Final answer:
Antidiuretic hormone (ADH), also known as vasopressin, is the major hormone involved in the regulation of extracellular fluids, which promotes water reabsorption and helps maintain blood volume and pressure.
Step-by-step explanation:
The major hormone involved in the regulation of extracellular fluids, promoting reabsorption of water, and assisting in the maintenance of blood volume and pressure is called antidiuretic hormone (ADH), also known as vasopressin.
ADH is secreted by the hypothalamus and acts on the kidneys, specifically making the collecting ducts permeable to water, therefore allowing water molecules to be reabsorbed from the urine back into the blood through osmosis. This action of ADH aids the body in conserving water and helps to maintain an adequate blood volume and pressure.
In contrast, another hormone called aldosterone also contributes to the regulation of electrolyte concentrations in extracellular fluids. Produced by the adrenal cortex, aldosterone enhances the reabsorption of sodium (Na+) and the secretion of potassium (K+) from the extracellular fluids of cells in kidney tubules.
This process not only helps in maintaining proper water balance due to the osmotic reabsorption of water but also contributes to the modification of blood volume and blood pressure.