A 52-year-old man is brought to the emergency department by emergency medical services after being found obtunded and lying on the floor by his wife. His wife reports that he had previously been healthy without any known medical problems. Vital signs reveal a normal temperature, blood pressure 96/52 mmHg, heart rate 120 beats/minute and respirations 16/minute. On physical examination, his mucous membranes are dry, and his skin exhibits decreased turgor. Laboratory analysis reveals that his plasma antidiuretic hormone (ADH) level is increased five times above the normal level.
Which of the following segments of the nephron is responsible for the majority of water reabsorption in this state?
- A) Cortical collecting duct
- B) Distal convoluted tubule
- C) Loop of Henle
- D) Medullary collecting duct
- E) Proximal convoluted tubule
E) Proximal convoluted tubule
The majority of water and solute reabsorption occurs in the proximal convoluted tubule (PCT), regardless of total body water status. In a state of severe dehydration, such as in this patient, antidiuretic hormone (ADH) will promote increased water reabsorption in the collecting duct of the nephron; however, the proximal convoluted tubule is still the primary site of water reabsorption.
Key Learning Point
The proximal convoluted tubule (PCT) is always responsible for the bulk of water reabsorption.