Cambridge O Level Biology 5090: Transport in humans (5090)

This study guide teaches Topic 11, Transport in humans, for Cambridge O Level Biology 5090. It is aligned to the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028, and covers every learning outcome in sections 11.1 to 11.4: the circulatory system, the heart, blood vessels and blood, including the heart-rate investigation in 11.2. The 5090 syllabus is not tiered, so every candidate studies all of it.

Use it with the revision notes for this topic and the practice questions. The course hub lists every topic and the printable checklist lets you tick off each outcome. Blood picks up oxygen in the lungs, covered in Human gas exchange, and absorbed food in the small intestine, covered in Human nutrition. To find your weak spots quickly, try one of the free 10-minute diagnostics.

What this topic covers

Syllabus ref	What you must be able to do
11.1.1–11.1.3	Describe the circulatory system and a double circulation; explain its low and high pressure circuits
11.2.1	Identify the muscular wall, septum, atria, ventricles, atrioventricular and semilunar valves and coronary arteries
11.2.2	Explain the relative thickness of the ventricle walls, and of atria compared with ventricles
11.2.3	Describe a heartbeat in terms of muscle contraction and valve action
11.2.4–11.2.5	State that arteries carry blood away and veins return it; state how heart activity is monitored
11.2.6	Investigate and explain the effect of physical activity on heart rate
11.2.7–11.2.8	Describe coronary heart disease and its risk factors; discuss diet and exercise in reducing risk
11.3.1–11.3.3	Name the main vessels; describe and identify arteries, veins and capillaries; relate structure to pressure
11.4.1–11.4.4	Identify blood cells; list blood components and their functions; describe exchange via tissue fluid

11.1 The circulatory system

The circulatory system is a system of blood vessels with a pump (the heart) and valves that make sure blood flows one way only.

Humans have a double circulation: blood passes through the heart twice for each complete circuit of the body.

• Pulmonary circulation: right side of the heart → lungs → left side of the heart.
• Systemic circulation: left side of the heart → body tissues → right side of the heart.

Why two circuits? The double circulation gives a low pressure circulation to the lungs and a high pressure circulation to the body tissues. Low pressure suits the delicate lung capillaries. High pressure pushes blood quickly round the long circuit to all the body organs, so oxygen and glucose reach respiring cells fast. Because blood returns to the heart after the lungs, it can be pumped out again at high pressure.

Worked example 1. Trace a red blood cell from the vena cava back to the vena cava.

vena cava → right atrium → right ventricle → pulmonary artery → lungs (collects oxygen) → pulmonary vein → left atrium → left ventricle → aorta → body organ (gives up oxygen) → vena cava

It passes through the heart twice.

11.2 The heart

Structures to identify (11.2.1)

Structure	What it is and does
Muscular wall	Cardiac muscle that contracts to pump blood
Septum	Wall of muscle dividing the left and right sides; keeps oxygenated and deoxygenated blood apart
Right and left atria	Upper, thin-walled chambers that receive blood from veins
Right and left ventricles	Lower, thick-walled chambers that pump blood into arteries
Atrioventricular valves	Between each atrium and its ventricle; stop backflow into the atria
Semilunar valves	At the bases of the aorta and pulmonary artery; stop backflow into the ventricles
Coronary arteries	Vessels on the outer surface of the heart, branching from the aorta, that supply the heart muscle itself with oxygen and glucose

On a front-view diagram, the left side of the heart is on the right of the page.

Wall thickness (11.2.2)

• Left ventricle wall thicker than right ventricle wall. The left ventricle pumps blood all round the body at high pressure; the right ventricle pumps blood only to the nearby lungs at low pressure. More muscle gives a stronger contraction.
• Atria walls thinner than ventricle walls. Atria only push blood a short distance, down into the ventricles. Ventricles must pump blood out of the heart into the arteries.

One heartbeat (11.2.3)

1. Atria contract. Blood is pushed through the open atrioventricular valves into the ventricles.
2. Ventricles contract. Pressure in the ventricles rises above that in the atria, so the atrioventricular valves close, preventing backflow into the atria.
3. Pressure rises above that in the arteries, so the semilunar valves open and blood is pushed into the aorta and pulmonary artery.
4. Ventricles relax. Pressure in the ventricles falls below that in the arteries, so the semilunar valves close, preventing backflow into the ventricles.
5. Blood from the veins fills the atria again, and the cycle repeats.

Valves open and close because of pressure differences; they are not muscles. Both atria contract together, then both ventricles.

Arteries and veins (11.2.4)

Blood is pumped away from the heart in arteries and returns to the heart in veins. This is about direction, not oxygen: the pulmonary artery carries deoxygenated blood.

Monitoring the heart (11.2.5)

The activity of the heart can be monitored by:

• an electrocardiogram (ECG), a trace of the electrical activity of the heart; each repeating pattern is one heartbeat
• pulse rate, counted at the wrist or neck, which gives beats per minute
• listening to the sounds of the valves closing, using a stethoscope; the two sounds of each beat are the atrioventricular valves closing, then the semilunar valves closing.

Worked example 2 (ECG). An ECG is printed at 25 mm per second. The peaks of two neighbouring beats are 20 mm apart. Calculate the heart rate.

• Time for one beat = 20 ÷ 25 = 0.8 s.
• Heart rate = 60 ÷ 0.8 = 75 beats per minute.

Physical activity and heart rate (11.2.6)

Method. Rest, then count the pulse for 15 s; repeat and take a mean. Do a fixed exercise (for example, step-ups) for a fixed time. Count the pulse immediately, then every minute until it returns to resting. Keep the person, exercise and counting time the same, and repeat.

Worked example 3. A student counts 19 beats in 15 s at rest and 33 beats in 15 s straight after exercise.

