Transport of fluids across the capillary

Question 1

Substances can move across the capillary wall by 3 main mechanisms. what are they?

Answer 1

diffusion, bulk flow and transcytosis

Question 2

what is diffusion?

Answer 2

the movement of a substance from an area of high concentration to an area of low concentration

Question 3

what substances move across the capillary via diffusion?

Answer 3

simple - lipid-soluble and smaller molecules across the pores in the capillary endothelium along a concentration gradient & other substances can diffuse through fenestrated capillary pores, such as glucose

facilitated - ions diffuse through specific ion channels

Question 4

what is bulk flow?

Answer 4

This describes the passive movement of water, and water soluble substances such as electrolytes like potassium and sodium, across the pores present in the capillary endothelium.

Question 5

describe the process of filtration in bulk flow

Answer 5

Volumes of fluid move from an area of higher pressure in a capillary bed to an area of lower pressure in the tissues

Question 6

describe the process of reabsorption in bulk flow

Answer 6

movement of fluid from an area of higher pressure in the tissues into an area of lower pressure in the capillaries

Question 7

what types of pressure interact to bring about either filtration or reabsorption in bulk flow?

Answer 7

hydrostatic pressure and oncotic pressure of the capillary blood plasma and tissue interstitial fluid.

Question 8

describe transcytosis

Answer 8

allows larger molecules and lipid-insoluble molecules to pass across the capillary endothelium via vesicles

Question 9

what substances are transported in transcytosis

Answer 9

proteins such as hormones

Question 10

what is hydrostatic pressure?

Answer 10

the pressure of any fluid enclosed in a space

Question 11

what is meant by BHP?

Answer 11

blood hydrostatic pressure: The pressure exerted by blood against the wall of a capillary

Question 12

what is the role of BHP in the arterial end of the capillary?

Answer 12

drives fluid out of capillaries and into the interstitial tissues.

Question 13

what is meant by IFHP

Answer 13

interstitial fluid hydrostatic pressure

Question 14

what happens to IFHP when fluid exist the capillaries and enters the tissues?

Answer 14

the pressure rises

Question 15

at the arterial end of the capillary, why is BHP higher than IFHP and what does this result in?

Answer 15

lymphatic vessels continually absorb excess fluid from the interstitial tissue space. Thus, fluid generally moves out of the capillary and into the interstitial fluid due to the hydrostatic pressure gradient across the capillary endothelium. This process is called filtration.

Question 16

at the arterial end, if hydrostatic pressure is high and blood capillary oncotic pressure is low, where does water move?

Answer 16

water moves out of the capillary in to the interstitial fluid. This suits the body as it promotes the movement of water, solutes and important electrolytes from the capillary to tissue fluid and body cells before removal by the lymphatics

Question 17

why is hydrostatic pressure less at the venous end of a capillary?

Answer 17

most water leaves the capillary at the arteriole end causing the BHP to drop down the capillary

Question 18

how does reabsorption occur if blood capillary oncotic pressure is constant?

Answer 18

its osmotic effect becomes greater at the venule end as the capillary water content reduces and plasma protein concentration relatively increases.

Question 19

Only 80% of interstitial water is reabsorbed back in to the venule end. what happens to the other 20%?

Answer 19

joins the lymphatic system

Question 20

Hydrostatic pressure is caused by …

Answer 20

water and fluid pressing on the capillary walls in blood plasma and interstitial fluid.

Question 21

Capillary hydrostatic pressure is greatest at …
what happens here?

Answer 21

the arteriole end of the capillary at approximately 36 mmHg causing most water to leave the capillary at this point to the interstitial fluid. This is termed filtration.

Question 22

Less water leaves towards the venule end of the capillary as…

Answer 22

most fluid has already been lost causing the hydrostatic pressure to drop. The colloid oncotic pressure also has a greater effect at the venule end of the capillary helping to draw water back in to the capillary. Hydrostatic pressure in the interstitial fluid is negligible at 0 mmHg as it is not constrained by walls like blood plasma and is not influenced by blood pressure in the same way as capillaries.

Question 23

The ??? has a greater effect at the venule end of the capillary helping to draw water back in to the capillary.

Answer 23

colloid oncotic pressure

Question 24

Describe hydrostatic pressure in the interstitial fluid

Answer 24

negligible at 0 mmHg as it is not constrained by walls like blood plasma and is not influenced by blood pressure in the same way as capillaries.

Question 25

What creates the colloid oncotic pressure in the capillary blood plasma?

Answer 25

plasma proteins, mainly albumin (70%)
They too large to pass out of the capillaries into the interstitial fluid. Albumin attracts Sodium ions and together these increase the solute concentration inside the blood plasma creating blood capillary oncotic pressure.

Question 26

describe the interaction of hydrostatic and oncotic pressures in the capillary driving fluid movement

Answer 26

1) At the arteriole end of the capillary, the blood hydrostatic pressure (BHP) in the capillary is about 35 mmHg.
2) BHP drops as the blood moves through the capillary (as it looses water) so that by the venous end, the pressure is about 16 mmHg.
3) In comparison, the plasma proteins remain suspended in the blood capillary, so the Blood Colloid Oncotic Pressure (BCOP) remains constant at about 26 mmHg throughout the length of the capillary, but its effect becomes pronounced towards the venule end as water leaves the capillary.
This is considerably below the interstitial fluid oncotic pressure (IFOP) which is 1 mmHg and negligible.
4) The net filtration pressure represents the interaction of pressures driving fluid out of the capillary at the arteriole end. It is equal to the difference between the BHP + IFOP and the BCOP. ((35 mmHg + 1 mmHg) - 26 mmHg = 10 mmHg).
5) The net reabsorption pressure represents the interaction of pressures driving fluid into the capillary at the venule end. It is equal to the difference between the BCOP - BHP+ IFOP. (26 mmHg - (16 mmHg - 1 mmHg) = -9 mmHg).
6) Therefore, more fluid leaves the capillary at the arterial end than enters at the venous end. Excess fluid enters the lymphatics.

Question 27

what happens if If Blood hydrostatic pressure rises across the capillary?

Answer 27

then more fluid and water will be pushed out of the capillary at the arteriole end. As the blood oncotic pressure remains constant it cannot oppose this and reabsorb sufficient fluid in to the capillary.
This causes tissue oedema and occurs with high blood pressure from cardiovascular disease or in the lower limbs when stood still for a prolonged time.
The Interstitial fluid oncotic pressure will increase following an injury or shock states. This because the capillary endothelium becomes more ‘leaky’ allowing large proteins to escape from the capillary in to the tissue spaces which means water is no longer pulled in to the capillary but rather pulled out in to the tissue spaces.