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.
-A positive pressure mechanical ventilator most likely inflates the lungs by directly:

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.
-A positive pressure mechanical ventilator most likely inflates the lungs by directly:

Quizplus · Accepted Answer

11ecba2d_12de_845d_a3bf_b31953f7054a_MD0008_00 Respiration occurs in concert with lung volume changes and is driven by pressure differentials between the lungs (alveoli) and the pleural (chest) cavity. The change in pressures during inspiration is initiated by different means in normal and mechanical ventilation. Normal inspiration is initiated by contraction of the diaphragm. As the diaphragm contracts, the volume of the pleural cavity increases and intrapleural pressure (IPP) decreases. The lungs expand to fill the pleural cavity and air flows into the lungs. Because normal inspiration is initiated by decreasing IPP, this pressure mechanism is referred to as negative pressure breathing. According to the passage, general anesthetics have a paralyzing effect. Therefore, the diaphragm cannot initiate inspiration by decreasing IPP (Choice B). Instead, the positive pressure ventilator directly pumps air into the lungs. This increases alveolar pressure, which causes the lungs to inflate (Choice D). (Choice A) An increased IPP would push against the lungs and oppose inhalation. In addition, the pleural cavity is separated from the external environment, and therefore a noninvasive mechanical ventilator would be unable to directly affect its pressure. Educational objective: In normal inspiration, the diaphragm contracts to reduce intrapleural pressure, which results in lung expansion. In positive pressure ventilation, an external pump directly increases alveolar pressure by pumping air into the lungs to inflate the lungs.

Passage When Present in the Bloodstream Above a Threshold Partial Pressure