Biomedical Engineering Reference
In-Depth Information
TABLE 14.4 Relationship between Arteriole Diameter, Capillary Hydrostatic Pressure, Transcapillary Fluid
Flow, and Arterial Pressure
Initial
Arterial
Pressure
Arteriole
Diameter
Change
Capillary
Hydrostatic
Effect
Venous
Return
Cardiac/Arterial
Output/ Pressure
Net Fluid
Low
constrict
reduced
into capillary
increase
increase
High
dilate
increased
out of capillary
decrease
decrease
produces a rise in the inlet hydrostatic pressure to capillaries, which results in a greater
pushing pressure for fluid moving out of the capillary and into the tissues and extracellular
fluid. With a net increase in fluid leaving the capillaries, there is a reduction in blood vol-
ume and in venous return. This results in a reduction in cardiac output and in arterial pres-
sure. Table 14.4 summarizes the cause and effect of changes in arteriole diameter.
When there is an increase in fluid leaving the capillary as a result of arteriole dilation,
then the extracellular fluid pressure rises. The increase in fluid volume will eventually
cause edema, a buildup of fluid that produces swelling and potential damage to cells and
tissues. To alleviate this condition, there is a network of lymphatic vessels that parallels
the circulatory system. The peripheral lymphatic vessels are similar in size to capillaries
and are networked alongside the capillaries. Their function is to collect excess fluid from
extracellular spaces and transport it along the lymphatic vessel network of larger and larger
vessels until the lymph reaches the vena cava, where it is returned to the bloodstream. The
lymph flow is normally quite small compared with the blood flow. Even the terminal
lymph flow is quite small (1 ml/hr) compared with the capillary axial blood flow (1 ml/sec).
The terminal lymphatic vessels located in close proximity to systemic capillaries are shown
in Figure 14.14.
Propulsion of lymph through the larger vessels is produced by contraction of the walls of
the vessels in a fashion similar to that provided for flow through the human intestines and
somewhat similar to the action of muscles on veins in the cardiovascular system. Lymph
vessels, like the venous system, have valves within the vessels to limit backflow and to pro-
mote forward flow along the lymphatic circulation. This is in contrast with flow through the
bloodstream, which is produced by a driving hydrostatic pressure gradient from the aorta
through arteries, arterioles, capillaries, venules, and veins. The lymph flow originates in the
terminal vessels and therefore has no embedded driving pressure gradient. The contraction
of the lymph vessels in concert with the added fluid volume (from all terminal lymph ves-
sels joining together within the network) produces the required lymph flow headed to the
vena cava.
14.1.4 Mass Transport in the Kidneys and Dialysis
As with the alveoli in the lungs, mass transfer in the kidneys is also processed via a large
array of extremely small elements. Each of these elements is a
. Although nephrons
and alveoli represent the essence of mass transfer in the body—small size and large overall
surface area—the nephron is quite different from an alveolus. The alveolus is solely
nephron
