Capillaries: the important exchange vessels

unit 3 week 11 lesson 3

what is the capillary wall

just a single cell thick and these cells are referred to as endothelial cells

why is having a thin wall in the capillary perfectly suitable

for exchange and can use diff ways to move substances across the capillary wall. all exchanges happens in capillaries

substance movement

movement may be from the plasma within a capillary out to the interstitial fluid surrounding a capillary or from the intersitial fluid back into the plasma

what do the substances encounter during their movement

substances passing thru the endothelial cell have to encounter two plasma membranes, the one closest to the capillary lumen, called the luminal membrane and the one closest to the intersitial fluid called the basolateral membrane

how do substances move across the epithelial cell

substances move across using transcellular transport, meaning the substance has to actually enter and then exit the endothelial cell

examples of transcellular transport

simple diffusion, diffusion using channels, faciliated diffusion, active transport, endocytosis and exocytosis

whats another way substances can move?

paracellular transport. substances can also move between two endothelial cells lining the capillaries thru small spaces called intercellular clefts. this type of movement is refered to as bulk flow

bulk flow

fluid and anything dissolved in it (ions, hormones, glucose, wastes, drugs) that are small enough to fit thru that intercellular cleft will flow thru it. Bulk flow isnt selective apart from size. this is one reason why ion concentrations between plasma and interstitial fluid are very similar. proteins cannot fit thru these clefts which is why interstitial fluid has diff protein concentrations than the plasma does.

what do intercellular clefts have 

proteins called tight junctions in the intercellular cleft (space between the endothelial cells) and they can vary in size, therefore affect the leakiness of that cleft

capillaries in the body

we have diff types of capillaries in our body: 

• continuous capillaries (most abundant), fenestrated capillaries (found in kidneys and intestines) and sinusoidal capillaries (of liver and spleen). These diff capillaries have diff permeability or leakiness to bulk flow

continuous capillary

eg in skin, muscles, lung, cns. are much less permeable than fenestrated capillaries. these small slits are found at the border where two endothelial cells meet. they can vary in permeability or leakiness. some have wider intercellular clefts between endothelial cells while others have none at all.

ex: brain is a great example of continuous capillaries with almost no permeability through intercellular clefts. This is important because if we had a lot of fluid leak out of capillaries in our brain, this could lead to problems as it is encased in a hard bony skull and can't expand

permeability of intercellular clefts

is related to proteins responsible for linking two endothelial cells together. these proteins form tight junctions. the tighter the tight junction, the narrower the intercellular cleft like in the blood brain barrier. the size of these clefts determine what substances can pass in between cells and which cant. cells and proteins typically cannot pass thru these clefts

bulk flow: filtration

when fluid and other substances move out of a capillary thru bulk flow, its called filtration. filtration will increase the amount of interstital fluid surrounding our tissue cells. 

edema

where excessive filtration was allowed to happen in a limb or organ. that limb or organ would continue to swell

how can edema occurs

• blood vessels are leakier and more fluid travels outside the vessel (thru infection/inflammation)

• low protein levels in the blood; proteins tend to hang onto fluid inside the blood vessels, so if u have very low protein, that fluid will leak out

• too much fluid in a blood vessel, it gets overloaded

starling forces

force involved in promoting or preventing filtration. can promote the return of fluid back into the capillary, process known as reabsorption

hydrostatic force

force that causes fluid to move

where does filtration and reabsorption occur in the vessels

tend to see filtration at the arteriole end (high bp) of the capillary and reabsorption (low bp) at the venule end of the capillary 

lymphatic system

its job is to return excess interstitial fluid back into circulation, back to heart and recirculate thru the body again. involves lymphatic vessels (lympathic capillary) that travels thruout the lymphatic system