Fundamentals of Human Nutrition/Digestion

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3.2 Digestion[edit]

Digestion is the process of breaking food into components small enough to be absorbed by the body (Whitney 72).

5.2.1 Phases[edit]

Food enters the mouth where it is tasted and the mechanical breakdown, by saliva, and chemical digestion begins. Secondly, the tongue moves the food back towards the pharynx, which is shared by the digestive and respiratory tracts. Thirdly, a valve-like flap called the epiglottis blocks the air passages during swallowing to allow the food to take the right path down the esophagus and into the stomach. Next, the food enters the stomach by passing through a sphincter, which is a muscle that encircle the tube of the digestive tract and acts like a valve. The food is then mashed up and mixed with acid to turn into a semi-liquid food mass called chime. The chime leaves the stomach and enters into the small intestine, where the main site of digestion and absorption of nutrients occurs. The small intestine also decreases the stomach motility and slows the secretion of gastric juices. From here, any materials that are no absorbed in the small intestine move on to the large intestine via the ileocecal valve. This valve does not let materials to re-enter the small intestine. In the large intestine, most water and vitamins are absorbed. Finally, anything that is not absorbed passes through the anus and exits as feces (Whitney 72, 73).

5.2.2 Mechanical processes[edit]

The chewing of food begins the mechanical process of digestion. It makes food easier to swallow and increases the surface area in contact with digestive juices. Moreover, the tongue uses its mechanics to push the food to the back of the throat to the pharynx. Next, the esophagus moves the food down to the stomach through rhythmic contractions of the smooth muscles, which is called peristalsis (Whitney 75). As the food moves through the digestive tract, peristalsis is a significant mechanical process.

5.2.3 Secretions[edit]

5.2.3.1 Salivary[edit]

The salivary glands moisten food and helps us taste and swallow the food we ingest (Whitney 76).. The enzyme, salivary amylase, that is secreted out of the salivary glands help with the digestion of starch and other carbohydrates, as well as cleanses the mouth, and protects teeth from decay (Whitney 76). Also, it lubricates the upper GI tract.

5.2.3.2 Gastric[edit]

Gastric acid is secreted in the stomach. It helps in digestion by creating the ideal pH for pepsin and gastric lipase and by stimulating pancreatic bicarbonate secretion. Additionally, the arrival of protein in the stomach further encourages gastric output (DiMarino).

5.2.3.3 Pancreatic[edit]

Secretes bicarbonate to neutralize intestinal contents. Also produces enzymes that digest carbohydrates.

5.2.4 Regulation[edit]

5.2.4.1 Hormones[edit]

Released in the blood to regulate activity in the GI tract. They are called enterogastrones. These include, gastrin, which is secreted by stomach, secretin, which is secreted by the duodenum, and the pancreatic secretions vary depending on the food content. These hormones, along with many more, serve to prepare different part of the gut for the arrival of food, in addition to regulation of the digestion of nutrients and the rate at which food moves through the system (Overview of Gastrointestinal Hormones)

5.2.4.2 Nervous system[edit]

Nerve cells help to regulate activity in the GI tract. The sight and smell of food, as well as the presence of food in the gut, stimulates nerves. Nerve signals cause muscle contractions that churn, mix and propel food through the gut at a rate that allows for the absorption of nutrients. Additionally, they stimulate or inhibit digestive secretions (The Enteric Nervous System). 3.3 Absorption Process of taking substances into the interior of the body.