Final answer:
Vitamin D is first activated by the liver and then by the kidney, turning into its active form calcitriol, which is essential for the absorption of calcium in the body and affects various bodily systems.
Step-by-step explanation:
Vitamin D Activation
Vitamin D must be activated first by the liver and then by the kidney before it can affect the body systems. The liver transforms vitamin D to 25(OH)D or calcidiol, which is the primary circulating form. Subsequently, in the kidneys, hydroxylation reactions occur, converting calcidiol to calcitriol (1,25(OH)D₂), the biologically active form of vitamin D.
This activated form of vitamin D is crucial for the absorption and reabsorption of calcium in the digestive tract and kidneys respectively, and it plays a pivotal role in maintaining normal serum concentrations of calcium and phosphate which are vital for various body functions including bone health and muscle contraction.