(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.
(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.
(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.
(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.
(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.
(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.
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+.
(C) Depolarization due to EK becoming less negative
In most cells, Vm 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 EK, according to the Nernst equation. Vm becomes less negative, or depolarize as a result.
(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 ...