📌 Snapshot
- How blood, lymph and a self starting heart keep every cell supplied, with the counts and values CUET repeats.
- Blood cells, plasma proteins, ABO and Rh groups and the clotting cascade, compared in tables.
- The four chambered heart, the pacemaker pathway, the cardiac cycle and what each ECG wave means.
- Double circulation, how nerves and hormones tune the heart, and the disorders to know.
📖 Detailed Notes
Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XI, Chapter 15.
What keeps a heart beating when nobody tells it to?
A single cubic millimetre of your blood, a speck about the size of a pinhead, holds about 5 million red cells. The heart that moves all this blood beats about 72 times a minute, over a lakh times a day, and it does so without waiting for a signal from your brain. Cut every nerve to a heart and it will still go on beating.
This chapter answers three questions. What is in the fluid being pumped? How do the pipes and the pump work? And how does the pump set its own rhythm? Every cell needs a continuous supply of nutrients and O₂ and a continuous removal of waste, and simple animals such as sponges and coelenterates just circulate the water around them. Complex animals use special body fluids instead: blood and lymph.
Blood: a fluid connective tissue
Blood is a connective tissue with a fluid matrix, the plasma, and formed elements floating in it. Plasma makes up about 55% of blood and formed elements about 45%.
Plasma is a straw coloured, viscous fluid. Roughly 90–92% of it is water and 6–8% is protein. It also carries minerals (Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻, Cl⁻), glucose, amino acids and lipids in transit, and clotting factors in inactive form. Plasma from which the clotting factors have been removed is called serum.
| Plasma protein | Job |
|---|---|
| Fibrinogen | Clotting |
| Globulins | Defence |
| Albumins | Osmotic balance |
Only fibrinogen is a clotting protein. The other two are common distractors.
The formed elements are three kinds of cells, and the counts are an exam favourite.
| Element | Count in healthy adult | Made in | Key facts |
|---|---|---|---|
| Erythrocytes (RBCs) | 5–5.5 million per mm³ in men | Red bone marrow | Biconcave, no nucleus in most mammals, haemoglobin 12–16 g per 100 mL blood, life span about 120 days, destroyed in the spleen |
| Leucocytes (WBCs) | 6,000–8,000 per mm³ | Bone marrow and lymphoid tissue | Colourless, nucleated, no haemoglobin, short lived |
| Platelets (thrombocytes) | 1,50,000–3,50,000 per mm³ | Fragments of megakaryocytes in the bone marrow | Release substances for clotting; low numbers mean excessive bleeding |
Why it works this way. RBCs carry no nucleus in most mammals, which leaves more room for haemoglobin, and their biconcave shape gives a large surface for gas exchange. The spleen, where old RBCs are destroyed, is called the "graveyard" of RBCs. Haemoglobin is the iron containing protein that makes blood red and carries O₂ and some CO₂ (see Breathing and Exchange of Gases).
WBCs split into two groups: granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes).
| WBC | Share of WBCs | Job |
|---|---|---|
| Neutrophils | 60–65% (most abundant) | Phagocytic; destroy foreign organisms |
| Lymphocytes | 20–25% | B and T types; responsible for immune responses |
| Monocytes | 6–8% | Phagocytic |
| Eosinophils | 2–3% | Resist infections; associated with allergic reactions |
| Basophils | 0.5–1% (least) | Secrete histamine, serotonin and heparin; involved in inflammation |
Do the shares add up? 60–65 + 20–25 + 6–8 + 2–3 + 0.5–1 comes to about 100.
Blood groups: a matching game
ABO grouping depends on two antigens, A and B, on the surface of RBCs. Plasma carries natural antibodies against the antigen your own cells lack. If a donor's antigen meets the matching antibody in the recipient, the RBCs clump.
| Group | Antigen on RBC | Antibody in plasma | Can receive from | Can donate to |
|---|---|---|---|---|
| A | A | Anti-B | A, O | A, AB |
| B | B | Anti-A | B, O | B, AB |
| AB | A and B | None | A, B, AB, O | AB |
| O | None | Anti-A and anti-B | O | A, B, AB, O |
The logic is simple. O has no antigens, so it gives to everyone (universal donor). AB has no antibodies, so it accepts from everyone (universal recipient). Always cross-match before a transfusion.
