Which body tissues are composed of the highest percentage of water?

A. By absorbing carbonic acid into tissues: Carbonic acid is not absorbed into tissues for storage; it exists in equilibrium with dissolved carbon dioxide. Retaining it would lead to a progressive decrease in systemic pH and tissue damage. The body must eliminate the source of the acid rather than sequestering it.

B. By increasing the respiratory rate to expel CO2: Central and peripheral chemoreceptors detect rising partial pressures of carbon dioxide and trigger the medullary respiratory center. This hyperventilation increases alveolar gas exchange, blowing off the excess carbon dioxide. This process reduces the concentration of carbonic acid and raises pH.

C. By producing more hydrogen ions: Increasing hydrogen ion production would exacerbate the acidotic state and further lower the blood pH. The goal of the respiratory system is to remove the chemical precursor of acidity. Hydrogen ions are a product of carbon dioxide retention, not a tool for stabilization.

D. By excreting carbonic acid through sweat: Sweating is a mechanism for thermoregulation and the excretion of small amounts of electrolytes and urea. It is not a significant pathway for the elimination of carbon dioxide or the regulation of acid-base balance. Respiratory gases must be exchanged via the pulmonary membrane.

A. The buffering ability of proteins is due to their charged amino acid side chains: Proteins contain carboxyl and amino groups that can donate or accept protons depending on the surrounding pH. Hemoglobin and plasma albumin are major contributors to the body's total buffering capacity. This allow proteins to stabilize both intracellular and extracellular fluids.

B. Proteins act only in acidic environments: Proteins are amphoteric molecules, meaning they can function as both acids and bases. They neutralize excess hydrogen ions in acidic states and release hydrogen ions in basic states. This versatility makes them effective across a wide spectrum of physiological pH disturbances.

C. Protein buffering is insignificant compared to other mechanisms: Intracellular proteins and hemoglobin represent one of the largest buffering pools in the human body. While the bicarbonate system is the primary extracellular buffer, proteins provide massive buffering power within the cells. They are a critical component of acid-base homeostasis.

D. Proteins buffer by converting to enzymes under pH stress: Enzymes are proteins with specific catalytic functions, but they do not "convert" into new forms to provide buffering. Buffering is an inherent chemical property of the protein structure itself. pH stress actually risks denaturing enzymes and destroying their functional capacity.