Digestive System by Lindun Lai and Yang Fuller



Digestion (di-jest'yun) is the mechanical and chemical breakdown of food into forms that cell membranes can absorb. Mechanical digestion breaks large pieces of food into manageable sizes for the body to absorb. Chemical digestion breaks food into simpler chemicals.

Digestion system consists of alimentary canal, which extends from the mouth to the anus, and includes the processes of ingestion, digestion, absorption, and elimination. In order to do the process, the alimentary canal and accessory organs work together to function as the digestive system.

Alimentary canal includes the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anal canal. See Figure 1.0.

Accessory organs includes salivary glands, liver, gallbladder, and pancreas. See Figure 1.0.

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Figure 1.0 Picture of the alimentary canal and accessory organs.


Structural Organs


The Mouth


The mouth is the start of the alimentary canal. Food is received and the teeth speeds up the process by chewing mechanically and then salivary gland produces saliva to mix with large solid particles to break down into smaller pieces and make it easier to swallow. To protect the human body, the tongue has rough projections called papillae. Some provide friction to grip food and others contain the taste buds. The taste buds determines which organic materials are safe to eat. Sweet organic materials are usually edible while some bitter things are considered poisonous.

The Pharynx and Esophagus


Broken down by the mouth, the food goes the pharynx (far'inks), which connects the food from the mouth to the esophagus. When swallowing, the epiglottis goes down and blocks the food from going into the lungs. Esophagus -sof'ah-gus) is a 25-cm tube that uses muscular contraction to deliver the food from the pharynx to the stomach.


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Figure 1.1 Picture of the mouth, pharnx, and esophagus.

The Stomach


Stomach (stunm'ak) receives the food from the esophagus and then mixes with gastric juice, which starts the chemical digestion of protein. The most important enzyme of the gastric juice is pepsin, which splits nearly all types of proteins. The mucous cells of the gastric glands secrete mucus. In addition to mucus, a more viscus and alkaline secretion in the gastric glands also coats the inside of the stomach, which protects the stomach from the very acidic hydrochloric acid (a pH of 2) that aid in breaking down food.

After a meal, the mixing process in the stomach wall help produce chyme (kīm), a semi-liquid paste of food particles and gastric juice. As chyme accumulates in the stomach, it allows the stomach to relax and then the pyloric sphincter is opened, which allows the chyme to go through the duodenum and eventually into the small intestine. This process takes anywhere from two to six hours.

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Figure 1.2 Picture of the stomach.

The Small Intestine, Pancreas, Liver, Gallbladder, and Gland Cells

Small Intestine (ˈsml in-ˈtes-tən) is the longest section of the alimentary canal. Most of the nutrients is absorbed in the small intestine into the bloodstream. The first 25 cm of the small intestine is the duoenum. Here acid chyme from the stomach mixes the digestive juices of the pancreas, liver, gollbladder, and gland cells. The pancreas produces hydrolytic enzymes that acts as buffers and offsets the acidity of the chyme. Then the pancreatic proteases are activated once they are in the duodenum. Another vital digestive organ is the liver, it aid in the production of bile which stores in the gall bladder. The gallbladder can hold up to 50 millileters of bile. Bile aid in the digestion and absorption of fats. Also other various digestive gland cells produce digestive enzymes to aid digestion. For example, pancreatic juce is secreted by pancreatic exocrine cells to aid in digesting nutrients.

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The Large Intestine

The Large Intestine (ˈlärj in-ˈtes-tən) is the end of the line for digestion. For the last 16 hours whatever that's left over from nutrients spends it's time here winding down to the anus. The large intestine is T shaped. One of the arm of the T is the cecum pouch. The cecum (ˈsē-kəm) joins the large intestine between the ileum and the colon. It functions to absorb the remaining water and salt from the undigested food. The cecum has a muscular wall that can speeds up digestion. The appendix is a branch of the cecum, it provide body defence against bacteria.
The colon (ˈkō-lən) is the last part of the digestive system. Its job is to recover water that has entered the alimentary canal as the solvent of various digestive juices. Also whatever water left from digestion, the colon will reabsorb about 90% of it. The final product of digestion is feces ('fē-(ˌ)sēz). Feces become more solid as they move along the colon by peristalisis. This slow process takes anywhere from 12 to 24 hours. Feces contain a lot of bacteria as well as cellulose and other undigested materials. Once at the end of the road, the feces enter the rectum (ˈrek-təm). Feces are stored in the rectum until they are excreted out into the anus.

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Sources


Reece, Campbell. Biology, Seventh Edition. 2005.
Shier, Butler, Lewis. Hole's Anatomy & Physiology, Eleventh Edition. 2007.
http://en.wikipedia.org/wiki/Small_intestine
http://en.wikipedia.org/wiki/Colon_(anatomy)