* Digestion: Solids are broken down into smaller molecules through the process of digestion. This begins in the mouth, where food is chewed and mixed with saliva, which contains enzymes that begin to break down carbohydrates. Food then travels to the stomach, where it is further broken down by stomach acid and enzymes. From the stomach, food moves to the small intestine, where it is further broken down by enzymes from the pancreas and bile from the liver. The small intestine is lined with villi, which are tiny finger-like projections that help to absorb nutrients from food. Any remaining solid waste is passed to the large intestine, where it is eventually expelled from the body as feces.
* Absorption: Once nutrients have been broken down into small molecules, they are absorbed into the bloodstream through the walls of the small intestine. This process is aided by the villi, which increase the surface area of the small intestine and allow for more absorption.
* Excretion: Any solid waste that is not absorbed in the small intestine is passed to the large intestine, where it is eventually expelled from the body as feces.
Liquids:
* Absorption: Liquids are absorbed into the bloodstream through the walls of the stomach and small intestine. The stomach has a limited capacity for absorbing liquids, so large amounts of liquid can cause discomfort or nausea. The small intestine is more efficient at absorbing liquids, and it can absorb large amounts of liquid without causing any problems.
* Excretion: Liquids that are not absorbed in the small intestine are passed to the large intestine, where they are eventually expelled from the body as urine.
Gases:
* Diffusion: Gases move through the body by diffusion, which is the movement of molecules from an area of high concentration to an area of low concentration. Oxygen diffuses into the bloodstream from the lungs, and carbon dioxide diffuses out of the bloodstream from the lungs.
* Respiration: Respiration is the process by which oxygen is taken into the body and carbon dioxide is released. Respiration occurs in the lungs, where alveoli (tiny air sacs) allow for the exchange of gases between the lungs and the bloodstream.