International A Level Biology / Mammalian transport systems

Why many animals need a heart and circulation

Edexcel International A Level Biology (YBI11), chapter 2: Mammalian transport systems, specification point 1.6. Free lesson.

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Large, active animals need a circulation because diffusion cannot supply deep cells quickly enough to meet their oxygen demand. A heart drives mass transport, and a double circulation pumps blood again after it passes through the lungs.

How size changes the surface area to volume ratio

As an organism gets bigger, its surface area to volume ratio gets smaller. Surface area grows with the square of length, but volume grows with its cube. This means a larger body has less surface area available for each unit of volume inside it. A cube has six square faces, so its surface area is 6 × side². Its volume is side³. Write the ratio in its simplest form, with the second number as 1.

Why diffusion cannot supply a large animal

A tiny single-celled organism, such as an amoeba, can take in oxygen by diffusion across its surface. Its cells are very close to that surface. In a large mammal, the deepest cells may be many centimetres away, and diffusion is far too slow over that distance. Mammals also have a high oxygen demand because they are active and keep warm. Diffusion is fast over very short distances, but slow over long distances. A large mammal's cells are far from the body surface and use oxygen quickly. For the question 'Why is diffusion too slow?', focus on distance. A small surface area to volume ratio…

What mass transport means

Mass transport is the bulk movement of a fluid, such as blood, carrying substances from one part of the body to another. It is driven by a pressure difference, created by the heart. Blood can travel close to cells in capillaries, so diffusion only has to cover the final short distance. For a precise definition, include both bulk movement and the pressure difference that drives it.

How single and double circulation differ

In a single circulation, blood passes through the heart once per circuit. A fish heart has one atrium and one ventricle. It pumps blood to the gill capillaries, where blood loses much of its pressure, then the blood continues to the body at low pressure. A mammal has a four-chambered heart with separate left and right sides. The right side pumps blood to the lung capillaries at lower pressure, protecting these delicate vessels. Blood returns to the heart and the left side pumps it again. It then reaches the body capillaries at high pressure. In a mammal's double circulation, blood passes…

Why double circulation suits active mammals

Blood loses pressure as it passes through the lung capillaries. Returning to the heart allows it to be pumped again, so it reaches the body at high pressure. This delivers oxygen and glucose quickly enough for the high rate of respiration in a large, active mammal. The pressure is lower in the lung capillaries, then raised again by the heart before blood goes to the body.

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