Physiology: Microcirculation

Question 1

What is the functions of the microcirculation?

Answer 1

Transport of nutrients to tissue cells

Transport of cellular waste products away from tissue cells

Question 2

Describe the design of the microcirculation.

Answer 2

Arteries branch several times before forming arterioles
Arterioles branch and then form capillaries
Blood enters the capillary through the arteriole and leaves via the venule

Question 3

What two things can arterioles give arise to?

Answer 3

Either terminate directly into capillaries
Or give rise to metarterioles which then give rise to capillaries
Metarterioles can directly connect to the venules as well

Question 4

What is the primary function of capillaries?

Answer 4

Gas exchange - very large surface area and low velocity of blood

Question 5

What is important to note about the structure of capillaries?

Answer 5

No smooth muscle is present
Thin walled tube of endothelial cells without smooth muscle cells
Covered by basement membrane
Endothelial cells contain vesicles (endocytotic and exocytotic vesicles)
Endothelial cells separated by water-filled spaces (intercellular cleft) - allows water and small lipid-insoluble molecules (glucose, amino acids, drugs)

Question 6

Why are capillaries known for nutritional blood flow?

Answer 6

6% of total circulating blood is flowing through capillaries, which performs all the functions - exchange of nutrients, metabolic end products and secretions.

Question 7

What is important to note about capillary structure?

Answer 7

True capillary has no smooth muscle - incapable of active constriction.
Capillaries do withstand high pressure considering their small size.

Question 8

What covers capillaries/are vascular mural cells?

Answer 8

Pericytes

Question 9

Name the three types of capillaries.

Answer 9

Continuous
Fenestrated
Discontinuous

Question 10

Describe the continuous capillary and where it is found.

Answer 10

Moderate permeability
Brain and nervous system (blood-brain-barrier, very tight)
Skeletal muscle, myocardium, lungs
Skin, fat and connective tissue

Question 11

Describe the fenestrated capillary and where it is found.

Answer 11

Rapid filtration - high water permeability
Exocrine gland e.g. salivary glands
Endocrine glands
Other high water turnover tissues e.g. kidney, synovial joints, anterior eye, choroid plexus (CSF), gut mucosa

Question 12

Describe the discontinuous capillaries and where they are found.

Answer 12

Endothelial gaps over 100nm wide (proteins pass through to go into circulation)
Liver
Spleen
Bone marrow

Question 13

Name the two main functions of capillaries.

Answer 13

Fluid exchange - regulation of plasma and interstitial fluid volumes
Solute exchange - nutrition of tissue, hormone and drug delivery

Question 14

Describe what allows fluid exchange to occur.

Answer 14

Due to pressure gradients across the wall

Obeys Starling’s Principle

Question 15

What allows solute exchange to occur?

Answer 15

Due to concentration gradient across the wall

Question 16

Which capillaries are more permeable?

Answer 16

Venous capillaries are more permeable than arterial capillaries

Question 17

What establishes diffusion at the capillary?

Answer 17

Cellular utilization or production of metabolites

Question 18

Describe how different substances diffuse through the cells.

Answer 18

Lipid soluble substances, oxygen and CO2 easily diffuse through endothelial cells

Ions and polar molecules are poorly soluble - pass through intercellular clefts

Proteins usually not allowed to diffuse through intercellular clefts

Question 19

Name the factors affecting diffusion across capillaries.

Answer 19

Type of molecule
Molecule size- low molecular weight = high permeability, high molecular weight = low permeability
Metabolic demand - e.g during exercise and recruitment of capillaries - increases TSA and shortens diffusion difference

Question 20

Describe the Starling Principle of fluid exchange.

Answer 20

Capillary exchange depends on hydrostatic pressure (exerted by the fluid in the blood, capillary pressure) and oncotic pressure (colloid osmotic pressure, COP) exerted by the proteins.

Question 21

Name the 4 pressure determining the filtration rate.

Answer 21

Capillary hydrostatic pressure - filtration
Osmotic - reabsorption
Arterial end of capillary favours filtration
Venous end of capillary favours absorption

Question 22

What causes oedema?

Answer 22

Increases imbalance between hydrostatic and colloid pressure
Starvation, malabsorption, liver failure, nephrotic syndrome

Question 23

What is the clinical significance of fluid exchange?

Answer 23

An excessive rate of capillary fluid filtration from plasma to Interstitium leads to fluid accumulation in the tissues = oedema

• lymphatic disorder
• cardiac failure
• renal failure
• pleural effusions
• ascites
• joint effusions

Question 24

An excessive movement of capillary fluid from the interstitium into the plasma leads is caused by:

Answer 24

Haemorrhage

Normal functioning of renal and intestinal mucosa