Beyond the textbook. For the exam: O is the universal donor and AB the universal recipient. In reality: the label applies to red cells only. O blood carries anti-A and anti-B in its plasma, which can harm a recipient if a large volume of whole blood is given, so hospitals usually give packed red cells and still cross-match. Why it matters: the table tells you which antigens meet which antibodies, and that is why it works, but real transfusions are matched as carefully as possible.
The Rh factor. Another antigen, similar to one found in Rhesus monkeys, is present on the RBCs of about 80% of humans, who are Rh positive. Those who lack it are Rh negative. An Rh negative person who receives Rh positive blood makes anti-Rh antibodies, so Rh must be matched too.
Rh incompatibility in pregnancy is the special case.
The sequence in one line: Rh⁻ mother + Rh⁺ foetus → at the first delivery, foetal blood leaks into mother's blood → mother makes anti-Rh antibodies → in a later pregnancy the antibodies cross the placenta → foetal RBCs are destroyed (erythroblastosis foetalis)
The baby suffers severe anaemia and jaundice, which can be fatal. The cure is prevention: anti-Rh antibodies are given to the mother immediately after the delivery of the first child.
Pause and predict: an Rh negative woman is carrying her first child, who is Rh positive. Is the child at risk during this pregnancy?
Check your answer: usually not. Her immune system meets foetal Rh positive blood in quantity only at delivery, so antibodies are made after the child is born. The danger is to a later Rh positive baby, which is why the antibody injection is given right after the first delivery.
Clotting: sealing a cut
Within seconds of an injury, a clot (coagulum) forms. It is a network of fibrin threads in which dead and damaged formed elements get trapped. A dark reddish brown scum forms on the surface when it is exposed to air.
The cascade in one line: injury → platelets and damaged tissue release factors → thrombokinase complex forms → prothrombin → thrombin → fibrinogen → fibrin clot
Thrombin is an enzyme that converts fibrinogen into fibrin. Thrombin itself comes from the inactive prothrombin, through the enzyme complex thrombokinase. Calcium ions are needed at the steps.
Pause and predict: a blood bank collects blood in a tube with a chemical that binds calcium ions. Will the blood clot in the tube?
Check your answer: no. Ca²⁺ is a critical part of the cascade, so with no free calcium the chain stops before fibrin forms.
Beyond the textbook. For the exam: thrombokinase converts prothrombin into thrombin. In reality: modern books call this enzyme complex prothrombin activator (prothrombinase). The cascade is longer than the textbook shows, and the clotting factors are numbered. Why it matters: if a question uses the modern term, you will still recognise the step.
Lymph: the second fluid
As blood flows through capillaries, water and small water soluble substances escape into the spaces between the tissue cells, leaving the larger proteins and most formed elements behind. This fluid is the interstitial (tissue) fluid. It has the same mineral composition as plasma, and the exchange of nutrients and gases between blood and cells goes through it.
A network of vessels called the lymphatic system collects this fluid and drains it back into the major veins. The fluid in these vessels is lymph. Lymph is colourless and contains specialised lymphocytes that carry out the immune responses. It is also an important carrier of nutrients and hormones. Fats are absorbed through lymph in the lacteals of the intestinal villi.
