Unit 2 AQA Biology - Tissue Fluid

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Tissue fluid
What is the role of tissue fluid?
It is the fluid which allows the exchange of substances between the blood and cells
What substances are found in tissue fluid?
Glucose, amino acids, fatty acids, salts and oxygen = all delivered to the cells.
Carbon dioxide and other waste substances = removed from the cells
How is it formed?
Tissue fluid is formed due to hydrostatic pressure.
Hydrostatic pressure:
As the capillaries are narrower than the arterioles, a pressure builds up which forces tissue
fluid out of the blood plasma, this is called hydrostatic pressure.
This pressure is resisted by:
Pressure of the tissue fluid on the capillaries (from the outside)
The lower water potential of the blood (caused by plasma proteins ­ too large to
leave the blood)
Overall, pressure pushes tissue fluid and small molecules out of the capillary, leaving cells and
large proteins behind, this is called ultrafiltration.
Return of the tissue fluid:
Most tissue fluid is returned to the blood plasma via the capillaries.
Hydrostatic pressure at the venule end of the capillary is higher outside the capillary
and tissue fluid is forced back in.
Osmotic forces (resulting from the proteins in the plasma) pull water back into
capillaries.
Remaining tissue fluid enters the lymph vessels ­ drain back into the veins close to the
heart.
Lymph:
Lymph is moved by:
Hydrostatic pressure
Contraction of body muscles (aided by valves in the lymph vessels)

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