Tissue fluid

Tissue fluid is the liquid that surrounds the cells of the body. It forms when blood pressure pushes water and small dissolved substances, such as glucose, amino acids, oxygen and ions, out of the capillaries. Cells take what they need from it and release waste into it. Most of the water then moves back into the capillaries by osmosis, and the rest drains away as lymph.

What is tissue fluid?

Capillaries come close to almost every cell, but blood never touches the cells directly. Exchange happens through tissue fluid, which fills the spaces between cells. It brings oxygen, glucose, amino acids, fatty acids and ions to the cells, and carries carbon dioxide and other wastes away.

Tissue fluid is made from blood plasma, so it contains the same small molecules. What it lacks is the big things: red blood cells, platelets and most plasma proteins stay inside the capillary because they are too large to get through.

How tissue fluid is formed

AQA A-level Biology asks for the formation of tissue fluid and its return to the circulatory system. It all happens in a capillary bed, between an arteriole and a venule:

At the arteriole end: fluid is forced out

1. The heart's contractions give the blood a high hydrostatic pressure, which is still fairly high when it reaches the arteriole end of a capillary.
2. Capillary walls are one cell thick, with small gaps between the cells.
3. The pressure pushes water and small molecules (glucose, amino acids, fatty acids, ions and oxygen) out through the gaps into the spaces between cells.
4. Red blood cells, platelets and plasma proteins are too large to pass through, so they stay in the blood.

Because the fluid is forced out under pressure and the large molecules are filtered off, this is a kind of pressure filtration.

At the venule end: water comes back

1. Along the capillary, the hydrostatic pressure falls, because fluid has left and friction slows the blood.
2. The plasma proteins still in the blood give it a lower water potential than the tissue fluid.
3. By the venule end, the pull of that water potential difference is bigger than the push of the hydrostatic pressure.
4. So water moves back into the capillary by osmosis. Carbon dioxide and other wastes diffuse in with it.

The lymphatic system

Not all of the tissue fluid gets back into the capillaries. The excess drains into tiny, blind-ended lymph vessels, where it is called lymph. Lymph vessels join into larger ones, and the lymph is moved along by the squeezing of nearby skeletal muscles, with valves stopping it from flowing backwards. It passes through lymph nodes, where white blood cells can respond to pathogens, and is finally emptied back into veins near the heart.

Arteries, veins and capillaries each have a structure that suits their job. Sort them out in this question from the Human Physiology course.

Oedema: when tissue fluid builds up

If more fluid leaves the capillaries than returns or drains away, tissue fluid builds up and the tissue swells. This is called oedema (edema in the US). Each cause links back to the two forces:

• High blood pressure: the hydrostatic pressure is higher all along the capillary, so more fluid is pushed out and less returns.
• Too little plasma protein: for example from a protein-poor diet or liver disease. The blood's water potential is not lowered as much, so less water is drawn back in.
• Blocked lymph vessels: the excess fluid cannot drain away.

Exam questions with answers

Explain how tissue fluid is formed.

At the arteriole end of a capillary, the blood has a high hydrostatic pressure. This forces water and small soluble molecules such as glucose and amino acids out through the capillary wall. The blood cells and the large plasma proteins are held back inside the capillary.

Explain why a low concentration of protein in the blood leads to swelling.

With less protein in the blood, the water potential of the blood is higher than normal, so it is closer to that of the tissue fluid. Less water returns to the capillaries by osmosis at the venule end, so more fluid stays in the tissues.

Check your understanding.

Common mistakes

• Getting water potential the wrong way round. The blood has the lower water potential, so water moves into it.
• Saying proteins "attract" or pump water. Water moves by osmosis, from the higher water potential of the tissue fluid to the lower water potential of the blood.
• Saying red blood cells leave the capillary. They stay in the blood; tissue fluid has none.
• Forgetting the lymph. Not all the fluid returns to the capillaries; the excess drains into lymph vessels.
• Saying hydrostatic pressure rises along the capillary. It falls from the arteriole end to the venule end.

Practise with a short timed paper

Tissue fluid is part of mass transport in the first year of A-level Biology. The daily A-level Biology: Year 1 paper is a short timed paper of 8 questions, with a new one every day. For the pump that drives it all, see the heart, and for exchange across thin walls, exchange surfaces.

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