A molecule can easily pass from one side of such a cell to the other, conducted through the cell membranes by special protein complexes that project through the membranes and provide passageways. The smallest capillaries are only 0.08 millimeter in diameter, and red blood cells, which collect and release oxygen, must pass through in single file. So do water molecules to some degree, along with small molecules dissolved in the blood fluid. As a result, the space around cells is bathed in fluid. Blood pressure maintains a balance between this fluid and the plasma in the capillaries, but there is nearly three times as much fluid outside the circulatory system as there is in blood.
Excess fluid is carried away by the lymphatic system .
The fluid that escapes from the capillaries, known as lymph, provides a route for the white blood cells of the immune system to travel to the cells. Thus when bacteria or viruses invade cells, the immune system can bring up its army and attack the invading germs. In hemorrhagic fevers capillaries break and cause bleeding as a major symptom. The red nose of the chronic alcohol abuser stems from a thinning of the capillaries caused by excessive drinking.