Closed Double Circulatory System
Overview🔄 Why "Double"? Why "Closed"?
Closed: blood is always contained within blood vessels — it never flows freely into body cavities.
Double: blood passes through the heart TWICE per complete circuit — once for the pulmonary loop and once for the systemic loop. This maintains high blood pressure throughout the body.
Pulmonary & Systemic Circuits — with major organs
Colour indicates oxygen status of blood in each vessel
🫁 Pulmonary Circuit
- Right ventricle → pulmonary artery → lungs
- In lungs: CO₂ released; O₂ absorbed (gas exchange)
- Lungs → pulmonary vein → left atrium
- Exception: pulmonary artery carries deoxygenated blood; pulmonary vein carries oxygenated blood
- Short, low-pressure circuit
🌍 Systemic Circuit
- Left ventricle → aorta → all body organs
- Oxygenated blood delivered to brain, intestine, liver, kidneys, muscles, etc.
- Body tissues → vena cava → right atrium
- Long, high-pressure circuit — left ventricle wall is much thicker
- Hepatic portal vein carries nutrient-rich blood from intestine → liver
| Organ | Artery supplying it | Vein draining it | Special note |
|---|---|---|---|
| Brain | Carotid artery | Jugular vein | Constant glucose & O₂ supply critical; no blood = brain damage within minutes |
| Small intestine | Mesenteric artery | Hepatic portal vein → liver | Absorbed nutrients go to liver first via hepatic portal vein before entering general circulation |
| Liver | Hepatic artery + hepatic portal vein | Hepatic vein → vena cava | Receives TWO blood supplies; detoxifies blood, regulates glucose |
| Kidneys | Renal artery | Renal vein | Filter 180 L of fluid per day; blood leaving renal vein has less urea |
| Lungs | Pulmonary artery (deoxy) | Pulmonary vein (oxy) | Only artery that carries deoxygenated blood; only vein carrying oxygenated blood |
⚠️ The Famous Exception
Arteries carry oxygenated blood — EXCEPT the pulmonary artery (carries deoxygenated blood from right ventricle to lungs).Veins carry deoxygenated blood — EXCEPT the pulmonary vein (carries oxygenated blood from lungs to left atrium).
The rule is really: arteries carry blood AWAY from the heart; veins carry blood TOWARD the heart.
Heart Structure
Internal & External❤️ A Four-Chambered Pump
The human heart is a cone-shaped, muscular organ about the size of a fist, located slightly left of centre in the chest. It has four chambers separated by valves that ensure one-way blood flow. The thick muscular wall (myocardium) contracts rhythmically without fatigue for a lifetime.
Internal Structure of the Human Heart
Viewed from the front — note: right side of heart appears on LEFT of diagram
| Structure | Location | Function |
|---|---|---|
| Pericardium | Tough outer sac surrounding the heart | Protects heart; contains lubricating fluid to reduce friction during beating |
| Myocardium | Thick muscular wall of the heart | Cardiac muscle — contracts rhythmically to pump blood; thickest in left ventricle |
| Endocardium | Smooth inner lining of chambers | Reduces friction as blood flows through chambers; continuous with vessel lining |
| Septum | Central wall dividing left and right sides | Prevents mixing of oxygenated and deoxygenated blood |
| Tricuspid valve | Between right atrium and right ventricle | Prevents backflow from right ventricle → right atrium during ventricular contraction |
| Bicuspid (mitral) valve | Between left atrium and left ventricle | Prevents backflow from left ventricle → left atrium during ventricular contraction |
| Semilunar valves | At base of aorta and pulmonary artery | Prevent backflow into ventricles when they relax after contracting |
| Chordae tendineae | "Heart strings" — attach valve flaps to papillary muscles | Prevent atrioventricular valves from inverting (turning inside out) during ventricular contraction |
| Coronary arteries | On outer surface of heart | Supply the heart muscle itself with oxygenated blood; blockage causes heart attack |
⚠️ Left vs Right Ventricle Wall Thickness
The left ventricle wall is 3× thicker than the right ventricle wall. Reason: the left ventricle pumps blood around the entire systemic circuit (high pressure, long distance). The right ventricle only pumps blood to the nearby lungs (low pressure, short distance). This is a classic IEB comparison question.
