1.16 - Vascular Filtration

Question 1

What are the three types of capillary?

Answer 1

Continuous
Fenestrated
Sinusoidal (Discontinuous)

Question 2

Describe Continuous capillaries

Answer 2

Lined by endothelial cells with delineating basal membrane

Pericytes between basal membranes

Question 3

Describe Fenestrated Capillaries

Answer 3

In kidneys, intestines, synovia, choroid plexus

Very permeable to water

Question 4

Describe Sinusoidal Capillaries

Answer 4

Found in areas where RBCs are produced or removed for processing
Liver, spleen & bone marrow
Discontinuity in basal membrane

Question 5

Describe the permeability of the vascular system

Answer 5

Vessels have semipermeable membrane
Permeability [cm/s] = capillary “diffusion” x concentration difference
Depends on properties of both membrane and solute:
Mostly, specific transporters contribute little to transcapillary exchange

Question 6

Describe the consequences of increased blood flow on solute transfer

Answer 6

The effect of increased blood flow on solute transfer will generally increase it, but the extent to which will depend on whether solute exchange in flow or diffusion limited.

Flow limited: If diffusion capacity > solute delivery rate,
blood (Ca) equilibrates with pericapillary fluid (Ci) before capillary end

Diffusion Limited: If diffusion capacity > solute delivery rate, no equilibration before capillary end (Cv)

Question 7

Describe the Starling principle on fluid exchange

Answer 7

Pressures determine solute flow.
Hydrolic Push = Capillary pressure (Pc) - Interstitial Pressure (Pi)
Osmotic Suction = πp (colloid osmotic pressure in plasma) - πi (colloid osmotic pressure in interstitial fluid)
Capillary filtration rate is proportional to Hydrolic push - osmotic suction)

Question 8

Draw a diagram showing the four starling forces working on a capillary

Answer 8

Pc = big arrow going out of capillary
Pi = medium arrow going into capillary
πp = smallish arrow going in to capillary
πi = small arrow going out of capillary

Question 9

What is the overall equation for the Starling principle?

Answer 9

Filtration pressure = (Pc-Pi) - o( πp - πi)

Question 10

What factors determine Pc?

Answer 10

Capillary Blood Pressure (Pc) –> pressure inside the lumen forces contents out
Determined by:
- Vascular Resistance
- Arterial Pressure
- Venous PRessure
- Gravity (greater in food than in brain)
- Distance along capillary

Question 11

What factors effect Pi?

Answer 11

Interstitial Fluid Pressure (Pi)
Determine by fluid volume and compliance of tissue
Can be slightly negative in some tissues (holds tissues together)

Question 12

What contributes to πp?

Answer 12

Plasma Colloid Osmotic Pressure (draws contents in to lumen)
Caused by impermeable protein in the plasma
80% of the pressure is caused by albumin (contributes disproportionately)
- Liver produces large amounts of albumin to help contribute to this pressure

Question 13

Describe πi?

Answer 13

Interstitial Colloid Osmotic Pressure
Is typically about 1/3 of plasma COP due to escaped plasma protein via pores and transcytosis
Average value ~8torr

Question 14

Describe the Fluid Balance along the capillary

Answer 14

Arterial end: Net outward force (~13torr) as Pc is high
Mid-capillary: Net outward force (0.3torr)
Venous end: Net inward force (~7torr) for absorption as
Pc is small

In most capillaries, amount of filtration ~ volume
returned by absorption
90% of fluid is reabsorbed –> Remainder in lymphatics (~2mL/min)

Question 15

How do the starling forces effect lymph flow?

Answer 15

Lymph flow increase if Pc increases, πp decreases, πi increases, capillary permeability increases

Lymph flow limited by Pi: >Patm, vessel diameter decreases (compression)–> Resistance increases