The heart and blood vessels: capillaries.

The idea

Capillaries are the smallest blood vessels and form networks that link arteries to veins. Their walls are one cell thick, so substances diffuse over a very short distance between blood and body cells. Capillaries have a narrow lumen, only about the width of a red blood cell, so blood flows slowly and red blood cells are squeezed close to the wall, shortening the diffusion path. Their total surface area is very large, and they penetrate almost every tissue, which speeds up exchange of oxygen, carbon dioxide, glucose, amino acids, urea and other substances. Blood entering capillary networks is at higher pressure in arteries and lower pressure in veins, and plasma loss at the arterial end helps deliver dissolved materials to tissue fluid.

What earns the marks

• Capillaries link arteries to veins and form extensive networks that reach almost all body tissues.
• Capillary walls are one cell thick, giving a short diffusion distance between blood and body cells.
• The lumen is narrow, roughly the width of a red blood cell, so blood flows slowly and cells pass close to the wall.
• Capillaries have a very large total surface area, which increases the rate of diffusion of substances.
• Substances exchanged include oxygen and glucose moving into cells and carbon dioxide and urea moving out of cells into the blood.
• The thin, permeable wall allows plasma and dissolved substances to pass into tissue fluid at the arterial end of the network.

Common mistakes and how to put them right

• Stating that capillaries have thick muscular walls.

  Put it right by describing walls that are one cell thick with no muscle or elastic tissue.

• Saying capillaries carry blood at high pressure like arteries.

  Put it right by explaining that pressure falls as blood passes through capillary networks.

• Describing capillaries as carrying only oxygen.

  Put it right by listing carbon dioxide, glucose, amino acids, urea and other dissolved substances that also move in or out.

Exam tips

• Always link each capillary feature to exchange, for example one-cell-thick wall and short diffusion distance.
• Use comparative language such as thinner, narrower and larger total surface area when contrasting capillaries with arteries and veins.
• Name the direction of movement of named substances, for example oxygen diffuses from blood into cells and carbon dioxide diffuses from cells into blood.