How does blood carry substances around the body, and how is the system adapted for its job?
Describe the role of the heart, blood vessels and blood in transport, compare arteries, veins and capillaries, and state the functions of the main blood components
A focused N(A)-Level answer on transport. The heart and blood vessels, how arteries, veins and capillaries differ, and the jobs of red cells, white cells, plasma and platelets.
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What this dot point is asking
SEAB wants you to describe how the heart, blood vessels and blood work together to transport substances, to compare arteries, veins and capillaries, and to state the functions of the main parts of the blood. The central idea is that the circulatory system delivers oxygen and food to every cell and carries waste away.
The answer
The heart and circulation
The heart is a muscular pump that pushes blood around the body through blood vessels. Blood carries:
- oxygen from the lungs and glucose from the gut to the body cells,
- carbon dioxide and other wastes away from the cells.
The heart pumps continuously, keeping the blood moving in one direction.
The three blood vessels
There are three main types of blood vessel, each suited to its job:
- arteries: carry blood away from the heart at high pressure. They have thick, muscular, elastic walls to withstand the pressure.
- veins: carry blood back to the heart at low pressure. They have thinner walls and contain valves to stop the blood flowing backwards.
- capillaries: tiny vessels that link arteries to veins. They have walls just one cell thick so substances can pass quickly between the blood and the body cells.
What blood is made of
Blood has four main parts:
- red blood cells: carry oxygen using a red pigment called haemoglobin,
- white blood cells: defend the body against germs (disease-causing microbes),
- plasma: the liquid part that carries dissolved substances such as glucose, carbon dioxide and wastes,
- platelets: help the blood to clot at a wound.
Adaptations of the red blood cell
Red blood cells are well adapted to carry oxygen: they contain haemoglobin that binds oxygen, they have a biconcave (dimpled disc) shape that gives a large surface area, and they have no nucleus, leaving more room for haemoglobin.
Examples in context
Example 1. Why a deep cut from an artery is more serious. Blood in an artery is at high pressure, so a cut artery bleeds rapidly in spurts, while a cut vein bleeds at low pressure. This is why arterial bleeding needs firm, fast pressure to control it.
Example 2. How exercise increases oxygen delivery. During exercise the heart beats faster and harder, pumping more blood each minute. This delivers oxygen and glucose to the working muscles more quickly and removes carbon dioxide faster, matching the higher demand of the active cells.
Try this
- Cue. State the function of plasma. The liquid part of blood that carries dissolved substances such as glucose, carbon dioxide and wastes.
- Cue. Explain why veins have valves. To stop the low-pressure blood flowing backwards.
- Cue. Name the part of the blood that fights disease-causing microbes. White blood cells.
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 marks(a) State the function of red blood cells. (b) Describe two ways a red blood cell is adapted to carry out this function.Show worked answer →
(a) Red blood cells carry oxygen around the body.
(b) Any two of: they contain haemoglobin, which binds oxygen; they have a biconcave (dimpled disc) shape that gives a large surface area for absorbing oxygen; they have no nucleus, leaving more room for haemoglobin.
What markers reward: carrying oxygen, and two correct adaptations such as haemoglobin, biconcave shape, and no nucleus.
Original3 marks(a) State one difference between an artery and a vein. (b) Explain why capillaries have very thin walls.Show worked answer →
(a) Arteries carry blood away from the heart at high pressure and have thick muscular walls; veins carry blood back to the heart at low pressure, have thinner walls, and contain valves.
(b) Capillaries have very thin walls (one cell thick) so that substances such as oxygen and glucose can pass quickly between the blood and the body cells.
What markers reward: a correct artery and vein difference, and thin capillary walls allowing fast exchange of substances.
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