Microcirculation Flashcards
What is the difference in the anatomy of arterioles vs meta-artioles?
Arterioles are heavily invested with smooth muscle cells whereas metarterioles are less so invested.
What are pre-capillary sphincters?
Precapillary sphincters are specialized vascular smooth muscle cells that regulate flow into individual capillaries at the take-off of capillaries from metarterioles.
Where are precapillary sphincters most abundant?
in the mesenteric circulation.
How do lipid soluble substances (CO2 and O2) diffuse from capillaries?
they are lipophilic and are therefore transported by simple diffusion, down their chemical gradients
How does water and small solutes move through capillaries? Proteins?
Water and small solutes move through clefts whereas proteins are essentially trapped in capillaries, except in capillaries that have fenestrae, as in liver.
What may happen to large molecules in capillaries?
they may be taken up by endothelial cells by endocytosis and transported across and expelled by exocytosis into interstitial fluid. Flux through this pathway is low
Eqn for osmotic pressure?
=RT(sigma)(deltaC), where
R=“reflection” coefficient, a term that is the inverse of permeability.
The greater the reflection coefficient, the less the capillary is permeable to that substance.
For example, large proteins such as albumin and hemoglobin are essentially reflected completely, i.e. the capillary is impermeable to these and therefore they generate a significant osmotic pressure, whereas water and NaCl have an R near zero
Normally sigma=1. When would it not be?
Increased capillary permeability for protein in inflammatory conditions and burns, for example, would decrease σ to a value of less than 1.
What is transcapillary flow defined by?
pressure differences across the capillary wall- governed by Starling’s eqn
What is Starling’s Eqn?
Jv= Kf*[(Pc-Pi)-(πc-πi)], where
Jv represents transcapillary flow and is a positive number when net flow occurs from the capillary to the interstitium (aka filtration). If Jv is negative, this indicates net flow from interstitium to capillary, indicating reabsorption.
Kf (termed the hydraulic conductance) is the filtration coefficient and is equivalent to the inverse of resistance in Ohm’s equation.
Pc is capillary hydrostatic pressure; Pi is interstitial hydrostatic pressure; πc is the capillary oncotic pressure; πi is the interstitial oncotic pressure.
Note about Starling’s Eqn.
P is a “positive”pressure, that is, it forces water OUT OF the compartment in which it occurs, whereas Π is a “negative” pressure and pulls water INTO the compartment in which it occurs
How does Pc change at the venous end of capillaries from the arterial end?
At the venous end of capillaries, there is reduced Pc compared to the arterial end, because of the loss of pressure during forward movement of fluid.
Note that the Pc is assumed to decrease linearly from arterial to venous end
However, all other pressures remain similar to those at the arterial end of the capillaries.
What is Kf regulated by?
- precapillary sphincters which block flow to capillaries thus shunting flow around them
- the type of capillary
- inflammation
How does the type of capillary affect Kf?
Fenestrated versus continuous endothelium; fenestrated endothelium has greater permeability and therefore a higher Kf
How does inflammation affect Kf?
Inflammatory mediators increase capillary permeability (e.g. histamine, bradykinin, some prosta-glandins, substance P, etc)
