Why is hydrochloric acid (HCI) not generated directly within the cytoplasm of parietal cells?
HCI is only formed in the bloodstream before entering the stomach
HCI is too acidic and would damage the parietal cells
The cytoplasm lacks the necessary enzymes for HCI production
It requires oxygen, which is absent in the cytoplasm
The Correct Answer is B
A. HCI is only formed in the bloodstream before entering the stomach: Gastric acid is never found in the systemic circulation. Blood pH is strictly maintained at 7.35 to 7.45. Acid production is localized exclusively to the secretory canaliculi of the gastric mucosa.
B. HCI is too acidic and would damage the parietal cells: The intracellular pH of 7.2 would be instantly neutralized by the high acidity of hydrochloric acid. To prevent cytoplasmic denaturation, hydrogen and chloride ions are secreted separately. They only combine within the lumen.
C. The cytoplasm lacks the necessary enzymes for HCI production: Parietal cells contain high concentrations of carbonic anhydrase and H+/K+ ATPase pumps. The machinery for acid production is present within the cell. The restriction is a matter of safety, not enzymology.
D. It requires oxygen, which is absent in the cytoplasm: Parietal cells are densely packed with mitochondria and require significant aerobic metabolism to power their proton pumps. Oxygen is abundant in the cytoplasm to support this high energy demand. Acid production is ATP-dependent.
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Correct Answer is D
Explanation
A. Aorta:This is the primary systemic artery that carries oxygenated blood away from the heart. It does not receive venous drainage from any organ. Blood flow in the aorta is always moving toward the peripheral tissues.
B. Superior vena cava:This large vein drains the head, neck, and upper extremities into the right atrium. It is located superior to the diaphragm and does not receive blood from the liver. Hepatic drainage occurs via the inferior vessel.
C. Hepatic artery:This vessel is a branch of the celiac trunk that delivers oxygenated blood to the liver parenchyma. It is an inflow vessel rather than an outflow vessel. It provides the liver with its nutrient and oxygen supply.
D. Inferior vena cava:This is the terminal vessel for all venous drainage below the diaphragm. The hepatic veins emerge from the posterior surface of the liver to join this vessel directly. It then returns the blood to the heart.
Correct Answer is C
Explanation
A. Ferment:Fermentation of undigested carbohydrates primarily occurs in the large intestine via anaerobic bacteria. The stomach is an acidic environment unsuitable for significant bacterial fermentation. Its primary role is mechanical and initial chemical breakdown.
B. Secrete:Secretion of gastric juice is a function of the mucosal glandular epithelium, not the muscularis externa. While muscle contraction helps mix secretions, the muscles themselves do not produce acid or enzymes. Secretion is an epithelial process.
C. Churn:The stomach possesses an inner oblique muscle layer in addition to circular and longitudinal layers. This allows for complex, multi-directional contractions that physically break down food into chyme. This vigorous mechanical mixing is unique to gastric physiology.
D. Absorb:The stomach has a limited capacity for absorption, primarily taking in lipid-soluble substances like alcohol and aspirin. Absorption is mainly a function of the villi in the small intestine. The muscular layers do not facilitate the absorptive process.
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