The Cardiac Cycle
Events & Blood Flow💓 One Complete Heartbeat ≈ 0.8 Seconds
The cardiac cycle is the sequence of events in one complete heartbeat. It involves the coordinated contraction (systole) and relaxation (diastole) of the atria and ventricles, driven by electrical signals, to ensure blood flows continuously in the correct direction.
📌 Key Terms
Systole = contraction of a heart chamber (pushes blood out)Diastole = relaxation of a heart chamber (allows blood to fill it)
Stroke volume = volume of blood pumped per beat (~70 mL)
Cardiac output = heart rate × stroke volume (~5 L/min at rest)
1
Atrial Diastole + Ventricular Diastole — Heart fills
Both atria and ventricles are relaxed. Blood flows passively from veins (vena cava and pulmonary veins) into the atria, then through the open atrioventricular valves (tricuspid and bicuspid) into the relaxed ventricles. Semilunar valves are closed — no backflow from arteries. About 70% of ventricular filling occurs passively.
2
Atrial Systole — Atria contract
Both atria contract simultaneously. This pushes the remaining ~30% of blood from the atria into the ventricles, completing ventricular filling. The atrioventricular valves are still open. Semilunar valves remain closed. Duration: ~0.1 seconds.
3
Ventricular Systole — Ventricles contract
Both ventricles contract simultaneously. Rising pressure in ventricles forces the atrioventricular valves SHUT (preventing backflow into atria — this produces the "lub" heart sound). When ventricular pressure exceeds arterial pressure, the semilunar valves are forced OPEN. Blood is ejected: right ventricle → pulmonary artery; left ventricle → aorta. Duration: ~0.3 seconds.
4
Ventricular Diastole — Ventricles relax
Ventricles relax. Ventricular pressure drops below arterial pressure — blood tries to flow back from arteries into ventricles. This causes the semilunar valves to SNAP SHUT (producing the "dub" heart sound). The cycle begins again. Duration: ~0.4 seconds.
Pressure Changes During the Cardiac Cycle
Relative pressure in left ventricle and aorta during one heartbeat
📌 Heart Sounds
"LUB" (first sound) = atrioventricular valves (tricuspid & bicuspid) closing at the START of ventricular systole"DUB" (second sound) = semilunar valves closing at the END of ventricular systole / start of diastole
A heart murmur = abnormal sound caused by backflow through a defective valve.
Mechanisms Controlling Heartbeat & Heart Rate
Myogenic Control⚡ The Heart's Own Pacemaker
The heart is myogenic — it generates its own electrical impulses without needing nerve signals to beat. However, the autonomic nervous system and hormones can increase or decrease the rate of beating to match the body's needs.
