Questions: There are two major types of water reabsorption that occurs in the kidney. In some area, water permeability cannot be adjusted because aquaporins are always present in the cell membranes. Water absorption in these areas is called water absorption.

The answer is the first one (A): obligatory water absorption.

Explanation:

• Obligatory water absorption occurs in parts of the kidney where water permeability is constant because aquaporins are always present in the cell membranes. This means that water reabsorption happens automatically and cannot be regulated by hormones or other factors.
• In contrast, facultative water absorption occurs in areas where water permeability can be adjusted, typically under the influence of hormones like antidiuretic hormone (ADH), which regulates the insertion of aquaporins into the cell membranes.

In summary, the type of water absorption where aquaporins are always present and water permeability cannot be adjusted is called obligatory water absorption.