Physiology: Microcirculation Flashcards
(24 cards)
What is the functions of the microcirculation?
Transport of nutrients to tissue cells
Transport of cellular waste products away from tissue cells
Describe the design of the microcirculation.
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
What two things can arterioles give arise to?
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
What is the primary function of capillaries?
Gas exchange - very large surface area and low velocity of blood
What is important to note about the structure of capillaries?
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)
Why are capillaries known for nutritional blood flow?
6% of total circulating blood is flowing through capillaries, which performs all the functions - exchange of nutrients, metabolic end products and secretions.
What is important to note about capillary structure?
True capillary has no smooth muscle - incapable of active constriction.
Capillaries do withstand high pressure considering their small size.
What covers capillaries/are vascular mural cells?
Pericytes
Name the three types of capillaries.
Continuous
Fenestrated
Discontinuous
Describe the continuous capillary and where it is found.
Moderate permeability
Brain and nervous system (blood-brain-barrier, very tight)
Skeletal muscle, myocardium, lungs
Skin, fat and connective tissue
Describe the fenestrated capillary and where it is found.
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
Describe the discontinuous capillaries and where they are found.
Endothelial gaps over 100nm wide (proteins pass through to go into circulation)
Liver
Spleen
Bone marrow
Name the two main functions of capillaries.
Fluid exchange - regulation of plasma and interstitial fluid volumes
Solute exchange - nutrition of tissue, hormone and drug delivery
Describe what allows fluid exchange to occur.
Due to pressure gradients across the wall
Obeys Starling’s Principle
What allows solute exchange to occur?
Due to concentration gradient across the wall
Which capillaries are more permeable?
Venous capillaries are more permeable than arterial capillaries
What establishes diffusion at the capillary?
Cellular utilization or production of metabolites
Describe how different substances diffuse through the cells.
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
Name the factors affecting diffusion across capillaries.
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
Describe the Starling Principle of fluid exchange.
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.
Name the 4 pressure determining the filtration rate.
Capillary hydrostatic pressure - filtration
Osmotic - reabsorption
Arterial end of capillary favours filtration
Venous end of capillary favours absorption
What causes oedema?
Increases imbalance between hydrostatic and colloid pressure
Starvation, malabsorption, liver failure, nephrotic syndrome
What is the clinical significance of fluid exchange?
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
An excessive movement of capillary fluid from the interstitium into the plasma leads is caused by:
Haemorrhage
Normal functioning of renal and intestinal mucosa
