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OpenStax. Anatomy and Physiology, OpenStax, 6 Mar. 2013, opentextbc.ca/anatomyandphysiology/chapter/23-7-chemical-digestion-and-absorption-a-closer-look/.

Pepsinogen - an Overview | ScienceDirect Topics. www.sciencedirect.com/topics/neuroscience/pepsinogen.

“Digestive System | Everything You Need to Know, Including Pictures.” InnerBody, www.innerbody.com/image/digeov.html.

“Erepsin.” Wikipedia, Wikimedia Foundation, 27 Mar. 2019, en.wikipedia.org/wiki/Erepsin.

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Now I enter the large intestine! That largest part of this, the colon, consists or four parts; the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. As the sandwich moves through the colon, water is being retained from the rest of me while bacteria from the colon is releasing vitamins from this waste. The last part of the large intestine is the rectum, which holds me until I am ready to be expelled through the anus, the external opening as an act of egestion.

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After this, the second part of the small intestine, the jejunum, is reached. Here, digestion is completed and absorption happens. The villi and microvilli, projections that increase surface area to allow for more absorption, in the jejunum have capillaries nearby that move around the body to carry the nutrients, except fats, of me with them. The fats, my cheese, is instead carried throughout the body in small vessels called lacteals.  The process continues and the final part of the small intestine, the ileum is reached. At this point in the digestion process, I am almost completely digested and the majority of absorption takes place.

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The liver creates bile that aids in digestion contains bile salts the surround the lipids, my cheese, and emulsifies fat into smaller droplets called micelles which allows the lipases to work more efficiently. This bile is stored by the gallbladder. A bile duct from the gallbladder is connected to the duodenum so bile can be secreted into it. When lipids enter the duodenum, the gallbladder is signalled to contract and secrete this bile.

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When I enter the duodenum, the chemical prosecretin located within the small intestine epithelial cells is changed to its active form of secretin. This activates the liver’s ability to create additional bile and the pancreas to produce both lipid and protein enzymes. Secretin’s main function is to encourage the pancreas to release bicarbonate ions which neutralize the acidity of the chyme, me, while it also regulates the digestive rate and ensures that additional food does not enter the duodenum until current digestion processes are complete.

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Along with these processes, the pancreas is releasing enzymes called lipases. The lipases enter the duodenum and also assist in breaking down lipids, my cheese, along with pancreatic amylase which is breaking down carbohydrates, my bread.

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The pancreas also releases trypsinogen, an inactive form of trypsin which digests protein. This enters the duodenum and encounters an enzyme called enterokinase that converts the trypsinogen into trypsin.

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The pancreas secretes enzymes that are administered to me, now as chyme. I enter the duodenum and stimulate the secretion of cholecystokinin (CCK), a hormone that activates pancreatic secretions in the form of enzymes needed for pH balance and digestions as well as stimulates for the stomach to slow down digestion to allow the small intestine to properly digest, through the pancreatic duct.  Erepsin, a mixture of enzymes that will digest my proteins, is secreted by the pancreas

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Once the stomach processes are complete, the second sphincter allows for me, now in the form of chyme, to enter to duodenum. This is the first part of the small intestine.

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Two cell types that line the stomach are also used in the form of chemical digestion. Gastrin, a hormone that activates the releasing of gastric juices, is discharged into the bloodstream and stimulates the expelling of hydrochloric acid, which acidifies the stomach, as well as mucus to line the stomach to protect it from enzymes. The hydrochloric acid converts released pepsinogen into pepsin which is an enzyme that can actively break down protein, such as my ham, into amino acids

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Lastly, the layer serosa ensures the stomach is in place and covers itself in fluid to reduce friction between itself and other organs.

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The third layer is the muscularis and hosts the smooth muscles of the stomach which mix gastric juices and me to produce chyme

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The next layer, called submucosa, is composed of connective tissue that contains the stomach nerves and blood vessels.

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There are four layers that make up the stomach. The first of these is the mucosa layer, which secretes gastric juices and contains epithelial cells that replicate quickly to heal damage to the stomach lining.

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I am now in the stomach! The stomach mechanically digests me through muscles within it. There are two sphincters located in the stomach and when I enter the stomach, these contract to allow digestion.

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I then enter the esophagus and this muscular tunnel moves me to the stomach through peristalsis, which is contractions in wave-like sequences initiated by me stretching the esophagus.

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After I have been chewed and mixed with saliva to form a bolus, I am pushed to the back of the mouth by tongue and swallowed. This forces me into the pharynx. While this is happening, the opening of the trachea is covered by a soft tissue flap called the epiglottis to prevent me from entering the lungs.

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While the teeth are chewing me, the salivary glands are producing salivary enzymes. Found in the saliva is amylase, which begins to break down carbohydrates such as my bread. Mucus is also contained within the saliva and lubricates me to make me easier to swallow.

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Hi! I am Sam the Sandwich! Today, my journey begins in the mouth.  Here, the incisor and canine teeth capture and rip me apart in an act of physical digestion.