Transport in Mammals: Question 5

Worked solution

Part (a): Formation of tissue fluid at the arteriole end

Blood arriving at a capillary bed from an arteriole is still under a relatively high hydrostatic pressure, generated ultimately by contraction of the heart. This pressure pushes fluid (water together with small dissolved substances such as glucose, amino acids and ions) outward through tiny gaps between the endothelial cells that make up the capillary wall.

This outward push is opposed by an inward-acting pressure created by the plasma proteins (such as albumin) dissolved in the blood plasma. These protein molecules are too large to pass through the capillary wall, so they remain inside the capillary; their presence lowers the water potential of the plasma relative to the fluid just outside the capillary, and this difference (often called the oncotic pressure) tends to draw water back into the capillary by osmosis.

At the arteriole end of the capillary bed, the outward-acting hydrostatic pressure is greater than the inward-acting oncotic pressure, so there is a net outward pressure. Fluid is therefore forced out of the capillary into the surrounding tissue spaces, but the plasma proteins and blood cells, being too large to cross the capillary wall, are left behind in the blood. This fluid that has left the capillary, containing water and small solutes but not the proteins or cells, is called tissue fluid.

Part (b): Capillary wall structure and its function

A capillary wall is built from a single layer of endothelial cells (a squamous epithelium), unlike the multi-layered walls of arteries and veins. This makes the wall extremely thin, so any substance moving between the blood and the surrounding cells (oxygen, glucose, carbon dioxide, urea and so on) has only a very short diffusion pathway to cross, allowing rapid exchange.

Between adjacent endothelial cells there are also narrow gaps (pores) in the wall. Under the hydrostatic pressure described in part (a), water and small dissolved solutes can be filtered directly through these gaps to form tissue fluid, while the much larger plasma proteins and blood cells cannot fit through and are retained inside the capillary.

Two further features of capillaries support efficient exchange. The lumen of a capillary is narrow, which slows the rate of blood flow through it, giving more time for substances to diffuse or filter across the wall before the blood moves on. Capillaries also form a dense, highly branched network running through almost every tissue, so the total surface area available for exchange (and for the formation of tissue fluid) is very large.

Final answers

• (a) At the arteriole end, hydrostatic pressure (from the pumping heart) exceeds the opposing oncotic pressure (due to plasma proteins too large to leave the capillary), giving a net outward pressure that forces fluid, but not proteins or cells, out of the capillary as tissue fluid.
• (b) The single-cell-thick endothelial wall gives a short diffusion pathway, gaps between endothelial cells allow filtration of fluid and small solutes (forming tissue fluid) while retaining proteins and cells, and the narrow lumen (slowing flow) together with the huge network of capillaries (large surface area) further support efficient exchange.