• Resting rate = 19 × (60 ÷ 15) = 76 beats per minute.
• After exercise = 33 × 4 = 132 beats per minute.
• Increase = 132 − 76 = 56; percentage increase = 56 ÷ 76 × 100 = 73.7%.

Explanation. Contracting muscles respire faster to release more energy. They need more oxygen and glucose, and produce more carbon dioxide. The heart beats faster (and more strongly) so blood flows faster to the muscles, delivering oxygen and glucose and removing carbon dioxide. The rate stays high after exercise, partly to carry lactic acid to the liver (Topic 10).

Coronary heart disease (11.2.7)

In coronary heart disease (CHD), a coronary artery becomes blocked, for example by fatty deposits in its wall or a blood clot. Less blood reaches part of the heart muscle, so it receives too little oxygen and glucose. That muscle cannot respire aerobically or contract properly and may die: a heart attack.

Possible risk factors (the syllabus list):

Risk factor	Notes
Diet	High in saturated (animal) fat and cholesterol, which add to fatty deposits; high in salt
Sedentary lifestyle	Little exercise
Stress	Long-term stress
Smoking	Chemicals in tobacco smoke damage artery walls and raise the risk of clots
Genetic predisposition	CHD in close relatives
Age	Risk rises with age
Gender	On average, men are affected at younger ages than women

Diet and exercise in reducing risk (11.2.8)

• Diet: eating less saturated fat and cholesterol reduces fatty deposits in the coronary arteries; less salt helps keep blood pressure down; a balanced energy intake avoids obesity, which adds to the risk.
• Exercise: regular exercise strengthens the heart muscle, helps control body mass and blood pressure, and helps keep blood cholesterol lower.
• Balance: diet and exercise reduce risk but cannot remove it; genetic predisposition, age and gender cannot be changed.

11.3 Blood vessels

The main vessels (11.3.1)

Vessel	Carries blood
Aorta	From the left ventricle to the body
Vena cava	From the body to the right atrium
Pulmonary artery	From the right ventricle to the lungs
Pulmonary vein	From the lungs to the left atrium
Hepatic artery	From the aorta to the liver
Hepatic portal vein	From the small intestine (gut) to the liver, carrying absorbed food
Hepatic vein	From the liver to the vena cava
Renal artery	From the aorta to a kidney
Renal vein	From a kidney to the vena cava

“Hepatic” means liver; “renal” means kidney.

Structure (11.3.2)

Feature	Artery	Vein	Capillary
Thickness of wall	Thick	Thin	One cell thick
Muscle and elastic tissue	Lots	Little	None
Diameter of lumen	Narrow	Wide	Very narrow (red cells pass in single file)
Valves	None (except at the heart)	Present	None

In a photomicrograph, an artery has a thick wall and small, round lumen; a vein has a thin wall and a large, often irregular lumen.

Structure and pressure (11.3.3)

• Arteries carry blood at high pressure in surges. The thick wall withstands the pressure; elastic tissue stretches with each surge and recoils, keeping blood moving; muscle helps keep the wall strong.
• Veins carry blood at low pressure. A thick wall is not needed. The wide lumen reduces resistance to flow, and valves prevent backflow.
• Capillaries carry blood at lower pressure than arteries. The wall one cell thick gives a short diffusion distance for exchange with tissues. The narrow lumen slows blood, allowing more time for exchange.

11.4 Blood

Components and functions (11.4.2 and 11.4.3)

Component	Function
Red blood cells	Oxygen transport
White blood cells: lymphocytes	Antibody production
White blood cells: phagocytes	Engulfing pathogens
Platelets	Clotting: soluble fibrinogen is converted to insoluble fibrin, forming a mesh that prevents blood loss and the entry of pathogens
Plasma	Transport of blood cells, ions, glucose, amino acids, hormones, carbon dioxide, urea, vitamins and plasma proteins

Identifying cells (11.4.1)

Cell	Appearance under the light microscope
Red blood cell	Small, round, pale centre, no nucleus; by far the most numerous
Lymphocyte	Larger than a red cell; large, round nucleus filling most of the cell, with a thin rim of cytoplasm
Phagocyte	Larger still; lobed or kidney-shaped nucleus with plenty of cytoplasm

Platelets appear as tiny fragments. See also Cell structure and organisation.

Worked example 4. On a photomicrograph at ×500, a red blood cell measures 3.5 mm across. Calculate its actual diameter in µm.

• Convert: 3.5 mm = 3500 µm.
• Actual size = image size ÷ magnification = 3500 ÷ 500 = 7 µm.

Exchange through tissue fluid (11.4.4)

1. Blood pressure at the arterial end of a capillary forces some plasma out through the capillary wall. This fluid, without the red blood cells and most plasma proteins, is tissue fluid, and it bathes every body cell.
2. Oxygen, glucose and amino acids diffuse from the blood into the tissue fluid, then from the tissue fluid into the cells.
3. Carbon dioxide and urea made by the cells diffuse the other way: cell → tissue fluid → blood.
4. Much of the tissue fluid passes back into the capillaries.

Tissue fluid is the go-between for blood and cells.

Common errors

• Writing “the heart pumps blood twice” for double circulation. Blood passes through the heart twice per complete circuit.
• Saying arteries carry oxygenated blood. They carry blood away from the heart.
• Explaining the thick left ventricle by “it pumps more blood”. Both ventricles pump the same volume; the left pumps at higher pressure over a longer circuit.
• Mixing up the hepatic portal vein (gut to liver) and the hepatic vein (liver to vena cava).
• Saying platelets “make fibrin” with no mention of fibrinogen, or calling fibrin soluble.
• Giving a 15 s pulse count instead of beats per minute.

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.