What is the most important factor in the maintenance of ECF volume?

Correct Answer: C

Rationale: The correct answer is C: salt balance. Salt balance is crucial for maintaining extracellular fluid (ECF) volume as salt, specifically sodium, plays a key role in regulating fluid balance in the body. When salt intake is high, the body retains more water to maintain osmotic balance, increasing ECF volume. On the other hand, low salt intake can lead to decreased ECF volume due to water loss. Vasopressin secretion (A) regulates water reabsorption in the kidneys, not salt balance. Free water balance (B) is important but does not directly impact ECF volume maintenance. Potassium balance (D) is more relevant to intracellular fluid balance and membrane potential, not ECF volume regulation.

Correct Answer: D

Rationale: The correct answer is D because osmolarity refers to the concentration of solutes in a solution. A hypertonic solution has a higher concentration of solutes compared to an isotonic solution, leading to water moving out of cells through osmosis. This definition directly relates to osmolarity. A, B, and C are incorrect as they do not directly relate to osmolarity. Choice A talks about vasopressin secretion in response to water deficit, B discusses urinary output in dehydration, and C is incomplete. These choices do not address the concept of osmolarity, which is about solute concentration in a solution.

Correct Answer: B

Rationale: Correct Answer: B (Ca2+) Rationale: In states of acidosis, the body retains Ca2+ in body fluids to help buffer the excess H+ ions and maintain pH balance. Ca2+ acts as a buffer by binding to excess H+ ions, reducing acidity. Kidneys increase Ca2+ retention to counteract the increased H+ secretion. This relationship helps stabilize the body's pH. Summary: A: Na+ is not directly involved in buffering excess H+ ions during acidosis. C: K+ is not specifically retained in response to increased H+ secretion. D: Cl- is not directly related to buffering and pH balance in acidosis.

Correct Answer: B

Rationale: The correct answer is B because each bicarbonate ion can carry two H+ ions, which helps maintain the pH balance in the extracellular fluid (ECF). When there is an excess of H+ ions, bicarbonate ions bind with them to form carbonic acid, which then dissociates into water and carbon dioxide. This helps prevent drastic changes in pH by removing excess H+ ions. Choice A is incorrect because bicarbonate and carbonic acid levels are tightly regulated in the body. Choice C is incorrect because carbonic acid is a weak acid and does not dissociate easily. Choice D is incorrect because the abundance of bicarbonate and carbonic acid in the plasma alone does not make the buffering system effective; it is the ability of bicarbonate ions to carry H+ ions that makes it effective.

Correct Answer: C

Rationale: The correct answer is C: haemoglobin. Haemoglobin acts as a buffer for H+ generated from CO at the tissue level by binding to the H+ ions, preventing drastic changes in blood pH. Hemoglobin can also release H+ ions when needed, helping to maintain pH balance. A: The HCO3- buffer system primarily buffers H+ generated from CO2 at the tissue level, not from CO. B: The phosphate buffer system mainly functions in the kidneys to regulate blood pH, not in buffering H+ from CO. D: Plasma proteins have a limited role in buffering H+ compared to haemoglobin.