Lecture 7 - Microcirculation

Question 1

What are the functions of the microcirculation?

Answer 1

• Transport of nutrients to tissue cells
• Transport of cellular waste products away from tissue cells.

The microcirculation of each organ is specifically organised to meet its onw special needs

Question 2

Describe the design of the microcirculation

Answer 2

Arteries branch several times before forming arterioles. Arterioles in turn branch 2-5 times reaching a diameter of 10-30microns before the feed into capillaries (4-10microns diameter).

Blood eneters the capillary through the arteriole and leaves through the venule.

Question 3

How do arterioles terminate into capillaries?

Answer 3

They can terminate into capillaries directly or to metaarterioles which can give rise to capillaries.

Metaarterioles can directly connect to the venules aswell.

Question 4

What is the primary function of capillaries? And what features do they have which allows them to do this?

Answer 4

The primary function of capillaries is exchange. They have a very large total cross-sectional area and very low velocity of blood flow; a capillary residence time (transit time) of about 0.5-2s. The slow velocity allows for more efficient diffusion of nutrients into the cell, and waste products being removed.

Not all the capillaries are open in an organ.

Question 5

Describe the structure of capillaries

Answer 5

They are a thin walled tube or endothelial cells without smooth muscle cells (have no tunica media).

The cells are not attached tightly to each other, they are seperated by water filled spaces called intercellular clefts - they allow movement of water and small lipid-insoluable solutes (glucose, aminoacids, drugs)

Endothelial cells contain vesicles and exocytotic/ endocytotic vesicles.

They are covered in a basement membrane, and are 500-1000microns long and 4-8microns wide with a total thickness of ~0.5microns (relative to wall thickness, they withstand more pressure than the aorta)

Question 6

At any given time 5% of total circulating blood volume is in the capillaries, what is this knwon as?

Answer 6

This 5% is known as nutritional blood flow, it allows for the exchange of nutriets, metabolic end products and secretions.

Question 7

Why can capillaries withinstand such high pressures? (relative to wall thickness)

Answer 7

Law of Laplace tells us.

T= wall tension (tendency for the vessl wall to split

P = transmural pressure

r = radius of vessel

A very small radius gives the capillary a very low wall tension, and this gives it high distensibility (very low tendency to split), this means it can withstand high pressures without splitting.

Question 8

Describe continuous capillaries

Answer 8

• They have moderate permeability
• Found in brain and nervous system (blood-brain barrier, very tight)
• Skeletal muscle, myocardium, lungs
• Skin, fat & connective tissue

Question 9

Describe fenestrated capillaries, and where they’re found

Answer 9

• They allow for rapid filtration due to their high water permeability
• They are found in exocrine glands (e.g. salivary gland)
• Endocrine glands
• Other areas with ‘high water turnover’ e.g. kidney, synovial joints, anterior eye, choroid plexus

Question 10

Describe discontinuous capillaries, and where they are found (sinusoidal)

Answer 10

They have endothelial gaps over 100nm wide

• found in liver
• spleen
• bone marrow

These allow for proteins to be filtered more easily due to the larger gaps

Question 11

What are the two main functions of capillaries, and how do they occur

Answer 11

Question 12

What is main way nutrients or drugs are transported across capillaries?

Answer 12

Via diffusion (98% is transported this way)

Ions and polar molecules are poorly soluble and pass through the water filled channels in the endothelial lining - intercellular cleft.

Whereas lipid soluble substances diffuse though endothelial cells.

the transcapillary diffusion gradient is because of the cellular utilization or production

Question 13

What initiates and facilitates diffusion of glucose, O2 and CO2 in capillaries?

Answer 13

Cellular utilization or production of metabolites establish diffusion at the capillary

At rest there is no gradient for O2, CO2 and glucose. But as the muscle cell starts to metabolise it uses up O2 and glucose, this creates a conc. gradient between the muscle cell and the interstitial fluid, which creates a conc gradient favouring movement of O2 and glucose into the muscle cell. This also favours O2 and glucose moving from the capillary into the interstitial fluid. CO2 is produced in the muscle cell, it moves into interstitial fluid down it’s conc. gradient, and then into the capillary down it’s gradient again.

Question 14

What factors affects diffusion across capillaries?

Answer 14

1. The type of molecule

2. Molecule size: Low molecular weight = high permeability

High molecular weight = low permeability

3. metabolic demand: During heavy exercise O2 transport from blood to muscle increases 40x: this is because of three factors

• increased tissue consumption rate
• causing an increase in conc. gradient
• And increased blood flow raises capillary conc.

4. Recruitment of capillaries

• Increases total surface area for diffusion
• Shortens diffusion distance

Question 15

Describe how metabolic demand affects diffusion across capillaries (in terms of O2)

Answer 15

there is a higher conc of O2 at the arterial end

Towards the venous end O2 is diffusedd from capillaries, and so the conc goes down

The tissue consumes more O2, and this increases the concentration gradient, this causes an increase in blood flow, and so the tissue will get more O2 from the circulation

O2 transport is increased depending on the need

Question 16

Describe how the recruit of capillaries effects diffusion

Answer 16

It increases total surface area for diffusion, and shortens the diffusion distance.

Question 17

What is the starling principle of fluid exchange?

Answer 17

Capillary blood flow is affected by plasma protein osmotic pressure as well as hydrostatic pressure

Question 18

In Starlings principle of fluid exchange which 4 pressures determine the filtration rate?

Answer 18

• Capillary
  
  • Capillary hydrostatic pressure outwards
  • Capillary osmotic force due to plasma protein conc. (mainly albumin)
• Interstitial fluid
  
  • ​Interstitial fluid hydrostatic pressure
  • Osmotic force due to interstitial fluid protein conc.

The Net filtration pressure can be calculated averaging all of these pressures.

Question 19

Describe the changes in hydrostatic pressure between the arterial and venus end of the capillary

Answer 19

Hydrostatic pressure decreases between the arterial end and venus end.

This means fluid is filtered out at the arterial end (since its greater than osmotic force in), and fluid filters into the capillarys (since the hydrostatic pressure outwards drops below the osmotic pressure inwards). Oncotic pressure stays the same between each end since proteins are filtered.

The fluid moves in the direction of the Net filtration pressure (+ve at start, and is -ve at the end)

Question 20

In relation to starlings principle of fluid exchange, why do we have a lymphatic system?

Answer 20

The outward force is always slightly higher than inward force.

The fluid is always being fliltered slightly more than can be absorbed.

Question 21

What happens when there’s a reduced oncotic pressure in the capillary?

Answer 21

Oedema due to increased filtration outwards - we are filtering more fluid into the interstitial space than can be absorbed by the lymphatics, this causes oedema

Question 22