Body Fluids and Circulation covers blood and its components, blood groups and clotting, lymph, the structure of the human heart, the cardiac cycle and cardiac output, double circulation, and common circulatory disorders. It is a high-yield NEET chapter where questions often test heart valves, ECG, blood pressure values, blood-group rules, and the calculation of cardiac output.
Table of Contents
- Key Concepts - Blood, blood groups, coagulation, lymph, heart, cardiac cycle, ECG, double circulation, disorders
- Weightage in Board & Entrance Exams
- Important Definitions
- Solved Examples
- Important Questions for Board Exams
- Quick Revision Points
Key Concepts
1. Body Fluids - Why Circulation Is Needed
Every cell needs a constant supply of nutrients and oxygen and a way to remove waste. To do this, the body uses fluids - chiefly blood and lymph - moved around by a transport system.
Blood is a specialised connective tissue (it has cells and a fluid matrix). Humans have a closed circulatory system, meaning blood flows inside vessels and is pumped by a muscular heart under high pressure.
2. Blood - Plasma and Formed Elements
Blood is made of a fluid part called plasma (about 55%) and cellular formed elements (about 45%).
Plasma
- About 90–92% water and 6–8% proteins.
- Major plasma proteins: fibrinogen (helps clotting), globulins (defence/antibodies), and albumins (osmotic balance).
- Also carries minerals (Na⁺, Ca²⁺, HCO₃⁻, Cl⁻), glucose, amino acids, lipids, and hormones.
- Plasma without clotting factors (fibrinogen removed) is called serum.
Formed Elements
| Cell | Key Facts | Function |
|---|---|---|
| Erythrocytes (RBCs) | 5–5.5 million/mm³; biconcave, no nucleus in mammals; contain haemoglobin; lifespan ~120 days | Transport of O₂ and CO₂ |
| Leucocytes (WBCs) | 6000–8000/mm³; nucleated, colourless; granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes) | Defence and immunity |
| Thrombocytes (platelets) | 1.5–3.5 lakh/mm³; cell fragments from megakaryocytes | Blood clotting |
Key idea: Neutrophils (60–65%) are the most abundant WBCs and are phagocytic; lymphocytes (20–25%) produce the immune response. RBCs are produced in the red bone marrow.
3. Blood Groups - ABO and Rh
Blood groups are based on the presence or absence of antigens (A and B) on the surface of RBCs and antibodies in the plasma.
| Blood Group | Antigen on RBC | Antibody in plasma | Can donate to | Can receive from |
|---|---|---|---|---|
| A | A | anti-B | A, AB | A, O |
| B | B | anti-A | B, AB | B, O |
| AB | A and B | none | AB | A, B, AB, O (universal recipient) |
| O | none | anti-A and anti-B | A, B, AB, O (universal donor) | O |
Rh Grouping
- The Rh antigen (Rh factor) is similar to one found in rhesus monkeys. People with it are Rh⁺; those without are Rh⁻.
- Erythroblastosis foetalis: if an Rh⁻ mother carries an Rh⁺ foetus, her body may make anti-Rh antibodies. In a later Rh⁺ pregnancy these can cross the placenta and destroy foetal RBCs. It is prevented by giving the mother anti-Rh antibodies after the first delivery.
4. Coagulation (Clotting) of Blood
When you get injured, blood forms a clot to stop bleeding. This is a defence mechanism called coagulation.
A clot is a network of threads of the protein fibrin in which dead and damaged blood cells are trapped.
Simplified Clotting Cascade
- Injured tissues and platelets release factors that form thrombokinase (thromboplastin).
- Thrombokinase, with Ca²⁺, converts inactive prothrombin → thrombin.
- Thrombin converts soluble fibrinogen → insoluble fibrin.
- Fibrin threads form a mesh that traps cells → the clot.
[DIAGRAM: Clotting cascade - Thrombokinase + Ca²⁺ → (Prothrombin → Thrombin) → (Fibrinogen → Fibrin) → clot.]
