Diffusion: (in animals) having an efficient blood supply

The idea

In animals, exchange surfaces such as alveoli in the lungs and villi in the small intestine are adapted to maximise diffusion. An efficient blood supply helps maintain a steep concentration gradient between the exchange surface and the blood. As blood flows through the capillaries, it carries away the substance that has diffused in, such as oxygen or glucose, and brings a fresh supply of blood with a lower concentration of that substance. This constant removal and replacement keeps the concentration difference large, so diffusion continues rapidly. For example, oxygen diffuses from the alveolus into the blood, and the blood is carried away by the circulation, keeping the oxygen concentration in the capillary low. A rich capillary network therefore supports fast, continuous exchange.

What earns the marks

• An efficient blood supply means a rich network of capillaries close to the exchange surface.
• Blood flow removes the substance that has diffused into the blood, helping to keep its concentration low.
• Blood flow brings a fresh supply of blood, helping to maintain a concentration difference across the exchange surface.
• Maintaining a steep concentration gradient increases the rate of diffusion.
• In alveoli, oxygen diffuses into the blood and is carried away, while carbon dioxide diffuses from the blood into the alveolus.
• In villi, blood carries absorbed glucose and amino acids away, helping to maintain a steep concentration gradient for further absorption.

Common mistakes and how to put them right

• Saying that blood supply provides energy for diffusion.

  Put it right: Diffusion is passive and the blood supply maintains a concentration gradient.

• Thinking that blood flow increases the concentration of the substance in the blood.

  Put it right: Blood flow removes the diffused substance and keeps its concentration low.

• Writing that an efficient blood supply means more blood cells.

  Put it right: Efficiency refers to a good capillary network and continuous flow that maintains the gradient.

Exam tips

• Always link an efficient blood supply to maintaining a steep concentration gradient, because that is the reason diffusion stays fast.
• Name the substance being transported, such as oxygen in the lungs or glucose in the small intestine, to make your explanation precise.
• Use the phrase concentration gradient correctly: a steep gradient means a large difference in concentration between two regions.