Microvascular Beds - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 202

there is a net increase in the movement of water out of the capillary. The inflow of water
on the venous side typically cannot make up for the extra loss of water along the arterial
side. Therefore, there is an accumulation of water within the interstitial space.
In some disease cases, the capillary wall begins to breakdown, which allows for the
movement of plasma proteins into the interstitial space. This causes an increase in the
interstitial osmotic pressure, preventing the movement of water into capillaries along the
venous side of the capillary. In a familiar case, bruising, there is swelling because the cap-
illary wall has become damaged. In other conditions, the plasma osmotic pressure can
decrease because there is either not enough protein being formed or the proteins are being
broken down within the cardiovascular system. In either case, due to the pressure changes
that disrupt the delicate pressure balance in the capillaries, fluid has a preference to stay
within the interstitial space.
Edema can also occur in specific organs in response to the inflammatory response, such
as pancreatitis. Particular organs can also develop edema based on certain biochemical
and biophysical phenomena that occur within that organ. For instance, cerebral edema
commonly occurs when there is an abnormal metabolic state. Corneal edema occurs in
conjunction with glaucoma. Cutaneous edema is common with venomous/poisonous
insect bites or plant contact, such as poison ivy.

END OF CHAPTER SUMMARY
6.1 The microcirculation includes all of the capillaries and the smallest arterioles that dictate
where blood flows in the vascular beds. Within the microcirculation, there is a transport of
nutrients, dissolved species, and water across the vascular wall. Therefore, mass balance
can no longer be accounted for by the inflow equaling the outflow. True capillaries have a
diameter in the range of 8 m. Flow through the capillaries is not continuous and is primar-

ily dictated by the need of the surrounding tissue. An intercellular cleft is located between
neighboring endothelial cells (typically 10 to 15 nm thick) and the vast majority of transport
is accounted by this channel. The flow through this channel is approximately 80 times faster
than the blood flow through the capillary.
6.2 Endothelial cells are typically 10

μ

m in diameter and range in their lengths. This cell type
lines all blood vessels, and they play a role in hemostasis, inflammation, and nutrient trans-
port. Vascular smooth muscle cells can perform work, similar to other muscle cells.
However, the regulation of contraction and the organization of the actin and myosin do not
mimic other muscle cells. Endothelial cells can communicate with other endothelial cells or
with smooth muscle cells via gap junctions.
6.3 There are two primary mechanisms for blood flow control. The first is the acute local con-
trol and the second is a long-term control. Acute control is rapid constriction or dilation in
response to stimuli. Long-term control is the formation of new blood vessels due to changes
in the length/diameter of existing blood vessels. The regulation of these mechanisms is pri-
marily through the need of the tissue although there is some disagreement as to the actual
mechanism that controls blood flow. There is no “command center” that continuously con-
trols blood flow through the capillaries.

μ