Note: Calcium ions (Ca²⁺) and vitamin K are essential for normal clotting. Heparin is a natural anticoagulant.
5. Lymph (Tissue Fluid)
As blood passes through capillaries, some plasma leaks out into the spaces between cells. This colourless fluid is lymph (tissue fluid).
- Lymph is plasma minus most large proteins and RBCs; it does contain WBCs (lymphocytes).
- It is collected by lymphatic vessels and returned to the blood.
- Functions: exchange of nutrients, gases and waste between blood and tissues; absorption of fats from the intestine (via lacteals); part of the immune system.
6. Human Circulatory System
The human circulatory (blood vascular) system consists of a muscular heart, a network of blood vessels, and the blood that flows through them.
- Arteries: carry blood away from the heart; thick, elastic walls; carry oxygenated blood (except the pulmonary artery).
- Veins: carry blood toward the heart; thinner walls with valves; carry deoxygenated blood (except the pulmonary vein).
- Capillaries: single-cell-thick vessels where exchange of materials occurs.
7. Structure of the Human Heart
The heart is a four-chambered, fist-sized muscular organ in the thoracic cavity, enclosed by a double-walled membrane called the pericardium.
- Two atria (upper, thin-walled) and two ventricles (lower, thick-walled). The left ventricle is the thickest because it pumps blood to the whole body.
- Tricuspid valve: between right atrium and right ventricle.
- Bicuspid (mitral) valve: between left atrium and left ventricle.
- Semilunar valves: at the openings of the aorta and pulmonary artery. Valves prevent backflow of blood.
[DIAGRAM: Human heart - right atrium, right ventricle, left atrium, left ventricle; tricuspid, bicuspid and semilunar valves; aorta, pulmonary artery, vena cava, pulmonary veins.]
Conducting System (Nodal Tissue)
- SA node (sinoatrial node): the pacemaker; generates ~70–75 impulses per minute on its own.
- AV node (atrioventricular node): picks up the impulse and passes it on.
- Bundle of His and Purkinje fibres: spread the impulse through the ventricles, making them contract.
8. Cardiac Cycle
The cardiac cycle is the sequence of events in one complete heartbeat. At 72 beats per minute, each cycle lasts about 0.8 seconds.
- Atrial systole: atria contract, pushing blood into ventricles (~0.1 s).
- Ventricular systole: ventricles contract; blood is pumped into the aorta and pulmonary artery (~0.3 s).
- Joint diastole: all chambers relax and fill with blood (~0.4 s).
Heart sounds: “Lubb” (first sound) is the closing of the tricuspid and bicuspid valves at the start of ventricular systole; “Dupp” (second sound) is the closing of the semilunar valves at the end of ventricular systole.
9. Cardiac Output
Cardiac output is the volume of blood pumped by each ventricle in one minute.
Cardiac Output = Stroke Volume × Heart Rate
- Stroke volume: volume of blood pumped per beat (~70 mL).
- At ~72 beats/min, cardiac output = 70 × 72 ≈ 5000 mL/min (about 5 litres).
- A trained athlete’s heart pumps more blood per beat, so it can maintain output at a lower heart rate.
10. Electrocardiograph (ECG)
An ECG is a graphical record of the electrical activity of the heart during a cardiac cycle.
- P wave: depolarisation (contraction) of the atria.
- QRS complex: depolarisation of the ventricles; ventricular contraction begins just after Q.
- T wave: repolarisation (relaxation/recovery) of the ventricles.
[DIAGRAM: ECG trace showing P wave, QRS complex and T wave along the baseline.]
Clinical use: Counting QRS complexes over a known time gives the heart rate; the shape of the waves helps doctors detect abnormalities.
11. Double Circulation
In humans, blood passes through the heart twice in one complete cycle around the body - this is double circulation. It keeps oxygenated and deoxygenated blood completely separate.
- Pulmonary circulation: right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium. (Deoxygenated blood goes to the lungs to be oxygenated.)
