Microcirculation and Capillary Function

Question 1

What does microcirculation include?

Answer 1

The terminal arterioles, capillaries, and post - capillary venules

Question 2

What is the function of microcirculation?

Answer 2

Transfer of gases, water, nutrients and other substances between blood and tissues

Question 3

How does microcirculation work involving temperature control?

Answer 3

To promote heat loss: decrease in sympathetic drive to AVA’s causes vasodilation, blood shunted to dermal venous plexus raising cutaneous flow and promoting heat loss

To conserve heat:
increase sympathetic drive, AVA’s constrict, less heat loss

AVA’s = arteriovenous anastomoses

Question 4

What does the density of the microcirculation depend on?

Answer 4

The supply of blood to each tissue (terminal arterioles). The tone of this is controlled by local factors

Question 5

What do capillaries and post - capillaries have to make them constrict?
What do these capillaries act as? What type of epithelium do they have?

Answer 5

They have pericytes because they do not have any smooth muscle

They acts as exchange vessels. They have continuous epithelium.

Question 6

What is the role of the lymphatic capillaries and where do they drain to?

Answer 6

The role is to absorb fluid and protein and return this back to the blood through the venous circulation.

Question 7

What is the process of vasomotion?

What is transit time?

Answer 7

• terminal arterioles constrict and relax periodically
• how long blood takes to flow through the capillaries (decrease in arteriolar tone causes transit time to fall (during exercise)

Question 8

What is the arrangement of smooth muscle and epithelium cells around a blood vessel?

Answer 8

Smooth muscle is wrapped around the vessel and epithelium cells.

Question 9

What affect does nitric oxide have on the blood vessels?

Answer 9

Nitric oxide

Question 10

What are the 3 types of capillary walls?

Answer 10

Continuous - cells are very packed together, continuous have 1-3 endothelial cells in each layer and surrounded by a basement membrane.
They are exchange vessels.
It is permeable to some substances but not others.
Efficient exchange due to short trans-capillary diffusion distance.

Fenestrated - Endothelium perforated by small fenestrae bridged by a fenestrae diaphragm.

Discontinuous - Endothelial gaps >100nm wise basal lamina is also interrupted

Question 11

Where are the 3 types of continuous, fenestrated, discontinous found?

Answer 11

Continuous - lung, skeletal muscle, myocardium, skin, connective tissue, fat

Fenestrated - kidneys, intestinal mucosa, joints

Discontinuous - bone marrow, spleen and liver

Question 12

What ways can substances permeate the capillary wall?

Answer 12

Transcellular
Intercellular
Fenestrated

Question 13

Movement of solutes across exchange vessel walls:

• Gases
• Small solutes
• Water
• Proteins

Answer 13

Gases: can move across the plasma membrane - very fast, diffusion
Small solutes: e.g. salts, glucose, amino acids: move via intercellular junctions or fenestrae
Water: about 90% via intercellular junctions, about 10% via water channels in the plasmalemma
Proteins: mainly vesicular transport, endothelial channels transiently formed by vesicular fusion - very slow, convection

Question 14

What substance is most permeable to the capilarries?

What is this compared against?

Answer 14

Oxygen (3000)

Water (1)

Question 15

What net volume leave the microcirculation in litres and returned to the bloodstream via the lymphatic system?

Answer 15

8 litres

Question 16

Fluid filtration depends on 2 pressure gradients, what are they?

Answer 16

1. Hydrostatic pressure gradient (primary force driving plasma filtration is the capillary blood pressure)
2. Osmotic pressure gradient - the primary force retaining fluid within capillaries is the osmotic pressure by plasma proteins (the oncotic pressure gradient tends to draw water into capillaries

Question 17

What does a positive value of plasma movement mean?

Answer 17

Outward movement

More fluid lost on the arterial side and on the venous side there is a small net resorption of fluid occurring.

Question 18

What does Starling’s equation mean?

Answer 18

The net movement of fluid across an exchange vessel is driven by overall force of these 2 pressure gradients.
Refers to to the reflection coefficient for plasma (0.8-0.95) which corrects for the endothelium as an imperfect barrier.
Less permeable solutes = have a higher coefficient

Question 19

Filtration is dynamic and varies —–

On average there is a small net—-pressure gradient.

Answer 19

Locally

Outward

Question 20

Filtered fluid is removed by the lymphatic system:

Answer 20

Lymphatic capillaries are slightly larger and blunted, allowing unidirectional flow .
Consist of continuous, overlapping endothelium with interrupted basal lamina and anchoring filaments including elastin. This allows for fluid to regularly move into the lymphatic vessel.
High internal oncotic pressure aids transfer, fluid moves in when interstitial pressure exceeds that inside the lymph

Question 21

Where is most of the fluid reabsorbed in the lymphatic system?

Answer 21

Most of it is reabsorbed at the lymph nodes

Question 22

When does oedema occur?

Answer 22

It occurs when net fluid volume leaving the capillaries and entering the interstitium exceeds uptake by the lymphatic system

Question 23

How does oedema develop?

Answer 23

Oedema occurs if the amount of fluid leaving the microcirculation increases due to …

1) an increased capillary hydrostatic pressure gradient
- an increase in CVP due to a decrease in cardiac output and or fluid retention
2) a decreased oncotic pressure gradient
3) lymphatic system being blocked