removed to reveal
(a) Continuous capillary
Continuous Capillary Basement membrane removed to reveal endothelial cells (a) Continuous capillary Endothelial Cells Plasma Narrow intercellular gap Basement membraneLecture 28
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
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.Fenestrated Capillary
(b) Fenestrated capillary
Fenestrated Capillary Fenestrations (pores) (b) Fenestrated capillary Plasma Wide intercellular gap white blood cell s.... FenestrationsMovement 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 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.
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.Starling Forces Across Capillaries
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Starling Forces Across Capillaries Interstitial fluid Abs tion tu tu tu eu Capillary Hydrostatic Oncotic pressure pressure gradient gradient Arteriole (b) VenuleIn most systemic capillaries, the balance of the force is such that filtration
occurs over a longer length than absorption. If there is more filtration than
absorption, there would be an excess of fluid in the interstitial space. This is
however prevented by the lymphatic system.
Veins conduct blood to the heart from the tissue. Their diameter is close to
those of the arteries but have thinner walls, so they have a larger lumen.
Veins are equipped with valves that allow unidirectional blood flow. Valves
are present in the peripheral veins but not in the central veins.
Veins function as a volume reservoir which is related to vessel compliance.
Because veins have thin walls and so are easily stretched, they have a very
high compliance. A small increase in pressure causes a large degree of
expansion, increasing volume. Veins can hold more blood than arteries. The
veins contain a greater volume of blood even if the pressure is low. We canVessel: Pressure Volume Relation
Vessel: Pressure Volume Relation Distending pressure Veins Arteries Ficlose a substantial amount of blood without losing pressure. Venous puli leads
to reduced veinous return and reduced blood pressure. For example, when
you stand up too quickly, most of the blood is in the legs and is not able to
go back fast enough to the brain, causing fainting. Fainting will allow blood
to return to the brain and the heart, causing stoke volume and cardiac input
At a given p