Circulatory patterns across animals
| Pattern | Where | How it works |
|---|---|---|
| Open | Arthropods, molluscs | Blood is pumped through large vessels into open spaces (sinuses) |
| Closed | Annelids, chordates | Blood stays in a network of vessels; flow is more precisely regulated |
Vertebrate hearts differ in how many chambers they have, and the more complete the separation, the more efficient the delivery of O₂.
| Animals | Chambers | Circulation |
|---|---|---|
| Fishes | 2 (one atrium, one ventricle) | Single: the heart pumps deoxygenated blood to the gills, oxygenated blood goes to the body and returns |
| Amphibians and reptiles (except crocodiles) | 3 (two atria, one ventricle) | Incomplete double: the ventricle mixes oxygenated and deoxygenated blood |
| Crocodiles, birds, mammals | 4 | Complete double: two separate pathways |
The human heart
The heart is mesodermal in origin and about the size of a clenched fist. It lies in the thoracic cavity between the two lungs, tilted slightly to the left. A double walled membranous bag, the pericardium, encloses it, with pericardial fluid between the layers.
It has four chambers: two upper atria and two lower ventricles. An inter-atrial septum separates the atria, and an inter-ventricular septum separates the ventricles. On each side a thick fibrous atrio-ventricular septum separates atrium from ventricle, but it has an opening that connects the two. Valves guard the openings and permit flow in one direction only: atria to ventricles to arteries.
| Valve | Guards | Note |
|---|---|---|
| Tricuspid (3 cusps) | Right atrium to right ventricle | Right AV valve |
| Bicuspid (mitral) | Left atrium to left ventricle | Left AV valve |
| Semilunar valve | Right ventricle to pulmonary artery, and left ventricle to aorta | Prevents backflow into the ventricles |
The heart's own wiring
Heart muscle is myogenic: the beat starts in the muscle itself, not in a nerve. A special part of the muscle, the nodal tissue, is autoexcitable, meaning it generates action potentials without any outside stimulus.
| Part | Position | Role |
|---|---|---|
| Sino-atrial node (SAN) | Upper right corner of the right atrium | Generates the maximum number of action potentials (70–75 per minute); initiates and maintains the rhythm, so it is the pacemaker |
| Atrio-ventricular node (AVN) | Lower left corner of the right atrium, near the AV septum | Receives the signal from the atria |
| AV bundle | From the AVN through the AV septum | Passes to the top of the inter-ventricular septum and divides into right and left bundles |
| Purkinje fibres | Throughout the ventricular muscle | Spread the signal to the ventricular walls |
The name bundle of His covers the AV bundle's right and left branches together with the Purkinje fibres. The signal travels in a fixed order.
The signal in one line: SAN → both atria contract → AVN → AV bundle → right and left bundles → Purkinje fibres → ventricles contract
Beyond the textbook. For the exam: the SAN is the pacemaker because it fires fastest. In reality: the other parts of the nodal system can fire too, but more slowly. If the SAN fails, the AVN takes over at a lower rate, which is why damage to the SAN lowers the heart rate and why an artificial pacemaker can be fitted. Why it matters: it shows that the "fastest one wins" rule is exactly why the SAN is in charge.
One heartbeat: the cardiac cycle
A cardiac cycle is the sequence of events from the start of one heartbeat to the start of the next. At 72 beats a minute, one cycle lasts 0.8 s.
| Stage | What happens | Valves |
|---|---|---|
| Joint diastole | All four chambers relaxed; blood flows from the venae cavae and pulmonary veins into the atria, then into the ventricles | AV valves open; semilunar valves closed |
| Atrial systole | SAN fires; both atria contract and push the last blood into the ventricles, which adds about 30% more flow | AV valves open |
| Ventricular systole | Signal reaches the ventricles; pressure rises and the ventricles contract (atria relax) | AV valves close, the first sound lub; then semilunar valves open and blood leaves |
| Ventricular diastole | Ventricles relax and pressure falls | Semilunar valves close, the second sound dub; then AV valves open and the cycle restarts |
The two heart sounds are the closing of valves. Lub is the closing of the AV valves at the start of ventricular systole, and dub is the closing of the semilunar valves at its end.
The amount of blood each ventricle pumps per beat is the stroke volume, about 70 mL. Multiply it by the heart rate and you get the cardiac output.
Worked example. Stroke volume 70 mL × 72 beats per minute = 5040 mL, so about 5000 mL (5 L) per minute. That is close to the whole blood volume of an adult, pumped round every minute.