- Systemic circulation: left ventricle → aorta → body tissues → vena cava → right atrium. (Oxygenated blood is delivered to the body.)
Hepatic portal system: a special vein carries blood from the intestine to the liver before it reaches the heart. Coronary circulation supplies blood to the heart muscle itself.
12. Regulation of Cardiac Activity
The heart beats on its own (myogenic), but its rate is fine-tuned by the body.
- A special neural centre in the medulla oblongata moderates cardiac function through the autonomic nervous system.
- Sympathetic nerves increase heart rate, force of contraction, and cardiac output.
- Parasympathetic nerves (vagus) decrease heart rate and cardiac output.
- The hormone adrenaline also increases the heart rate (the “fight or flight” response).
13. Disorders of the Circulatory System
| Disorder | Description |
|---|---|
| Hypertension (high BP) | BP higher than 140/90 mmHg (normal ~120/80). Leads to heart disease and affects the kidneys and brain. |
| Coronary Artery Disease (CAD) | “Atherosclerosis” - narrowing of coronary arteries by deposits of calcium, fat, cholesterol and fibrous tissue, reducing blood flow to the heart muscle. |
| Angina (angina pectoris) | Chest pain due to insufficient oxygen reaching the heart muscle, often when the heart works harder. |
| Heart failure | The heart is unable to pump enough blood to meet the body’s needs (congestive heart failure when fluid collects in the lungs). |
Note: Do not confuse heart failure (heart cannot pump enough), cardiac arrest (heart stops beating), and heart attack (heart muscle is damaged, e.g. by a blocked coronary artery).
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Weightage in Board & Entrance Exams
| Exam | Typical Weightage | Most-Tested Areas |
|---|---|---|
| CBSE Board (Class 11) | 6–8 marks | Blood composition, blood groups, cardiac cycle, double circulation |
| NEET | 2–3 questions | Blood groups & Rh, ECG waves, cardiac cycle, conducting system, disorders |
| Other entrance (AIIMS-pattern) | 1–2 questions | Clotting cascade, cardiac output, regulation of heart |
[TABLE: Question-type split - VSA (1 mark): definitions, valves, ECG waves; SA (2–3 marks): blood groups, clotting, cardiac cycle; LA (5 marks): heart structure with diagram, double circulation.]
Important Definitions
| Term | Definition |
|---|---|
| Plasma | Straw-coloured fluid matrix of blood (~55%), 90–92% water with proteins, ions and nutrients |
| Serum | Plasma from which fibrinogen (clotting factors) has been removed |
| Haemoglobin | Red, iron-containing pigment in RBCs that transports O₂ and CO₂ |
| Universal donor | Blood group O - no antigens on RBCs, can donate to all groups |
| Universal recipient | Blood group AB - no antibodies in plasma, can receive from all groups |
| Coagulation | Formation of a fibrin clot to stop bleeding from a wound |
| Lymph | Colourless tissue fluid containing WBCs but lacking RBCs and large proteins |
| Cardiac cycle | One complete sequence of atrial systole, ventricular systole and diastole (~0.8 s) |
| Cardiac output | Volume of blood pumped by a ventricle per minute = stroke volume × heart rate |
| Double circulation | Blood passing through the heart twice per body circuit (pulmonary + systemic) |
Solved Examples
Example 1
A person has stroke volume 75 mL and heart rate 80 beats/min. Calculate the cardiac output.
Answer: Cardiac output = stroke volume × heart rate = 75 × 80 = 6000 mL/min = 6 litres/min.
Example 2
Why is blood group O called the universal donor?
Answer: Group O RBCs carry neither antigen A nor antigen B, so when O blood is transfused there are no surface antigens for the recipient’s antibodies to attack. Hence O can be donated to all ABO groups.
Example 3
In one cardiac cycle of 0.8 s, how is the time distributed among the phases?
