3.4 Tissue fluid

Question 1

What is tissue fluid?

Answer 1

• A watery liquid that contains glucose, amino acids, fatty acids and ions in solution. It supplies these to the cells, while also removing any waste materials

Question 2

How is tissue fluid formed?

Answer 2

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

Question 3

What is hydrostatic pressure?

Answer 3

• It’s when the blood pumped by the heart passes along the arteries, then the narrower arterioles, and finally, the even narrower capillaries
• Pumping by the heart creates hydrostatic pressure.
• This causes tissue fluid to move out of the blood plasma

Question 4

What is the outward pressure of tissue fluid opposed by?

Answer 4

• Hydrostatic pressure of the tissue fluid outside the capillaries, which resists outward movement of liquid
• The lower water potential of the blood, due to the plasma proteins, that causes water to move back into the blood within the capillaries

Question 5

What is ultrafiltration?

Answer 5

• The combined effect of these forces creates an overall pressure that pushes tissue fluid out of the capillaries at the arterial end
• The 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
• This filtration under pressure is called ultrafiltration

Question 6

How is tissue fluid returned to the circulatory system?

Answer 6

• The loss of the tissue fluid from the capillaries reduces the hydrostatic pressure inside them
• 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 fluid around it
• Therefore fluid is forced back into the capillaries by the higher hydrostatic pressure outside them
• Additionally, the plasma has lost water and still contains proteins, so therefore has a lower water potential than the tissue fluid
• Water leaves the tissue by osmosis

Question 7

What is the other method of return of tissue fluid?

Answer 7

• Not all can return to the capillaries
• the remainder is carried back via the lymphatic system

Question 8

How are the contents of the lymphatic system moved?

Answer 8

• Hydrostatic pressure of the tissue fluid that has left the capillaries
• Contraction of body muscles that squeeze the lymph vessels - valves in the vessels ensure that the fluid inside them moves away from the tissues in the direction of the heart

Question 9

What is the function of tissue fluid?

Answer 9

• It supplies all of these substances to the tissues
• It receives carbon dioxide and other waste materials from the tissues
• It is the immediate environment of cells and is where they live

Question 10

What is the lymphatic system?

Answer 10

• This is a system of vessels that begin in the tissues
• Initially they resemble capillaries, but 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