Final answer:
When blood volume is low, the first physiological response is the secretion of renin by the kidneys, which then initiates a series of reactions to increase blood pressure and volume.
Step-by-step explanation:
When the blood volume is low, the first action that occurs is B. Renin is secreted by the kidneys. Renin is a critical enzyme in the regulation of blood pressure and volume.
When blood pressure drops, the kidneys secrete renin which acts on a plasma protein called angiotensinogen to form angiotensin I. Subsequently, angiotensin I is converted to angiotensin II by the enzyme angiotensin-converting enzyme (ACE). Angiotensin II is a potent vasoconstrictor and stimulates the release of aldosterone from the adrenal glands and antidiuretic hormone (ADH) from the pituitary gland, leading to sodium reabsorption and water retention, thus increasing blood pressure and volume.
The other options, such as the secretion of aldosterone and the release of ADH, occur later in this physiological response known as the renin-angiotensin-aldosterone system (RAAS).
When blood volume is low, the action that occurs first is the secretion of renin by part of the juxtaglomerular complex. Renin acts on angiotensin to form angiotensin I, which is then converted to angiotensin II. Angiotensin II stimulates the release of aldosterone and antidiuretic hormone (ADH). Aldosterone promotes the reabsorption of sodium by the kidneys, while ADH promotes water reabsorption in the kidneys.