Cambridge O Level Biology 5090: Transport in humans (5090) – Revision Notes

For full explanations and worked examples, use the study guide for this topic. These notes condense Topic 11, Transport in humans, of the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028: every learning outcome in sections 11.1 to 11.4, including the heart-rate investigation. The syllabus is not tiered, so all of it applies to every candidate.

When you are ready, move on to the practice questions. The course hub and the printable checklist cover the rest of the course, and the free 10-minute diagnostics show where to focus. Oxygen loading in the lungs is revised in Human gas exchange.

Key definitions

Term	Meaning
Circulatory system	Blood vessels with a pump (heart) and valves that ensure one-way flow of blood
Double circulation	Blood passes through the heart twice for each complete circuit
Artery	Vessel carrying blood away from the heart
Vein	Vessel returning blood to the heart
Capillary	Vessel with a wall one cell thick, where exchange with tissues happens
Coronary heart disease (CHD)	Blockage of the coronary arteries, so heart muscle gets too little oxygen
Tissue fluid	Fluid from plasma that leaves capillaries and bathes body cells

11.1 Circulation in one line each

• Pulmonary circuit: right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium. Low pressure.
• Systemic circuit: left ventricle → aorta → body organs → vena cava → right atrium. High pressure.
• Why both? Low pressure protects the lungs; high pressure delivers blood quickly to all body tissues.

11.2 The heart

Labels to know

muscular wall · septum · right atrium · left atrium · right ventricle · left ventricle · atrioventricular valves · semilunar valves · coronary arteries

• Septum: separates left and right sides, so oxygenated and deoxygenated blood do not mix.
• Atrioventricular valves: between atria and ventricles.
• Semilunar valves: where the aorta and pulmonary artery leave the ventricles.
• Coronary arteries: supply the heart muscle itself.

Wall thickness: the two comparisons

Comparison	Thicker	Reason
Left vs right ventricle	Left	Pumps blood round the whole body at high pressure; right pumps only to the nearby lungs
Atria vs ventricles	Ventricles	Pump blood out of the heart into arteries; atria push blood only into the ventricles

Method in steps: one heartbeat

1. Atria contract → blood through open AV valves into ventricles.
2. Ventricles contract → AV valves close (stops backflow to atria).
3. Semilunar valves open → blood into aorta and pulmonary artery.
4. Ventricles relax → semilunar valves close (stops backflow to ventricles).
5. Atria refill from the veins.

Rule: a valve opens when pressure behind it is higher, and closes when pressure in front is higher.

Monitoring heart activity

• ECG: trace of electrical activity; one repeating pattern = one beat.
• Pulse rate: beats per minute, at wrist or neck.
• Valve sounds: heard with a stethoscope; first sound = AV valves closing, second = semilunar valves closing.

Convert	Do this
Pulse count over t seconds → beats per minute	count × (60 ÷ t)
ECG time per beat (s) → beats per minute	60 ÷ time per beat
ECG distance → time	distance ÷ paper speed

Heart rate and physical activity

• Investigate: resting pulse (repeat, mean) → fixed exercise for fixed time → pulse at once and each minute until back to resting. Control person, exercise, duration, counting time.
• Explain: muscles respire faster → need more oxygen and glucose, make more carbon dioxide → heart beats faster (and harder) → faster delivery and removal.

Coronary heart disease

• What: coronary artery blocked (fatty deposits, clot) → heart muscle short of oxygen and glucose → cannot respire and contract → may die (heart attack).
• Risk factors: diet, sedentary lifestyle, stress, smoking, genetic predisposition, age, gender.
• Diet: less saturated fat, cholesterol and salt; avoid excess energy intake.
• Exercise: strengthens heart muscle; helps control body mass, blood pressure and cholesterol.
• Limit: genes, age and gender cannot be changed.

11.3 Blood vessels

The nine named vessels

Vessel	From → to
Aorta	Left ventricle → body
Vena cava	Body → right atrium
Pulmonary artery	Right ventricle → lungs
Pulmonary vein	Lungs → left atrium
Hepatic artery	Aorta → liver
Hepatic portal vein	Small intestine → liver
Hepatic vein	Liver → vena cava
Renal artery	Aorta → kidney
Renal vein	Kidney → vena cava

Must-know distinctions: artery, vein, capillary

	Artery	Vein	Capillary
Blood pressure	High	Low	Lower than arteries, falling along the capillary
Wall	Thick	Thin	One cell thick
Muscle and elastic tissue	Much	Little	None
Lumen	Narrow	Wide	Very narrow
Valves	No	Yes	No
Structure linked to pressure	Thick, elastic wall withstands and evens out surges	Wide lumen eases flow; valves stop backflow	Thin wall gives short diffusion distance

11.4 Blood

Components and functions

Component	Function
Red blood cells	Oxygen transport
Lymphocytes	Produce antibodies
Phagocytes	Engulf pathogens
Platelets	Clotting: soluble fibrinogen → insoluble fibrin; stops blood loss and entry of pathogens
Plasma	Transports blood cells, ions, glucose, amino acids, hormones, carbon dioxide, urea, vitamins, plasma proteins

Spotting cells on a slide

• Red cell: no nucleus, pale centre, very many.
• Lymphocyte: big round nucleus, thin rim of cytoplasm.
• Phagocyte: lobed nucleus, lots of cytoplasm, largest.

