Tissue fluid and the lymph systemEdexcel A-Level Biology B: Revision notes

Tissue fluid is the fluid that surrounds the cells of the body. It is formed from blood plasma that leaks out of the capillaries, so it contains water, dissolved oxygen, glucose, amino acids, fatty acids and ions, but not the large plasma proteins or most blood cells, which are too large to pass through the capillary wall. (Some white blood cells can squeeze out.)

Tissue fluid is the link between the blood and the cells. Oxygen and nutrients diffuse from it into the cells, and carbon dioxide and waste diffuse the other way, so the cells never touch the blood directly.

Hydrostatic pressure is the pressure exerted by a liquid, here the blood pressure caused by the pumping of the heart. At the arteriole end of a capillary it is relatively high (for example 4.0 kPa), and it forces fluid out through the thin walls (ultrafiltration).

As fluid leaves and as the blood meets the friction of the very narrow capillaries, the hydrostatic pressure falls. At the venule end it is lower (for example 1.5 kPa).

Oncotic pressure is the tendency of water to move into the capillary by osmosis because of the plasma proteins. The proteins stay in the blood, so the plasma has a lower water potential than the tissue fluid, and water is drawn back in. In the example the oncotic pressure is 3.3 kPa all along the capillary.

Net filtration pressure = hydrostatic pressure − oncotic pressure:

• arteriole end: 4.0 − 3.3 = +0.7 kPa (net movement out)
• venule end: 1.5 − 3.3 = −1.8 kPa (net movement in)

A negative value means the fluid is reabsorbed. Most of the fluid (about 90%) returns to the capillaries this way.

Not all the tissue fluid is reabsorbed at the venule end. The excess drains into lymph capillaries, small blind-ended vessels in the tissues, where it is called lymph. These join larger lymph vessels, which have valves to prevent backflow. The lymph is moved along by the contraction of surrounding skeletal muscles and by breathing movements, and it eventually empties into a large vein near the heart.

The lymph system therefore returns excess fluid and any leaked plasma proteins to the blood, which maintains blood volume and prevents a build-up of tissue fluid.

Swelling of the tissues due to excess tissue fluid is called oedema. It occurs when:

• the oncotic pressure of the plasma falls, for example in severe protein deficiency, so less fluid is reabsorbed
• the hydrostatic pressure rises, for example when standing for a long time, in heart failure or in raised blood pressure, so more fluid is forced out and less is reabsorbed
• the lymph vessels are blocked or removed, so excess fluid and protein cannot drain away.

When reasoning in an exam, decide which pressure has changed, then say what that does to the net movement of fluid at the arteriole and venule ends.