Microcirculation

Question 1

What is the mechanism of gas exchange across the capillary wall?

Answer 1

• The exchange happens based on the type of solubility the molecules have, lipid or water-soluble.
• If it’s a lipid-soluble substance, it can cross through the endothelial wall easily as the wall is lipid-based. E.g. oxygen and carbon dioxide.
• If it’s a water-soluble substance, it cannot go through the endothelial lining directly, and this is where the pores come in handy and are used as ‘entrances’ for these substances. E.g. water, glucose, amino acids, sodium, and potassium.
• Small proteins can get exchanged through the wall through by the ‘vesicular transport’, keep in mind the protein has to be SMALL.

Question 2

Describe the structure of the capillary bed

Answer 2

• One layer of endothelium
• Located between the arterial and venules
• Precapilary sphincters are present in the metarterioles before reaching the true capillaries, which regulates the amount of blood entering the capillaries
• Thoroughfare channel allowed blood to bypass the true capillary minimizing the gas exchange

Question 3

What are the major types of capillaries?

Answer 3

1) Continuous (muscles, lungs, and the brain)

2) Fenestrated (GIT, kidneys, gallbladder, and endocrine glands)

3) Sinusoid (liver, bone marrow, spleen, and lymphoid tissue)

Question 4

What is Fick’s first law of diffusion?

Answer 4

• Gases and solutes mainly exchange across the capillary wall by simple diffusion

J (net rate of diffusion) = D (Diffusion coefficient) * A (Surface area of the membrane) * (Dc “concentration difference across the membrane” * Dx “Thickness of the membrane”)

J = DA * (Dc/Dx)

• The only variable that has an inverse relationship is the thickness of the membrane (Dx), which is usually affected in pathologies

Question 5

The diffusion coefficient depends on which factors?

Answer 5

1) Size of the molecule

2) Viscosity of the medium

• The higher the more diffusion

Question 6

What are the mechanisms by which the capillaries increase the exchange rate?

Answer 6

1) Arteriolar dilation

• Increases the concentration gradient

2) Opening of the precapillary sphincters

• Increase available SA
• Decrease the diffusion distance

Question 7

What are the starling pressures/forces?

Answer 7

They are driving forces of fluid exchange across the capillary wall which are:

1) Hydrostatic pressure

2) Osmotic pressure

• Fluid is driven by their sums

Question 8

How to determine the filtration using the starling forces?

Answer 8

• Basically, you sum the hydrostatic pressure of the capillary with the osmotic pressure of the interstitial fluid then minus (-) it from the sum of the osmotic pressure of the capillary and the hydrostatic pressure of the interstitial fluid
• If the total is negative then it is an absorption if the total is positive then it is filtration
• Filtration occurs near the arterioles while absorption occurs near the venules, if there was any left overs it will be taken by the lymphatics
• To determine the fluid movement you multiply the sum with the hydraulic conductance (water permeability ml/min mmHg)

Fluid movement = Kf * (“Pc + Io” - “Po + Ic”)

Question 9

What regulates the capillary hydrostatic pressure?

Answer 9

Blood pressure

Question 10

What regulates the capillary oncotic pressure?

Answer 10

Protein concentration

Question 11

What determines the hydraulic conductance Kf?

Answer 11

• It is the water permeability of the capillary wall, which determines the magnitude of fluid movement for a given pressure
• It depends on the anatomical characteristics of the capillary (size of cleft, fenestration, etc)
• Lowest in the cerebral capillaries and highest in glomerular capillaries
• It increases in capillary injuries (like toxins and burns)

Question 12

What are the conditions that affect the capillary hydrostatic pressure?

Answer 12

1) Increases in arteriolar dilation and vasoconstriction

2) Decreases in arteriolar constriction when there is a hemorrhage for example

Question 13

What are the conditions that affect the capillary oncotic pressure?

Answer 13

1) It increases if we lose fluid but not protein (like in dehydration and diarrhea)

2) Decreases due to less protein in the plasma (due to malnutrition, malabsorption, liver or renal disease)

Question 14

What are the conditions that affect the interstitial hydrostatic pressure?

Answer 14

In respiratory distress syndrome, we will have a more negative thoracic pressure which will decrease the hydrostatic pressure, which will cause more filtration

Question 15

What are the conditions that affect the interstitial osmotic pressure?

Answer 15

It will increase in case of Impaired lymphatics like a tumor in the lymph nodes or if the permeability of the capillary increases like in burns

Question 16

How is the structure of the lymphatics related to its function?

Answer 16

Their endothelial arrangement is such that it allows for unidirectional movement of fluid to go in but not out

• Were the movement of the fluid in it is dependent on the surrounding smooth muscles and its valves
• The rate of lymph movement per day is 1-3 liters while blood is 7200L

Question 17

What is edema?

Answer 17

When there is a increased volume in the interstitial fluid due to increased filtration and decreased absorption by the lymphatics or other causes

Question 18

What are the things that can impair the lymphatic drainage?

Answer 18

1) Long-standing

2) Removal of lymph nodes

3) Infection of the lymph nodes

Question 19

What is lymphedema?

Answer 19

• Seen in stage-2 breast cancer where it is related to the edema of the right upper extremity
• Uterine cancer lymph edema is seen in the left lower extremity

Question 20

What is elephantiasis?

Answer 20

It could be due to a mosquito-born parasitic infection of the lymph nodes