Magnification: actual size = image size ÷ magnification (convert mm to µm by × 1000 first).

Tissue fluid in four steps

1. Pressure forces plasma out of capillaries → tissue fluid around cells.
2. Oxygen, glucose, amino acids: blood → tissue fluid → cells (diffusion).
3. Carbon dioxide, urea: cells → tissue fluid → blood.
4. Much of the fluid returns to the capillaries.

More must-know distinctions

Pair	The difference
Pulmonary artery vs pulmonary vein	Artery: right ventricle to lungs, deoxygenated blood. Vein: lungs to left atrium, oxygenated blood
Hepatic portal vein vs hepatic vein	Portal vein: gut to liver, rich in absorbed food. Hepatic vein: liver to vena cava
AV valves vs semilunar valves	AV: close when ventricles contract. Semilunar: open when ventricles contract, close when they relax
Lymphocyte vs phagocyte	Lymphocyte: round nucleus, makes antibodies. Phagocyte: lobed nucleus, engulfs pathogens
Plasma vs tissue fluid	Plasma: liquid part of blood, inside vessels. Tissue fluid: plasma that has left capillaries, around cells, with few proteins and no red cells
Fibrinogen vs fibrin	Fibrinogen: soluble, always in plasma. Fibrin: insoluble threads of the clot

Small worked reminders

Pulse to heart rate. 26 beats in 20 s → 26 × (60 ÷ 20) = 78 beats per minute. Always convert to per minute before comparing.

Percentage change. Resting 78, after exercise 117: (117 − 78) ÷ 78 × 100 = 50%. Divide by the starting (resting) value.

ECG. Peaks 15 mm apart at 25 mm per second: 15 ÷ 25 = 0.6 s per beat → 60 ÷ 0.6 = 100 beats per minute.

Recovery time. Read the first time at which the rate equals the resting value again, counting from the end of the exercise.

Explaining in a chain. For “explain” answers, link each step with “so”: more exercise → faster respiration in muscle → more oxygen and glucose needed → heart beats faster → more blood delivered per minute.

Quick self-test

1. Define a double circulation.
2. Which side of the heart pumps blood to the lungs?
3. Name the valves that stop blood flowing back from the aorta into the ventricle.
4. Give the reason the right ventricle wall is thinner than the left.
5. A pulse count is 21 beats in 15 s. Calculate the heart rate in beats per minute.
6. One beat on an ECG lasts 0.6 s. Calculate the heart rate.
7. Name the vessel that carries absorbed glucose from the small intestine to the liver.
8. Give two structural features of a vein linked to low blood pressure.
9. Name the soluble protein converted during clotting, and its product.
10. An image of a white blood cell is 24 mm wide; the cell is 12 µm wide. Calculate the magnification.
11. Name the white blood cell with a lobed nucleus, and give its function.
12. Name three of the risk factors for CHD that diet and exercise cannot change.

Answers

1. Blood passes through the heart twice for each complete circuit of the body.
2. The right side.
3. Semilunar valves.
4. It pumps blood only to the lungs, which are close, at low pressure.
5. 21 × (60 ÷ 15) = 84 beats per minute.
6. 60 ÷ 0.6 = 100 beats per minute.
7. Hepatic portal vein.
8. Wide lumen; valves (also: thin wall with little muscle).
9. Fibrinogen → fibrin.
10. 24 mm = 24 000 µm; 24 000 ÷ 12 = ×2000.
11. Phagocyte; engulfs pathogens.
12. Genetic predisposition, age, gender.

Where marks are usually lost

• Defining double circulation as “two circuits” without saying blood passes through the heart twice per complete circuit.
• Stating that the left ventricle is thicker “because it pumps more blood” instead of “to a higher pressure, round the whole body”.
• Putting events of a heartbeat in the wrong order, or opening the semilunar valves before the AV valves close.
• Writing that arteries carry oxygenated blood and veins deoxygenated blood, which the pulmonary vessels contradict.
• Giving a pulse rate per 15 s or per 30 s and calling it the heart rate.
• Confusing the hepatic portal vein with the hepatic vein.
• Saying capillaries have thin walls “so blood can pass through”. Substances diffuse through; blood cells stay inside.
• Writing that platelets “clot the blood” without fibrinogen and fibrin, or saying fibrin is soluble.
• Listing CHD risk factors with no link to blocked coronary arteries and starved heart muscle.

Official syllabus

Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), Topic 11: Transport in humans, published by Cambridge Assessment International Education.