Chapter in a nutshell: The circulatory system transports materials around the body using blood (plasma + RBCs, WBCs, platelets), pumped by a four-chambered heart through arteries, veins and capillaries. Humans have double circulation (pulmonary + systemic). Blood also clots to seal wounds, carries immunity, and is classified into ABO and Rh blood groups for safe transfusion. The lymphatic system is the accessory circulation.
1. Blood โ Composition
Blood is a fluid connective tissue (โ 5 L in an adult). It has two parts:- Plasma (โ 55%): pale-yellow fluid (90% water) carrying proteins (albumin, globulin, fibrinogen), nutrients, hormones, wastes, salts and gases.
- Blood cells (โ 45%):
| Cell | Other name | Features | Function |
|---|---|---|---|
| RBC | erythrocyte | biconcave, no nucleus (mature), has haemoglobin; made in bone marrow; life ~120 days | carries Oโ (and some COโ) |
| WBC | leucocyte | larger, nucleated, amoeboid | defence/immunity (phagocytosis, antibodies) |
| Platelets | thrombocytes | tiny cell fragments, no nucleus | help in blood clotting |
2. Blood Vessels
| Feature | Arteries | Veins | Capillaries |
|---|---|---|---|
| Direction | carry blood away from heart | carry blood to the heart | connect arteries to veins |
| Walls | thick, elastic, muscular | thinner, less elastic | one-cell-thick |
| Valves | absent | present (prevent backflow) | absent |
| Blood | usually oxygenated* | usually deoxygenated* | exchange site |
| Lumen | narrow | wide | very narrow |
3. Structure of the Human Heart
A muscular (cardiac muscle) organ in the thorax, enclosed in the pericardium, with four chambers:- Two upper atria (right and left) โ receive blood.
- Two lower ventricles (right and left) โ pump blood out; the left ventricle has the thickest wall (pumps to the whole body).
- Valves prevent backflow:
- The septum separates the right (deoxygenated) and left (oxygenated) sides, preventing mixing.
4. Working of the Heart (Cardiac Cycle)
- Diastole: chambers relax and fill with blood.
- Systole: chambers contract and pump blood out.
- Sequence: atria contract โ blood to ventricles โ ventricles contract โ blood to pulmonary artery (right) and aorta (left).
- Heartbeat (~72/min) produces "lub" (AV valves close) and "dub" (semilunar valves close) sounds. The pulse is the rhythmic expansion of an artery with each beat. A pacemaker (SA node) sets the rhythm.
5. Double Circulation
Blood passes through the heart twice in one complete cycle:- Pulmonary circulation: right ventricle โ pulmonary artery โ lungs (gets Oโ) โ pulmonary vein โ left atrium.
- Systemic circulation: left ventricle โ aorta โ body tissues โ venae cavae โ right atrium.
6. Blood Clotting (Coagulation)
When a vessel is cut, platelets release thromboplastin, which (with Caยฒโบ and vitamin K) converts prothrombin โ thrombin; thrombin converts soluble fibrinogen โ insoluble fibrin threads that trap cells and form a clot, sealing the wound.7. Blood Groups & Transfusion
- ABO system: A, B, AB, O โ based on antigens on RBCs and antibodies in plasma.
- Rh factor: Rhโบ (antigen present) or Rhโป (absent).
| Group | Antigen | Antibody |
|---|---|---|
| A | A | anti-B |
| B | B | anti-A |
| AB | A & B | none โ universal recipient |
| O | none | anti-A & anti-B โ universal donor |
8. Lymphatic System (accessory circulation)
Lymph is a colourless fluid formed from tissue fluid; it has WBCs (no RBCs), flows in lymph vessels, is filtered by lymph nodes, and returns fluid + proteins to the blood. Functions: drains excess tissue fluid, absorbs fats from the intestine, and provides immunity.9. Worked / Structured Examples (ICSE pattern)
Q1. Name the blood vessel that carries oxygenated blood from the lungs to the heart. Solution: The pulmonary vein. Q2. Why is the wall of the left ventricle thicker than the right? Solution: It pumps blood to the whole body (systemic circulation) at high pressure; the right pumps only to the nearby lungs. Q3. Which valve prevents backflow from the left ventricle to the left atrium? Solution: The bicuspid (mitral) valve. Q4. A person of blood group O can donate to whom? Solution: To all groups โ O is the universal donor. Q5. Name the pigment in RBCs and its role. Solution: Haemoglobin โ carries oxygen (forms oxyhaemoglobin).10. Key Terms โ Quick Glossary
| Term | One-line definition |
|---|---|
| Plasma | fluid part of blood carrying cells & substances. |
| Haemoglobin | iron-pigment in RBCs that carries Oโ. |
| Artery / vein | vessel carrying blood away from / to the heart. |
| Atrium / ventricle | upper receiving / lower pumping heart chamber. |
| Tricuspid / bicuspid valve | right / left atrioventricular valve. |
| Double circulation | blood passes through the heart twice per cycle. |
| Pulmonary circulation | heart โ lungs circuit. |
| Systemic circulation | heart โ body circuit. |
| Clotting | fibrin-mesh sealing of a wound. |
| Universal donor / recipient | group O / group AB. |
| Lymph | colourless tissue fluid in lymph vessels. |
11. Common Mistakes to Avoid
- Saying all arteries carry oxygenated blood โ the pulmonary artery carries deoxygenated blood.
