When nutrients pass through the digestive tract wall into the bloodstream?

This is the process of Absorption. It is the process b which digested nutrients pass through the wall of small intestine into the blood stream. Absorption involves the physiological processes, active transport and diffusion. Active transport involves use of energy supplied by the cells to move molecules across a membrane (against a concentration gradient). Diffusion on the other hand involve movement of molecules from wehere they are highly concentrated to where they are less concentrated. 

The answer is joseph is observing the colour of the reaction mixture to see whether proteins are present in the given solution.