Why is tetany seen with hyperventilation:
**Core Concept**
Tetany is a condition characterized by muscle cramps, spasms, and stiffness, often resulting from an imbalance of electrolytes, particularly calcium, in the body. The underlying mechanism involves the regulation of calcium levels through the action of the enzyme carbonic anhydrase and the buffering capacity of the blood.
**Why the Correct Answer is Right**
Hyperventilation leads to an excessive loss of carbon dioxide (CO2) from the lungs, which in turn causes a decrease in blood carbonic acid levels. This triggers a compensatory increase in bicarbonate (HCO3-) levels through the action of carbonic anhydrase, an enzyme that catalyzes the reversible reaction between CO2 and H2O to form carbonic acid. The increased HCO3- levels lead to a left shift of the calcium-binding curve, resulting in a decrease in ionized calcium levels. This decrease in ionized calcium levels triggers the release of acetylcholine from the motor endplate, leading to muscle hyperexcitability and tetany.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not address the underlying mechanism of tetany in hyperventilation. While respiratory alkalosis is a consequence of hyperventilation, it is not the direct cause of tetany.
**Option B:** This option is incorrect because it suggests that tetany is caused by a decrease in blood pH. However, the actual mechanism involves a decrease in ionized calcium levels, which is not directly related to pH changes.
**Option C:** This option is incorrect because it suggests that tetany is caused by a direct effect of CO2 on muscle cells. While CO2 does play a role in the regulation of calcium levels, its effect is indirect, mediated through the action of carbonic anhydrase and the buffering capacity of the blood.
**Clinical Pearl / High-Yield Fact**
Remember that hyperventilation can lead to tetany due to the complex interplay between CO2 levels, bicarbonate levels, and ionized calcium levels. This phenomenon is a classic example of how a seemingly innocuous action (hyperventilation) can have a profound effect on the body's electrolyte balance.
**Correct Answer:** C.