Microcirculation

Question 1

What is the difference in the anatomy of arterioles vs meta-artioles?

Answer 1

Arterioles are heavily invested with smooth muscle cells whereas metarterioles are less so invested.

Question 2

What are pre-capillary sphincters?

Answer 2

Precapillary sphincters are specialized vascular smooth muscle cells that regulate flow into individual capillaries at the take-off of capillaries from metarterioles.

Question 3

Where are precapillary sphincters most abundant?

Answer 3

in the mesenteric circulation.

Question 4

How do lipid soluble substances (CO2 and O2) diffuse from capillaries?

Answer 4

they are lipophilic and are therefore transported by simple diffusion, down their chemical gradients

Question 5

How does water and small solutes move through capillaries? Proteins?

Answer 5

Water and small solutes move through clefts whereas proteins are essentially trapped in capillaries, except in capillaries that have fenestrae, as in liver.

Question 6

What may happen to large molecules in capillaries?

Answer 6

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

Question 7

Eqn for osmotic pressure?

Answer 7

=RT(sigma)(deltaC), whereR=“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

Question 8

Normally sigma=1. When would it not be?

Answer 8

Increased capillary permeability for protein in inflammatory conditions and burns, for example, would decrease σ to a value of less than 1.

Question 9

What is transcapillary flow defined by?

Answer 9

pressure differences across the capillary wall- governed by Starling’s eqn

Question 10

What is Starling’s Eqn?

Answer 10

Jv= Kf*[(Pc-Pi)-(πc-πi)], whereJv 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.

Question 11

Note about Starling’s Eqn.

Answer 11

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

Question 12

How does Pc change at the venous end of capillaries from the arterial end?

Answer 12

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 endHowever, all other pressures remain similar to those at the arterial end of the capillaries.

Question 13

What is Kf regulated by?

Answer 13

• precapillary sphincters which block flow to capillaries thus shunting flow around them- the type of capillary- inflammation

Question 14

How does the type of capillary affect Kf?

Answer 14

Fenestrated versus continuous endothelium; fenestrated endothelium has greater permeability and therefore a higher Kf

Question 15

How does inflammation affect Kf?

Answer 15

Inflammatory mediators increase capillary permeability (e.g. histamine, bradykinin, some prosta-glandins, substance P, etc)

Question 16

What things regulate capillary hydrostatic pressure?

Answer 16

a. Altered arteriolar resistance – has relatively major effectb. Altered venous pressure- has relatively major effect; is associated with disease statesc. Altered arterial pressure- has relatively minor effectd. Altered venous resistance –has relatively minor effect

Question 17

How does arteriolar resistance regulate capillary pressure?

Answer 17

-decrease arteriolar resistance increases blood flow into capillaries and expands capillary volume. This increases Pc

Question 18

How does venous pressure regulate capillary pressure?

Answer 18

increased venous pressure decreases blood flow out of capillaries and therefore increases Pc

Question 19

How much fluid is removed from interstitium via capillary reabsorption vs lymph removal?

Answer 19

Fluid removal mostly occurs via lymph flow although a small amount of fluid is removed by capillary absorption.

Question 20

What is lymph flow regulated by?

Answer 20

Pi- there is a dramatic increase in lymph flow in response to elevated Pi. can you draw the curve?

Question 21

Describe the relationship between lymph flow (y) and Interstitial fluid pressure (x)?

Answer 21

starts off close to linear from (0,0) and begins to go vertical quickly at (4,10) up to about (5,20) and then goes completely horizontal (i.e. 1000,20)the horizontal portion of the graph represents when fluid pressure rises above a certain level the lymphatic pump is no longer able to cope, which may lead to edema.

Question 22

What happens when a decrease in plasma protein concentration occurs?

Answer 22

net oncotic pressure decreases. As a result, there is increased filtration. We assume that all other pressures are unchanged.

Question 23

What things could cause plasma protein conc to decrease?

Answer 23

-liver disease and starvation-kidney disease-intravenous infusion; fluid shift

Question 24

What things could cause plasma protein conc to increase (i.e. loss of plasma volume)?

Answer 24

-dehydration-diarrhea-vomiting-prolonged sweating

Question 25

What is interstitial oncotic pressure regulated by?

Answer 25

The levels of interstitial protein are regulated by protein entering from capillaries and protein removed by lymphatic drainage. Because fluid entering the interstitial space is essentially protein free, increased lymph flow will lower the protein concentration

Question 26

How does lymph drain?

Answer 26

Flip valves open more often when interstitial pressure increases. Moreover, spontaneous contraction of smooth muscle cells creates suction. In addition to active propulsion, lymphatics are compressed by skeletal muscle activity.

Question 27

Edema does not occur until capillary pressure has risen substantially by about ____.

Answer 27

17-20 mm Hg.