Final answer:
Aldosterone, a hormone produced by the adrenal cortex, is responsible for the increased Na+ reabsorption by principal cells in the renin-angiotensin-aldosterone pathway, critical for regulating blood pressure.
Step-by-step explanation:
The hormone that causes increased Na+ reabsorption by principal cells in the renin-angiotensin-aldosterone pathway is aldosterone. Aldosterone is stimulated by the renin-angiotensin system when there is a need to increase blood pressure. It acts on the principal cells in the kidney by promoting the manufacture of luminal Na+ and K+ channels and Na+/K+ ATPase pumps. This action results in higher sodium reabsorption from the forming urine, which in turn allows water to follow sodium passively back into the blood, thereby increasing blood volume and pressure. While ADH primarily regulates water recovery, aldosterone plays a vital role in ensuring sodium and water balance, and hence blood pressure stability.