Tissue Fluid

Question 1

What is tissue fluid

Answer 1

• Tissue fluid bathes all of the cells in the body.
• It is a watery liquid that contains glucose, amino acids, fatty acids, ions in solution and oxygen.
• It supplies all of these substances to the tissues and in return it receives carbon dioxide and other waste materials from the tissues.
• It is the means by which materials are exchanged between blood and cells.

Question 2

Summarise briefly how tissue fluid is formed and what this means

Answer 2

• Tissue fluid is formed from blood plasma, and the composition of blood plasma is controlled by various homeostatic systems.
• As a result tissue fluid provides a mostly constant environment for the cells it surrounds.

Question 3

Explain how tissue fluid is formed

Answer 3

1) The heart pumping creates a pressure, called hydrostatic pressure at the arterial end of the capillaries.
2) This hydrostatic pressure causes tissue fluid to move out of the blood plasma.
3) This outward pressure is, however, opposed by two forces:
- the hydrostatic pressure of the tissue fluid outside of the capillaries, which resists the outward movement of liquid.
- the lower water potential of the blood, due to plasma proteins, that causes water to move back into the blood within the capillaries.
4) The combined effect of all of these forces is to create an overall pressure that pushes tissue fluid out of the capillaries at the arterial end.
5) This pressure is only enough to force small molecules out of the capillaries, leaving all cells and proteins in the blood because these are too large to cross the membranes.
6) This type of filtration is called ultrafiltration.

Question 4

Explain how tissue fluid returns to the circulatory system

Answer 4

• The loss of tissue fluid from the capillaries reduces the hydrostatic pressure within them.
• As a result, by the time the blood has reached the venous end of the capillary network its hydrostatic pressure is usually lower than that of the tissue outside it.
• Therefore tissue fluid is forced back into the capillaries by the higher hydrostatic pressure outside them.
• In addition the plasma has lost water and still contains proteins. It therefore has a lower water potential than the tissue fluid.
• As a result, water leaves the tissue via osmosis down a water potential gradient.

Question 5

Explain how the rest of the tissue fluid that can’t return via the capillaries returns to the circulatory system

Answer 5

• Via the lymphatic system.
• This is a system of vessels that begin in the tissues.
• They initially resemble capillaries but have dead ends.
• They then gradually merge into larger vessels that form a network throughout the body.
• These larger vessels drain their contents back into the bloodstream via two ducts that join veins close to the heart.

Question 6

What do we call the contents of the lymphatic system

Answer 6

Lymph

Question 7

What are the contents of the lymphatic system moved by

Answer 7

1) The hydrostatic pressure of the tissue fluid that has left the capillaries.
2) contraction of body muscles that squeeze the lymph vessels- valves in the lymph vessels ensure that the fluid inside them moves away from the tissues and towards the heart.