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OCR GCSE Biology B · J257
OCR J257 Check the specification (PDF) (opens in a new tab)
Every living cell needs a supply of substances such as oxygen, water and molecules from food. Cells use some of these substances in respiration and to make new cell material. They also produce wastes, including carbon dioxide, which must be removed. A cell deep inside the body is too far from the lungs or digestive system to exchange these substances directly. The connects those organs to the cells: the heart pumps blood through blood vessels, and the blood carries substances between them.
Blood travels in two linked circuits. The right side of the heart pumps blood to the lungs. There it passes through tiny vessels called capillaries, which lie beside the air-filled alveoli. Oxygen diffuses from the air into the blood, while carbon dioxide diffuses from the blood into the air to be breathed out. Blood returns from the lungs to the left side of the heart. The left side pumps it around the rest of the body, where capillaries bring it close to cells. Blood then returns to the right side of the heart, and the journey repeats.
The blood links gas exchange, absorption from the digestive system and waste removal to the cells that need supplies.
The heart keeps blood moving; the blood does not carry substances straight from an alveolus to every cell without passing through the heart. Arteries take blood away from the heart and veins bring it towards the heart. Capillaries are the places where substances move between blood and its surroundings.
The digestive system breaks food down into small, soluble molecules. Water and dissolved food molecules are absorbed from the digestive system into blood in capillaries. Circulation then delivers them to cells throughout the body. For example, a cell can use glucose from food together with oxygen from the lungs in respiration.
As blood passes body cells, oxygen and useful dissolved molecules move from the blood towards the cells. Carbon dioxide produced by cells moves into the blood and is carried back to the lungs. Other waste substances also enter the blood. Urea, a waste made when excess amino acids are broken down, is carried in the blood to the kidneys. The kidneys filter urea from the blood into urine, which leaves the body. This is the circulatory system’s link with the excretory system. The lungs also remove a waste product—carbon dioxide—but their gas-exchange role is to bring oxygen into the blood as well as remove carbon dioxide.
The lungs, digestive system and kidneys are not arranged one after another along a single blood vessel. Each exchanges substances with blood flowing through its own vessels. In this way the circulation links the body’s exchange and excretory organs with cells that need supplies and produce wastes.
Heart + blood vessels + blood transport substances between organs and cells. Right heart → lungs → left heart → body → right heart. Arteries carry blood away from the heart; veins return it. Exchange occurs at capillaries.
When describing a substance’s journey, name where it enters or leaves the blood and where the blood carries it next.
Do not describe the gut, kidneys and other body tissues as stops in one long sequence: each receives blood through the body’s circulation.
Remember that arteries carry blood away from the heart and veins carry it towards the heart; their names do not tell you how much oxygen the blood contains.
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Circulatory system
The organ system made up of the heart, blood vessels and blood, which transports substances around the body.
Capillary
A tiny blood vessel where substances can move between the blood and surrounding tissues or air in the lungs.
Alveolus
The tiny air sac in a lung where oxygen enters the blood and carbon dioxide leaves it.
Absorption
The movement of digested, dissolved food molecules and water from the digestive system into the blood.
Excretion
The removal of waste products made by the body’s cells.
Put your knowledge into practice — try past paper questions for Biology B
Circulatory system
The organ system made up of the heart, blood vessels and blood, which transports substances around the body.
Capillary
A tiny blood vessel where substances can move between the blood and surrounding tissues or air in the lungs.
Alveolus
The tiny air sac in a lung where oxygen enters the blood and carbon dioxide leaves it.
Absorption
The movement of digested, dissolved food molecules and water from the digestive system into the blood.
Excretion
The removal of waste products made by the body’s cells.