Electrical Conduction System of the Heart
⚡ Intrinsic Control (Myogenic)
- SAN (sinoatrial node) — "pacemaker"; located in wall of right atrium; generates electrical impulse spontaneously ~72×/min
- Impulse spreads across both atria → atrial systole
- AVN (atrioventricular node) — receives impulse from atria; delays it by ~0.1 seconds (allows ventricles to finish filling)
- Bundle of His — carries impulse from AVN down the septum
- Purkinje fibres — spread impulse through ventricular walls from apex upward → ventricular systole
🧠 Extrinsic Control (Nervous & Hormonal)
- Medulla oblongata (cardiovascular centre in brain) adjusts rate via two nerves:
- Sympathetic nerve — speeds up heart rate (e.g. during exercise, fear); releases noradrenaline at SAN
- Vagus nerve (parasympathetic) — slows heart rate (at rest/recovery); releases acetylcholine at SAN
- Adrenaline (hormone from adrenal gland) — increases heart rate + force of contraction during "fight or flight"
- Baroreceptors in aorta/carotid — detect blood pressure changes; signal medulla to adjust rate
| Factor | Effect on heart rate | Mechanism |
|---|---|---|
| Exercise | Increases | Muscles produce more CO₂ → blood pH drops → detected by chemoreceptors → sympathetic nerve stimulates SAN |
| Adrenaline | Increases | Directly stimulates SAN; increases stroke volume; prepares body for fight-or-flight |
| Rest / sleep | Decreases | Vagus nerve releases acetylcholine → slows SAN firing rate |
| High body temp | Increases | Increased metabolic rate → more O₂ needed → heart beats faster |
| High blood pressure | Decreases | Baroreceptors detect high pressure → signal medulla → vagus nerve slows heart |
Blood Vessels
Arteries · Veins · Capillaries🩸 Three Types, Three Jobs
The 100 000 km of blood vessels in the human body are not all the same. Arteries, veins and capillaries each have a structure perfectly matched to their function — from the thick elastic walls of the aorta to the single-cell-thick walls of capillaries where exchange actually happens.
Arteries
Away from heart
- Thick, muscular and elastic walls
- Small lumen (narrow channel)
- No valves (high pressure maintains flow)
- Blood under HIGH pressure; flows in pulses
- 3 layers: tunica intima, tunica media (thick), tunica externa
- Carry blood AWAY from heart
- Usually carry oxygenated blood (except pulmonary artery)
Veins
Toward heart
- Thin walls, less muscle and elastic tissue
- Large lumen (wide channel)
- Valves present — prevent backflow
- Blood under LOW pressure; flows steadily
- Assisted by skeletal muscle contractions + breathing
- Carry blood TOWARD the heart
- Usually carry deoxygenated blood (except pulmonary vein)
Capillaries
Exchange vessels
- Wall = ONE CELL THICK (endothelium only)
- Extremely narrow lumen — RBCs pass in single file
- No muscle layer; no valves
- Blood under very low pressure; flows slowly
- Exchange of O₂, CO₂, glucose, urea between blood and tissue fluid
- Linked to lymph capillaries; excess tissue fluid drained as lymph
Cross-section Comparison — Artery, Vein & Capillary
⚠️ Why Veins Have Valves But Arteries Don't
Arteries receive blood at HIGH pressure directly from the heart — this pressure keeps blood moving forward, so no valves are needed. Veins carry blood at LOW pressure back to the heart — gravity and low pressure could allow blood to pool and flow backward. Valves in veins (especially leg veins) prevent backflow and ensure one-way flow toward the heart. Varicose veins occur when vein valves become damaged and fail.
Blood & Lymph as Tissues
Components & Functions🔬 Blood — A Liquid Connective Tissue
Blood is classified as a connective tissue — it has cells suspended in a liquid matrix (plasma). An adult human has about 5 litres of blood, consisting of plasma (~55%) and cellular components (~45%). Each component has a highly specific structure matched to its function.
