Ganong's Review of Medical Physiology, 23rd Edition

ECF
Cytoplasm
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)
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 transport
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 secondary
active transport cited in the preceding paragraph involve
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 because
of a difference in pressure on the two sides.
ONCOTIC PRESSURE
β
3Na +
1
2K +
The structure of the capillary wall varies from one vascular bed
to another. However, in skeletal muscle and many other organs,
2
4
3
Ouabain
α
5
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology 49
Countertransport
3Na +
Na +
H +
Ca 2+
FIGURE 2–19 Composite diagram of main secondary effects
of active transport of Na + and K + . Na,K ATPase converts the chemical
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 membrane
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.)
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 colloids
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 exert
an osmotic pressure of about 25 mm Hg. The colloid osmotic
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 hydrostatic
and oncotic pressures controls exchanges across the
capillary wall is considered in detail in Chapter 32.
TRANSCYTOSIS
Active transport
2K +
Ouabain
ATP
3Na + Na +
Na +
ADP + Pi
Cl− Na +
H +
K +
K +
Cl− Vm = −70 mV
Na + 140 meq/L
K + 4 −
Cl− Na
− − − −
+ + + +
105 −
+ 15 meq/L
K + 150 −
Cl− 7 −
Na +
K + , 2Cl− Sugars
or amino
acids
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 indicates
that small amounts of protein are transported out of capillaries
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.
Cotransport