Pause and predict: an athlete at rest has a heart rate of 50 beats per minute and a stroke volume of 100 mL. What is the cardiac output?
Check your answer: 100 × 50 = 5000 mL per minute, the same as in an ordinary person. A trained heart pumps more blood per beat, so it can beat slowly and still deliver the same output at rest. During exercise, an athlete's output rises far above the ordinary person's, because the body can alter both stroke volume and heart rate.
ECG: the heart's electrical diary
An electrocardiograph records the electrical activity of the heart. To get a standard ECG, a patient is connected through three electrical leads (one to each wrist and one to the left ankle) that continuously monitor the heart. The trace has a repeating pattern of waves.
| Wave | What it shows | What follows |
|---|---|---|
| P wave | Depolarisation (electrical excitation) of the atria | Atrial contraction |
| QRS complex | Depolarisation of the ventricles | Ventricular contraction starts shortly after Q, so this is the beginning of systole |
| T wave | Return of the ventricles from excited to normal state (repolarisation) | The end of the T wave marks the end of systole |
Counting the QRS complexes in a given time gives the heart rate. A deviation from the standard shape can point to an abnormality or a disease. Atrial repolarisation also occurs, but it is hidden under the large QRS complex, so there is no separate wave for it.
The route of the blood
Arteries carry blood away from the heart and veins carry it back. Both have three layers: the inner tunica intima (squamous endothelium), the middle tunica media (smooth muscle and elastic fibres, thinner in veins) and the outer tunica externa (fibrous connective tissue with collagen).
Because a mammal's heart has a complete wall between its right and left halves, blood passes through the heart twice in each round trip. This is double circulation.
Pulmonary circulation in one line: right ventricle → pulmonary artery (deoxygenated) → lungs → pulmonary veins (oxygenated) → left atrium
Systemic circulation in one line: left ventricle → aorta → arteries → arterioles → capillaries → tissues → venules → veins → venae cavae → right atrium
The pulmonary artery carries deoxygenated blood and the pulmonary veins carry oxygenated blood. Remember artery and vein by direction relative to the heart, not by oxygen content.
Two special circuits are worth knowing. The hepatic portal system carries blood from the intestine to the liver through the hepatic portal vein before it joins the systemic circulation. The coronary system supplies blood to and from the heart muscle itself, and only to it.
Tuning the heart
The beat starts in the nodal tissue, but the rate can be changed. A neural centre in the medulla oblongata adjusts the heart through the autonomic nervous system.
| Signal | Effect on the heart |
|---|---|
| Sympathetic nerves | Increase heart rate, strength of ventricular contraction and cardiac output |
| Parasympathetic nerves | Decrease heart rate, speed of conduction of the action potential and cardiac output |
| Adrenal medullary hormones (adrenaline, noradrenaline) | Increase cardiac output |
Disorders of the circulatory system
| Disorder | What it is |
|---|---|
| Hypertension | Blood pressure above the normal 120/80 mm Hg (systolic/diastolic). Repeated readings of 140/90 or higher indicate hypertension. It harms the heart, brain and kidney |
| Coronary artery disease (atherosclerosis) | Deposits of calcium, fat, cholesterol and fibrous tissue narrow the lumen of the coronary arteries |
| Angina (angina pectoris) | Acute chest pain when not enough O₂ reaches the heart muscle |
| Heart failure | The heart cannot pump blood effectively enough for the body's needs; called congestive heart failure because lung congestion is a main symptom |
Do not confuse heart failure with cardiac arrest (the heart stops beating) or a heart attack (heart muscle suddenly damaged by an inadequate blood supply).