Plasma
~55% of blood volume
- ~90% water; straw-coloured liquid
- Transports: glucose, amino acids, fatty acids, urea, hormones, CO₂, plasma proteins
- Plasma proteins: albumin (osmotic pressure), fibrinogen (clotting), antibodies (immunity)
- Tissue fluid is formed by plasma leaking out of capillaries
Red Blood Cells
Erythrocytes · ~5 million/mm³
- Biconcave disc shape — large surface area for O₂ diffusion
- No nucleus — more space for haemoglobin
- Contain haemoglobin (Hb) — binds O₂ in lungs, releases in tissues
- Flexible — squeeze through narrow capillaries
- Lifespan: ~120 days; made in red bone marrow
White Blood Cells
Leucocytes · ~7 000/mm³
- Have a nucleus; larger than RBCs; irregular shape
- Phagocytes — engulf and destroy bacteria/pathogens by phagocytosis
- Lymphocytes — produce antibodies (specific immune response); B and T cells
- Produced in bone marrow and lymph nodes
Platelets
Thrombocytes · ~250 000/mm³
- Cell fragments (not whole cells); no nucleus
- Essential for blood clotting
- Clotting cascade: damage → platelets aggregate → fibrinogen → fibrin mesh → clot
- Prevent excessive blood loss; seal wounds
- Lifespan: ~10 days
📌 Haemoglobin & Oxygen Transport
In the lungs (high O₂): Hb + 4O₂ → oxyhaemoglobin (HbO₂) — bright redIn body tissues (low O₂): oxyhaemoglobin → Hb + O₂ released — dark red/purple
CO₂ is transported mainly dissolved in plasma as bicarbonate ions (HCO₃⁻), not by haemoglobin (which binds CO only as carboxyhaemoglobin — poisonous).
Lymph
Tissue Fluid| Feature | Blood Plasma | Tissue Fluid | Lymph |
|---|---|---|---|
| Origin | Circulates in blood vessels | Filtered from plasma at capillaries | Excess tissue fluid collected by lymph capillaries |
| Proteins | High (albumin, fibrinogen, antibodies) | Very low (too large to leak through capillary walls) | Very low (same as tissue fluid) |
| Red blood cells | Present | Absent | Absent |
| White blood cells | Present | Very few | Many lymphocytes |
| Fat | Some | Small amount | High (especially after meals — lacteal absorption) |
| Colour | Red (RBCs) / straw (plasma) | Colourless | Milky white / colourless |
The Lymphatic System
Structure & Functions💧 The Blood System's Partner
The lymphatic system is a one-way drainage network that works alongside the blood circulatory system. It collects excess tissue fluid (now called lymph), filters it through lymph nodes, and returns it to the blood. It is also a critical part of the immune system.
Relationship Between Capillaries, Tissue Fluid and Lymph
🏗️ Structure of Lymphatic System
- Lymph capillaries — blind-ended, one-way; drain tissue fluid from spaces between cells
- Lymph vessels — merge to form larger vessels; have valves (similar to veins) to prevent backflow
- Lymph nodes — oval structures along lymph vessels; contain lymphocytes; filter lymph to remove pathogens
- Thoracic duct & right lymphatic duct — major lymph vessels that drain into subclavian veins (returning lymph to blood)
- Spleen — largest lymphoid organ; filters blood; destroys old RBCs; stores lymphocytes
- Thymus — where T lymphocytes mature
⚙️ Functions of the Lymphatic System
- Fluid balance: returns excess tissue fluid to the blood — prevents oedema (fluid accumulation in tissues)
- Immunity: lymph nodes filter pathogens; lymphocytes produce antibodies; major role in immune response
- Fat absorption: lacteals (lymph capillaries in small intestine villi) absorb digested fats and fat-soluble vitamins (A, D, E, K) — transported as milky lymph called chyle
- Transport: returns plasma proteins (too large for blood capillaries to reabsorb) back to circulation
| Feature | Blood Circulatory System | Lymphatic System |
|---|---|---|
| Pump | Heart — active pump | No pump — relies on skeletal muscle contractions, breathing, peristalsis |
| Flow direction | Circular — continuous loop | One-way only — toward the heart (subclavian veins) |
| Valves | Heart valves + vein valves | Valves throughout lymph vessels |
| Fluid carried | Blood (plasma + cells) | Lymph (tissue fluid + lymphocytes + fats) |
| Red blood cells | Present | Absent |
| Capillary type | Closed at both ends (loop) | Blind-ended at one end (one-way drainage) |
⚠️ Oedema — What Happens When Balance Breaks Down
If lymph vessels are blocked (e.g. by parasites in lymphatic filariasis / elephantiasis, or by tumour), lymph cannot drain back to the blood. Tissue fluid accumulates and the tissue swells dramatically — this is called oedema. The same happens if blood protein levels drop (plasma loses osmotic pull to reabsorb tissue fluid) — common in severe malnutrition (kwashiorkor).
