Explain tissue fluid formation
Describe capillary filtration and explain the role of tissue fluid in exchange between blood and cells.
AS Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; microscope recognition, biological drawing and supervised practical competence require practical work, while human subject-teacher review remains pending.
Before you begin
- Know that mass transport moves substances between exchange surfaces and cells, and that diffusion occurs down a concentration or partial-pressure gradient.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the main force promoting tissue-fluid formation at the arterial capillary end?
What you will learn
- State the functions of tissue fluid
- Describe how tissue fluid forms in capillary networks and returns to the circulation
Build the explanation
Tissue fluid surrounds body cells and provides a local medium for exchange. Dissolved oxygen and nutrients move from the blood across capillary walls into tissue fluid and then to cells; carbon dioxide and some wastes move in the opposite direction. At the arterial end of a capillary, hydrostatic blood pressure is relatively high and forces water and small dissolved substances out through the permeable wall. Large plasma proteins and blood cells generally remain in the capillary.
Apply it carefully
As blood flows along the capillary, hydrostatic pressure falls. At the venous end, the water-potential effect of retained plasma proteins helps draw much of the water back into the blood. Not all tissue fluid returns directly to capillaries; excess enters lymphatic vessels and is eventually returned to the circulation. Avoid claiming that all plasma leaves or that large proteins diffuse freely through the wall.
Pause and explain
What usually happens to excess tissue fluid that does not return directly to capillaries?
Worked example
Why is tissue fluid formed more readily at the arterial end of a capillary?
Show the worked solution
- Compare the pressure there with pressure farther along.
- Higher hydrostatic pressure pushes water and small solutes out across the capillary wall.
Answer Hydrostatic pressure is higher at the arterial end, so filtration forces water and small dissolved substances out into the surrounding tissues.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe two functions of tissue fluid.
Give me a hint
Relate the fluid to nearby cells.
Compare my reasoning
- It provides a medium for exchange around cells.
- It carries oxygen and nutrients towards cells and carbon dioxide/waste away.
Tissue fluid surrounds cells and forms an exchange medium: oxygen and nutrients move from blood through tissue fluid to cells, while carbon dioxide and some wastes move towards the blood or lymph.
Look for these in your work
- Cell surroundings stated
- Exchange direction examples correct
Explain why blood cells and most large plasma proteins do not leave the capillary with tissue fluid.
Give me a hint
Compare particle size with capillary-wall permeability.
Compare my reasoning
- The capillary wall is selectively permeable.
- Blood cells and large proteins are too large to pass through the pores in normal filtration.
The capillary wall permits water and small solutes to pass, but blood cells and most large plasma proteins are too large to cross during normal tissue-fluid formation and remain in the blood.
Look for these in your work
- Selective permeability stated
- Large components retained
A capillary's hydrostatic pressure decreases along its length. Explain the likely fluid movements at the arterial and venous ends, then account for excess fluid.
Give me a hint
Separate filtration from reabsorption and lymph drainage.
Compare my reasoning
- Higher arterial-end pressure favours filtration out.
- Lower pressure and the osmotic effect of retained proteins favour return at the venous end; excess enters lymphatics.
At the arterial end, higher hydrostatic pressure forces water and small solutes out to form tissue fluid. As pressure falls, the osmotic effect of retained plasma proteins favours water return at the venous end. Excess fluid that is not reabsorbed drains into lymphatic vessels.
Look for these in your work
- Arterial filtration explained
- Venous return explained
- Lymph drainage included
Common mistakes
- Saying all plasma components leave the capillary
- Omitting the fall in hydrostatic pressure along the network
- Claiming all tissue fluid returns directly to capillaries
What can you explain now?
What is the main force promoting tissue-fluid formation at the arterial capillary end?
Compare with the explanation
Blood hydrostatic pressure
The higher pressure drives filtration through the permeable capillary wall.
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Make it stick
What drives fluid out at the arterial end, what favours return, and where does excess fluid go?
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