Body Fluids and Circulation Class 11 Biology: Blood, Heart, Cardiac Cycle and ECG

What Body Fluids and Circulation is about

Weightage: This chapter is part of Unit V, Human Physiology, which carries 18 of the 70 theory marks in the school annual exam together with Breathing and Exchange of Gases, Excretory Products and their Elimination, Locomotion and Movement, Neural Control and Coordination and Chemical Coordination and Integration.Source: CBSE Biology (044) syllabus 2026-27, Class XI course structure, Unit V Human Physiology

Key concepts

Blood: plasma and formed elements

NCERT section 15.1 Blood (15.1.1 Plasma; 15.1.2 Formed Elements)

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

Plasma (straw-coloured, viscous; about 55% of blood):

• 90-92% water; proteins 6-8%: fibrinogen (clotting), globulins (defence mechanisms), albumins (osmotic balance).
• Small amounts of minerals (Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻, Cl⁻), glucose, amino acids, lipids (always in transit).
• Clotting factors are present in inactive form. Plasma without clotting factors = serum.

Formed elements (about 45%):

• RBCs (erythrocytes): most abundant; 5-5.5 million per mm³ in a healthy adult man. Formed in red bone marrow in adults. In most mammals they lack a nucleus and are biconcave. Contain red, iron-containing haemoglobin: about 12-16 g per 100 mL of blood. They transport respiratory gases. Average life span 120 days; destroyed in the spleen (graveyard of RBCs).
• WBCs (leucocytes): colourless (no haemoglobin), nucleated, fewer: 6000-8000 per mm³, generally short-lived. Two main types:
• Granulocytes: neutrophils (most abundant WBC, 60-65%; phagocytic), eosinophils (2-3%; resist infections, allergic reactions), basophils (least, 0.5-1%; secrete histamine, serotonin, heparin; inflammatory reactions).
• Agranulocytes: lymphocytes (20-25%; B and T types, responsible for immune responses), monocytes (6-8%; phagocytic).
• Platelets (thrombocytes): cell fragments from megakaryocytes (special bone marrow cells); 1,50,000-3,50,000 per mm³. They release substances for coagulation. Too few platelets cause clotting disorders and excessive blood loss.

Worked example

A blood sample is allowed to clot, and the clear straw-coloured fluid above the clot is collected. (a) Name this fluid and state how it differs from plasma. (b) Which plasma protein will be missing from it, and why? (c) If a disease lowers a person's plasma albumin sharply, which function of blood is disturbed? (3 marks)

Try it first, then show the solutionSolution

1. (a) Serum: plasma without the clotting factors. (1)
2. (b) Fibrinogen: during clotting it is converted by thrombin into fibrin threads, which stay in the clot. (1)
3. (c) Albumins maintain osmotic balance, so fluid balance between blood and tissues is upset. (1)

Answer: Serum (plasma minus clotting factors); fibrinogen is used up as fibrin; albumin loss upsets osmotic balance.

Blood groups: ABO and Rh

NCERT section 15.1.3 Blood Groups (15.1.3.1 ABO grouping; 15.1.3.2 Rh grouping)

Blood of different people may look the same but differs chemically, so blood must be matched before transfusion.

ABO grouping depends on two surface antigens on RBCs, A and B, and natural antibodies in plasma:

• Group A: antigen A; antibodies anti-B; can receive from A, O.
• Group B: antigen B; antibodies 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 from O only; can donate to all → universal donor.

Rule: the donor's RBC antigens must not meet the matching antibody in the recipient's plasma, or the RBCs clump (agglutinate).

Rh grouping:

• The Rh antigen (similar to one in Rhesus monkeys) is present on RBCs of nearly 80% of humans: Rh positive (Rh+ve); others are Rh negative (Rh−ve).
• An Rh−ve person exposed to Rh+ve blood forms antibodies against Rh antigens, so Rh must also be matched.

