The circulatory system is a closed loop that keeps every cell in your body a few microns away from a capillary. Heart structure, cardiac cycle, BP regulation and the double circulation concept are tested every year in CBSE Class 10, 11 and NEET. This is one of the highest-weightage physiology topics.
The heart beats about 100,000 times per day, pumping about 7,500 litres of blood. It does this without any conscious effort, controlled by its own pacemaker and modulated by the nervous system. Understanding the anatomy (four chambers, valves, blood vessels), the physiology (cardiac cycle, conduction system, blood pressure) and the pathology (hypertension, heart failure) covers the entire chapter.
Core Concepts
Types of circulatory systems
| Type | Blood path | Example |
|---|---|---|
| Open | Blood enters body cavities (sinuses), not always in vessels | Insects, most molluscs |
| Closed | Blood stays in vessels throughout | Vertebrates, earthworms |
| Single | Blood passes through the heart once per full loop | Fish (2-chambered heart) |
| Double | Blood passes through the heart twice per loop | Birds, mammals (4-chambered heart) |
In double circulation: Pulmonary circuit = right heart → lungs → left heart. Systemic circuit = left heart → body → right heart. Keeping oxygenated and deoxygenated blood separate ensures efficient oxygen delivery.
Human heart anatomy
Four chambers: two atria (upper, thin-walled, receiving chambers) and two ventricles (lower, thick-walled, pumping chambers).
- Right atrium receives deoxygenated blood from the body via superior and inferior vena cava
- Right ventricle pumps deoxygenated blood to the lungs via pulmonary artery
- Left atrium receives oxygenated blood from the lungs via pulmonary veins
- Left ventricle pumps oxygenated blood to the body via aorta
The left ventricle has the thickest wall — it must generate enough pressure to push blood through the entire systemic circulation, from head to toes.
Valves prevent backflow:
- Tricuspid (right AV valve) — between right atrium and ventricle (3 cusps)
- Bicuspid/Mitral (left AV valve) — between left atrium and ventricle (2 cusps)
- Pulmonary semilunar — between right ventricle and pulmonary artery
- Aortic semilunar — between left ventricle and aorta
Cardiac cycle
One complete heartbeat lasting about 0.8 seconds at 72 bpm. Three phases:
| Phase | Duration | Events |
|---|---|---|
| Atrial systole | 0.1 s | Atria contract, push blood into ventricles through open AV valves |
| Ventricular systole | 0.3 s | Ventricles contract, AV valves close (first heart sound — “lub”), blood pushed into arteries through semilunar valves |
| Joint diastole | 0.4 s | All chambers relax, semilunar valves close (second heart sound — “dub”), blood fills atria and then ventricles passively |
Heart rate ~72 bpm × stroke volume ~70 mL = ~5 L/min at rest. During exercise, CO can increase to 20-25 L/min through increases in both HR and SV.
Conduction system
The heart generates its own rhythm — it is myogenic (not neurogenic). The conduction pathway:
- SA node (sinoatrial node) — in the right atrium wall. The natural pacemaker, fires at ~72 impulses/min.
- AV node (atrioventricular node) — at the junction of atria and ventricles. Delays the signal by ~0.1 s so atria finish emptying before ventricles contract.
- Bundle of His — runs through the interventricular septum, divides into left and right bundle branches.
- Purkinje fibres — spread the impulse through the ventricular myocardium, causing coordinated contraction from the apex upward (squeezing blood toward the arteries).
NEET loves asking: “Where is the SA node located?” (right atrial wall) and “Why is there a delay at the AV node?” (to allow complete atrial emptying before ventricular contraction). Also know that the heart is myogenic — crustacean hearts are neurogenic, fish and mammalian hearts are myogenic.
Blood pressure
Systolic pressure — peak pressure during ventricular contraction, normally ~120 mmHg. Diastolic pressure — minimum pressure during relaxation, normally ~80 mmHg.
Measured using a sphygmomanometer and reported as systolic/diastolic (e.g., 120/80 mmHg).
Hypertension: persistent BP above 140/90 mmHg. Risk factors: obesity, salt intake, stress, smoking, genetics. Complications: heart attack, stroke, kidney damage.
Regulation of BP:
- Baroreceptors in carotid sinus and aortic arch detect pressure changes → signal the medulla → adjust heart rate and vessel diameter via sympathetic/parasympathetic nerves
- RAAS (renin-angiotensin-aldosterone system) — long-term regulation via kidney
- ADH — water retention increases blood volume and BP
- ANF — from heart atria, promotes Na excretion, lowers BP
Blood vessels
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Wall | Thick, elastic, muscular | Thin, less muscular | Single cell thick (endothelium only) |
| Lumen | Narrow | Wide | Microscopic |
| Valves | None (except pulmonary) | Present | None |
| Pressure | High | Low | Low |
| Direction | Away from heart | Toward heart | Connects arterioles to venules |
| Function | Carries blood under pressure | Returns blood to heart | Exchange of gases, nutrients, wastes |
Blood composition
Plasma (~55% of blood volume): water, proteins (albumin, globulins, fibrinogen), glucose, electrolytes, hormones, waste products.
Formed elements (~45%):
- RBCs (erythrocytes) — biconcave, no nucleus, contain haemoglobin, ~5 million/mm, lifespan 120 days, made in bone marrow
- WBCs (leucocytes) — immune defence, ~5000-8000/mm. Types: neutrophils, lymphocytes, monocytes, eosinophils, basophils
- Platelets (thrombocytes) — blood clotting, ~250,000/mm, cell fragments from megakaryocytes
ECG (Electrocardiogram)
A recording of the electrical activity of the heart from the body surface.
| Wave | Represents |
|---|---|
| P wave | Atrial depolarisation (atrial systole) |
| QRS complex | Ventricular depolarisation (ventricular systole) |
| T wave | Ventricular repolarisation (relaxation) |
The P-Q interval represents AV node delay. A normal ECG has a regular rhythm, consistent intervals, and no abnormal waves. Irregularities indicate arrhythmias, blocks or other cardiac pathology.
