The structure of capillaries and the importance of capillary beds as…

The idea

A capillary wall is a single endothelial cell thick, giving a very short diffusion pathway, and there are gaps between those cells through which fluid can pass. A capillary bed is a dense branching network, so the total surface area is enormous and the total cross-sectional area is large, which slows the blood and allows time for exchange. At the arteriole end, the hydrostatic pressure of the blood is high enough to force water and small solutes such as glucose, amino acids and ions out through the gaps: this is tissue fluid. Plasma proteins are too large to leave, so they remain in the capillary and keep its water potential low, that is more negative. Along the capillary the hydrostatic pressure falls, so at the venule end water re-enters by osmosis down the water potential gradient. Excess tissue fluid drains into the lymphatic system and returns to the blood.

What earns the marks

• High hydrostatic pressure at the arteriole end forcing water and small solutes out of the capillary.
• Plasma proteins remaining in the capillary, so the water potential of the blood is lower than that of the tissue fluid.
• Water returning at the venule end by osmosis down a water potential gradient.
• Excess tissue fluid draining into the lymphatic system and returning to the blood.
• Explaining that a fall in plasma protein raises the water potential of the blood, so less water returns by osmosis and tissue fluid accumulates.

Common mistakes and how to put them right

• Reversing the mechanism, so tissue fluid is said to form by osmosis and return by hydrostatic pressure.

• Saying red blood cells and plasma proteins leave the capillary to form tissue fluid.

• Saying water potential becomes more or less without saying higher or lower, so the direction of the gradient is unclear.

• Leaving out the lymphatic system, so the fluid that leaves the capillary has nowhere to go.

• Using the abbreviation WP without defining it.

  Put it right: Write water potential in full or use the symbol.

Exam tips

• Answer in a fixed order: hydrostatic pressure out, water potential and osmosis in, lymph for the remainder.
• When explaining osmosis, state the direction of the water potential gradient explicitly; equivalent wording is normally accepted unless a question-specific scheme says otherwise.
• For swelling of tissues, state the direction of the change in water potential before the direction of water movement.