The digestive system is that system which exists within all organisms to allow nutritive particles to be taken in, broken down, and either used by the body or removed from the body as waste. Digestive systems range from the very simple, primitive systems of the once-celled organisms, to the complex, multi-organ systems used by vertebrates (organisms with backbones).
Single-celled organisms engulf food particles with their outer membrane. The food particles are brought inside the cell; hence this type of digestion is referred to as intracellular. Food particles are ultimately engulfed by bags of potent chemicals within the cell (called digestive vacuoles) which break down the particles into usable components. Waste products may be re-packaged into little bags, and passed back out through the membrane(interestingly enough, this type of digestion is used by the immune system's white blood cells in mammals; white blood cells protect the body from infection by engulfing and digesting foreign invaders such as viruses and bacteria).
More complex organisms may have a mouth which leads to a large, open body cavity. Sponges, for example, carry water into this cavity, where any tiny food particles are extracted and distributed throughout the cells of the sponge for digestion. The water and any waste is then sent back out through the mouth.
Flatworms have a blind gut. This is a tube within which digestion takes place. It is equipped with a mouth opening, but is otherwise totally sealed off. Food enters the mouth and is partially digested by chemicals released into the gut. Because this type of digestion occurs within the cavity of the gut, and not within a cell, it is referred to as extracellular digestion. Once the food has been broken down through this extracellular digestion, the smaller bits can be absorbed by the cells which line the gut. Waste products are passed back out through the mouth.
As organisms become more complex, their digestive systems become increasingly complex. Higher up the evolutionary ladder, the blind sac eventually leads to a separate opening for the passage of waste products, the primitive anus. This is evident in various worms called nematodes. In higher animals, outpouchings from the digestive tube become specialized organs such as the liver, pancreas, and gallbladder.
The basic schemata for vertebrate (organisms with backbones) alimentary canals or digestive tracts involves structures which are responsible for receiving food, conducting and storing food, breaking food down and absorbing its useful nutrients, and absorbing water and eliminating wastes (defecating).
The mouth is the opening responsible for receiving food. Within the mouth, food is often broken down into smaller bits by the mechanical actions of chewing with teeth and by the chemical action of substances found within saliva. Saliva helps to lubricate food, allowing it to pass into the esophagus.
The esophagus serves to conduct the food to its next destination. In some animals (certain birds and reptiles), an outpouching from the esophagus called the crop is used for storage of food. It may also contain small rocks, which help to grind tough bits of food, such as seeds.
The stomach is a muscular pouch which works to mix the food, usually with a highly acidic combination of chemicals which will further breakdown the food. The stomach sends food into the small intestine, where a variety of alkaline chemicals produced by the liver and pancreas continue the process of digestion. Enzymes and bacteria along the border of the small intestine aid in the digestive process. As food becomes broken down into its component nutrients, these nutrients are absorbed into the bloodstream.
The components of food which are not absorbed continue passing down the large intestine, which works to absorb water from this mass. Ultimately, the mass of waste (feces), appropriately dehydrated by the absorption of water from it, arrives at the rectum, where feces are stored. When a sufficient quantity has accumulated, the feces are expelled through the anus.
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