Capillary Filtration Flashcards
Briefly describe capillary structure.
- 0.5 - 1mm long
- Between 5 – 8μ in diameter
- High density of capillaries allows for large surface area for exchange of materials
- Capillary wall 0.2μ thickness
Is pressure higher at the arterial end or the venous end of the capillary?
ARTERIAL END
How is capillary flow non-uniform? What causes it to be non-uniform?
- Vasomotor tone - can be influenced by neural and humoral factors
- Controlled at arteriolar level
- No pre-capillary sphincter
- Capillary recruitment to alter blood flow in response to specific circumstances
Describe the following structures of the capillary
- Endothelial tube
- Vacuoles
- Pericytes
- Intercellular junctions
Describe the gap junctions and tight junctions in capillaries.
How are the following transported across capillaries
- O2 and CO2
- Water
- Solutes
- Macromolecules
- O2 and CO2 - Diffusion
- Water - Filtration
- Solutes - Diffusion
- Macromolecules - Transcytosis
Describe transcytosis.
- Energy requiring method of transport
- Transendothelial vacuolar transport
- Transports macromolecules e.g ferritin
Describe endothelial glycocalyx.
-
- Fibrillar glycoproteins
- Covers intercellular gaps
The Starling forces are major determinants of capillary filtration. Identify them.
- Hydrostatic pressure in the capillary (Pc)
- Hydrostatic pressure in the interstitium (Pi)
- Oncotic pressure in the capillary (pc)
- Oncotic pressure in the interstitium (pi)
How does increasing venous pressure lead to swelling?
- Increased venous pressure leads to raised hydrostatic capillary pressure
- Increased fluid filtration rate into interstitium
- Tissue swelling (oedema)
What was proposed in the traditional model of fluid exchange?
- At steady state with absorption and filtration balanced
- Plasma colloid osmotic pressure leads to absorption as hydrostatic pressure decreases along capillary
What has been proposed in the revised model of fluid exchange?
Osmotic pressure exerted by the endothelial glycocalyx is responsible for the opposition to hydrostatic pressure.
Describe the relationship between the Starling forces in the revised model of forces and fluid exchange.
- Hydrostatic capillary pressure, Pc is opposed by interstitial pressure Pi
- Interstitial osmotic pressure Πi is opposed by the osmotic pressure exerted by the endothelial glycocalyx, Πg
Write out the revised Starling equation.
Jv = Lp .A .[(Pc- Pi) – σ(Π𝑖−Π𝑔)]
What do the symbols in the revised Starling equation refer to?
Jv: Fluid Filtration Rate
Lp: Hydraulic Conductivity
A: Capillary surface area
Pc: Hydrostatic Capillary Pressure
Pi: Hydrostatic pressure in the interstitium
Π𝑝: Plasma colloid osmotic pressure
Π𝑔: Glycocalyx osmotic pressure
σ: Reflection coefficient
What does reflection coefficient refer to?
- Describes the capillary’s permeability to proteins.
- Between 0 and 1 - if coefficient closer to 1 means more impermeable to proteins
What factors influence Lp?
- Number of open Endothelial Junctions
- Size of intercellular cleft gaps
- Permeability of glycocalyx
The endothelial cells contain actin and myosin. Suggest how capillary permeability would be affected if these two were to interact.
- Change in conformation of intercellular junctions
- Alters capillary permeability
Identify the 3 types of capillary.
- Continuous
- Fenestrated
- Discontinuous
Describe fenestrated capillaries.
- Allows for rapid water filtration
- Doesn’t filter macromolecules
- EXAMPLES - Salivary gland and renal glomerulus
Describe discontinuous capillaries.
- Allows for loss of macromolecules and cells from circulation
- EXAMPLE - Bone marrow and spleen/liver sinusoids
Describe oedema.
- Tissue swelling due to fluid accumulation
- Occurs when venous pressure increases (not arterial)
