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PSL201Y1 Lecture Notes - Hydrostatics, Extracellular Fluid, Oncotic Pressure

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Christopher Perumalla

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Lecture 28
1. Capillaries
There are about 10-40 billion capillaries per body, most cells being within 1
mm of capillary. They have very thin walls. It is at the level of the capillaries
that exchange between blood and tissue occurs. There are two types of
capillaries: continuous and frenestrated. Most solutes can enter the
capillaries by simple diffusion however in the brain, the blood brain barrier
limits the diffusion of solutes across the membrane.
Continuous capillaries: They are the most common and have small gaps
between endothelial cells. They allow small water soluble molecules to
move through.
Fenestrated capillaries: There are large gaps between the endothelial
cells forming pores or fenestrations. They allow proteins and in some
cases blood cells to move through.

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Movement of fluid across the capillary walls
The fluid moves in and out of capillaries by bulk flow based on pressure
gradients. The movement from capillary into interstitial space is called
filtration while movement from interstitial space into capillary is called
Starling forces across capillary walls
There are two forces for bulk flow: hydrostatic and osmotic pressures.
The hydrostatic pressure gradient is the difference between the
hydrostatic pressure of the fluid inside the capillaries and the hydrostatic
pressure of the fluid outside of the capillaries. In the capillaries the
pressure of the blood inside is generally greater than the pressure of the
interstitial fluid outside. The hydrostatic pressure gradient is directed
therefore outwards, it tends to drive the water out of the capillaries into
the interstitial fluid.
Osmotic pressure is exerted by the proteins that are left behind in the
capillaries. Because the concentration of protein in the plasma is higher
than the concentration of proteins in the interstitial fluid, the osmotic
pressure is directed inwards. It is the difference of pressure at the

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capillaries between the arterioles and the venules that allows the
substance to leave or enter the capillaries.
The amount and the kind of substances that leave the capillaries is highly
regulated and is dependent of hydrostatic and osmotic pressure gradients.
Pressure gradients
The hydrostatic pressure gradient is higher towards the arteriole ends
because this is where the blood is coming from. However, when it reaches
the venous (venule) end, the pressure is not as high as it is before.
Osmotic pressure tends to drive water back into the capillaries because there
are a lot of proteins sitting inside the capillaries. There is very little
difference of protein concentration from the arteriole end to the venule end.
Thus, the osmotic pressure is constant. Hydrostatic pressure results in
filtration across the capillaries while osmotic pressure results in absorption
across the capillaries. At the arteriole end of the capillaries, the outwardly
directed hydrostatic pressure gradient is greater than the inwardly directed
osmotic pressure gradient. Flow of fluid goes from capillaries into the
interstitial fluid at the arteriole end because the pressure is higher here
compared to osmotic pressure. However, at the venule end of the capillary,
the hydrostatic pressure gradient is smaller than the osmotic pressure
gradient so fluid flows inwardly.
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