Erythroblastosis foetalis (Rh incompatibility in pregnancy):
1. Mother Rh−ve, foetus Rh+ve.
2. First pregnancy: the two bloods are separated by the placenta, so the foetus is usually safe. But at delivery, small amounts of foetal Rh+ve blood may reach the mother, and she starts making anti-Rh antibodies.
3. Later pregnancies with an Rh+ve foetus: these antibodies can leak into foetal blood and destroy foetal RBCs, which can be fatal or cause severe anaemia and jaundice in the baby.
4. Prevention: give anti-Rh antibodies to the mother immediately after the first delivery.

Worked example

Why is a person with group O called a universal donor and one with group AB a universal recipient? (2 marks)

Try it first, then show the solutionSolution

1. Group O RBCs carry neither A nor B antigen, so they are not clumped by anti-A or anti-B in any recipient. (1)
2. Group AB plasma has neither anti-A nor anti-B antibodies, so it does not attack donated RBCs of any ABO group. (1)

Answer: O lacks antigens; AB lacks antibodies.

Coagulation of blood and lymph

NCERT section 15.1.4 Coagulation of Blood; 15.2 Lymph (Tissue Fluid)

Coagulation (clotting): the response to injury that prevents excessive loss of blood.

• A dark reddish-brown scum forms at the injury after a while: the clot or coagulum, mainly a network of threads called fibrins, in which dead and damaged formed elements are trapped.
• Fibrins form from inactive fibrinogen by the enzyme thrombin.
• Thrombin forms from inactive prothrombin with the help of an enzyme complex, thrombokinase.
• Thrombokinase forms by a cascade of enzyme reactions involving several inactive plasma factors.
• An injury stimulates platelets to release factors that activate the clotting cascade; injured tissues also release such factors.
• Calcium ions play a very important role.

Flow chart:
Injury → platelets and tissue release factors → cascade → thrombokinase → prothrombin → thrombin → fibrinogen → fibrin threads → clot (with Ca²⁺ needed).

Lymph (tissue fluid):

• As blood passes through capillaries, some water and small water-soluble substances move out into the spaces between tissue cells, leaving larger proteins and most formed elements in the vessels. This fluid is interstitial (tissue) fluid, with the same mineral distribution as plasma.
• Nutrients and gases are exchanged between blood and cells through this fluid.
• The lymphatic system (a network of vessels) collects this fluid and drains it back to major veins; the fluid inside is lymph.
• Lymph: a colourless fluid with specialised lymphocytes responsible for immune responses. It also carries nutrients and hormones.
• Fats are absorbed through lymph in the lacteals of intestinal villi.

Worked example

Write the sequence of events from injury to the formation of a clot, naming the enzymes involved and one ion needed. (3 marks)

Try it first, then show the solutionSolution

1. Injury → platelets and damaged tissue release factors → cascade forms thrombokinase. (1)
2. Thrombokinase converts prothrombin into thrombin. (1)
3. Thrombin converts soluble fibrinogen into fibrin threads that trap blood cells to form a clot; calcium ions are essential. (1)

Answer: Thrombokinase → thrombin → fibrin; Ca²⁺ needed.

Circulatory pathways and the human heart

NCERT section 15.3 Circulatory Pathways (15.3.1 Human Circulatory System)

Open vs closed circulation:

• Open: in arthropods and molluscs; the heart pushes blood through a few large vessels that empty into open body spaces called sinuses, where it bathes the organs directly.
• Closed: in annelids and chordates; blood always flows within a closed network of vessels. It is more advantageous because flow can be regulated more precisely.

Vertebrate hearts:

• Fishes: 2-chambered (one atrium, one ventricle); the heart pumps deoxygenated blood, which is oxygenated by the gills and goes to the body: single circulation.
• Amphibians and reptiles (except crocodiles): 3-chambered (two atria, one ventricle). Left atrium gets oxygenated blood (from gills/lungs/skin), right atrium gets deoxygenated blood, but they mix in the single ventricle: incomplete double circulation.
• Crocodiles, birds and mammals: 4-chambered (two atria, two ventricles), no mixing: double circulation.