Answer: Atrial systole ≈ 0.1 s, ventricular systole ≈ 0.3 s, joint diastole ≈ 0.4 s. Total = 0.8 s, which gives a heart rate of about 75 beats per minute.
Example 4
An Rh⁻ mother is pregnant with an Rh⁺ child for the second time. What complication may arise and why?
Answer: Erythroblastosis foetalis. Anti-Rh antibodies formed in the mother during the first Rh⁺ pregnancy cross the placenta in the second and destroy the foetal RBCs, causing anaemia and jaundice in the baby.
Example 5
Name the wave of the ECG that represents ventricular repolarisation and the one that represents atrial depolarisation.
Answer: Ventricular repolarisation = T wave; atrial depolarisation = P wave.
Example 6
Trace the path of a drop of deoxygenated blood from the right ventricle until it returns oxygenated to the left atrium.
Answer: Right ventricle → pulmonary artery → lungs (gas exchange, becomes oxygenated) → pulmonary veins → left atrium. This is the pulmonary circulation.
Important Questions for Board Exams
1-Mark Questions (VSA)
- Name the pigment that gives blood its red colour.
- Which node is called the pacemaker of the heart?
- What is serum?
- Name the valve present between the left atrium and the left ventricle.
- Which blood group is the universal recipient?
2–3-Mark Questions (SA)
- Differentiate between blood and lymph on the basis of composition and function.
- Explain the events of the clotting (coagulation) of blood with the help of a simple cascade.
- What is double circulation? How is it advantageous?
- Distinguish between heart failure, cardiac arrest, and heart attack.
5-Mark Questions (LA)
- Describe the structure of the human heart with a labelled diagram, including its chambers and valves.
- Explain the cardiac cycle in detail, mentioning the duration of each phase and the origin of heart sounds.
- Describe ABO and Rh blood grouping. Explain erythroblastosis foetalis and how it is prevented.
Quick Revision Points
- Blood = plasma (~55%) + formed elements (~45%); plasma is 90–92% water
- RBCs: biconcave, no nucleus (mammals), carry O₂; WBCs defend; platelets clot
- ABO groups based on antigens A/B on RBCs; O = universal donor, AB = universal recipient
- Rh⁻ mother + Rh⁺ foetus → erythroblastosis foetalis (in second pregnancy)
- Clotting: thrombokinase + Ca²⁺ → prothrombin→thrombin → fibrinogen→fibrin → clot
- Heart: 4 chambers; tricuspid (right), bicuspid/mitral (left), semilunar at arteries
- SA node = pacemaker (~70–75/min); AV node → Bundle of His → Purkinje fibres
- Cardiac cycle ≈ 0.8 s; Lubb = AV valves close, Dupp = semilunar valves close
- Cardiac output = stroke volume × heart rate ≈ 70 × 72 ≈ 5 L/min
- ECG: P = atrial depolarisation, QRS = ventricular depolarisation, T = ventricular repolarisation
- Double circulation = pulmonary + systemic; medulla + autonomic nerves regulate the heart
- Disorders: hypertension (>140/90), CAD/atherosclerosis, angina, heart failure
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Blood: Plasma + Formed Elements
Blood is a fluid connective tissue: ~55% plasma and ~45% cells.
Serum = plasma − clotting factors (no fibrinogen)
- Plasma proteins: fibrinogen (clotting), globulins (immunity), albumins (osmotic balance)
- RBCs ~5-5.5 million/mm3 · WBCs ~6000-8000/mm3 · platelets ~1.5-3.5 lakh/mm3
- Haemoglobin lives inside the RBC, not free in plasma
Red Blood Cells (Erythrocytes)
Biconcave, enucleate carriers of oxygen, packed with haemoglobin.
Mammalian RBCs are enucleate (no nucleus when mature)
- Erythropoiesis = RBC formation in red bone marrow
- Spleen is the ‘graveyard of RBCs’
- WBC abundance: N > L > M > E > B (neutrophils most, basophils least)
ABO Blood Groups
Group is set by A/B antigens on RBCs and antibodies in plasma.
