# Digestion --- **Digestion** is the process of the [[digestive system]] of breaking down of [[nutrients]] that we eat. It is not a singular process, but rather a combination of several interrelated processes. It may go without saying, but each different [[nutrients|macronutrient]]: ([[carbohydrates|carbs]], [[lipids]] and [[protein|proteins]]) need to be broken down by their own specific [[enzyme]]. Each of these of enzyme is produced in one or more distinct segments of the [[digestive system|gastrointestinal tract]]. The [[carbohydrate digestion]] happens largely in the [[mouth]] and [[small intestine]] by digestive enzymes digestive enzymes called *amylases*. The [[protein digestion]] happens largely in the [[stomach]] and [[small intestine]] by digestive enzymes called *proteases*. [[Lipid digestion]] is mostly done in the [[small intestine]]. It is complicated by the fact that lipids are [[polar and nonpolar|hydrophobic]], and thus cannot be deposited directly into the bloodstream. First they must be emulsified by the action of the [[polar and nonpolar|amphipathic]] bile salts found in [[bile]], which create lipoprotein called [[lipoprotein|chylomicrons]]. These exit the [[enterocytes]] into the [[lymphatic system|lymph]] where they then enter the [[blood]] where they eventually reach the [[liver]] and from there are processed. ![[digestive enzymes.png]] ## Mechanical and Chemical Digestion **Mechanical digestion** is the physical breaking down of food particles into smaller pieces. It occurs in the [[mouth]] by chewing and [[stomach]] by the contraction of its muscles. **Chemical digestion** is the breaking down of food by using [[digestive enzymes]] produced in the [[salivary glands]] the [[stomach]], the [[pancreas]], and the [[liver]]. ## Propulsion & Mixing The contents of the GI tract must be moved along from start to end, and they also need to be thoroughly mixed up, so we can gain all the nutrients we can from it. There are two main ways the digestive system physically acts on it's contents to move it around. The material being digested is mixed up by a process called **segmentation**, where the stuff is squeezed so that some of it moves forward, and some of it moves back. This action is repeated multiple times so that the material is churned up and mixed together. The material being digested is moved forward by a process called [[peristalsis]], which uses the rhythmic contraction of smooth muscle to squeeze the contents forward like toothpaste from a tube. ![[GI motility.png]] ## Stages of Digestion There are three stages of digestion that are initiated and concluded depending on where the food material is in the [[gastrointestinal tract|GI tract]]. The **cephalic phase** begins with the **cephalic reflex**, which is triggered by the sight, thought, smell of food, or even the sound of food being prepared. The [[medulla oblongata]] relays signals along the [[vagus nerve]] to the [[stomach]] which triggers it to increase the force of its contractions, and to begin releasing its gastric secretions. The **gastric phase** begins when food enters the stomach. Mechanoreceptors detect the stretch of material entering the stomach, and chemoreceptors detect [[protein]] or high [[pH]]. The signals are relayed to the [[medulla oblongata]] which relays back down the [[vagus nerve]] to continue to increase both the force of the stomach contractions and produce secretions. The [[gastric gland|G-cells]] to secrete [[gastric gland|gastrin]], which also has those effects, as well as causing contraction of the [[stomach|pyloric sphincter]]. The **intestinal phase** begins when acidic [[stomach|chyme]] enters the [[duodenum]], and typically stays in the [[small intestine]] for about 12 hours. Chemoreceptors on the intestinal wall detect the low [[pH]] and send those signals to the [[medulla oblongata]], which then *decreases* its signals along the [[vagus nerve]] and this reduces the force of the stomach's contractions and secretions. Acidic chyme also has the effect of releasing [[duodenum|secretin]] which inhibits the stomach's secretions; fatty chyme causes the release of [[cholecystokinin]], which decreases the force of contractions in the stomach. ___