Digestive System

Mouth:

 

The mouth is where digestion begins using both mechanical and chemical elements to break down food. The mechanical properties consist of chewing and ingestion through our teeth and tongue. Through mastication (chewing) the teeth crush food into smaller pieces in which the tongue can position and push down the throat. Our teeth consist of incisors (sharp front teeth) to cut, canines (pointed) to tear, and molars (flat) to crush the food. Saliva is the main component of chemical breakdown. This is produced from three glands in the mouth: parotoid, sublingual, and submandibular. In saliva there consists of enzymes such as amylase (breaks down starch) as well as lysozymes (protect from infections), mucus, and bicarbonate. As the mouth chews the food is formed into a bolus which can be pushed down the pharynx and esophagus. 

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Pharynx and esophagus:

 

The pharynx is a muscular tube that has three parts connecting it from the mouth to the esophagus. The upper portion is the nasopharynx which contains a soft palate to prevent food from backing up or entering nasal cavities. The middle portion is the oropharynx and the bottom that connects to the esophagus and trachea is the laryngopharynx. The epiglottis is used to cover the trachea and prevent food from entering it and muscles push the bolus down into the esophagus. The esophagus transports food all the way to the stomach. It contains mucosa and smooth muscles to secrete mucus and push food down. The consistent contractions of muscles in the esophagus is called peristalsis. Finally, the ring or valve that prevents acid reflux is the lower esophageal sphincter. 

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Stomach:

 

The stomach uses both mechanical and chemical breakdown of food to digest it. First smooth muscle tissue churn the food through relaxing and contracting muscles. This creates the liquid substance chyme that can be absorbed by other organs. The chemical breakdown is led by gastric juices containing hydrochloric acid, pepsin, and mucus. These break down proteins and kill harmful pathogens. In addition, the mucus acts as a barrier for mucosal cells preventing them from damage from the acids. The chyme is emptied into the small intestine through the pyloric sphincter in which high fat meals are emptied slowly while carbohydrate and liquid rich meals empty quickly. Hormones in the small intestine like secretin and CCK tell the stomach to slow down the emptying of chyme. 

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Small intestine:

 

This is the longest part of the digestive tract. The first part of the small intestine, the duodenum contains pancreatic juices as well as bile to break down carbs, proteins, and fats. The middle portion, the jejunum, contains villi and microvilli to increase surface area for nutrient absorption. In the final portion, the ileum recycles bile acids back to the liver. 

 

Large intestine:

 

The large intestine or colon is the smaller tube connecting the small intestine to the rectum. The large intestine absorbs water, electrolytes, and vitamins. This is also where fermentation occurs. Bacteria break down sugars and complex carbohydrates through anaerobic respiration producing energy. Finally, the large intestine forms feces as well as stores them in the rectum. 
 

Rectum and anus:

 

The rectum is where feces are stored and the anus is where it is expelled through a process called defecation. The anus contains muscles to control the release of waste. 

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Absorption of nutrients

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Liver:

 

The liver is found on the right side of the abdomen and produces bile. Bile is important for emulsifying fats. In addition, the liver can convert glucose to glycogen and break down fats and proteins. Nutrients such as glycogen as well as iron and vitamins are stored in the liver. Finally, the liver detoxifies the blood as well from alcohol, drugs, and waste. 

 

Gallbladder:

 

The gallbladder is pear shaped and can be found below the liver. It is used to store the bile produced from the liver. It absorbs water and electrolytes to concentrate the bile and when fatty food enters the small intestine the gallbladder contracts to release the bile. 

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Pancreas:

 

Pancreas are located underneath the stomach. It produces different enzymes such as amylase, lipase, protease, and bicarbonate that can break down food. In addition, pancreas have hormonal effects as well. Insulin, used to decrease blood sugar by storing glucose as glycogen in cells, and glucagon, used to increase blood by promoting the release of glucose from the liver, are both produced by the pancreas. 

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