Page 31 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

!
united to form a close electrical and metabolic their structure (! p. 9A and B) and transport
unit (syncytium), as is present in the function. Hence, the apical membrane (facing
epithelium, many smooth muscles (single- exterior) of an epithelial cell has a different set
unit type, ! p. 70), the myocardium, and the of transport proteins from the basolateral
glia of the central nervous system. Electric membrane (facing the blood). Tight junctions
coupling permits the transfer of excitation, (described below) at which the outer phos-
e.g., from excited muscle cells to their adjacent pholipid layer of the membrane folds over,
cells, making it possible to trigger a wave of ex- prevent lateral mixing of the two membranes
(! D2).
citation across wide regions of an organ, such
Fundamentals and Cell Physiology municate in this manner (electric synapses). cellular transport takes place between cells.
Whereas the apical and basolateral mem-
as the stomach, intestine, biliary tract, uterus,
ureter, atrium, and ventricles of the heart. Cer-
branes permit transcellular transport, para-
tain neurons of the retina and CNS also com-
Certain epithelia (e.g., in the small intestinal
and proximal renal tubules) are relatively per-
Gap junctions in the glia (! p. 338) and
meable to small molecules (leaky), whereas
epithelia help to distribute the stresses that
occur in the course of transport and barrier ac-
others are less leaky (e.g., distal nephron,
tivities (see below) throughout the entire cell
colon). The degree of permeability depends on
the strength of the tight junctions (zonulae "
community. However, the connexons close
2+
occludentes) holding the cells together (! D).
when the concentration of Ca (in an extreme
concentration increases too rapidly ( ! C3). In
permeability (sometimes cation-specific) are
essential functional elements of the various
other words, the individual (defective) cell is
1 case, due to a hole in cell membrane) or H + The paracellular pathway and the extent of its
epithelia. Macromolecules can cross the bar-
left to deal with its own problems when neces-
sary to preserve the functionality of the cell rier formed by the endothelium of the vessel
community. wall by transcytosis (! p. 28), yet paracellular
transport also plays an essential role, es-
Transport through Cell Layers pecially in the fenestrated endothelium.
Multicellular organisms have cell layers that Anionic macromolecules like albumin, which
are responsible for separating the “interior” must remain in the bloodstream because of its
from the “exterior” of the organism and its colloid osmotic action (! p. 208), are held back
larger compartments. The epithelia of skin and by the wall charges at the intercellular spaces
gastrointestinal, urogenital and respiratory and, in some cases, at the fenestra.
tracts, the endothelia of blood vessels, and neu- Long-distance transport between the
roglia are examples of this type of extensive various organs of the body and between the
barrier. They separate the immediate extra- body and the outside world is also necessary.
cellular space from other spaces that are Convection is the most important transport
greatly different in composition, e.g., those mechanism involved in long-distance trans-
filled with air (skin, bronchial epithelia), port (! p. 24).
gastrointestinal contents, urine or bile
(tubules, urinary bladder, gallbladder),
aqueous humor of the eye, blood (endothelia)
and cerebrospinal fluid (blood–cerebrospinal
fluid barrier), and from the extracellular space
of the CNS (blood–brain barrier). Nonetheless,
certain substances must be able to pass
through these cell layers. This requires selec-
tive transcellular transport with import into
the cell followed by export from the cell. Un-
like cells with a completely uniform plasma
membrane (e.g., blood cells), epi- and en-
18 dothelial cells are polar cells, as defined by
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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