Human heart:

• Mesodermally derived, in the thoracic cavity between the two lungs, slightly tilted to the left; about the size of a clenched fist.
• Protected by a double-walled membranous bag, the pericardium, enclosing pericardial fluid.
• Four chambers: two small upper atria and two larger lower ventricles.
• Inter-atrial septum (thin, muscular) separates the atria; inter-ventricular septum (thick-walled) separates the ventricles; the atrio-ventricular septum (thick fibrous tissue) separates the atrium and ventricle of the same side.
• Valves:
• Tricuspid valve (three muscular flaps/cusps): right atrium → right ventricle.
• Bicuspid (mitral) valve: left atrium → left ventricle.
• Semilunar valves: at the openings of the right ventricle into the pulmonary artery and the left ventricle into the aorta.
• Valves allow flow in one direction only (atria → ventricles → pulmonary artery/aorta) and prevent backflow.
• Cardiac muscle forms the entire heart; the walls of the ventricles are much thicker than the atria.

Nodal tissue (specialised cardiac musculature):

• Sino-atrial node (SAN): in the right upper corner of the right atrium.
• Atrio-ventricular node (AVN): in the lower left corner of the right atrium, close to the AV septum.
• AV bundle (bundle of His): from the AVN, passes through the AV septum, divides into right and left bundles along the inter-ventricular septum, giving minute Purkinje fibres throughout the ventricular musculature.
• Nodal musculature can generate action potentials without external stimuli (autoexcitable). The SAN generates the maximum number, 70-75 per minute, so it initiates and maintains the rhythm: the pacemaker. The heart normally beats 70-75 times per minute (average 72).

Worked example

Arrange in the order in which they are excited during one heartbeat: ventricular muscle, AV node, Purkinje fibres, SA node, bundle of His, atrial muscle. Which of them sets the rate, and why must the impulse pass through the AV node and bundle of His to reach the ventricles? (3 marks)

Try it first, then show the solutionSolution

1. SA node → atrial muscle → AV node → bundle of His → Purkinje fibres → ventricular muscle. (1½)
2. The SA node generates the most action potentials per minute (70-75), so it sets the rhythm as the pacemaker. (½)
3. Atria and ventricles are separated by the thick fibrous atrio-ventricular septum; the bundle of His, arising from the AV node, is the path through this septum, branching along the inter-ventricular septum to the Purkinje fibres. (1)

Answer: SAN → atria → AVN → bundle of His → Purkinje fibres → ventricles; SAN sets the rate; the AV bundle is the route across the fibrous AV septum.

Cardiac cycle, cardiac output and heart sounds

NCERT section 15.3.2 Cardiac Cycle

Cardiac cycle: the sequence of events in one heartbeat, repeated cyclically: systole (contraction) and diastole (relaxation) of atria and ventricles.

Steps (start with all four chambers relaxed: joint diastole):
1. Joint diastole: tricuspid and bicuspid valves open; semilunar valves closed. Blood from the pulmonary veins and venae cavae flows into the left and right atria and then into the ventricles.
2. Atrial systole: the SAN fires, both atria contract, pushing more blood into the ventricles; this increases ventricular filling by about 30%.
3. The impulse reaches the ventricles via AVN and bundle of His; ventricular systole begins while the atria relax (atrial diastole).
4. Ventricular pressure rises, closing the tricuspid and bicuspid valves (preventing backflow into atria).
5. Pressure rises further, the semilunar valves open, and blood flows into the pulmonary artery (right) and aorta (left).
6. Ventricular diastole: ventricles relax, pressure falls, semilunar valves close (preventing backflow), then the AV valves open as atrial pressure exceeds ventricular, and the cycle repeats.

Numbers:

• Heart beats 72 per minute, so one cycle lasts 60 ÷ 72 ≈ 0.8 s.
• Stroke volume: blood pumped by each ventricle per beat: about 70 mL.
• Cardiac output = stroke volume × heart rate = 70 × 72 ≈ 5000 mL (5 litres) per minute.
• Both stroke volume and heart rate can change, so cardiac output changes (it is much higher in athletes).

Heart sounds (heard with a stethoscope):

• First sound, 'lub': closure of the tricuspid and bicuspid valves.
• Second sound, 'dub': closure of the semilunar valves.
• These sounds have clinical diagnostic significance.