Exam traps
| The trap | Why it's tempting | The truth |
|---|---|---|
| "RBCs number 6000–8000 per mm³." | All three counts look alike | RBC 5–5.5 million · WBC 6000–8000 · platelets 1.5–3.5 lakh |
| "Serum has the clotting factors but not the cells." | Plasma and serum sound the same | Serum is plasma minus clotting factors |
| "AB is the universal donor." | AB has everything | AB has no antibodies, so it is the universal recipient. O is the universal donor |
| "Erythroblastosis foetalis occurs in the first pregnancy." | The mother and baby are Rh mismatched at once | Antibodies form after the first delivery, so the later baby is at risk |
| "Tricuspid valve is on the left." | Left feels like the main side | Tricuspid is right AV; bicuspid (mitral) is left |
| "Lub is the closing of the semilunar valves." | Lub sounds like the first beat of the blood leaving | Lub is the AV valves closing; dub is the semilunar valves closing |
| "The QRS complex is ventricular relaxation." | The complex looks like the end of the beat | QRS is ventricular depolarisation (start of systole); the T wave is repolarisation |
| "Atherosclerosis is caused by too much fibrinogen." | Fibrinogen is a blood protein linked to clots | The cause is deposits of calcium, fat, cholesterol and fibrous tissue |
60-second revision
| Topic | Lock this in |
|---|---|
| Blood | Plasma 55% (water 90–92%, protein 6–8%) · formed elements 45% |
| Plasma proteins | Fibrinogen (clot) · globulin (defence) · albumin (osmotic balance) |
| Counts | RBC 5–5.5 million · WBC 6000–8000 · platelets 1.5–3.5 lakh per mm³ |
| WBC order | Neutrophil 60–65 > lymphocyte 20–25 > monocyte 6–8 > eosinophil 2–3 > basophil 0.5–1 |
| Blood groups | O = universal donor · AB = universal recipient · Rh⁺ about 80% |
| Clotting | Thrombokinase → prothrombin to thrombin → fibrinogen to fibrin · Ca²⁺ needed |
| Hearts | Fish 2 · amphibians and most reptiles 3 · crocodiles, birds, mammals 4 |
| Nodal tissue | SAN (pacemaker, 70–75/min) → AVN → AV bundle → Purkinje fibres |
| Cardiac cycle | 0.8 s · stroke volume 70 mL · cardiac output about 5 L/min |
| Heart sounds | Lub = AV valves close · dub = semilunar valves close |
| ECG | P = atrial depolarisation · QRS = ventricular depolarisation · T = ventricular repolarisation |
| Blood pressure | Normal 120/80 · hypertension 140/90 or higher |
Where this chapter connects
- Before: Breathing and Exchange of Gases explains how haemoglobin loads and unloads the O₂ and CO₂ this chapter transports.
- Next: Excretory Products and their Elimination shows how the kidney filters this same blood, about a fifth of each ventricle's output.
- Control link: Neural Control and Coordination covers the autonomic nervous system (sympathetic and parasympathetic) that tunes the heartbeat.
- Tissue link: Structural Organisation in Animals places blood among the connective tissues and describes circulation in the cockroach, earthworm and frog.
- Class XII link: Human Health and Disease uses lymphocytes, antibodies and the lymphoid organs in immunity.
🎯 Practice MCQs
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Q1. Match List I with List II. List I: (i) RBCs (ii) WBCs (iii) Platelets (iv) Haemoglobin. List II: (p) 12–16 g per 100 mL of blood (q) 6000–8000 per mm³ (r) 5–5.5 million per mm³ (s) 1,50,000–3,50,000 per mm³
▸ Show answer & explanation
Answer: A
A healthy adult man has 5–5.5 million RBCs, 6000–8000 WBCs and 1,50,000–3,50,000 platelets per mm³ of blood, and 12–16 g of haemoglobin per 100 mL. Options B and C swap two of the counts, and option D shifts every match by one place.
Q2. Match List I with List II. List I: (i) Fibrinogen (ii) Globulin (iii) Albumin (iv) Serum. List II: (p) Osmotic balance (q) Plasma without clotting factors (r) Defence (s) Clotting
▸ Show answer & explanation
Answer: D
Fibrinogen is needed for clotting, globulins for defence and albumins for osmotic balance, while serum is plasma from which the clotting factors have been removed. The other options exchange at least two of these functions.
