Biomedical Engineering Reference
In-Depth Information
(on the arterial side) and within the range of 12 mmHg (on the venous side). The intersti-
tial pressure is approximately negative 3 mmHg (this is a gauge pressure), but can
increase to positive 4 to 5 mmHg when the space between the cells constricts. Cell constric-
tion occurs due to different tissue loading conditions. This increase would act to inhibit
water movement out of the capillary.
The osmotic pressure within capillaries averages to approximately 28 mmHg. This pres-
sure is developed from all of the compounds within the capillary that cannot diffuse
across the endothelial cell barrier. The major contributor to this is albumin, which solely
develops an osmotic pressure of approximately 22 mmHg within the capillary. The globu-
lins in the capillary contribute the majority of the remaining portion of the osmotic pres-
sure. The total protein concentration within the interstitial space is greater than the protein
concentration within the capillaries. This suggests that the osmotic pressure of the intersti-
tial space should be larger than the plasma osmotic pressure. However, because the vol-
ume of the interstitial space is significantly larger than that of the capillary, the total
osmotic pressure of the interstitial fluid is approximately 7 mmHg. The osmotic pressure
does not change between the arterial and venous side of the capillary because proteins do
not leave the blood vessel and the blood volume remains relatively constant.
The total driving force for water out of the capillary is simply the summation of all of
these pressure values, according to Starling's theory. On the arterial side of the capillary,
the summation of the forces that tend to aid in fluid movement out of the capillary are
P I
1 Π
25 mmHg
2 ð 2
3 mmHg
Þ 1
7 mmHg
35 mmHg
P B
2
5
5
I
The only force that tends to prevent fluid movement out of the capillary is the plasma
osmotic pressure:
Π
28 mmHg
5
B
Therefore, the total driving force of fluid out of capillaries on the arterial side is approxi-
mately 7 mmHg. By using this same analysis for the venous side of the capillary, the bal-
ance of the forces shifts to aid in water movement back into the capillary:
P B 2
P I 2 Π B 1 Π I 5
15 mmHg
2 ð 2
3 mmHg
Þ 2
28 mmHg
7 mmHg
3 mmHg
1
52
Therefore, along the venous side of the capillary there is a net re-absorption of water that
was lost along the arterial side. This change from water loss within the capillary to water
gain within the capillary is primarily governed by the change in the capillary hydrostatic
pressure. The other pressures (osmotic and interstitial hydrostatic) are relatively constant
throughout the capillary length. Also, these hydrostatic and osmotic pressure values (as
well as the permeability) are tightly controlled by the body, because slight changes in any
of them can cause either edema (increased water within the interstitial space; swelling) or
dehydration, by having a significant effect on the movement of water across the capillary
wall.
The velocity of fluid within the interstitial space is relatively slow compared to the car-
diovascular system, and most of the fluid does not have the ability to move freely. Due to
the high concentration of charged proteins (collagens/proteoglycans) within the interstitial
space, water becomes associated with the proteins through hydrogen bonding. Because
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