Worked example

During ventricular systole, state the position (open/closed) of the AV valves and semilunar valves, and give the cause of the 'lub' sound. (2 marks)

Try it first, then show the solutionSolution

1. AV (tricuspid and bicuspid) valves closed; semilunar valves open once ventricular pressure is high enough. (1)
2. 'Lub' is caused by closure of the tricuspid and bicuspid valves at the start of ventricular systole. (1)

Answer: AV closed, semilunar open; lub = AV valve closure.

Electrocardiograph (ECG) and double circulation

NCERT section 15.3.3 Electrocardiograph (ECG); 15.4 Double Circulation; 15.5 Regulation of Cardiac Activity

ECG: a graphical record of the electrical activity of the heart during a cardiac cycle, obtained by connecting the patient to an electrocardiograph. For a standard ECG, three electrical leads are attached (one to each wrist and one to the left ankle); detailed evaluation uses multiple leads on the chest.

Waves:

• P wave: electrical excitation (depolarisation) of the atria, leading to atrial contraction.
• QRS complex: depolarisation of the ventricles, which starts ventricular contraction. Contraction starts shortly after Q; systole begins here.
• T wave: the ventricles recovering to their resting electrical state (repolarisation); systole is over when the T wave ends.
• Heart rate can be found by counting the number of QRS complexes in a given time.
• Each person's ECG has a characteristic shape; any deviation suggests a possible abnormality, so it has great clinical significance.

Double circulation (blood passes through the heart twice in one complete round of the body):

• Pulmonary circulation: right ventricle → pulmonary artery (deoxygenated blood) → lungs → pulmonary veins (oxygenated blood) → left atrium.
• Systemic circulation: left ventricle → aorta → arteries, arterioles, capillaries of tissues → venules, veins and vena cava (deoxygenated) → right atrium. It supplies nutrients, O₂ and other substances to tissues and takes away CO₂ and harmful substances.
• Hepatic portal system: the hepatic portal vein carries blood from the intestine to the liver before it reaches the systemic circulation.
• Coronary system: vessels that supply blood exclusively to and from the cardiac musculature.

Regulation of cardiac activity:

• Normal activities are regulated intrinsically by nodal tissue: the heart is myogenic.
• A special neural centre in the medulla oblongata moderates cardiac function through the autonomic nervous system:
• Sympathetic nerves increase heart rate, strength of ventricular contraction and cardiac output.
• Parasympathetic nerves decrease heart rate, speed of conduction and cardiac output.
• Adrenal medullary hormones can also increase cardiac output.

Worked example

What does each wave of a normal ECG represent? How would you find the heart rate from an ECG? (3 marks)

Try it first, then show the solutionSolution

1. P wave: atrial depolarisation (leading to atrial contraction). (1)
2. QRS complex: ventricular depolarisation (start of ventricular contraction); T wave: ventricular repolarisation (end of systole). (1)
3. Count the QRS complexes in a known time, e.g. per minute. (1)

Answer: P atria, QRS ventricles depolarise, T ventricles repolarise; count QRS complexes.

Disorders of the circulatory system

NCERT section 15.6 Disorders of Circulatory System

High blood pressure (hypertension):

• Blood pressure higher than normal (120/80 mm Hg). 120 is the systolic (pumping) pressure; 80 is the diastolic (resting) pressure.
• If repeated checks show 140/90 or higher, the person has hypertension.
• It leads to heart diseases and also affects vital organs such as the brain and kidney.

Coronary artery disease (CAD), often called atherosclerosis:

• Affects vessels supplying blood to the heart muscle.
• Caused by deposits of calcium, fat, cholesterol and fibrous tissue, which narrow the lumen of arteries.

Angina (angina pectoris):

• Acute chest pain when not enough oxygen reaches the heart muscle.
• Can occur at any age, but is more common in the middle-aged and elderly; due to conditions that affect blood flow.

Heart failure:

• The heart does not pump blood effectively enough to meet the body's needs.
• Also called congestive heart failure, because congestion of the lungs is a main symptom.
• Not the same as cardiac arrest (heart stops beating) or a heart attack (heart muscle suddenly damaged by inadequate blood supply).

