Which of the following heart valves separates the right atrium from the right ventricle?

A. Vitamin B12 is not synthesized in the skin. It is primarily absorbed through the digestive system from dietary sources such as meat and dairy products.
B. Vitamin D is synthesized in the skin when exposed to sunlight, specifically ultraviolet B (UVB) radiation, which triggers the production of vitamin D3, a precursor to the active form of vitamin D.
C. Vitamin A is obtained from dietary sources, such as fruits and vegetables, and is not synthesized in the skin.
D. Vitamin K is primarily absorbed from food and produced by bacteria in the intestines, not synthesized in the skin.

A. The stomach plays a key role in digestion by breaking down food mechanically and chemically but does not play a significant role in nutrient absorption.

B. The large intestine is primarily involved in water and electrolyte absorption, as well as the formation of feces. It does not significantly contribute to nutrient absorption.

C. While some digestion begins in the mouth, particularly with enzymes in saliva, nutrient absorption does not occur there.

D. The small intestine is the primary site of nutrient absorption in the digestive system. Its walls are lined with villi and microvilli that increase surface area for the absorption of nutrients such as carbohydrates, proteins, fats, vitamins, and minerals.