Alimentation and Elimination: The Principles of Gastrointestinal Digestion

Abstract for Section 9.1

The function of the human gastrointestinal (GI) tract is a continual supply of water, electrolytes, and nutrients to the human body; this provides the energy necessary for the uphold of our body’s integrity and the performance of higher functions like talking, moving objects, or solving intellectual problems. To acquire this energy the body has to move the food through the GI tract, secrete digestive juices to digest the food, and absorb the nutrients during their passage through the GI tract. The anatomical and physiological basics of these processes are going to be reviewed in this chapter. The regulation of these processes is only partly under our voluntary control, which becomes particularly clear in times of emotional stress, where many individuals experience a “queasy feeling.”

The GI tract does not only comprise organs like the stomach or the small intestine, also the oral cavity with the parotid and salivary glands belongs to the human GI tract. About 9 l of fluid in 1 day is secreted and reabsorbed in the human GI tract; its length is on average 9 m, which equates to the smaller side of a volleyball field. The surface of the GI tract estimates a football field; it harbors up to one thousand different bacteria species. It is estimated that up to 10¹⁸ bacteria are inside the human GI tract. The number of the bacteria and the surface of the human GI tract make the presence of immune-competent cells throughout the GI tract necessary; these immune-competent cells are distributed all over the GI tract in aggregations like the tonsils, adenoids, or the Peyer’s patches.

Abstract for Section 9.2

Like humans, also all other animals are dependent on the intake of organic material and therefore called heterotrophs. The organic material is originally produced with the help of photosynthesis by autotrophs (e.g., plants and algae) that are solely dependent on inorganic material like carbon dioxide and water. In all heterotrophic species the digestive system is highly adapted according to the food. Three general groups can be distinguished: carnivores that feed on meat of prey, herbivores that feed on plants only, and omnivores consuming both meat and plants.

The main functions of the digestive system are ingestion of food, digestion (physical and chemical disassembly), transportation and temporal storage, absorption of nutrients, and subsequent elimination of undigested wastes. These functions are fulfilled by different parts of the digestive tract that are adapted to the dietary habits of the respective species: mouthparts, esophagus, stomach, and intestine. These organs are supported by salivary glands, pancreas, and liver.

As in the human, also for animals there are essential nutritional components that are to be provided with the food. Vitamin C, for example, is essential for guinea pigs, but not for most other mammals. An additional essential amino acid for cats is taurin. Cats can get a retinal degeneration if fed on dog food due to lack of taurin.