EXCHANGE FUNCTION OF
Understanding the concepts presented in this chapter will enable the student to:
Describe the principal mechanisms by which gases, fluid, electrolytes, and
macromolecules move across the capillary endothelium.
Name three different types of capillaries; know the organs in which they are
found, and describe their differences in permeability to macromolecules and fluid.
Describe the factors that determine oxygen’s rate of exchange between the
microcirculation and tissue.
Explain the relationship between oxygen content of blood, percent saturation,
and partial pressure of oxygen.
Describe the relationship between oxygen delivery to a tissue, oxygen extrac-
tion, and oxygen consumption.
Describe the mechanisms responsible for the movement of fluid across capillaries.
Describe the relationship between interstitial fluid volume, interstitial hydro-
static pressure, and interstitial compliance.
Describe how changes in capillary hydrostatic pressure, plasma oncotic pres-
sure, capillary permeability, and lymphatic function can lead to tissue edema.
The microcirculation consists of small arteries,
arterioles, capillaries, venules, small veins, and
small lymphatic vessels found within organs
and tissues (see Chapter 5, Fig. 5.1) and has
the following important functions:
1. Small arteries and arterioles are the principal
sites of resistance within the systemic cir-
culation and therefore play a major role in
the regulation of arterial blood pressure and
blood flow within organs (see Chapter 5).
2. Venules and small veins have an important
capacitance function and therefore deter-
mine the distribution of blood volume
within the body.
3. The microcirculation allows passage of leu-
kocytes from the blood into the extravascular
space, which is important in inflammation
4. The microcirculation is where gases, circu-
lating substances (e.g., nutrients, hormones,
therapeutic drugs), metabolic wastes from
the tissues, fluid, and thermal energy are
exchanged between the blood and tissues.
Within the microcirculation, capillaries are
quantitatively the most important site for
exchange because of their physical structure
(small volume-to-surface area ratio and thin
walls), large number, and enormous surface area
available for exchange. This chapter focuses on
the exchange function of capillaries.
MECHANISMS OF EXCHANGE
Fluid, electrolytes, gases, and small and laige
molecular weight substances transverse the cap-
illary endothelium by several different mecha-
nisms: diffusion, bulk flow, vesicular transport,
and active transport (Fig. 8.1). Some substances