Why do we need a transport system?
Food absorbed in the intestine, oxygen taken in by the lungs and hormones made by glands must reach every one of trillions of cells. Wastes such as CO₂ and urea must be carried away. Diffusion alone is far too slow over such distances, so humans have a circulatory system: a pump (heart), pipes (blood vessels) and a fluid (blood), plus a second drainage fluid, lymph.
The human heart: four chambers and valves
The heart is about the size of your fist and sits between the lungs, slightly to the left. A wall called the septum divides it into right and left halves so that oxygen-rich and oxygen-poor blood never mix.
- Atria (upper, thin-walled): receive blood.
- Ventricles (lower, thick muscular walls): pump blood out. The left ventricle has the thickest wall because it pumps blood to the whole body.
- Valves: flaps that open only one way, so blood cannot flow backward when the chambers contract.
Double circulation
Pulmonary circulation: right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium.
Systemic circulation: left ventricle → aorta → body → vena cava → right atrium.
Blood goes through the heart twice in one full circuit, so it is called double circulation. Keeping oxygenated and deoxygenated blood apart gives the body a very efficient oxygen supply, which birds and mammals need to keep a constant, warm body temperature.
Other animals: fish have a two-chambered heart and blood passes it only once per circuit (single circulation). Amphibians and most reptiles have three chambers and tolerate some mixing.
Blood vessels: arteries, veins and capillaries
| Arteries | Veins | Capillaries |
|---|---|---|
| Carry blood away from the heart | Bring blood back to the heart | Join arteries to veins |
| Thick, elastic, muscular walls (high pressure) | Thinner walls, wider space inside | Wall only one cell thick |
| No valves (except at the start) | Valves stop backflow | Exchange food, O₂, CO₂ and wastes with cells |
Exceptions to remember: the pulmonary artery carries oxygen-poor blood and the pulmonary vein carries oxygen-rich blood. The rule is about direction, not colour.
Blood pressure is the force blood exerts on vessel walls, measured with a sphygmomanometer. Normal is about 120/80 mm Hg (systolic/diastolic). Long-term high blood pressure (hypertension) can burst an artery.
Blood and lymph
Blood is a fluid connective tissue:
- Plasma: the liquid part; carries food, CO₂ (dissolved), salts, hormones and nitrogen wastes.
- Red blood cells: carry oxygen using haemoglobin.
- White blood cells: fight germs.
- Platelets: plug leaks and help blood clot at a cut, so we do not lose too much blood.
Lymph (tissue fluid): some plasma, proteins and blood cells leak out of capillaries into the spaces between cells. This colourless fluid, with less protein than plasma, drains into lymph vessels and finally back into large veins. Lymph carries digested fat absorbed from the intestine (via lacteals in villi), returns extra fluid to blood and carries lymphocytes that fight infection.
Exam tip: the heart diagram, double circulation, artery vs vein table, role of platelets and lymph are asked almost every year.
Key formulas and definitions
- Pulmonary: right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium
- Systemic: left ventricle → aorta → body → vena cava → right atrium
- Arteries: Away from heart; veins: back to heart; capillaries: exchange
- Blood = plasma + RBC + WBC + platelets
- Lymph = tissue fluid; carries absorbed fat, drains fluid back to blood
- Normal blood pressure ≈ 120/80 mm Hg (systolic/diastolic)
Worked examples
1. Why does the left ventricle have a thicker wall than the right ventricle?
The right ventricle only pumps blood to the nearby lungs. The left ventricle has to push blood through the aorta to the whole body, up to the head and down to the toes, so it needs much more muscle and force.
2. The pulmonary vein carries oxygen-rich blood. Then why is it called a vein?
Vessels are named by the direction of flow, not by the oxygen in them. The pulmonary vein brings blood back to the heart (left atrium), so it is a vein.
3. Why is separating right and left sides of the heart useful for mammals and birds?
It stops oxygen-rich and oxygen-poor blood from mixing, so every organ gets blood full of oxygen. These animals need lots of energy to keep their body warm, and a rich oxygen supply makes that possible.
4. Why do veins have valves but arteries mostly do not?
In arteries blood is under high pressure from the heartbeat and always flows forward. In veins pressure is low, and blood from the legs must rise against gravity; valves stop it from falling back.
5. A child with a cut stops bleeding in a few minutes, but a person with very few platelets keeps bleeding. Explain.
Platelets gather at the cut and start clotting, forming a plug that seals the leak. With too few platelets, clotting is slow and bleeding continues longer.
6. How is lymph different from blood?
Lymph has no red blood cells, so it is colourless, and it has less protein. It flows in lymph vessels in one direction only, towards the heart, and carries absorbed fat and infection-fighting lymphocytes.
Common mistakes
- Saying all arteries carry oxygen-rich blood. The pulmonary artery carries oxygen-poor blood.
- Drawing the heart as if it were your own: in diagrams the heart's right side is on the viewer's left.
- Writing that the atria have thicker walls. The ventricles, especially the left one, are the thickest.
- Thinking lymph is the same as plasma. Lymph has less protein, no RBCs and flows in separate vessels.