern, where blood passes through the heart twice in one complete round of the body, is called double circulation.

Meaning of double circulation. It consists of two circuits:

1. Pulmonary circulation: the right ventricle pumps deoxygenated blood into the pulmonary artery; it goes to the lungs, where it is oxygenated, and returns by the pulmonary veins to the left atrium.
2. Systemic circulation: the left ventricle pumps oxygenated blood into the aorta; it is carried by arteries, arterioles and capillaries to the tissues, and the deoxygenated blood is collected by venules, veins and vena cava and emptied into the right atrium. The systemic circulation provides nutrients, O₂ and other essential substances to tissues and takes CO₂ and other harmful substances away for elimination.

Two special pathways. The 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 before it is delivered to the systemic circulation. The coronary system of vessels is present exclusively for blood supply to and from the cardiac musculature.

Significance of double circulation

1. Complete separation of oxygenated and deoxygenated blood. The blood going to the tissues is fully oxygenated, so the tissues receive the maximum oxygen supply; the deoxygenated blood is sent only to the lungs.
2. Efficient delivery of oxygen to the body. This supports the high metabolic rate needed by warm-blooded (endothermic) animals such as birds and mammals.
3. Higher pressure in the systemic circuit. Because the blood passes through the heart a second time, the left ventricle can raise its pressure and drive blood to all body tissues, while the pulmonary circuit works at lower pressure (general physiology).
4. Precise regulation. A closed system allows the flow of blood to be more precisely regulated, as NCERT notes for closed circulation.

Conclusion: Double circulation is the passage of blood twice through the heart in one complete cycle, via the pulmonary and systemic circuits; its significance is the complete separation of oxygenated and deoxygenated blood, giving efficient oxygen supply to the tissues.

Concept. These four pairs deal with body fluids (blood vs lymph), circulatory patterns in animals, phases of the heartbeat, and waves of the electrocardiogram. A good answer gives the definition of each member and the points of contrast in a table.

(a) Blood and lymph

(b) Open and closed circulatory system

(c) Systole and diastole

(d) P-wave and T-wave

Extra depth. Remember the links: ventricular systole begins just after Q and ends at the end of the T wave; the QRS complex (not mentioned in this question) lies between P and T and represents ventricular depolarisation.

Conclusion: Blood is a red fluid with RBCs and platelets while lymph is colourless; open circulation uses sinuses while closed circulation uses vessels; systole is contraction and diastole is relaxation; and the P-wave marks atrial depolarisation while the T-wave marks ventricular repolarisation.

Concept. All vertebrates possess a muscular chambered heart, but the number of chambers and the degree of separation of oxygenated and deoxygenated blood increase from fishes to mammals. This gradual change is an evolutionary adaptation to life on land and to higher metabolic needs.

1. Fishes: 2-chambered heart (one atrium and one ventricle). The heart pumps out deoxygenated blood, which is oxygenated by the gills and supplied to the body parts, from where the deoxygenated blood returns to the heart. Blood passes through the heart only once in a complete circuit — single circulation.
2. Amphibians and reptiles (except crocodiles): 3-chambered heart (two atria and a single ventricle). The left atrium receives oxygenated blood from the gills/lungs/skin and the right atrium receives deoxygenated blood from the other body parts, but the two get mixed in the single ventricle, which pumps out mixed blood — incomplete double circulation.
3. Crocodiles, birds and mammals: 4-chambered heart (two atria and two ventricles). Oxygenated and deoxygenated blood received by the left and right atria pass to the ventricles of the same sides and are pumped out without mixing; two separate pathways exist — double circulation.

Extra depth. The trend shows increasing efficiency of oxygen delivery: separation of the two kinds of blood gives the tissues maximum oxygen, which supports the high metabolic rate of warm-blooded birds and mammals. In fishes the gills take away much of the pressure so the blood flow to the body is slower.

Conclusion: In vertebrates the heart evolved from a 2-chambered single-circulation heart in fishes through a 3-chambered incomplete double circulation in amphibians and reptiles to a 4-chambered heart with complete double circulation in crocodiles, birds and mammals.

Concept. The word myogenic means “originating in muscle” (myo = muscle, genic = origin). A heart is myogenic if the impulses that start the heartbeat arise within the cardiac muscle itself and not from nerves, whereas a neurogenic heart (as in some invertebrates) depends on nerve impulses.