Disorders
| Disorder | Description |
|---|---|
| Atherosclerosis | Plaque buildup in arteries, narrows lumen |
| Heart attack (MI) | Blocked coronary artery → cardiac muscle death |
| Heart failure | Heart cannot pump effectively |
| Angina pectoris | Chest pain from reduced coronary blood flow |
| Arrhythmia | Abnormal heart rhythm (too fast, too slow, irregular) |
Worked Examples
It must generate enough pressure (~120 mmHg) to push blood through the entire systemic circuit — from the aorta to the capillaries in the toes and the brain, and back through the veins. The right ventricle only pushes blood to the nearby lungs (~15-25 mmHg), so its wall is thinner. The pressure requirement directly determines the muscle mass needed.
Toe capillary → venules → veins → inferior vena cava → right atrium → tricuspid valve → right ventricle → pulmonary semilunar valve → pulmonary artery → lung capillary. Then: pulmonary vein → left atrium → bicuspid valve → left ventricle → aortic semilunar valve → aorta → arteries → body.
At rest: HR = 72, SV = 70 mL → CO = 5 L/min. During exercise: HR can reach 180, SV can reach 130 mL → CO = 180 × 0.13 = 23.4 L/min — almost 5 times the resting output. This massive increase is needed to supply oxygen and glucose to working muscles.
The P wave represents atrial depolarisation — the atria have thin walls and less muscle mass. The QRS complex represents ventricular depolarisation — the ventricles have much thicker walls. More muscle = more electrical activity = taller wave on the ECG.
Common Mistakes
Writing that arteries always carry oxygenated blood. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs. The definition of an artery is based on direction (away from heart), not oxygenation.
Thinking the SA node is in the ventricle. It is in the upper wall of the right atrium. The AV node is at the atrio-ventricular junction, not in the ventricle either.
Saying the heart is neurogenic in mammals. The mammalian heart is myogenic — it generates its own rhythm via the SA node. The nervous system modulates the rate but does not initiate it. Crustacean hearts are the classic neurogenic example.
Confusing systole and diastole. Systole = contraction (higher pressure). Diastole = relaxation (lower pressure). Blood pressure is reported as systolic/diastolic.
Forgetting that coronary arteries supply the heart muscle itself. The heart does not absorb oxygen from the blood passing through its chambers — it has its own dedicated blood supply. Blockage of coronary arteries causes heart attack.
Exam Weightage and Strategy
Body Fluids and Circulation carries 5-7 marks in CBSE Class 11 boards. NEET asks 2-3 questions per year from this chapter — typically on heart anatomy, cardiac cycle timing, blood pressure regulation, ECG waves, and blood vessel differences. This is one of the most diagram-heavy chapters.
Memorise the cardiac cycle as three timed boxes — 0.1 s atrial systole, 0.3 s ventricular systole, 0.4 s diastole (total 0.8 s). Know which valves are open and closed in each phase. Also memorise the ECG wave assignments (P = atrial, QRS = ventricular, T = repolarisation). These cover the most common PYQ patterns.
Practice Questions
Q1. Why does exercise increase cardiac output?
During exercise, muscles need more O and glucose. The sympathetic nervous system increases both heart rate and stroke volume. Adrenaline from the adrenal medulla amplifies this. Venous return increases due to muscular pumping (skeletal muscles compress veins, pushing blood back to the heart). Result: CO can increase from 5 L/min to 20-25 L/min, meeting the metabolic demand.
Q2. What causes the “lub-dub” heart sounds?
“Lub” (S1) — closing of the AV valves (tricuspid and bicuspid) at the start of ventricular systole. This is a lower-pitched, longer sound. “Dub” (S2) — closing of the semilunar valves (pulmonary and aortic) at the start of diastole. This is a higher-pitched, shorter sound. Murmurs (abnormal sounds) indicate valve defects — stenosis (narrowing) or regurgitation (leaking).
Q3. Explain the significance of double circulation.
Double circulation separates oxygenated and deoxygenated blood completely (in the 4-chambered heart). Blood is pumped at high pressure through the systemic circuit and at lower pressure through the pulmonary circuit. This is more efficient than single circulation (in fish, where blood loses pressure passing through gill capillaries before reaching the body). Double circulation allows mammals and birds to maintain high metabolic rates and body temperature.
FAQs
Why do veins have valves but arteries do not?
Arteries carry blood under high pressure from the heart — the pressure itself prevents backflow. Veins carry blood at low pressure, often against gravity (legs to heart). Valves prevent blood from pooling. Failure of venous valves causes varicose veins.
What is the difference between a heart attack and cardiac arrest?
A heart attack (myocardial infarction) is a blockage in a coronary artery that cuts blood supply to part of the heart muscle, causing tissue death. Cardiac arrest is when the heart suddenly stops beating effectively (usually due to an electrical malfunction like ventricular fibrillation). A heart attack can trigger cardiac arrest, but they are different events.
What is blood type and why does it matter?
Blood type (A, B, AB, O) is determined by antigens on the RBC surface and antibodies in the plasma. Mismatched transfusion causes agglutination (clumping) and haemolysis. Type O is the universal donor (no antigens). Type AB is the universal recipient (no antibodies). Rh factor (+ or -) adds another layer — Rh incompatibility between mother and fetus can cause erythroblastosis fetalis.
Circulation is the chapter where anatomy, physiology and physics meet. Think of the heart as a pump, the vessels as pipes, and the pressure drop as the driving force.