How does the heart and blood system carry oxygen, food and wastes around the body?
Describe the human circulatory system including the structure of the heart, the blood vessels and the components of blood and their functions
A focused answer to the O-Level Combined Science outcome on human transport. The double circulation and the heart, the structure and function of arteries, veins and capillaries, and the components of blood.
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What this dot point is asking
SEAB wants you to describe the human circulatory system: the double circulation and the heart's structure, the three types of blood vessel and how each suits its job, and the components of blood and their functions. Labelled diagrams of the heart and the artery-vein-capillary comparison are common exam tasks.
The answer
A double circulation
Humans have a double circulation: blood passes through the heart twice for each complete circuit. One loop carries blood from the heart to the lungs and back (to pick up oxygen); the other carries blood from the heart to the rest of the body and back. This keeps oxygenated and deoxygenated blood separate and delivers oxygen at high pressure.
The heart
The heart is a muscular pump with four chambers: two upper atria and two lower ventricles. The right side pumps deoxygenated blood to the lungs; the left side pumps oxygenated blood to the body. The left ventricle has a thicker, more muscular wall because it must pump blood at high pressure all around the body. Valves between the chambers and in the vessels stop blood flowing backwards.
The blood vessels
There are three types, each suited to its role:
- Arteries carry blood away from the heart at high pressure; they have thick, muscular, elastic walls and a narrow lumen.
- Veins carry blood back to the heart at low pressure; they have thinner walls, a wide lumen and valves to prevent backflow.
- Capillaries are tiny vessels with walls one cell thick; they allow the exchange of substances (oxygen, glucose, carbon dioxide, wastes) between blood and the body's cells.
The components of blood
Blood is made of:
- Red blood cells: carry oxygen using haemoglobin, which forms oxyhaemoglobin; they have no nucleus and a biconcave shape to carry more oxygen.
- White blood cells: defend the body against disease (by engulfing pathogens or making antibodies).
- Platelets: help the blood to clot at a wound.
- Plasma: the liquid that carries cells, dissolved food (glucose, amino acids), carbon dioxide, urea and other substances.
Examples in context
Example 1. Why a heart attack is dangerous. The heart muscle itself is supplied by coronary arteries; if one is blocked, that part of the muscle is starved of oxygen and can die. Understanding that arteries deliver oxygenated blood at high pressure explains why a blockage in these vessels is so serious.
Example 2. Anaemia and tiredness. A person with too few red blood cells or too little haemoglobin (anaemia) cannot carry enough oxygen, so they feel tired and breathless. This connects the function of red blood cells directly to how the body feels when transport of oxygen is reduced.
Try this
Q1. State the function of red blood cells and the substance they use to do it. [2 marks]
- Cue. Red blood cells transport oxygen using haemoglobin (which forms oxyhaemoglobin).
Q2. Explain why the wall of the left ventricle is thicker than the wall of the right ventricle. [2 marks]
- Cue. The left ventricle pumps blood at high pressure all around the body, so it needs more muscle; the right ventricle only pumps to the nearby lungs.
Q3. Name the type of blood vessel that allows exchange of substances with the cells and state one feature that suits this. [2 marks]
- Cue. Capillaries; their walls are one cell thick, giving a short diffusion distance for exchange.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksCompare the structure of an artery with the structure of a vein, giving two structural differences, and explain how each difference is related to its function.Show worked answer →
Difference 1: an artery has a thick, muscular and elastic wall, while a vein has a thinner wall. The thick elastic wall of the artery withstands and smooths the high pressure of blood pumped from the heart; the vein carries blood at low pressure so does not need thick walls.
Difference 2: a vein has valves along its length, while an artery (away from the heart) does not. The valves in veins prevent the backflow of the low-pressure blood, keeping it moving towards the heart.
(Other valid point: arteries have a narrower lumen than veins.)
Markers reward two correct structural differences, each linked to function (thick elastic artery wall for high pressure; valves in veins to prevent backflow at low pressure).
Original4 marksBlood is made of several components. Name the component that transports oxygen and describe how it is adapted for this role, and name the component that defends the body against disease.Show worked answer →
The component that transports oxygen is the red blood cell. It is adapted by containing the red pigment haemoglobin, which combines with oxygen to form oxyhaemoglobin. It has no nucleus, leaving more room for haemoglobin, and a biconcave shape giving a large surface area for absorbing oxygen.
The component that defends the body against disease is the white blood cell (for example by engulfing pathogens or producing antibodies).
Markers reward red blood cells with haemoglobin (and adaptations such as no nucleus / biconcave shape) for oxygen transport, and white blood cells for defence against disease.
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