Exam Tips & Memo Answers
IEB Style📝 Write Like a Top Candidate
The circulatory system is one of the most tested topics in IEB Grade 11. Questions often combine structure with function, ask you to trace the path of blood, or apply your knowledge to clinical situations. Here are the most commonly examined questions with full memo answers.
❓ Trace the path of a red blood cell from the right atrium to the brain. Name ALL structures in order. (5 marks)
✅ Memo Answer
✓Right atrium → (tricuspid valve) → right ventricle
✓→ (semilunar valve) → pulmonary artery → lungs (gas exchange)
✓→ pulmonary vein → left atrium → (bicuspid valve) → left ventricle
✓→ (semilunar valve) → aorta
✓→ carotid artery → brain capillaries → brain
❓ Explain why the left ventricle wall is thicker than the right ventricle wall. (2 marks)
✅ Memo Answer
✓The left ventricle pumps blood into the aorta and around the entire systemic circuit (whole body) — a long, high-resistance pathway requiring high pressure.
✓The right ventricle only pumps blood to the nearby lungs via the pulmonary artery — a short, low-resistance pathway requiring much less pressure. Therefore the left ventricle needs a much thicker (more powerful) muscular wall.
❓ Explain the role of the SAN in initiating and coordinating the heartbeat. (4 marks)
✅ Memo Answer
✓The sinoatrial node (SAN) in the right atrium wall spontaneously generates an electrical impulse ~72 times per minute — it is the heart's natural pacemaker.
✓The impulse spreads across both atria, causing atrial systole (both atria contract).
✓The impulse reaches the AVN (atrioventricular node), which delays it by ~0.1 seconds — allowing the ventricles to finish filling.
✓The impulse passes down the Bundle of His and Purkinje fibres, causing ventricular systole from the apex upward — efficiently ejecting blood into the arteries.
⚠️ Language Precision
- Pulmonary artery = deoxygenated; pulmonary vein = oxygenated
- Arteries carry blood AWAY; veins carry blood TOWARD (not about O₂ content)
- Valves PREVENT BACKFLOW — not "stop blood flowing"
- The heart is MYOGENIC — generates its own impulse
- Capillaries are ONE CELL thick — not "very thin walled"
- Lymph returns to blood at the SUBCLAVIAN VEIN — not "back to the heart"
📌 Quick Recall Checklist
- ✅ Double circuit: pulmonary (lungs) + systemic (body)
- ✅ Hepatic portal vein: intestine → liver
- ✅ Renal artery/vein → kidneys
- ✅ Carotid artery/jugular vein → brain
- ✅ LUB = AV valves closing; DUB = semilunar valves
- ✅ SAN → AVN → Bundle of His → Purkinje fibres
- ✅ Artery: thick wall, small lumen, no valves
- ✅ Vein: thin wall, large lumen, has valves
- ✅ Lymph → subclavian vein (returns to blood)
Test Yourself
Quiz🎯 Circulatory System Quiz
IEB and CAPS style questions. Select your answer — if incorrect, the correct answer is highlighted immediately with a full explanation.
Q1
Blood in the pulmonary artery is moving FROM the heart TO the lungs. What is the oxygen status of this blood, and why is this an exception to the usual rule?
Q2
During ventricular systole, what causes the "LUB" heart sound, and what happens to the semilunar valves at this moment?
Q3
A patient's blood test shows a very low red blood cell count. Which symptom would you expect, and why?
Q4
Why do veins have valves but arteries do not?
Q5
A patient has a blocked hepatic portal vein. Which TWO organs are most directly affected, and what process is disrupted?
Q6 — Application
A marathon runner finishes a race. His heart rate is 180 bpm. Explain the sequence of events that caused his heart rate to increase from 72 bpm during the race.