Explain how tissue fluid is formed at the arterial end of capillaries.

Hydrostatic pressure forces plasma and dissolved substances out through gaps between endothelial cells into the surrounding tissues.

Define hydrostatic pressure in the context of capillaries.

Hydrostatic pressure is the pressure exerted by a fluid; in capillaries it tends to push fluid out into tissues.

What is oncotic pressure?

Oncotic pressure is the osmotic pressure exerted by plasma proteins, which tends to draw water back into capillaries.

What is the net direction of fluid movement at the arterial end of a capillary, and why?

Net outward movement occurs because hydrostatic pressure exceeds oncotic pressure, forming tissue fluid.

At the venous end of a capillary, which pressure is greater: oncotic pressure or hydrostatic pressure?

Oncotic pressure is greater than hydrostatic pressure at the venous end.

Explain why there is net inward movement of fluid at the venous end of capillaries.

Hydrostatic pressure has dropped along the capillary length, so oncotic pressure (due to plasma proteins) now exceeds it, drawing fluid back into the blood by osmosis.

OCR A-Level Biology H420

Cell Structure

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Biological Molecules

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Transport in Animals

Tissue fluid and lymph

1 thing to know
Components 01 and 03

Overview

Hydrostatic and oncotic pressure at each end of a capillary, how tissue fluid forms and returns, and how plasma, tissue fluid and lymph differ.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Process

Explain tissue fluid formation and return.

Questions from this subtopic

Each one with the model answer underneath.

1
Explain how tissue fluid is formed at the arterial end of capillaries.
Model answer
Hydrostatic pressure forces plasma and dissolved substances out through gaps between endothelial cells into the surrounding tissues.
2
Define hydrostatic pressure in the context of capillaries.
Model answer
Hydrostatic pressure is the pressure exerted by a fluid; in capillaries it tends to push fluid out into tissues.
3
What is oncotic pressure?
Model answer
Oncotic pressure is the osmotic pressure exerted by plasma proteins, which tends to draw water back into capillaries.
4
What is the net direction of fluid movement at the arterial end of a capillary, and why?
Model answer
Net outward movement occurs because hydrostatic pressure exceeds oncotic pressure, forming tissue fluid.
5
At the venous end of a capillary, which pressure is greater: oncotic pressure or hydrostatic pressure?
Model answer
Oncotic pressure is greater than hydrostatic pressure at the venous end.
6
Explain why there is net inward movement of fluid at the venous end of capillaries.
Model answer
Hydrostatic pressure has dropped along the capillary length, so oncotic pressure (due to plasma proteins) now exceeds it, drawing fluid back into the blood by osmosis.

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