The process by which needed nutrients enter the bloodstream of an animal is called

Integration, in a biological context, is more related to the nervous system. It refers to the process by which the nervous system processes and interprets sensory information and decides what actions to take. It has nothing to do with the process of nutrient uptake into the bloodstream.

Osmosis is a specific type of passive transport in which water molecules move across a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration. While osmosis is crucial for maintaining cellular hydration, it does not directly involve the transfer of nutrients into the bloodstream.

Assimilation is a broader term for the combination of processes that convert nutrients into usable substances within an organism's body. Absorption is a part of this larger process, but assimilation also includes digestion, which is the breakdown of food into smaller molecules, and incorporation of these molecules into the body's cells.