1. Normal activities of the heart are regulated intrinsically — auto-regulated — by specialised cardiac muscles called the nodal tissue, distributed in the heart (SAN, AVN, AV bundle and Purkinje fibres).
2. The nodal musculature is auto-excitable. It has the ability to generate action potentials without any external stimuli.
3. The SAN (pacemaker) generates the maximum number of action potentials (70–75 per minute) and initiates and maintains the rhythmic contractile activity of the heart; the impulse spreads through the atria, AVN, AV bundle, bundle of His and Purkinje fibres to the ventricles.
4. Nerves only modify, they do not initiate. A neural centre in the medulla oblongata can moderate cardiac function through the autonomic nervous system (sympathetic nerves increase and parasympathetic nerves decrease the heart rate), and adrenal medullary hormones can increase the cardiac output, but the heartbeat itself continues even if these are removed.

Extra depth. This is why a heart removed from the body can continue to beat for some time, and why a transplanted heart works although its nerves are cut.

Conclusion: The human heart is myogenic because its rhythmic contractions are initiated and regulated by its own auto-excitable nodal tissue (SAN, AVN, bundle) and not by nerves, which only moderate its rate.

Concept. A pacemaker is a structure that sets the pace (rate and rhythm) of a rhythmic activity. In the heart, the nodal tissue is a patch of specialised cardiac musculature that can generate action potentials without any external stimulus (auto-excitability). The nodal system has several parts — SAN, AVN, AV bundle and Purkinje fibres — and the number of action potentials each part can generate per minute is different.

1. Location. The sino-atrial node (SAN) is a patch of nodal tissue in the right upper corner of the right atrium.
2. Highest rate of impulse generation. Of all the parts of the nodal system, the SAN generates the maximum number of action potentials, i.e. 70–75 per minute; the other parts discharge at slower rates (general physiology).
3. It initiates the heartbeat. Because the SAN fires first and fastest, its impulse spreads over both atria, causing atrial systole, and then reaches the AVN, AV bundle, bundle of His and Purkinje fibres, forcing the ventricles to contract. The slower parts are always “reset” by the SAN before they can fire on their own.
4. It maintains the rhythm. The SAN is responsible for initiating and maintaining the rhythmic contractile activity of the heart; therefore the heart normally beats 70–75 times per minute (average 72).

Extra depth. If the SAN fails, other nodal regions can take over at a lower rate, and an artificial pacemaker may be implanted to set the pace. Sympathetic nerves increase and parasympathetic nerves decrease the SAN’s rate, which is how the nervous system changes the heart rate.

Conclusion: The SAN is called the pacemaker because it is the part of the auto-excitable nodal tissue that generates the maximum number of action potentials (70–75/min), thereby initiating and setting the rhythm of the heartbeat.

Concept. The atria and ventricles of the heart are separated by a thick fibrous atrio-ventricular septum, which does not conduct the action potential. The only path through which excitation can pass from the atria to the ventricles is therefore the specialised nodal tissue: the AVN, the AV bundle and its branches.

1. Position. The atrio-ventricular node (AVN) is a mass of nodal tissue in the lower left corner of the right atrium close to the atrio-ventricular septum. The AV bundle (bundle of His) continues from the AVN, passes through the AV septa and emerges on the top of the inter-ventricular septum, where it divides into a right and a left bundle.
2. Transmission of the impulse to the ventricles. After the SAN generates an action potential and the atria contract, the action potential is conducted to the ventricular side by the AVN and AV bundle; the bundle of His then transmits it through the entire ventricular musculature (via Purkinje fibres), causing ventricular systole.
3. Co-ordination between atria and ventricles. The conduction through AVN introduces a short delay, so the ventricles contract only after the atria have contracted and filled them (atrial systole increases ventricular filling by about 30%).
4. Synchronous contraction of the ventricles. The branches of the bundle and the Purkinje fibres distribute the impulse quickly to all parts of the ventricular muscle so that both ventricles contract together.
5. Without them the impulse would not reach the ventricles and the atria and ventricles would beat independently (heart block).

Conclusion: The AVN and AV bundle form the only conducting pathway from atria to ventricles; by passing the SAN’s impulse to the ventricular muscle through the bundle branches and Purkinje fibres, they co-ordinate atrial and ventricular systole.

Concept. The heart works as a pump, and each beat consists of a regular series of electrical and mechanical events. These repeat cyclically about 72 times a minute.

Cardiac cycle. The sequential events in the heart which are cyclically repeated are called the cardiac cycle. It consists of the systole and diastole of both the atria and ventricles.

