Human Physiology, 14th edition (2016)

148 Chapter 6

In addition, there is bulk movement of fluid through

pores in epithelial membranes. The presence of pores,

and their size, depends on the extent to which junctional

complexes surround each epithelial cell in the membrane.

For example, the epithelial cells that compose the walls of

many blood capillaries (the thinnest of blood vessels) have

pores between them that can be relatively large, permit-

ting filtration of water and dissolved molecules out of the

capillaries through the paracellular route. In the capillaries

of the brain, however, such filtration is prevented by tight

junctions, so molecules must be transported transcellularly.

This involves the cell transport mechanisms previously

described, as well as the processes of endocytosis and exo-

cytosis described next.

Bulk Transport

Polypeptides and proteins, as well as many other molecules, are

too large to be transported through a membrane by the carriers

described in previous sections. Yet many cells do secrete these

molecules—for example, as hormones or neurotransmitters—

by the process of exocytosis. As described in chapter 3, this

involves the fusion of a membrane-bound vesicle that contains

these cellular products with the plasma membrane, so that the

membranes become continuous ( fig. 6.23 ).

Figure 6.22 Junctional complexes provide a barrier between adjacent epithelial cells. Proteins penetrate
the plasma membranes of the two cells and are joined to the cytoskeleton of each cell. Junctional complexes consist of three
components: tight junctions (zonula occludens), adherens junctions (zonula adherens), and desmosomes (macula adherens).
Epithelial membranes differ, however, in the number and arrangement of these components, which are illustrated in ( a ) and shown
in an electron micrograph in ( b ).

(b)

Zonula

occludens

Zonula

adherens

Macula adherens

(Desmosome)

(a)

CLINICAL APPLICATION

Acute gastroenteritis (inflammation of the stomach

and intestine, as from cholera), and its resultant diarrhea

produce an excessive loss of Na^1 and water, malnutri-

tion, and metabolic acidosis, causing an estimated 1.5 to

2.5 million deaths of young children each year. Because

rehydration through intravenous therapy is often not practi-

cal, the World Health Organization (WHO) developed a sim-

pler, more economical treatment called oral rehydration

therapy (ORT). In the late 1940s, ORT consisted of a bal-

anced salt solution that was later supplemented by glu-

cose to serve as an energy source. Quite by accident, this

led to the discovery that the presence of glucose aids the

intestinal absorption of Na^1 and water. We now know that

the intestinal Na^1 /glucose transporter absorbs two sodium

ions with each glucose molecule and that water follows by

osmosis. The ORT solution of WHO/UNICEF contains spe-

cific concentrations of different electrolytes and glucose,

and has an osmolality of 245 mOsm. The Na^1 and glucose

should have equal molarity concentrations in the rehydrat-

ing solutions for effective cotransport; sodas and juices

have too high a glucose and too low a Na^1 concentration

for this purpose. It has been estimated that oral rehydra-

tion therapy saves the lives of more than a million small

children each year.