Final answer:
The uptake of nutrients from the gastrointestinal tract into the bloodstream is known as absorption, primarily occurring in the small intestine through various mechanisms, including active transport and endocytosis.
Step-by-step explanation:
The uptake of nutrients across the epithelial cells of the gastrointestinal tract and into the bloodstream is called absorption. This is an essential process where digested food substances enter the body's internal environment. Nutrient absorption takes place primarily in the small intestine, where most nutrients pass through the mucosal epithelium and into the bloodstream or lymphatic system.
Various mechanisms of absorption include active transport, passive diffusion, facilitated diffusion, co-transport, and endocytosis. Each of these mechanisms facilitates the movement of different types of nutrients at the cellular level. For instance, water-soluble nutrients that cannot permeate the hydrophobic cell membrane require special transport proteins or mechanisms to be absorbed.