Chapter 1

### Chapter 1. General Physiology

The uptake of a novel drug by hepatocytes occurs down an electrochemical gradient. Uptake is independent of other solutes, and the rate of uptake is saturated at high extracellular drug concentrations. Which membrane transport process is most likely to account for all these characteristics of drug uptake?

A Antiport with Cl

B Cotransport with Na+

C Facilitated diffusion

D Primary active transport

E Simple diffusion through ion channels

F Simple diffusion through plasma membrane

C. Facilitated diffusion only occurs passively down the solute electrochemical gradient; it occurs via uniporters, with no dependence on other solutes and is saturable.

A segment of normal human intestine was perfused in vitro. The lumen was perfused with 150 mM NaCl. The bathing solution contained 300 mM mannitol (an inert sugar). The reflection coefficients of NaCl and mannitol were 0.9 and 1.0, respectively. Assume that NaCl dissociates completely into Na+ and Cl; mannitol does not dissociate in solution.

When compared to the osmolarity of the luminal solution, which of the following best describes the osmolarity of the bathing solution?

A Isosmotic

B Hyperosmotic

C Hyposmotic

A. Lumen osmolarity = 2 × 150 = 300 mOsm/L; bathing solution osmolarity = 1 × 300 = 300 mOsm/L. The solutions are therefore isosmotic.

A segment of normal human intestine was perfused in vitro. The lumen was perfused with 150 mM NaCl. The bathing solution contained 300 mM mannitol (an inert sugar). The reflection coefficients of NaCl and mannitol were 0.9 and 1.0, respectively. Assume that NaCl dissociates completely into Na+ and Cl; mannitol does not dissociate in solution.

Under the conditions described, what net water movement is expected to occur?

A Water moves into the intestine from the bathing solution

B Water moves out of the intestine

C No net water transport occurs

B. Effective osmolarity (tonicity) = osmolarity × reflection coefficient. Lumen tonicity = 300 × 0.9 = 270 mOsm/L; bath tonicity = 300 × 1.0 = 300 mOsm/L. The luminal solution is hypotonic with respect to the bathing solution; water therefore moves out of the intestine by osmosis.

A study of a secretory airway epithelial ...

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