Biomedical Engineering Reference
In-Depth Information
As can be seen, the transport of ions across a cell membrane is affected by their charges and
relative diffusivities as much as by their concentration gradients. Note that the diffusivities
of all ions are normalized to that of potassium (1).
14.1.3 Mass Transport in Systemic Capillaries
Capillaries, the smallest and most numerous of the blood vessels, form the connection
between the vessels that carry blood away from the heart (arteries) and the vessels that
return blood to the heart (veins). The primary function of capillaries is the exchange of
materials between the blood and tissue cells. Gas exchange occurs across capillary walls
in a manner similar to that which occurs across alveoli and pulmonary capillaries. A partial
pressure gradient exists for each gas (primarily oxygen and carbon dioxide) that controls
the gas exchange process. As with pulmonary capillaries, the gas transfer across systemic
capillaries occurs across the entire membrane wall, since the gases are lipid soluble.
However, unlike the pulmonary capillaries, mass transfer across systemic capillaries also
includes liquids and ions (dissolved or free floating within the liquid) that travel across
pores in the capillary walls. In addition to forming the connection between the arteries
and veins, systemic capillaries have a vital role in the exchange of gases, nutrients, and met-
abolic waste products between the blood and the tissue cells. Substances pass through the
capillary wall by diffusion, filtration, and osmosis. Oxygen and carbon dioxide move across
the capillary wall by diffusion regulated by the partial pressure differences. Fluid move-
ment across a capillary wall via the pores is determined by a combination of hydrostatic
and osmotic pressure. The net result of the capillary microcirculation created by hydro-
static and osmotic pressure is that substances leave the blood at one end of the capillary
and return at the other end, as the hydrostatic pressure drops along the length of the capil-
lary, and thus the pressure difference (hydrostatic-osmotic) is different from the beginning
of the capillary (arteriole side) to the end (venule side).
Thus, the driving mechanism for this mass transfer is twofold. The pressure (called the
hydrostatic pressure
,
which is a pulling pressure that results from the concentration difference of substances that
cannot fit through the pores. These are the nondiffusible components. These two pressure
differences acting across the tube-like pores of the capillary are shown in Figure 14.9.
The capillaries form a network of minute vessels between arterioles and venules so as to
maximize mass transfer by (a) shortening the distance for mass transfer and (b) maximizing
the overall surface area for mass transfer (Figure 14.10). The anatomy of capillaries is well
suited to the task of efficient exchange. Capillary walls are composed of a single layer of
endothelial cells. The thin nature of the capillary wall facilitates efficient diffusion of oxygen
and carbon dioxide, as well as containing short pore lengths to facilitate the bulk motion of
liquids and dissolved ions (Figure 14.11).
As was just stated, the hydrostatic pressure in a capillary is higher on the arteriole end of
the capillary and lower on the venule end. This is due to the normal pressure gradient
along a pipe or tube that produces the axial (down the vessel) flow rate. In fact, a pressure
drop is required in order to propel the blood downstream. As a result, the higher hydro-
static pressure near the arteriole end is greater than the osmostic pressure, and the net pres-
sure is outward—pushing fluid out of the capillary. As the hydrostatic pressure drops
) is a pushing pressure. Opposite in direction is the
osmotic pressure
