Fluid movement across a capillary wall

Question 1

What are the two forces battling each other to control the movement of fluids in/out of a capillary?

Answer 1

1. hydrostatic pressure created by blood pressure (wants to squeeze fluid out of capillary)
2. oncotic pressure created by proteins in the blood (wants to keep fluid in capillary)

Question 2

In a normal/healthy person, what is the hydrostatic pressure at the arteriole end of the capillary? At the venous end?

Answer 2

Entering the arteriole side of the capillary bed, hydrostatic pressure is 32 mmHg and due to resistance of the capillary drops to 15 mmHg on the venous end.
~~17 mmHg drop~~

Question 3

What is the oncotic pressure at the arterial end of the capillary? At the venous end? (normally)

Answer 3

Entering the arteriole side of the capillary bed, the oncotic pressure is 25 mmHg. There’s no change in blood proteins during the journey through the capillary, so the pressure at the venous end is ALSO 25 mmHg. NO CHANGE.

Question 4

What happens to fluid movement along the capillary?

Answer 4

~At arterial end, the hydrostatic pressure (wants to squeeze fluid out) is 32 and the oncotic pressure (wants to pull fluid in) is 25.
Hydrostatic 32 > oncotic 25 so fluid leaves the capillary (“filtration”) at the beginning/arteriole end.
~As blood moves along, the hydrostatic pressure goes down due to resistance, until hydrostatic = oncotic in the middle and there’s no net fluid movement.
~Then hydrostatic pressure keeps falling, and oncotic stays the same, so at the venous end of the capillary, hydrostatic (wants to squeeze fluid out) 15 < oncotic (wants to pull fluid in) 25 so fluid is ABSORBED.

Question 5

What happens in a hypertensive patient?

Answer 5

Hypertension causes a higher hydrostatic pressure at the beginning/arteriole side of the capillary bed.
The hydrostatic pressure STILL FALLS THE SAME AMOUNT, 17 mmHg, as the blood traverses the capillary toward the venous end.
So higher pressure on the arterial end = more filtration and less absorption = edema.

Question 6

What happens in hypotension?

Answer 6

Hypotension causes a lower hydrostatic pressure at the beginning/arteriole side of the capillary bed. The hydrostatic pressure still falls the same amount as the blood goes through the capillary toward the venous end (17 mmHg).
So lower pressure on the arterial end = less filtration and more absorption = helps restore blood volume