lecture 15 - microcirculation

Question 1

What are the 2 functions of the microcirculation?

Answer 1

Transport of nutrients/o2 to tissues, transport of cellular waste products away from tissues

Question 2

What does blood enter and then leave capillaries through?

Answer 2

Enters via an arteriole, leaves through a venule

Question 3

Why do capillaries have a very low velocity of blood flow

Answer 3

To allow sufficient capillary residency time for exchange to occur

Question 4

What are the layers of a capillary?

Answer 4

Single layer of endothelial cells surrounded by a basement membrane

Question 5

What is the name for the spaces between adjoining endothelial cells in a capillary?

Answer 5

Intercellular clefts

Question 6

What is the function of the intercellular clefts of capillaries?

Answer 6

They are water filled spaces that allow water and lipid insoluble solutes (glucose, amino acids, drugs, etc.) to pass through

Question 7

How are capillaries able to undergo constriction?

Answer 7

Although the have no smooth muscle, pericytes (vascular mural cells) on the outside of the capillary can generate constriction

Question 8

What are the 3 classes of capillary?

Answer 8

Continuous, fenestrated, sinusoidal/discontinuous

Question 9

Why does the brain have continuous capillaries?

Answer 9

To maintain the blood-brain barrier, there must be less permeability

Question 10

Why do the kidneys have fenestrated capillaries?

Answer 10

Because they have high water turnover, but still need to make sure large molecules and proteins are not filtered out.

Question 11

Why does the liver have discontinuous capillaries?

Answer 11

Because it produces lots of proteins, which are large in size and must squeeze into the circulation via large gaps in the capillaries

Question 12

What maintains and alters the gradient which allows diffusion across capillaries?

Answer 12

Cellular utilisation or production of metabolites determines the diffusion gradient, and therefore how much of the solute will be able to diffuse based on how much is needed.

Question 13

What factors determine the rate of diffusion of gases across a membrane (Fick’s Law)?

Answer 13

Surface area of membranes, membrane thickness, pressure difference across membrane

Question 14

What are the 3 key factors that affect diffusion across capillaries?

Answer 14

1.) Molecule type (solubility, polarity, etc.)
2.) Molecule size (molecular weight)
3.) Metabolic demand (O2 demand, capillary recruitment)

Question 15

What is the relatinship between molecular weight and diffusion across a capillary?

Answer 15

Molecules with low molecular weight have higher permeability

Question 16

What are the 2 pressures that capillary exchange depends on?

Answer 16

Hydrostatic pressure, colloid osmotic pressure

Question 17

What is hydrostatic pressure exerted by?

Answer 17

Fluid in the blood or interstitial fluid

Question 18

What is colloid osmotic pressure exerted by?

Answer 18

Proteins in the plasma or interstitium

Question 19

If capillary pressure os greater than oncotic pressure, what will be the net change to plasma volume?

Answer 19

Net loss from plasma (more filtration than absorption)

Question 20

If capillary pressure is less than oncotic pressure, what will be the net change to plasma volume?

Answer 20

Net gain to plasma (more absorption than filtration)

Question 21

What types of clinical conditions would lead to capillary pressure exceeding oncotic pressure, and therefore a net loss from the plasma?

Answer 21

Inflammatory response, heart failure, hepatic failure

Question 22

What types of clinical conditions would lead to oncotic pressure exceeding capillary pressure, and therefore a net gain to the plasma?

Answer 22

Haemorrhage/shock

Question 23

Why does the inflammatory response lead to net loss from plasma?

Answer 23

Vessels become leaky, increasing capillary pressure and driving filtration

Question 24

Why does heart failure cause a net loss from plasma?

Answer 24

Poor right heart function causes blood to back up in the veins, which increases capillary pressure resulting in peripheral oedema.

Question 25

Why does haemorrhage/shock cause processes that cause a net gain in plasma?

Answer 25

Decreased blood volume decreases capillary pressure, driving reuptake/absorption from the interstitium

Question 26

Why does hepatic failure cause a net loss from plasma?

Answer 26

Decreased production of proteins such as albumin reduces the colloid osmotic pressure of the blood, reducing capillary pressure and driving loss from the plasma and into the interstitium.