Vitamin D deficiency is most commonly due to insufficient exposure to sunlight, as the skin synthesizes vitamin D when exposed to UVB radiation. Other factors include poor dietary intake and impaired conversion or absorption in the liver and kidneys.

The cholinergic system, acting via muscarinic receptors, increases peristalsis and relaxes the sphincters in the gastrointestinal tract to facilitate the movement of food and waste through the digestive system.

The majority of glucose reabsorption occurs in the proximal convoluted tubule, where glucose is actively transported back into the bloodstream.

Oxytocin causes contraction of the myoepithelial cells surrounding the mammary alveoli, which presses milk into the larger ducts and leads to the milk let-down reflex. This is essential for milk ejection during lactation.

In dark-skinned patients with high serum bilirubin levels, the skin turns yellow due to the accumulation of bilirubin.