[7.6] (2/2) tissue fluid and its formation

Question 1

what does tissue fluid contain?

Answer 1

• glucose
• oxygen
• amino acids
• fatty acids
• ions in solution

Question 2

what is the role of tissue fluid?

Answer 2

• to supply useful substances to tissues
• in return, it receives carbon dioxide and other waste materials from the tissues
• therefore, tissue fluid is the means by which materials are exchanged between blood and cells

Question 3

what relationship does tissue fluid have with cells?

Answer 3

• tissue fluid bathes all the cells of the body
• it is the immediate environment of cells ie. effectively where they live

Question 4

what is the effect of tissue fluid’s relationship with plasma?

Answer 4

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

Question 5

what happens as blood passes through capillaries?

Answer 5

• some plasma leaks out through gaps in the walls of the capillary to surround the cells of the body
• this results in the formation of tissue fluid

Question 6

how does the composition of plasma and tissue fluid compare?

Answer 6

• their compositions are virtually the same but tissue fluid contains far fewer proteins
• this is because proteins are too large to fit through gaps in the capillary walls and so remain in the blood

Question 7

what is hydrostatic pressure?

Answer 7

pressure created from the pumping of the heart

Question 8

what occurs at the arterial and venule end of the capillary?

Answer 8

• there is a higher hydrostatic pressure at the arterial end which causes tissue fluid to move out of the blood plasma
• lower higher hydrostatic pressure at the venule end of capillary so less fluid is pushed out of capillary

Question 9

what does how much liquid leaving the plasma to form tissue fluid depend on?

Answer 9

two opposing forces:

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

Question 10

what happens when there is no water potential gradient?

Answer 10

• when the water potential gradient between the capillary and tissue fluid are the same, at the arterial end, water flows back into the capillary from the tissue fluid
• however, overall, more fluid leaves the capillary than returns, leaving tissue fluid behind to bathe cells

Question 11

when does tissue fluid return to the blood plasma?

Answer 11

• once tissue fluid has exchange metabolic materials with the cells it bathes, it is returned to the circulatory system
• most tissue fluid returns to the blood plasma directly via the capillaries

Question 12

how does tissue fluid return to the circulatory system via the capillaries?

Answer 12

1. loss of tissue fluid from capillaries reduces the hydrostatic pressure inside them
2. by the time the blood has reached the venous end of the capillary, its hydrostatic pressure is lower than the tissue fluid outside it
3. tissue fluid is forced back into the capillary due to higher hydrostatic pressure outside them
4. plasma has lost water but still contains proteins so has a lower water potential than tissue fluid
5. water leaves tissue by osmosis down a water potential gradient

Question 13

what happens to the tissue fluid that does not return through the capillaries?

Answer 13

• carried back via the lymphatic system
• contain lymph capillaries, which resemble capillaries but have dead ends
• lymph capillaries gradually merge into larger vessels that form a network throughout the body (lymphatic system)
• these larger vessels drain their contents back into the bloodstream via two ducts that join veins close to the heart

Question 14

how and why are plasma proteins returned to the blood?

Answer 14

• any plasma proteins that have escaped from the blood are returned to the blood via the lymph capillaries
• if plasma proteins were not removed from tissue fluid they could lower the water potential of the tissue fluid and prevent the reabsorption of water into the blood in the capillaries

Question 15

what happens to large molecules that are not able to pass through the capillary wall?

Answer 15

• they enter the lymphatic system as lymph
• lymph capillaries have closed ends and large pores that allow large molecules to pass through

Question 16

how are the contents of the lymphatic system moved?

Answer 16

• hydrostatic pressure of the tissue fluid that has left the capillaries
• 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 towards that heart