Worked example

Distinguish between heart failure, cardiac arrest and heart attack. (3 marks)

Try it first, then show the solutionSolution

1. Heart failure: the heart pumps, but not effectively enough for the body's needs; lungs become congested. (1)
2. Cardiac arrest: the heart stops beating. (1)
3. Heart attack: heart muscle is suddenly damaged because of inadequate blood supply. (1)

Answer: Weak pumping vs stopped heart vs sudden muscle damage.

Body Fluids and Circulation in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Blood Groups and Coagulation of Blood
• Cardiac Cycle, Cardiac Output and ECG

Body Fluids and Circulation: key facts

Cardiac output

$\text{cardiac output}=\text{stroke volume}\times \text{heart rate}\approx 70\text{mL}\times 72\approx 5000\text{mL/min}$

About 5 litres per minute at rest.

Duration of cardiac cycle

$t=\frac{60\text{s}}{\text{beats per minute}}=\frac{60}{72}\approx 0.8\text{s}$

Shorter cycle at higher heart rate.

Clotting cascade

$\text{prothrombin}\stackrel{\text{thrombokinase}}{\to }\text{thrombin};\text{fibrinogen}\stackrel{\text{thrombin}}{\to }\text{fibrin}$

Calcium ions essential.

Conduction pathway

$\text{SAN}\to \text{atria}\to \text{AVN}\to \text{bundle of His}\to \text{bundle branches}\to \text{Purkinje fibres}$

SAN fires 70-75 times per minute (pacemaker).

Blood composition

$\text{blood}=\text{plasma (55\%)}+\text{formed elements (45\%)};\text{serum}=\text{plasma}-\text{clotting factors}$

Plasma is 90-92% water, 6-8% proteins.

Blood pressure

$\text{normal}=120/80\text{mm Hg};\text{hypertension}\ge 140/90\text{mm Hg}$

Systolic/diastolic.

Body Fluids and Circulation: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Body Fluids and Circulation

The question patterns to expect, and the method that works for each.

Tricks to remember Body Fluids and Circulation

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Never Let Monkeys Eat Bananas

  WBCs from most to least common: Neutrophils (60-65%), Lymphocytes (20-25%), Monocytes (6-8%), Eosinophils (2-3%), Basophils (0.5-1%).

  Use it when: WBC order.

• Try before you Buy

  Tricuspid on the right, Bicuspid on the left (blood reaches the right side first).

  Use it when: Valve sides.

• Lub = AV shut, Dub = semilunar shut

  L for 'lower' doors (AV valves) close first; D for 'departure' doors (semilunar) close second.

  Use it when: Heart sounds.

• P-QRS-T: Atria Push, Ventricles Squeeze, Ventricles Tidy up

  P atrial depolarisation, QRS ventricular depolarisation, T ventricular repolarisation.

  Use it when: ECG waves.

• 70 × 72 ≈ 5000

  Stroke volume 70 mL × 72 beats ≈ 5 L/min; 60/72 ≈ 0.8 s per cycle.

  Use it when: Cardiac numericals.

• O gives, AB takes

  O: no antigens, universal donor. AB: no antibodies, universal recipient.

  Use it when: Blood transfusion.

Body Fluids and Circulation: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Body Fluids and Circulation

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying group O blood contains no antibodies

  Mixing up antigens and antibodies.

  ✓Do this: Group O has no antigens but has both anti-A and anti-B antibodies; AB has no antibodies.

  Usually costs: 1 mark.

• ✗Mistake: Stating that Rh problems occur in the first pregnancy

  Not knowing when the mother is sensitised.

  ✓Do this: The mother is usually sensitised at the first delivery; antibodies harm later Rh+ve foetuses.

  Usually costs: 1 mark.

• ✗Mistake: Putting the AV node in the left atrium or ventricle

  Guessing positions.

  ✓Do this: Both SAN (right upper corner) and AVN (lower left corner) are in the right atrium.

  Usually costs: ½-1 mark.

• ✗Mistake: Saying the T wave shows atrial repolarisation

  Waves are matched to the wrong chambers.

  ✓Do this: T = ventricular repolarisation; atrial repolarisation is hidden in the QRS complex.

