In the context of cell signaling and kidney function, where does facultative reabsorption of water primarily occur?

• A. Collecting duct
• B. Bowman's capsule
• C. Loop of Henle
• D. Proximal convoluted tubule

Answer: (A)

Verified step by step guidance

1. Understand the concept of facultative reabsorption: Facultative reabsorption refers to the process where water reabsorption is regulated based on the body's needs, often influenced by hormones like antidiuretic hormone (ADH). This is distinct from obligatory reabsorption, which occurs regardless of the body's hydration status.
2. Review the anatomy of the nephron: The nephron is the functional unit of the kidney and consists of several parts, including Bowman's capsule, the proximal convoluted tubule, the Loop of Henle, the distal convoluted tubule, and the collecting duct. Each part has specific roles in filtration, reabsorption, and secretion.
3. Identify the role of the collecting duct: The collecting duct is the site where facultative reabsorption of water primarily occurs. ADH increases the permeability of the collecting duct to water, allowing more water to be reabsorbed into the bloodstream when the body is dehydrated.
4. Eliminate incorrect options: Bowman's capsule is involved in filtration, not reabsorption. The proximal convoluted tubule and Loop of Henle are primarily responsible for obligatory reabsorption of water and solutes, not facultative reabsorption.
5. Conclude that the correct answer is the collecting duct, as it is the primary site for hormonally regulated water reabsorption in response to the body's hydration needs.