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Key Stage 4: Heart and circulatory system

Author: Pete Sewell

Circulatory system
Every cell in the body operates like a tiny factory. It takes in raw materials, processes them in some way, then gets rid of the waste products. The raw materials are delivered to the cells by the blood, which circulates around the body. For example, oxygen is carried in the red blood cells of the blood, where it is combined with haemoglobin to form oxyhaemoglobin. The products of digestion, too, are carried in the blood to the body cells. All the substances delivered to the cells are used in the numerous reactions that take place there, eg respiration, enzyme production and hormone production. The waste products of these reactions are then removed from the cells and carried away in the blood. Therefore the circulatory system acts as a transport system, taking substances to and from the body cells.

It is only larger and more complex organisms that require a circulatory system to transport substances around the body. The amoeba, which is a single-celled animal, does not require a transport system because it is able to exchange all the substances it needs directly across its thin cell membrane. This organism lives in water and is often found in the mud at the bottom of ponds. Food and dissolved oxygen pass from the water directly into the amoeba by a process called diffusion. Waste products are eliminated from the amoeba by the same process. The amoeba can do this because it:

  • has a thin permeable membrane
  • lives in an aqueous (water) environment
  • has a large surface area to volume ratio.

In the following pages you will learn about the human circulatory system.

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