- Confusing tricuspid (right) with bicuspid/mitral (left) valves.
- Mixing up atria (receive) and ventricles (pump).
- Calling group A the universal donor โ it is O (donor) and AB (recipient).
- Forgetting that veins have valves but arteries do not.
- Saying lymph contains RBCs โ it does not (only WBCs).
12. Likely Exam Questions (with crisp answers)
- Name the components of blood. โ Plasma + RBCs, WBCs, platelets.
- State the function of RBCs. โ Transport oxygen (via haemoglobin).
- What is the role of platelets? โ Help in blood clotting.
- Differentiate arteries and veins. โ Arteries carry blood away (thick walls, no valves); veins carry blood to the heart (thinner, valves present).
- Name the four chambers of the heart. โ Right atrium, right ventricle, left atrium, left ventricle.
- Which valve lies between the right atrium and right ventricle? โ Tricuspid valve.
- What is double circulation? โ Blood passing through the heart twice โ pulmonary and systemic circuits.
- Why is double circulation advantageous? โ It keeps oxygenated and deoxygenated blood separate for efficient supply.
- Define cardiac cycle. โ One complete sequence of heart relaxation (diastole) and contraction (systole).
- What causes the "lub" and "dub" heart sounds? โ Closing of the atrioventricular valves (lub) and semilunar valves (dub).
- Why is group AB the universal recipient? โ Its plasma has no anti-A or anti-B antibodies.
- Name the process by which blood seals a wound. โ Clotting (coagulation).
- What is lymph and one of its functions? โ Colourless tissue fluid; provides immunity / drains excess fluid / absorbs fats.
- Which blood vessel carries deoxygenated blood from the heart to the lungs? โ Pulmonary artery.
- Name the pacemaker of the heart. โ The SA (sino-atrial) node.
13. Functions of Blood
- Transport: oxygen (RBC/haemoglobin), COโ, digested food, hormones, and nitrogenous wastes (urea) to the kidneys.
- Regulation: distributes heat (body temperature) and maintains water/pH balance (buffering).
- Protection: WBCs fight infection (phagocytosis, antibodies); platelets clot wounds.
14. The Heart's Blood Supply & Rate
- The heart muscle is fed by its own coronary arteries; a block causes a heart attack.
- Heart rate โ 72 beats/min at rest; rises with exercise/fear (adrenaline).
- Blood pressure is the force of blood on artery walls โ normal โ 120/80 mm Hg (systolic/diastolic). Persistently high = hypertension.
15. Portal & Special Circulations
- Hepatic portal vein: carries blood rich in absorbed food from the intestine to the liver before it returns to the heart.
- Vena cavae: the superior and inferior venae cavae return deoxygenated blood from the body to the right atrium; the aorta is the largest artery, leaving the left ventricle.
16. Common Disorders
| Disorder | Cause/Feature |
|---|---|
| Hypertension | persistently high blood pressure |
| Anaemia | low haemoglobin/RBC โ fatigue |
| Atherosclerosis | fatty deposits narrowing arteries |
| Heart attack | blocked coronary artery |
17. More Worked / Structured Examples
Q6. Trace the path of a drop of blood from the right ventricle until it returns oxygenated to the left atrium. Solution: Right ventricle โ pulmonary artery โ lungs โ pulmonary vein โ left atrium. Q7. Why do veins need valves but arteries do not? Solution: Venous blood is at low pressure and flows against gravity; valves stop it flowing backward. Arterial blood is at high pressure, so no valves are needed. Q8. What is normal human blood pressure, and what do the two numbers mean? Solution: โ 120/80 mm Hg โ systolic (ventricles contract) over diastolic (ventricles relax). Q9. Why can a person with group O receive blood only from group O? Solution: O plasma has both anti-A and anti-B antibodies, which would react with A or B antigens.18. More Exam Questions (with crisp answers)
- State three functions of blood. โ Transport of substances, temperature regulation, protection/immunity.
- Name the largest artery and the vein returning blood to the heart. โ Aorta; venae cavae.
- What is blood pressure and its normal value? โ Force of blood on artery walls; โ 120/80 mm Hg.
- Define hypertension. โ Persistently raised blood pressure.
- Which vein carries food-rich blood to the liver? โ Hepatic portal vein.
- Why is RBC count higher than WBC count? โ RBCs must continuously transport large amounts of oxygen to all tissues.
- Name the membrane around the heart. โ Pericardium.
- What happens at a capillary? โ Exchange of gases, nutrients and wastes between blood and tissues.
- Differentiate systole and diastole. โ Systole = contraction (pumping); diastole = relaxation (filling).
- Name the universal donor and universal recipient blood groups. โ O (donor); AB (recipient).
- Which chamber receives oxygenated blood from the lungs? โ The left atrium.
- Why does the pulmonary artery carry deoxygenated blood? โ It takes blood from the right ventricle to the lungs for oxygenation.