Final answer:
Around 70% of the water in the tubular fluid is reabsorbed by the proximal convoluted tubule (PCT), a key segment of the nephron in the kidney.
Step-by-step explanation:
Approximately 70% of water in the tubular fluid is reabsorbed in the proximal convoluted tubule (PCT). The PCT is responsible for a significant amount of solute and water reabsorption from the filtered fluid collected by Bowman's capsule. It is highly efficient at this process due to its structure, which includes simple cuboidal cells with prominent microvilli that greatly increase the surface area, facilitating absorption and secretion of solutes. Furthermore, these cells contain a high concentration of mitochondria to provide the ATP needed for active transport. Substantial reabsorption of ions, water, nutrients occurs in the PCT, along with the removal of toxins and adjustment of filtrate pH.