13. Capillary Exchange & Localised Control of Circulation

Question 1

Define Capillary Exchange?

Answer 1

the exchange of materials from capillary blood vessels into the surrounding tissues.

The processes that facilitate capillary exchange are principally by diffusion, bulk transport and transcytosis.

Question 2

What are the cellular mechanisms responsible for transport processes?

Answer 2

Diffusion (O2, CO2, Lipid-soluble molecules)

Bulk Flow (water, electrolytes, small molecules)

Transcytosis (macromolecules)

Active transport (ions, small molecules)

Question 3

Where does Bulk Flow occur?

Answer 3

Bulk flow occurs between the intercellular clefts ant the gaps between the endothelial cells.

Question 4

Describe the overall process of the Net Filtration Pressure.

Answer 4

Capillary Hydrostatic Pressure (CHP) - “blood pressure”, adds a force to the wall of capillaries. “pushing” water & solutes out of capillaries into the interstitial fluid.

But this is opposed by osmotic forces from interstitial fluid - Blood Colloid Osmotic Pressure (BCOP).

Hence the overall drive - the net filtration pressure, is determined by both hydrostatic and osmotic pressure values.

Question 5

How does blood pressure in capillaries change?

Answer 5

Capillary blood pressure declines from the arterial end to the venous end.

Question 6

What is capillary hydrostatic pressure (CHP)?

Answer 6

CHP is the force that drives fluid out of capillaries and into the tissues.

CHP forces water & solute through gaps between adjacent endothelial cells in continuous capillaries

Question 7

what counteracts capillary hydrostatic pressure (CHP)?

Answer 7

CHP is counteracted by a hydrostatic pressure gradient generated by the interstitial fluid.

Question 8

What is Net colloid osmotic pressure?

Answer 8

The pressure created by the concentration of colloidal proteins in the blood.

Question 9

Describe the process of capillary exchange.

Answer 9

• At arteriole end there is a positive net filtration pressure (CHP >BCOP), = blood is carrying a pressure wave and forcing fluid out of capillaries. - filtration
• The opposite occurs at venous end (BCOP >CHP). - reabsorption
• Max filtration pressure > than maximal absorption pressure = transition point is located towards the venous end of capillary and not in centre.
  = more filtration takes place along capillary than absorption

Question 10

Why is Elevated Hydrostatic Pressure Problematic?

Answer 10

• High BP results = poor capillary exchange
• If CHP ↑ or ↓ BCOP fluid balance is altered and there is net exit into peripheral tissue:
  = Fluid collects in extremities
  = Systemic oedema

Question 11

How is the pulmonary circulatory system different to other parts of the body?
(O2 levels)

Answer 11

• ↓ O2 + CO2 ↑ = constriction of pulmonary arterioles
• Under O2 rich conditions, pulmonary vessels dilate and blood flow increases to absorb O2.
• When O2 is depleted, pulmonary vessels constrict to shunt O2 to the alveoli cells.

This is different to other parts of the body where low O2 signals promote vasodilation to enhance O2 delivery.

Question 12

Describe Pulmonary Capillary Exchange.

Answer 12

• Pulmonary vascular resistance is very low.
• Pulmonary arterial BP is lower than the systemic BP
• Increased distensibility means these vessels can readily accommodate increased cardiac output without significant increases in pressure
• Importantly, fluid can also be continually absorbed along the entire capillary length. The prevents any interference with gas diffusion.