Final answer:
Aldosterone, released from the adrenal cortex, conserves water by promoting the reabsorption of sodium and excreting potassium, thus helping maintain blood pressure and volume.
Step-by-step explanation:
The hormone aldosterone, which is released during exercise in the heat, plays a crucial role in the body's fluid and electrolyte balance. When released from the adrenal cortex, aldosterone promotes the reabsorption of sodium (Na+) by the nephron, which in turn helps in the conservation and retention of water. This action is vital for the maintenance of blood pressure and volume. Aldosterone also has an influence on potassium (K+) levels by promoting its excretion from the body.
When angiotensin II levels rise or when there is an increase in plasma K+ levels, aldosterone is secreted. It acts primarily on the distal tubules of the kidney to enhance Na+ reabsorption and K+ secretion. The reabsorption of Na+ leads to the passive recovery of water, which increases plasma volume and helps maintain a normal blood pressure. Aldosterone's regulation involves a negative feedback loop where increased osmolarity of extracellular fluid (ECF) following sodium absorption inhibits further aldosterone release.