Microcirculation

Question 1

List the major types of vessels and their morphological differences, including diameter and wall thickness. Describe the types and properties of the three major types of capillaries, including general mechanisms that regulate capillary flow.

Answer 1

Continuous capillaries are most common with interendothelial junctions 10 – 15 nm wide as in skeletal muscle. These junctions are absent in brain capillaries, which have narrow tight junctions that form the blood-brain barrier.

Fenestrated capillaries often surround exocrine glands or epithelial membranes such as the small intestine. Their endothelial cells have conduits that permit the flow of fluid and solutes across the capillary endothelial membrane.

Discontinuous or sinusoidal capillaries are found in liver sinusoids. In addition to fenestrations, they have large gaps between their endothelial cells.

Question 2

Define convective transport, list the factors that determine its rate

Answer 2

Convective transport = solutes carried along with the flow of blood

Transport rate of B = flow rate * concentration of B

Transport rate of B = Q x [B]

Question 3

State Fick’s principle: given data calculate the removal of a solute from the blood as it passes through the circulation

Answer 3

Fick’s principle is an equation that helps understand the production or utilization of a substance by a tissue.

Transcapillary efflux rate (mass/time) = Q (flow rate) x ([X]arterial-[X]venous)

If positive = tissue is using substance since it is higher in the arteries coming into the tissue

If negative = tissue is producing the substance & shooting it into the venous system

Question 4

Describe how capillary wall permeability determines the transport of solutes of differing size. List the factors that determine transcapillary fluid (water) movement: given data predict the flow direction of transcapillary fluid movement.

Answer 4

Pnet = (Pc - Pif) - (πc - πif)

Pc = capillary hydrostatic pressure = +25mmHg (make sure to only put value for arterial or venous end)

Pif = interstitial fluid hydrostatic pressure
*remember hydrostatic pressure = blood pressure

πc = capillary osmotic pressure

πif = capillary interstitial fluid pressure

Pnet positive = water driven out of capillary = capillary filtration pressure

Pnet = 0 = no net transcapullary water movement so capillary filtration pressure = capillary reabsorption pressure

Pnet negative = water flow into capillary = capillary reabsorption pressure

Question 5

Fick’s law of diffusion

Answer 5

Explains the rate of diffusion of molecules:

J = DA x (Cp - Ci)/l

J = flux (mol/min)
D = diffusion coefficient (cm/sec)
A = area of membrane (cm^2)
l = membrane thickness (cm)
Cp = concentration plasma (moles/cm^3)
Ci = concentration interstitial (moles/cm^3)

Question 6

Lymphatic system

Answer 6

Is a pathway for larger molecules to reenter the circulatory system

Question 7

Define edema & state what promotes edema formation

Answer 7

Edema = excess salt and water in the interstitial space

Anything that promotes capillary filtration promotes edema formation.

The capillary filtration coefficient is a measure of the hydraulic conductance across the capillary and is increased by anything that increases capillary permeability.

An increase in venous pressure will raise capillary hydrostatic pressure and promote filtration.

If the concentration of plasma proteins decreases, more fluid will be filtered = more edema.

Question 8

Area of a circle

Answer 8

πr^2

Question 9

Diameter

Answer 9

Diameter = 2 x radius

Question 10

Discuss the Fåhraeus-Lindqvist effect

Answer 10

The apparent viscosity of blood also depends on the diameter of the tube through which the blood is flowing, a phenomenon called the Fåhraeus-Lindqvist effect. At tube diameters less than 0.3 mm (arterioles, capillaries, and venules), the apparent viscosity = hematocrit of blood decreases. The viscosity of plasma or saline is independent of tube diameter.

It provides a major explanation for the Fahraeus-Lindqvist effect, whereby dynamic hematocrit is lower in smaller tubes because of erythrocyte skimming and also the greater velocity of the red blood cells than the plasma in the small tubes.

This is because erythrocytes move over the center of the vessel, leaving plasma at the wall of the vessel.

Question 11

Define diffusion, list the factors that determine its rate

Answer 11

Water flow = Kf x Net pressure

Kf is the filtration coefficient for the capillary

Diffusion refers to the process by which molecules intermingle as a result of their kinetic energy of random motion.

Diffusion occurs efficiently @ capillary walls, it is a powerful mechanism of material exchange, & substances move from high to low concentration gradients.

Question 12

Describe the relationship between plasma proteins & osmotic pressure

Answer 12

Plasma proteins create an osmotic potential (plasma colloid osmotic pressure) that holds fluid in the vasculature.

Question 13

Describe the role of albumin in blood pressure

Answer 13

Serum albumins are important in regulating blood volume by maintaining the colloid osmotic pressure of the plasma (blood compartment).

Question 14

Will decreased plasma concentration of albumin cause edema?

Answer 14

decreased plasma concentration of albumin will cause edema since it reduces πc = capillary osmotic pressure

Question 15

Is the velocity of blood flow higher in the veins or venules?

Answer 15

It is higher in the veins than in the venules

Question 16

What is the best way to increase local blood flow, by dilating arterioles or venules?

Answer 16

Dilate arterioles because they actually have smooth muscle (they are the major controller), venules have no muscle so cannot produce same effect.

Question 17

Define microcirculation

Answer 17

Microcirculation is the circulation of the blood in the smallest blood vessels, present in the vasculature embedded within organ tissues. This contrasts with macrocirculation, which is the circulation of blood to and from the organs. The microcirculation is composed of terminal arterioles, capillaries and venules which drain capillary blood.

Question 18

Pressure in the interstitial fluid promotes ______.

Answer 18

Pressure in the interstitial fluid promotes reabsorption

Question 19

An increase in venous pressure will raise _______ and promote _____.

Answer 19

An increase in venous pressure will raise capillary hydrostatic pressure and promote filtration.

Question 20

Capillary filtration promotes ______ formation.

Answer 20

Capillary filtration promotes edema formation.

Question 21

If the concentration of plasma proteins decreases, filtration _______.

Answer 21

If the concentration of plasma proteins decreases, filtration increases.