Exchange at the capillaries 3.2.3

Question 1

What does blood consist of?

Answer 1

• Plasma : contains dissolved substances including oxygen, carbon dioxide, glucose, minerals, hormones etc.
• Plus cells - RBC’s, WBC’s and platelets

Question 2

What is tissue fluid?

Answer 2

It is similar to blood plasma however does not contain most of the cells found in blood.
- It does contain dissolved nutrients/oxygen

Question 3

How is tissue fluid formed?

Answer 3

From plasma leaking from the capillaries:
- At the arterial end, the hydrostatic pressure of the blood is relatively high. This forces fluid out through the capillary walls - forming tissue fluid

Question 4

How are substances exchanged between the blood and the cells in capillary beds?

Answer 4

They first diffuse into tissue fluid that surrounds all cells, and then diffuse from there to the cells.

Question 5

How is tissue fluid returned to blood?

Answer 5

At the venous end of the capillaries, the blood pressure is lower so fluid is no longer forced out.
This allows the tissue fluid to return to blood via lymph capillaries, returning to the blood in the subclavian vein.

Question 6

Define hydrostatic pressure

Answer 6

The pressure that a fluid exerts when pushing against the sides of a vessel or container.

Question 7

Define oncotic pressure

Answer 7

The pressure created by the osmotic effects of the solutes.
It causes the movement of water from a solution with a high water potential to one with a low water potential.

Question 8

What is oedema?

Answer 8

It occurs when blood pressure is elevated above normal, causing more fluid than normal to be forced out of the capillaries. As a result swelling occurs due to fluid accumulating in the tissues.

Question 9

What is pulmonary oedema?

Answer 9

Happens during failure of the left ventricle, which leads to an accumulation of tissue fluid in the lung due to the backup of blood in the pulmonary capillaries (raising blood pressure).
This reduces gas exchange.