  Usually costs: 1 mark.

• ✗Mistake: Writing that the pulmonary artery carries oxygenated blood

  'Arteries carry oxygenated blood' is over-generalised.

  ✓Do this: Pulmonary artery: deoxygenated blood to lungs; pulmonary veins: oxygenated blood to the left atrium.

  Usually costs: 1 mark.

• ✗Mistake: Confusing heart failure with heart attack

  Everyday use of the words.

  ✓Do this: Failure = ineffective pumping; attack = sudden muscle damage from poor blood supply; arrest = heart stops.

  Usually costs: 1 mark.

NCERT exercise guide: Body Fluids and Circulation

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Body Fluids and Circulation

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   The average life span of human RBCs is:
   (a) 12 days (b) 60 days (c) 120 days (d) 1 year

   Hint

   Destroyed in the spleen.

   Answer

   (c) 120 days

2. Q2 · 1 mark · MCQExam level

   The 'dub' sound of the heart is produced by closure of the:
   (a) tricuspid valve (b) bicuspid valve (c) semilunar valves (d) AV node

   Hint

   Second sound.

   Answer

   (c) semilunar valves

3. Q3 · 2 marks · Very short answerExam level

   In a cockroach, blood pumped by the heart leaves the vessels and bathes the organs directly; in an earthworm, blood never leaves the vessels. Name the type of circulation in each, the spaces the cockroach's blood flows into, and one advantage of the earthworm's type. (2 marks)

   Hint

   Arthropods vs annelids.

   Answer

   Cockroach (an arthropod): open circulation, blood emptying into open spaces called sinuses; earthworm (an annelid): closed circulation (1). In a closed system blood flow can be regulated more precisely (1).

4. Q4 · 2 marks · Very short answerHarder exam level

   Give the functions of eosinophils and lymphocytes. (2 marks)

   Hint

   Allergy; immunity.

   Answer

   Eosinophils (2-3%) resist infections and are associated with allergic reactions (1). Lymphocytes (20-25%), B and T types, are responsible for immune responses (1).

5. Q5 · 2 marks · Very short answerHarder exam level

   In a patient the bundle of His is damaged, but the SA node and atria work normally. Predict the effect on (a) contraction of the atria, and (b) the timing of ventricular contraction, and explain. (2 marks)

   Hint

   The AV bundle is the only route across the AV septum.

   Answer

   (a) The atria still contract normally, because the SA node's impulse spreads over them directly (½). (b) The impulse can no longer travel from the AV node through the AV septum and along the bundle branches to the Purkinje fibres, so the ventricles stop following the SA node's rhythm; they contract out of step with the atria, driven only by their own slower autoexcitable tissue (1½).

6. Q6 · 2 marks · Very short answerHarder exam level

   How does the autonomic nervous system regulate cardiac output? (2 marks)

   Hint

   Medulla centre.

   Answer

   A centre in the medulla oblongata acts through the ANS: sympathetic nerves increase heart rate, strength of ventricular contraction and cardiac output (1); parasympathetic nerves decrease heart rate, conduction speed and output. Adrenal medullary hormones also raise output (1).

7. Q7 · 3 marks · Short answerHarder exam level

   Describe the formation and functions of lymph. (3 marks)

   Hint

   Capillary leak.

   Answer

   Water and small solutes leave capillaries into tissue spaces, leaving large proteins and most cells behind: interstitial fluid (1). The lymphatic system collects it as lymph and returns it to the major veins (1). Lymph carries lymphocytes for immunity, nutrients and hormones, and absorbs fats via lacteals (1).

8. Q8 · 3 marks · Short answerHarder exam level

   Describe the structure of the human heart. (3 marks)

   Hint

   Chambers, septa, valves.

   Answer

   Mesodermal, fist-sized, in the thorax tilted left, covered by pericardium with pericardial fluid; two atria and two thicker ventricles (1). Inter-atrial (thin), inter-ventricular (thick) and atrio-ventricular (fibrous) septa (1). Tricuspid (right) and bicuspid (left) valves between atria and ventricles; semilunar valves at the pulmonary artery and aorta (1).

