What is tissue fluid?

It is the fluid that bathes the cells of a tissue, formed from plasma that has been forced out through the capillary wall.

What is meant by the hydrostatic pressure of the blood in a capillary?

It is the pressure the blood exerts outwards against the capillary wall, generated by the pumping of the heart.

Define oncotic pressure.

It is the pressure produced by the plasma proteins lowering the water potential of the blood, which tends to draw water back into the capillary by osmosis.

What happens at the arteriolar end of a capillary bed?

The hydrostatic pressure is greater than the oncotic pressure, so there is a net outward force and water with small dissolved solutes is forced out through the gaps in the capillary wall.

What happens at the venule end of a capillary bed?

The hydrostatic pressure has fallen while the oncotic pressure is unchanged, so there is a net inward force and most of the fluid is drawn back into the capillary.

OCR A-Level Biology B H422

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Transport Systems in Mammals

Tissue fluid and capillary exchange

3 things to know
Components 01, 02 and 03

Overview

Recall questions on tissue fluid and what it lacks, hydrostatic and oncotic pressure at the arteriolar and venule ends of a capillary bed, why each changes or stays constant, the lymphatic system, and oedema caused by protein deficiency or high blood pressure.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Name two components of the blood in a capillary that are absent from tissue fluid.

Key term

Why does hydrostatic pressure fall along the length of a capillary?

Key term

Which system returns excess tissue fluid to the blood?

Questions from this subtopic

Each one with the model answer underneath.

1
What is tissue fluid?
Model answer
It is the fluid that bathes the cells of a tissue, formed from plasma that has been forced out through the capillary wall.
2
Name two components of the blood in a capillary that are absent from tissue fluid.
Model answer
Red blood cells and plasma proteins.
3
What is meant by the hydrostatic pressure of the blood in a capillary?
Model answer
It is the pressure the blood exerts outwards against the capillary wall, generated by the pumping of the heart.
4
Define oncotic pressure.
Model answer
It is the pressure produced by the plasma proteins lowering the water potential of the blood, which tends to draw water back into the capillary by osmosis.
5
What happens at the arteriolar end of a capillary bed?
Model answer
The hydrostatic pressure is greater than the oncotic pressure, so there is a net outward force and water with small dissolved solutes is forced out through the gaps in the capillary wall.
6
What happens at the venule end of a capillary bed?
Model answer
The hydrostatic pressure has fallen while the oncotic pressure is unchanged, so there is a net inward force and most of the fluid is drawn back into the capillary.

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