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Peristalsis

This image shows the peristaltic movement of food. In the left image, the food bolus is towards the top of the esophagus and arrows pointing downward show the direction of movement of the peristaltic wave. In the center image, the food bolus and the wave movement are closer to the center of the esophagus and in the right image, the bolus and the wave are close to the bottom end of the esophagus.
Peristalsis moves food through the digestive tract with alternating waves of muscle contraction and relaxation.

Digestion includes both mechanical and chemical processes. Mechanical digestion is a purely physical process that does not change the chemical nature of the food. Instead, it makes the food smaller to increase both surface area and mobility. It includes mastication    , or chewing, as well as tongue movements that help break food into smaller bits and mix food with saliva. Although there may be a tendency to think that mechanical digestion is limited to the first steps of the digestive process, it occurs after the food leaves the mouth, as well. The mechanical churning of food in the stomach serves to further break it apart and expose more of its surface area to digestive juices, creating an acidic “soup” called chyme    . Segmentation , which occurs mainly in the small intestine, consists of localized contractions of circular muscle of the muscularis layer of the alimentary canal. These contractions move their contents back and forth while continuously subdividing, breaking up, and mixing the contents.

In chemical digestion    , starting in the mouth, digestive secretions break down complex food molecules into their chemical building blocks (for example, proteins into separate amino acids). These secretions vary in composition, but typically contain water, various enzymes, acids, and salts. The process is completed in the small intestine.

Food that has been broken down is of no value to the body unless it enters the bloodstream and its nutrients are put to work. This occurs through the process of absorption    , which takes place primarily within the small intestine. There, most nutrients are absorbed from the lumen of the alimentary canal into the bloodstream through the epithelial cells that make up the mucosa. Lipids are absorbed into lacteals and are transported via the lymphatic vessels to the bloodstream. Remember that digestion and absorption are different processes . The details of these processes will be discussed later.

In defecation    , the final step in digestion, undigested materials are removed from the body as feces.

In some cases, a single organ is in charge of a digestive process. For example, ingestion occurs only in the mouth and defecation only in the anus. However, most digestive processes involve the interaction of several organs and occur gradually as food moves through the alimentary canal ( [link] ).

Digestive processes

This image shows the different processes involved in digestion. The image shows how food travels from the mouth through the major organs. Associated textboxes list the different processes such as propulsion, chemical and mechanical digestion and absorption near the organs where they take place.
The digestive processes are ingestion, propulsion, mechanical digestion, chemical digestion, absorption, and defecation.

Some chemical digestion occurs in the mouth. Some absorption can occur in the mouth and stomach, for example, alcohol and aspirin.

Regulatory mechanisms

The nervous and endocrine systems mechanisms work to maintain the optimal conditions in the lumen needed for digestion and absorption. These regulatory mechanisms, which stimulate digestive activity through mechanical and chemical activity.

Neural controls

The walls of the alimentary canal contain a variety of sensors that help regulate digestive functions. For example, these receptors can sense when the presence of food has caused the stomach to expand, whether food particles have been sufficiently broken down, how much liquid is present, and the type of nutrients in the food (lipids, carbohydrates, and/or proteins). Another example is that the sight, smell, and taste of food initiate long reflexes that begin with a sensory neuron delivering a signal to the brain. The signal stimulates cells in the stomach to begin secreting digestive juices in preparation for incoming food.

Hormonal controls

A variety of hormones are involved in the digestive process. The main digestive hormone of the stomach is gastrin, which is secreted in response to the presence of food. Gastrin stimulates the secretion of gastric acid by the parietal cells of the stomach. Other GI hormones are produced and act upon the gut and its accessory organs. Hormones produced by the duodenum include secretin , which stimulates a watery secretion of bicarbonate by the pancreas; cholecystokinin (CCK) , which stimulates the secretion of pancreatic enzymes and bile from the liver and release of bile from the gallbladder; and gastric inhibitory peptide , which slows gastric secretion and gastic emptying.These GI hormones are secreted by specialized epithelial cells, called endocrinocytes , located in the lining of the stomach and small intestine. These hormones then enter the, through the bloodstream which they can reach their target organs.

Chapter review

The digestive system ingests and digests food, absorbs released nutrients, and excretes food components that are indigestible. The six activities involved in this process are ingestion, motility, mechanical digestion, chemical digestion, absorption, and defecation. These processes are regulated by neural and hormonal mechanisms.

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Source:  OpenStax, Digestive system. OpenStax CNX. Feb 23, 2015 Download for free at http://legacy.cnx.org/content/col11761/1.1
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