Figure 624

The transepithelial transport of most organic solutes (X) involves their movement into a cell through a secondary active transport driven by the downhill flow of sodium. The organic substance then moves out of the cell at the blood side down a concentration gradient, by means of facilitated diffusion. Shown below the cell is the concentration profile of the transported solute across the epithelium.

side to lumen during secretion. In our example, the movement of sodium from the lumen into the epithelial cell occurs by diffusion through sodium channels in the luminal membrane (Figure 6-23). Sodium diffuses into the cell because the intracellular concentration of sodium is kept low by the active transport of sodium out of the cell across the basolateral membrane on the opposite side, where all of the Na,K-ATPase pumps are located. In other words, sodium moves downhill into the cell and then uphill out of it. The net result is that sodium can be moved from lower to higher concentration across the epithelium.

Figure 6-24 illustrates the active absorption of organic molecules across an epithelium. In this case, entry of an organic molecule X across the luminal plasma membrane occurs via a secondary active transporter linked to the downhill movement of sodium into the cell. In the process, X moves from a lower concentration in the luminal fluid to a higher concentration in the cell. Exit of the substance across the basolateral membrane occurs by facilitated diffusion, which moves the material from its higher concentration in the cell to a lower concentration in the extracellular fluid on the blood side. The concentration of the substance may be considerably higher on the blood side than in the lumen since the blood-side concentration can approach equilibrium with the high intracellular concentration created by the luminal membrane entry step.

Although water is not actively transported across cell membranes, net movement of water across an epithelium can be achieved by osmosis as a result of the active transport of solutes, especially sodium, across the epithelium. The active transport of sodium, as described above, results in a decrease in the sodium concentration on one side of an epithelial layer (the lumi-nal side in our example) and an increase on the other. These changes in solute concentration are accompanied by changes in the water concentration on the two sides since a change in solute concentration, as we have seen, produces a change in water concentration. The water concentration difference produced will cause water to move by osmosis from the low-sodium to the high-sodium side of the epithelium (Figure 6-25). Thus, net movement of solute across an epithelium is accompanied by a flow of water in the same direction. If the epithelial cells are highly permeable to water, large net movements of water can occur with very small differences in osmolarity.

Lumen side

Lumen side

Epithelial cell

Tight junction

Epithelial cell

Blood side

Tight junction

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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