Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 2Overview of Cellular Physiology in Medical Physiology 49

about 24% of the energy utilized by cells, and in neurons it
accounts for 70%. Thus, it accounts for a large part of the
basal metabolism. A major payoff for this energy use is the
establishment of the electrochemical gradient in cells.

TRANSPORT ACROSS EPITHELIA

In the gastrointestinal tract, the pulmonary airways, the renal
tubules, and other structures, substances enter one side of a cell
and exit another, producing movement of the substance from
one side of the epithelium to the other. For transepithelial trans-
port to occur, the cells need to be bound by tight junctions and,
obviously, have different ion channels and transport proteins in
different parts of their membranes. Most of the instances of sec-
ondary active transport cited in the preceding paragraph in-
volve transepithelial movement of ions and other molecules.

THE CAPILLARY WALL

FILTRATION

The capillary wall separating plasma from interstitial fluid is
different from the cell membranes separating interstitial fluid
from intracellular fluid because the pressure difference across
it makes filtration a significant factor in producing movement
of water and solute. By definition, filtration is the process by
which fluid is forced through a membrane or other barrier be-
cause of a difference in pressure on the two sides.

ONCOTIC PRESSURE

The structure of the capillary wall varies from one vascular bed
to another. However, in skeletal muscle and many other organs,

water and relatively small solutes are the only substances that

cross the wall with ease. The apertures in the junctions between

the endothelial cells are too small to permit plasma proteins and

other colloids to pass through in significant quantities. The col-

loids have a high molecular weight but are present in large

amounts. Small amounts cross the capillary wall by vesicular

transport, but their effect is slight. Therefore, the capillary wall

behaves like a membrane impermeable to colloids, and these ex-

ert an osmotic pressure of about 25 mm Hg. The colloid osmot-

ic pressure due to the plasma colloids is called the oncotic

pressure. Filtration across the capillary membrane as a result of

the hydrostatic pressure head in the vascular system is opposed

by the oncotic pressure. The way the balance between the hy-

drostatic and oncotic pressures controls exchanges across the

capillary wall is considered in detail in Chapter 32.

TRANSCYTOSIS

Vesicles are present in the cytoplasm of endothelial cells, and

tagged protein molecules injected into the bloodstream have

been found in the vesicles and in the interstitium. This indi-

cates that small amounts of protein are transported out of cap-

illaries across endothelial cells by endocytosis on the capillary

side followed by exocytosis on the interstitial side of the cells.

The transport mechanism makes use of coated vesicles that

appear to be coated with caveolin and is called transcytosis,

vesicular transport, or cytopempsis.

FIGURE 2–18 Na+–K+ ATPase. The intracellular portion of the
α subunit has a Na+-binding site (1), a phosphorylation site (4), and
an ATP-binding site (5). The extracellular portion has a K+-binding
site (2) and an ouabain-binding site (3). (From Horisberger J-D et al:
Structure–function relationship of Na–K-ATPase. Annu Rev Physiol 1991;53:565.
Reproduced with permission from the Annual Review of Physiology, vol. 53.
Copyright © 1991 by Annual Reviews)

3

2

1

4

5

2K+

β

α

3Na+

ECF

Cytoplasm

Ouabain

FIGURE 2–19 Composite diagram of main secondary effects

of active transport of Na+ and K+. Na,K ATPase converts the chemi-

cal energy of ATP hydrolysis into maintenance of an inward gradient

for Na+ and an outward gradient for K+. The energy of the gradients is

used for countertransport, cotransport, and maintenance of the mem-

brane potential. Some examples of cotransport and countertransport

that use these gradients are shown. (Reproduced with permission from Skou

JC: The Na–K pump. News Physiol Sci 1992;7:95.)

Active transport

2K+

Ouabain

3Na+

Na+

Na+

H+

H+

K+

K+

Ca2+

ATP

3Na+ Na+

Na+

Na+

ADP + Pi

Cl−

K+, 2Cl−

Cotransport

Countertransport

Sugars

or amino

acids

Vm = −70 mV

Na+ 140 meq/L

K+ 4 −

Cl− 105 −

++++

−−−−

Na+ 15 meq/L

K+ 150 −

Cl− 7 −

Cl−