1. Joint diastole: all four chambers are relaxed; the tricuspid and bicuspid valves are open and blood flows from the pulmonary veins and vena cava into the ventricles; semilunar valves are closed.
2. Atrial systole: the SAN generates an action potential and both atria contract together, increasing the flow of blood into the ventricles by about 30%.
3. Ventricular systole: the impulse passes via AVN, AV bundle and bundle of His to the ventricles; the ventricles contract, the atria relax, the tricuspid and bicuspid valves close and the semilunar valves open to eject blood into the pulmonary artery and aorta.
4. Ventricular diastole: ventricles relax, pressure falls, semilunar valves close and the atrioventricular valves open again — return to joint diastole.

A healthy heart beats 72 times per minute, so the duration of a cardiac cycle is 0.8 s (60/72).

Cardiac output. The volume of blood pumped out by each ventricle per minute is the cardiac output. During each cardiac cycle each ventricle pumps out about 70 mL of blood, the stroke volume (beat volume).

Cardiac output = Stroke volume × Heart rate

= 70 mL × 72 per minute = about 5040 mL ≈ 5 litres per minute

Extra depth. The body can alter both the stroke volume and the heart rate and thereby the cardiac output; for example, the cardiac output of an athlete is much higher than that of an ordinary man, because the athlete’s heart pumps a larger stroke volume. During exercise sympathetic stimulation and adrenal hormones raise the cardiac output.

Conclusion: The cardiac cycle is the cyclically repeated sequence of systole and diastole of atria and ventricles (0.8 s, 72 per minute), and cardiac output is the volume of blood pumped by each ventricle per minute, equal to stroke volume (70 mL) × heart rate (72) ≈ 5 litres.

Concept. During each cardiac cycle two prominent sounds are produced which can easily be heard through a stethoscope. They arise from the vibrations set up in the blood and heart walls when the valves of the heart close, and they are not caused by the opening of valves or the flow of blood alone.

1. First heart sound (“lub”). It is associated with the closure of the tricuspid and bicuspid valves (the atrio-ventricular valves) at the start of ventricular systole, when the rising ventricular pressure attempts to push blood back into the atria. It is relatively longer and louder (general physiology).
2. Second heart sound (“dub”). It is associated with the closure of the semilunar valves guarding the pulmonary artery and aorta at the end of ventricular systole, when the ventricles relax and the pressure in them falls. It is shorter and sharper.

Clinical significance. NCERT states that these sounds are of clinical diagnostic significance. Abnormal sounds called murmurs, extra sounds or changes in their intensity indicate defective or leaking valves and other heart disorders.

Conclusion: The two heart sounds heard through a stethoscope are the first sound (lub) caused by closure of the tricuspid and bicuspid valves and the second sound (dub) caused by closure of the semilunar valves; both have clinical diagnostic significance.

Concept. An electrocardiogram (ECG) is a graphical representation of the electrical activity of the heart during a cardiac cycle. It is obtained with an electrocardiograph. For a standard ECG the patient is connected to the machine with three electrical leads (one to each wrist and one to the left ankle) that continuously monitor the heart’s activity; for a detailed evaluation multiple leads are attached to the chest region.

Explanation of the segments — each peak in the ECG is identified with a letter from P to T that corresponds to a specific electrical activity of the heart:

1. P-wave: represents the electrical excitation (depolarisation) of the atria, which leads to the contraction of both the atria (atrial systole).
2. QRS complex: represents the depolarisation of the ventricles, which initiates the ventricular contraction. The contraction starts shortly after Q and marks the beginning of the systole.
3. T-wave: represents the return of the ventricles from excited to normal state (repolarisation). The end of the T-wave marks the end of systole.

Uses of ECG. By counting the number of QRS complexes in a given period one can determine the heart rate. Since the ECGs of different individuals have roughly the same shape for a given lead configuration, any deviation from this shape indicates a possible abnormality or disease, so the ECG is of great clinical significance.

Extra depth. The atria also repolarise, but this is hidden by the large QRS complex and not seen separately on a standard ECG (general knowledge). A faster heartbeat shortens the distance between successive R peaks.

Conclusion: A standard ECG records the heart’s electrical activity: the P-wave shows atrial depolarisation, the QRS complex ventricular depolarisation (start of systole) and the T-wave ventricular repolarisation (end of systole), and deviations from the normal shape reveal cardiac abnormalities.