Q3. Which of the following statements about leucocytes are correct? I. Neutrophils are the most abundant WBCs and are phagocytic. II. Basophils secrete histamine, serotonin and heparin. III. Eosinophils are agranulocytes. IV. Lymphocytes are of two types, B and T.
▸ Show answer & explanation
Answer: B
Statements I, II and IV are correct. Eosinophils, like neutrophils and basophils, are granulocytes, so statement III is false. The agranulocytes are lymphocytes and monocytes.
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Q4. Which of the following statements about human blood groups is correct?
▸ Show answer & explanation
Answer: D
Group AB has both antigens and no antibodies, so it accepts blood from all four groups. Group O has no antigens but carries both antibodies, so it is the universal donor and not the recipient. That makes options A, B and C incorrect.
Q5. An Rh negative woman marries an Rh positive man and carries an Rh positive child. Which of the following is correct?
▸ Show answer & explanation
Answer: A
Foetal blood reaches the mother in quantity at the first delivery, so she makes anti-Rh antibodies only then. In a later pregnancy these cross the placenta and destroy foetal RBCs. Option B is wrong because the first child is usually safe. Option C reverses the Rh status of mother and foetus. In option D, the antibodies are given to the mother immediately after the delivery of the first child, not before conception.
Q6. Arrange the following events of blood clotting in the correct order. I. Thrombokinase complex forms. II. Injury releases factors from platelets and tissues. III. Fibrinogen is converted into fibrin. IV. Prothrombin is converted into thrombin.
▸ Show answer & explanation
Answer: A
Injury releases factors, which build the thrombokinase complex. This converts prothrombin into thrombin, and thrombin converts fibrinogen into the fibrin threads of the clot. The other options put thrombin formation before its activator or the fibrin step before thrombin.
Q7. Assertion (A): The sino-atrial node is called the pacemaker of the heart. Reason (R): The sino-atrial node generates the maximum number of action potentials per minute and initiates and maintains the rhythmic activity of the heart.
▸ Show answer & explanation
Answer: A
The SAN fires about 70–75 times a minute, faster than any other part of the nodal tissue, so it sets the rhythm for the whole heart. That is exactly why it is called the pacemaker.
Q8. Match List I (ECG component) with List II (event). List I: (i) P wave (ii) QRS complex (iii) T wave. List II: (p) Repolarisation of the ventricles (q) Depolarisation of the atria (r) Depolarisation of the ventricles
▸ Show answer & explanation
Answer: C
The P wave marks atrial depolarisation, the QRS complex marks ventricular depolarisation and the start of systole, and the T wave marks ventricular repolarisation. The other options misplace the QRS complex or the T wave.
Q9. A healthy resting adult has a stroke volume of about 70 mL and a heart rate of 72 beats per minute. The cardiac output is approximately
▸ Show answer & explanation
Answer: B
Cardiac output equals stroke volume multiplied by heart rate, so 70 mL × 72 is about 5000 mL, or 5 L, per minute. The other options are wrong by a factor of ten.
Q10. Which statement about the human circulatory system is INCORRECT?
▸ Show answer & explanation
Answer: D
Atherosclerosis is caused by deposits of calcium, fat, cholesterol and fibrous tissue that narrow the coronary arteries, and not by fibrinogen. Statements A, B and C are all correct.
Q11. Which of the following correctly pairs a vertebrate with its heart and circulation?
▸ Show answer & explanation
Answer: C
Birds and mammals, like crocodiles, have a four chambered heart with two separate circulatory pathways. Fishes have a two chambered heart with single circulation, crocodiles are the exception among reptiles with four chambers, and amphibians have three chambers with incomplete double circulation.
Q12. Which of the following pairs of disorder and description is correct?
▸ Show answer & explanation
Answer: C
Angina is chest pain from an inadequate O₂ supply to heart muscle. Hypertension means repeated readings of 140/90 or higher, and 120/80 is normal. Sudden stopping of the heart is cardiac arrest. Pumping failure with lung congestion is congestive heart failure, whereas a heart attack is sudden damage to heart muscle from an inadequate blood supply.
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