Microvascular Beds - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 189

necessarily large (although this is a relatively large force on the cell itself), but due to the
high viscous forces compared to the small particle size (as related to the inertial forces),
there is a large drag on the cells.
Similar to the pressure distribution within the microcirculation, there is a temporal vari-
ation in the velocity distribution based on the cardiac cycle as well as the propagation of
the velocity throughout the cardiovascular system. The velocity can change on the order
of a few millimeters per second within the capillaries. For most calculations, however, this
is a relatively small change and is typically neglected. This allows for a fairly valid
assumption of steady flow within the microcirculation. As with the pressure distribution,
the relative changes based on vessel diameter are more important to consider than are the
temporal changes.

6.6 INTERSTITIAL SPACE PRESSURE AND VELOC ITY

The interstitial space is the area between cells, which is typically composed of fluid and
proteins (such as collagen and/or proteoglycans). The proteins that are present within the
interstitial space function to “connect” cells together into a uniform tissue that can with-
stand mechanical loads as one entity. The fluid within the interstitial space has a very sim-
ilar composition as plasma, except for the much lower concentration of plasma proteins.
Plasma proteins are unable to diffuse through the capillary membrane under normal con-
ditions and therefore do not enter the interstitial space.
In Chapter 7, we will see that the pressure within the interstitial space is critical to
determine the transport of fluid and nutrients between the blood vessels and the extravas-
cular space. The flow rate of water across the blood vessel wall is directly proportional to
the pressure (osmotic and hydrostatic) within the blood vessel and the interstitial space
( Figure 6.5 ). This is summarized by Starling's theory:

m _ K p ð P B P I

2 Π 1 Π

Þ ð 6 : 8 Þ

5 2 B I

where m is the volumetric flow rate of water. K p is a permeability constant. P B is the

hydrostatic pressure within the capillary, and this tends to force water out of the blood
vessel. P I is the hydrostatic pressure within the interstitial space. This tends to inhibit the
movement of water out of the blood vessel (if it is positive), but can aid in fluid movement
(if it is negative).

_

Π B is the osmotic pressure within the capillary, and this tends to promote
the movement of water into the blood vessel. Π I is the osmotic pressure within the intersti-

tial space and this tends to promote the movement of water out of the blood vessel. As
previously discussed, the capillary hydrostatic pressure is within the range of 25 mmHg

FIGURE 6.5 The capillary and interstitial fluid hydrostatic pres-
sure and the colloidal osmotic pressure (for plasma and interstitial
space) affect the movement of fluid within microvascular beds. The
capillary hydrostatic pressure and the interstitial osmotic pressure
generally aid in water movement out of the capillary. The interstitial
hydrostatic pressure and the interstitial plasma pressure generally
aid in water movement into the capillary.

Capillary
pressure

Plasma osmotic
pressure

Interstitial fluid
pressure

Interstitial osmotic
pressure