Lecture 25

Question 1

Describe the pre capillary sphincter

Answer 1

When we have little tissue demand for metabolites, in our capillaries there is contraction of the sphincters - so little perfusion of the capillary beds. So blood flows through the met arterioles rather than through the true capillaries. By changing the resistance in pre-capillary sphincters we can change the perfusion.

Question 2

Describe auto-regulation of blood flow within tissues?

Answer 2

The nerves do not control the pre-capillary sphincters, it is the metabolites in the local tissue. If an area has decreased oxygen levels the sphincter will dilate and let blood flow through. Lactic acid will cause the sphincters to dilate.

Question 3

Describe capillaries?

Answer 3

Where exchange occurs - pressure and flow to the capillaries is controlled nicely. Capillaries are small (8microm in diameter - about the size of the RBC). There tends to be fenestrations and gaps to allow fluid and oxygen to diffuse.

Question 4

How can we have transcapillary exchange?

Answer 4

1. Diffusion (concentration gradient).
2. Filtration (pressure gradient).
3. Large molecule movement process (pinocytosis).

Question 5

Describe diffusion in transcapillary exchange?

Answer 5

Diffusion is dependent: on how diffusible the compound is (i.e. oxygen is less diffusible than carbon dioxide), the area and the thickness of the capillary and the concentration gradient (ability of molecule to move from high to low).

Question 6

Describe filtration in transcapillary exchange?

Answer 6

Bulk flow of fluid across the membrane, out of the capillary into the interstitial fluid. It is pressure dependent: pressure inside and outside of the capillary and the osmotic pressure. Fluid movement is dependent on: the permeability of the capillary, the hydrostatic pressure (blood pressure of inside the capillary and outside the capillary - pressure gradient), the colloid osmotic pressure (typically the osmotic pressure is higher inside the capillaries due to the big proteins left inside the capillary after fluid has moved out so this draws fluid back into the capillary).

Question 7

Describe the flow of fluid in the capillaries?

Answer 7

There is a pressure gradient inside the capillary as you move along it. The hydrostatic pressure is 35mmHg and it drops to 17mmHg at the venous end. The osmotic pressure doesn’t change as you go down the capillary. The driving force at the start of the capillary is 10mmHg (fluid goes out) and at the other end the net pressure is -8mmHg (fluid comes in).

Question 8

Describe capillary pressure?

Answer 8

Capillary pressure is the major driver for fluid coming in or out. If we increase the resistance before the capillary bed the pressure will drop inside the capillary so net filtration will drop. If you increase the resistance after the capillary bed the pressure inside the capillary will increase and therefore there will be an increase in fluid moving out of the capillary. Anything that increases arterial or venous pressure will increase your capillary pressure.

Question 9

What happens to filtration and absorption in the capillary when there is vasodilation?

Answer 9

You are decreasing the resistance around the arterial end. This is vasodilation at the capillary end - more blood coming in, arterial pressure is higher so capillary pressure is higher so more filtration and less reabsorption. Could occur if you have hypertension. High bp you will see more fluid moving out of the capillary beds.

Question 10

What happens to filtration and absorption in the capillary when there is dehydration?

Answer 10

The osmotic pressure is increased as there is less water in our arteries to start off with, so there is a drive for reabsorption (less filtration).

Question 11

What happens to filtration and absorption in the capillary when there is vasoconstriction?

Answer 11

There is a lower arterial pressure (pre-capillary sphincter could be closed off or there is high resistance at the arterial end), this will decrease filtration and there will be more reabsorption.

Question 12

What happens to filtration and absorption in the capillary when there is increased venous pressure?

Answer 12

There is less gradient for reabsorption - more fluid moving out of the blood vessel.

Question 13

What happens to filtration and absorption in the capillary when there is hypoproteinemia?

Answer 13

There are low proteins in the capillaries, do decrease osmotic pressure. So there is more filtration and less reabsorpti

Question 14

Describe heart failure?

Answer 14

Reduced CO, reduced EF, kidneys aren’t being perfused well, retention of fluid, increased pre-load in the heart, central venous pressure increases, increased capillary pressure (both in periphery and pulmonary circulation) and increased net filtration (oedema - will get a dimple in oedema area). Patients will tend to sleep sitting up.

Question 15

Describe large molecule movement processes?

Answer 15

Molecules move via the cells via pinocytosis.

Question 16

Describe lymphatic vessels in the cardiovascular system?

Answer 16

24L of fluid is moving out via filtration and 20.4L is moving back in. The major roles of the lymphatic vessels is to remove the excess fluid back to your heart. There is absorption via the gut and removal of RBC’s from tissue

Question 17

Describe lymphatic drainage?

Answer 17

Trying all the time to get all the excess fluid back to the heart. The terminal ends of the lymphatic vessels are tethered by collagen fibres, when interstitial fluid increases the fluid can be moved back into the lymph vessels. There are valves to prevent back flow. Thin walled - lymph will contract.