Biomedical Engineering Reference
In-Depth Information
Endothelial
cell
Plasmalemmal
vesicle
Intercellular
cleft
Basement
membrane
FIGURE 6.2
Cross section of a capillary, showing possible transport mechanisms across the capillary wall.
Recall that the capillary is composed of a single endothelial cell (in thickness) surrounded by a small basement
membrane. There are typically two to three endothelial cells that form a capillary in circumference. In between
endothelial cells, there is an intercellular cleft which is responsible for the majority of transport between the blood
and the extravascular space. Plasmalemmal vesicles are responsible for the transport of larger molecules, how-
ever, this is a slow transport mechanism. Adapted from Guyton and Hall (2000).
gap of about 10 to 15 nm in thickness. This endothelial cell
endothelial cell adhesion is
primarily carried out by gap junctions, cadherins, and other cell membrane adhesion pro-
teins. These clefts represent a very small proportion of the total capillary surface area
(about 1/1000 of the total available surface area), but due to the permeability of nutrients
across this barrier, some of the important transport molecules can diffuse very rapidly
through the intercellular cleft. Furthermore, flow through the intercellular cleft is approxi-
mately 80 times faster than blood flow through the capillary itself, and therefore, a rapid
equilibrium is reached between the blood and the interstitial fluid. This type of intercellu-
lar cleft is present in most tissues. Two exceptions are the brain and the liver, where the
intercellular cleft is much smaller and much larger, respectively. In the brain, these tight
junctions only allow small molecules to pass (such as O
2
, glucose), preventing the entrance
of potential dangerous and unwanted compounds into the brain tissue. In the liver, the
pores are so large that even plasma proteins can pass through them. Because the liver par-
tially acts to detoxify blood, it requires access to all of the blood components. In general,
only metabolically important compounds are transported through the intercellular cleft,
except for within the liver capillary beds.
The second method of transport across the endothelial cell is via the plasmalemmal
vesicles. These vesicles form on both the vascular side and the interstitial side of the endo-
thelial cell via a cellular invagination process. Through this process, fluid and molecules
are trapped within the vesicle and then are slowly transported to the opposing endothelial
cell surface. Once the plasmalemmal vesicles reach the opposing endothelial cell surface,
they fuse with the cell membrane and release the contents into the vascular space or the
interstitial space. This is clearly not an effective method to transport nutrients/wastes to
every cell within the body because it is slow and requires a great amount of energy to
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