3. Tissue Fluid

Question 1

What is tissue fluid

Answer 1

the fluid that surrounds cells in tissues

It is made up of substances that leave the blood eg oxygen, water and nutrients.

Cells then take in oxygen and nutrients and release metabolic wastes into it eg carbon dioxide and urea

Question 2

Formation of tissue fluid

Answer 2

Contraction of the heart muscle sends blood along arteries, arterioles and capillaries which creates a pressure called hydrostatic pressure. This acts outwards, forcing tissue fluid out of the capillaries.

Red blood cells and large plasma proteins are too big to leave through the capillary walls.

Tissue fluid is formed at the arterial end of the capillary bed. As the blood is forced from the artery into narrower vessels hydrostatic pressure forces tissue fluid out of the plasma

Question 3

hydrostatic pressure is opposed by 2 other forces

Answer 3

The hydrostatic pressure of the tissue fluid outside the capillaries.

The lower water potential of the blood. This is due to the plasma proteins which remain in the blood

Question 4

Ultra-filtration

Answer 4

The overall effect of these forces is to force small molecules out of the capillaries leaving all the cells and proteins in the blood. Filtration occurs under pressure

Question 5

Journey of tissue fluid relating to HSP

Answer 5

Tissue fluid is formed at the arterial end of the capillary bed. As the blood is forced from the artery into narrower vessels hydrostatic pressure forces tissue fluid out of the plasma

Question 6

Tissue fluid vs plasma

Answer 6

Question 7

Return of the tissue fluid

Answer 7

Once metabolites have been exchanged with the cells the tissue fluid has to be returned to the blood. This is done via the capillaries mainly.

Loss of tissue fluid from the capillaries reduces the hydrostatic pressure within them. Therefore by the time the blood reaches the venous end the pressure outside is greater than the pressure inside. Coupled with osmotic forces (due to the proteins in the blood) fluid moves back in.

The remainder of the tissue fluid is returned to the blood system via the lymphatic system.

Question 8

The lymph is moved by

Answer 8

Hydrostatic pressure of the tissue fluid that has left the capillaries

Contraction of body muscles that squeeze the lymph vessels. These vessels contain valves to keep the lymph moving in the right direction.

Question 9

Lymph

Answer 9

Water
Oxygen
Nutrients e.g glucose
White blood cells
Chylomicrons
It does not contain red blood cells (too big) or big protein molecules

Question 10

Elephantiatis

Answer 10

If the lymph vessels are blocked, the flow of fluid in the lymph vessels will stop

The tissue fluid will build up in the tissues.

Question 11

Explain how high blood pressure may cause oedema (4)

Answer 11

If blood pressure is high, the hydrostatic pressure at the arterial end of the capillary bed will be high.

This means that a lot of tissue fluid will be formed.

Not all of it will be able to return to the capillary

The rest will remain in the tissues and cause swelling

Question 12

Describe and explain four ways in which the structure of a capillary adapts for the exchange of substances between blood and the suurrounding tissue

Answer 12

• Single cell thick so reduces diffusion distance
• small diameter so short diffusion distance
• narrow lumen so reduces flow rate so more time for diffusion
• red blood cells in contact with wall so more time for diffusion

Question 13

Hypoalbuminemia is a condition where the level of a protein called albumen in the blood is very low. It causes an increase in tissue fluid, which can lead to swelling.

Explain how hypoalbuminemia causes an increase in tissue fluid. (2)

Answer 13

The water potential of the blood in the capillary is high because there is less albumin in the blood.

This means that less water is absorbed back by osmosis at the venule end of the capillary bed

This leads to an increase in tissue fluid.