Final answer:
The primary defense against hyperosmolality and hypernatremia is the release of antidiuretic hormone (ADH) which prompts the kidneys to reabsorb more water, along with a stimulated thirst mechanism to increase fluid intake. The correct option is d.
Step-by-step explanation:
The major defense against hyperosmolality and hypernatremia is the release of antidiuretic hormone (ADH), which prevents water loss by causing the kidneys to reabsorb more water. The thirst mechanism also plays a key role in defending against hyperosmolality by prompting increased water intake. Meanwhile, the sodium-potassium pump is fundamental in maintaining cellular osmotic balance, but it is not the major defense against hyperosmolality and hypernatremia at the systemic level. Aldosterone secretion regulates sodium and potassium levels but is also not the primary defense against hyperosmolality; its main role is connected to blood volume and pressure. When the body experiences dehydration or physiological stress leading to an increase in blood osmolarity, the hypothalamus signals for more ADH to be released. The hormone then acts on the kidneys to promote water reabsorption, reducing urine volume and thereby decreasing blood osmolarity toward normal levels.