Answers

CHAPTER 10

1. (E) Simple diffusion

   Small lipid soluble gas molecules such as N₂O readily diffuse through biological membranes. The inability to increase blood N₂O concentration above that in the lung alveolus is consistent with simple diffusion and the absence of any primary or secondary active transport process.

2. (C) inspired N₂O concentration increased

   According to Fick’s law of diffusion, the rate of simple diffusion of N₂O is proportional to its membrane permeability and to its concentration gradient. The N₂O concentration gradient is increased if the inspired N₂O concentration is increased, but is decreased if alveolar ventilation or pulmonary blood flow are decreased. The rate of simple diffusion is slower at colder temperatures or if the diffusion distance is increased.

3. (D) Opening of Na⁺ channels

   When a depolarizing stimulus causes the threshold membrane potential (of about −55 mV) to be exceeded, voltage sensitive Na⁺ channels rapidly open, resulting in fast depolarization.

4. (C) Net inward Na⁺ current

   At 0 mV during repolarization, Na⁺ channels are inactivating but many remain open. K⁺ channels are opening rapidly at this time. The electrochemical driving force is inward for Na⁺ and outward for K⁺. Therefore, there is net outward K⁺ flow and net inward Na⁺ flow at this time. A significant Na⁺ conductance must still be present to account for a membrane potential of 0 mV.

5. (E) would not be triggered

   The stimulus is applied during the absolute refractory period. No second action potential is possible because too many Na⁺ channels are now inactivated.

6. (A) Increased preload

   Curves intersect the x-axis when there is zero shortening (“isometric” contraction). The total force on the muscle at this point is indicative of preload; therefore curve C has the largest preload. If a total load is selected on the x-axis and a vertical line is drawn, the curves may be compared; increased preload results in faster muscle shortening.

7. (F) H⁺/K⁺-ATPase

   Primary active transport has a direct dependence on adenosine triphosphate hydrolysis and would be inhibited under anaerobic conditions. Secretion moves H⁺ out of the cell; dependence on external K⁺ suggests an exchange with H⁺.

8. (C) Depolarization due to E_K becoming less negative

   In most cells, V_m is a function of [K⁺]_o because K⁺ channels provide the dominant membrane conductance at rest. Acutely increasing [K⁺]_o results in a less negative value for E_K, according to the Nernst equation. V_m becomes less negative, or depolarize as a result.

9. (E) reduced time for Ca²⁺ reuptake into sarcoplasmic reticulum

   The question describes temporal summation, in which the muscle is tetanized. The basis of this effect is accumulation of Ca²⁺ in the sarcoplasm with repeated stimulation due to insufficient time for Ca²⁺ reuptake by sarcoplasmic reticulum. A plateau is reached ...