9. Q9 · 5 marks · Long answerHarder exam level

   (a) What is double circulation? Trace the path of blood through it. (b) Describe the coronary and hepatic portal systems. (5 marks)

   Hint

   Two circuits.

   Answer

   (a) Blood passes through the heart twice in one round, in separate pulmonary and systemic circuits without mixing (1). Pulmonary: right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium (1). Systemic: left ventricle → aorta → arteries, arterioles, capillaries → venules, veins, venae cavae → right atrium, delivering O₂ and nutrients and removing CO₂ (1). (b) Coronary system: vessels supplying blood only to and from the heart muscle (1). Hepatic portal system: hepatic portal vein carries blood from the intestine to the liver before systemic circulation (1).

10. Q10 · 3 marks · Short answerHOTS / competency

    Suppose the SA node of a patient stops working, but the heart keeps beating at about 40-50 per minute. Suggest what has taken over and why a pacemaker device may be fitted. (3 marks)

    Hint

    Other nodal tissue is also autoexcitable.

    Answer

    Other nodal tissue (e.g. the AV node) can generate action potentials without external stimuli, but at a lower rate, so it takes over (1). The slower rate reduces cardiac output (output = stroke volume × rate), so the person may feel weak or dizzy (1). An artificial pacemaker supplies regular impulses at a normal rate (about 72 per minute) to restore output (1).

11. Q11 · 3 marks · Short answerHOTS / competency

    A patient's platelet count is 40,000 per mm³. Predict what might happen after a small cut and explain using the clotting mechanism. (3 marks)

    Hint

    Normal is 1.5-3.5 lakh.

    Answer

    Normal count is 1,50,000-3,50,000 per mm³, so this is very low (1). Platelets release factors that start the clotting cascade leading to thrombokinase, thrombin and fibrin (1). With too few platelets, clot formation is slow and weak, so the cut may bleed for a long time (excessive blood loss) (1).

12. Q12 · 1 mark · Assertion-reasonHOTS / competency

    Assertion (A): Blood in the left ventricle is pumped at higher pressure than blood in the right ventricle.
    Reason (R): The left ventricle has a thicker muscular wall because it pumps blood to the whole body.
    (a) Both A and R are true and R is the correct explanation of A.
    (b) Both A and R are true but R is not the correct explanation of A.
    (c) A is true but R is false.
    (d) A is false but R is true.

    Hint

    Systemic vs pulmonary.

    Answer

    (a)

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What percentage of blood is plasma?Show answer

   About 55%.

2. 2.What is the RBC count of a healthy adult man?Show answer

   5-5.5 million per mm³.

3. 3.Which organ is the graveyard of RBCs?Show answer

   Spleen.

4. 4.Which WBCs secrete histamine, serotonin and heparin?Show answer

   Basophils.

5. 5.From which cells do platelets arise?Show answer

   Megakaryocytes in the bone marrow.

6. 6.What percentage of humans are Rh positive?Show answer

   Nearly 80%.

7. 7.Which enzyme converts fibrinogen to fibrin?Show answer

   Thrombin.

8. 8.Where are fats absorbed into lymph?Show answer

   Lacteals in the intestinal villi.

9. 9.Which animals have open circulation?Show answer

   Arthropods and molluscs.

10. 10.How much does atrial systole add to ventricular filling?Show answer

    About 30%.

11. 11.What does the QRS complex represent?Show answer

    Depolarisation of the ventricles.

12. 12.Which nerves decrease the heart rate?Show answer

    Parasympathetic nerves.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you give the composition of plasma and the counts of RBCs, WBCs and platelets?
• ☐Can you list the five WBC types with percentages and functions?
• ☐Can you build the ABO transfusion table and explain Rh incompatibility?
• ☐Can you write the clotting cascade?
• ☐Can you draw the heart with chambers, valves, vessels and nodal tissue?
• ☐Can you describe the cardiac cycle with valve positions?
• ☐Can you label a normal ECG and explain each wave?
• ☐Can you define hypertension, CAD, angina and heart failure?

Body Fluids and Circulation: frequently asked questions