AB recipient = no antibodies; O donor = no antigens
- Match donor cells to recipient’s plasma antibodies to avoid agglutination
- Rh⁻ mother + Rh⁺ foetus → erythroblastosis foetalis in the 2nd Rh⁺ pregnancy
- First Rh⁺ pregnancy only sensitises the mother
Coagulation (Clotting Cascade)
An injury triggers a relay that ends in a fibrin clot.
Vitamin K needed for prothrombin; heparin is a natural anticoagulant
- Platelets + damaged tissue release thromboplastin to start it
- Ca2⁺ ions are essential at several steps
- Soluble fibrinogen becomes insoluble fibrin threads that trap cells
Lymph
Fluid that filters out of capillaries to bathe the body’s cells.
Colourless; lacteals carry fat-rich lymph called chyle
- Middle-man for exchange of nutrients, O2 and wastes between blood and cells
- Fats absorbed via lacteals (lymph route); glucose/amino acids via blood
- Lymph nodes filter pathogens; lymphocytes aid immunity
Structure of the Human Heart
A four-chambered, myogenic muscular pump in the pericardial sac.
Left ventricle is the thickest wall (pumps to whole body)
- Heart is mesodermal; oxygenated and deoxygenated blood never mix
- LAB hook: Left Atrioventricular is Bicuspid → right is tricuspid
- Myogenic = beat originates in heart muscle, not nerves
Pacemaker & Impulse Spread
The SA node fires the beat and the impulse spreads in a fixed order.
SA node = primary pacemaker; AV node is the backup (~40-50/min)
- SA node sits in the right atrium wall
- Ordered spread makes atria contract first, then ventricles
- P wave = atrial depolarisation, QRS = ventricular, T = ventricular repolarisation
Cardiac Cycle & Output
One full heartbeat (~0.8 s at 72 bpm) made of three phases.
Phases: atrial systole 0.1 s + ventricular systole 0.3 s + joint diastole 0.4 s
- LUBB (S1) = AV valves close; DUP (S2) = semilunar valves close
- Stroke volume ≈ 70 mL of blood per ventricle per beat
- Rearrange: Heart Rate = CO ÷ SV
Double Circulation, BP & Disorders
Blood passes through the heart twice per circuit via two loops.
Normal BP ≈ 120/80 mm Hg; persistent ≥140/90 = hypertension
- Pulmonary artery = only artery with deoxygenated blood; pulmonary vein = only vein with oxygenated blood
- BP regulated by the cardiac centre in the medulla oblongata + adrenaline
- Disorders: hypertension, CAD/atherosclerosis, angina pectoris, heart failure
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Frequently Asked Questions
Blood is a fluid connective tissue made of plasma (about 55 percent, mostly water with proteins like fibrinogen, globulins and albumins) and formed elements (about 45 percent), which are red blood cells (RBCs), white blood cells (WBCs) and platelets. Plasma without clotting factors is called serum.
Cardiac output is the volume of blood pumped by a ventricle per minute. It equals stroke volume multiplied by heart rate. With a typical stroke volume of about 70 mL and a heart rate of 72 beats per minute, cardiac output is roughly 5 litres per minute.
Group O has no A or B antigens on its RBCs, so it can be given to any recipient without triggering an antigen-antibody reaction, making it the universal donor. Group AB has both A and B antigens but no anti-A or anti-B antibodies in its plasma, so it can receive any blood type, making it the universal recipient.
The pulmonary artery is the only artery that carries deoxygenated blood, from the right ventricle to the lungs. The pulmonary vein is the only vein that carries oxygenated blood, from the lungs back to the left atrium. This is a frequent NEET trap because it reverses the usual artery and vein rule.
The first heart sound LUBB is produced when the bicuspid and tricuspid (atrioventricular) valves close at the start of ventricular systole. The second sound DUP is produced when the semilunar valves close at the start of ventricular diastole.