Biomedical Engineering Reference
In-Depth Information
Venules
Capillary Bed
Arterioles
FIGURE 7.7 A capillary network. A sphincter is usually located at the arteriole's entry to the capillary bed
that restricts the blood flow. Not all capillaries carry blood all the time, and blood flow is based on the needs of
the tissue. For instance, less than 10 percent of the capillaries in muscle carry blood during rest.
TABLE 7.3
Percentage of Plasma Volumes in the Circulatory System
Volume
Percentage
Heart
7%
Pulmonary System
9%
Arteries
13%
Arterioles and Capillaries
7%
Veins and Venules
64%
larger and larger veins until it reaches the heart. Table 7.3 provides the volume percentages
in each of the circulatory system compartments. Total plasma volume is approximately 3 L,
and total interstitial volume is 11 L.
Plasma proteins are the only substances that are impermeable 11 to the capillary wall,
which creates an osmotic pressure. The osmotic pressure created by plasma proteins is
called the colloid osmotic pressure, named to differentiate it from the osmotic pressure
across the cell membrane. The concentration of proteins in the plasma is 7.5 gm/dl and in
the interstitial volume is 3 gm/dl. By far, the protein with the greatest contribution to the
colloid osmotic pressure in the plasma/capillary space is albumin (21.8 mm Hg), followed
by globulins (6 mm Hg), and then fibrinogen (0.2 mm Hg), giving a total colloid pressure of
28 mm Hg to move fluids inward. The interstitial colloid osmotic pressure due to all pro-
teins within this space is 8 mm Hg. Thus, the total colloid osmotic pressure (difference
between plasma/capillary and interstitial volume) is 20 mm Hg, a resultant force that
moves fluid inward.
11 Actually, some plasma proteins slowly leak through the capillary wall into the interstitial compartment,
which are then returned to the plasma from lymphatic circulation.
Search WWH ::




Custom Search