Passage
When present in the bloodstream above a threshold partial pressure, a general anesthetic blocks the sensation of pain, induces a state of unconsciousness, and causes muscle paralysis. Many anesthetic agents are administered in their gaseous phase and are absorbed into the bloodstream through the lungs. The rate of diffusion V_gas of an anesthetic across the alveolar membrane with a surface area A and a thickness T is described by the Fick law of diffusion:
Vgas=DAΔPTEquation 1where D is the diffusion coefficient specific for the gas and ΔP is the partial pressure difference across the membrane.An inhaled anesthetic is described by its blood-gas partition coefficient, the ratio of the anesthetic's concentration in the blood to its concentration in the lungs when the partial pressures are equal. The concentration C of an anesthetic gas dissolved in blood is described by the Henry law of solubility:
C=kHPgasEquation 2where k_H is the solubility constant specific for the gas and P_gas is the partial pressure of the gas.Due to the paralytic effects of general anesthetics, mechanical ventilators are usually necessary to assist a patient's breathing during surgery. Positive-pressure ventilators are noninvasive and use external pumps to induce inspiration by forcing air into the lungs through a ventilation mask. Despite the paralytic effects of general anesthesia, patients are often capable of expiration without the active aid of a mechanical ventilator.
-Which of the following graphs best depicts the anesthetic partial pressure along the length of the pulmonary capillaries when it is first administered?

Question

Passage
When present in the bloodstream above a threshold partial pressure, a general anesthetic blocks the sensation of pain, induces a state of unconsciousness, and causes muscle paralysis. Many anesthetic agents are administered in their gaseous phase and are absorbed into the bloodstream through the lungs. The rate of diffusion V_gas of an anesthetic across the alveolar membrane with a surface area A and a thickness T is described by the Fick law of diffusion:

Vgas=DAΔPTEquation 1where D is the diffusion coefficient specific for the gas and ΔP is the partial pressure difference across the membrane.An inhaled anesthetic is described by its blood-gas partition coefficient, the ratio of the anesthetic's concentration in the blood to its concentration in the lungs when the partial pressures are equal. The concentration C of an anesthetic gas dissolved in blood is described by the Henry law of solubility:

C=kHPgasEquation 2where k_H is the solubility constant specific for the gas and P_gas is the partial pressure of the gas.Due to the paralytic effects of general anesthetics, mechanical ventilators are usually necessary to assist a patient's breathing during surgery. Positive-pressure ventilators are noninvasive and use external pumps to induce inspiration by forcing air into the lungs through a ventilation mask. Despite the paralytic effects of general anesthesia, patients are often capable of expiration without the active aid of a mechanical ventilator.
-Which of the following graphs best depicts the anesthetic partial pressure along the length of the pulmonary capillaries when it is first administered?

Quizplus · Accepted Answer

11ecba2d_12dd_e81b_a3bf_dd2adfb75bbc_MD0008_00 Gas exchange between the alveolar sacs and the pulmonary capillaries occurs due to passive diffusion and is driven by partial pressure differentials (difference) across the membrane from higher to lower partial pressures. The rate of diffusion (V_gas) of the anesthetic is described by the Fick law of diffusion (Equation 1) and is directly proportional to the partial pressure difference ΔP between the alveoli and the capillaries: 11ecba2d_12de_0f2c_a3bf_fd3d4c51278a_MD0008_00 When the anesthetic is first administered, its partial pressure is greater in the lungs, and therefore it will diffuse into the capillaries. The capillary partial pressure will build up along the length of the capillary as the blood flows through it (Choice D). The alveolar partial pressure does not change significantly because the volume of gas in the lungs is much greater than the volume of gas in the capillaries. As the partial pressure in the capillary increases, the partial pressure difference (ΔP) across the membrane driving diffusion will decrease. The rate of diffusion will decrease and will eventually reach zero when the capillary and alveolar partial pressures are equilibrated. This behavior is similar to that of oxygen exchange in the alveoli. (Choices A and B) The graph should reflect an initial increase in partial pressure as the anesthetic diffuses into the capillaries and then levels off as the partial pressures in the lungs and capillaries reach equilibrium. Educational objective: The diffusion of a gas across a membrane is directly proportional to its partial pressure difference (Fick law of diffusion). Gases diffuse passively along the partial pressure gradient to reach equilibrium.

Passage When Present in the Bloodstream Above a Threshold Partial Pressure