Final answer:
Sodium reabsorption in amphibians primarily occurs in the proximal convoluted tubule, with the loop of Henle and the collecting ducts providing additional regulation and fine-tuning of water and sodium balance.
Step-by-step explanation:
In amphibians, sodium reabsorption primarily occurs in the proximal convoluted tubule of the nephron.
This process is vital for the regulation of sodium balance and the conservation of water.
The proximal convoluted tubule reabsorbs a significant portion of filtered sodium along with other substances such as glucose and amino acids.
Subsequent fine-tuning of sodium and water balance occurs in other sections of the nephron, including the distal convoluted tubule and the collecting ducts.
The loop of Henle, which consists of both an ascending and a descending limb, plays a crucial role in creating a concentration gradient in the medulla of the kidney, enabling the reabsorption of water in the collecting ducts.
However, the direct reabsorption of sodium occurs predominantly in the proximal convoluted tubule and the ascending loop of Henle, not in the descending limb.
Additionally, structures like Bowman's capsule are involved in the initial filtration of blood to form urine, but not in reabsorption.