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Digestion is how the body obtains energy and material for growth from the food we eat. There are four main stages in digestion:
Physical/Mechanical digestion, Chemical Digestion, Absorption and Elimination.
The main organs in the digestive system are the mouth, the esophagus, the stomach, the liver, the pancreas, the gall bladder, the small intestine and the large intestine. Food doesn’t pass through the liver, pancreas and the gall bladder, but these organs perform functions necessary for our digestion.
So it all starts in the mouth, when we eat saliva from our salivary glands moisten the food and we chew and grind the mouth down with our teeth. This helps to increase the surface area of food for chemical digestion.
Food is ground into a manageable ball called a bolus so it can easily pass through our digestive system. Saliva contains an important enzyme called amylase which digests starch into simpler sugars. Then we swallow, and when we do this the uvula (flap of skin at the back of the mouth) and epiglottis ensure that food goes into the esophagus to our stomach, and not to our lungs.
The esophagus is a tube that carries food to the stomach, and this happens through peristalsis- involuntary constrictions of tubular muscle to push food along. The esophageal sphincter controls entrance into the stomach.
The stomach is a muscular J-shaped sac that has three main functions:
The stomach contracts once every 20 seconds to mix food into a soupy mixture called chyme. Once chyme is the right consistency and at the right acidic level, it is released out of the lower stomach and enters the small intestine through the pyloric sphincter.
Did you know that... very few substances are actually absorbed in the stomach, just some salt, alcohol, water and some anti inflammatory medications like Aspirin.
The Epithelial lining of the stomach contains over 35 million glands that release 2-3 liters of gastric juice each day. Parietal cells in the stomach secrete hydrochloric acid and Chief cells secrete pepsinogen. Pepsinogen is an inactive enzyme that is activated in the presence of hydrochloric acid to form pepsin. Pepsin digests proteins into smaller chains. Chief cells in the stomach lining also secrete Rennin, which is an enzyme that aids in the digestion of milk proteins.
The stomach also secretes a type of mucus called mucin which keeps acid of the stomach walls and prevents holes from being eaten through the stomach wall. Gastrin is a hormone that is stimulated by the entry of food into the stomach. It causes the gastric glands to release more gastric juices and increased movement of the stomach walls.
The liver is the largest internal organ of the body. It produces bile to assist with lipid (fat) digestion. Bile is a greenish-yellow fluid mixture that emulsifies large fat globules and breaks them up physically.
Bile is stored in the gall bladder where it carried via the hepatic duct to the small intestine. The liver has additional functions: managing sugar, and detoxification.
The liver converts excess glucose into glycogen and excess sugar into fats. Glycogen is converted back to glucose when the body needs more energy.
The liver breaks down poisonous material that enter the blood stream in a process called detoxification. The liver will detoxify materials such as alcohol, nicotine, caffeine, poisons and excess hormones.
The Pancreas is a large organ located below the stomach and liver and on top of the small intestine. It secretes:
The small intestine is about 6 meters long, 'small' refers to the size of its diameter compared to the large intestine. The contents of the stomach are emptied into the small intestine.
Here the majority of digestion and absorption occur. The small intestine contains a rich supply of capillaries in order to provide the blood an opportunity to pick up the digested nutrients.
There are three sections of the small intestine:
The Duodenum, Jejunum and the Ileum.
Various intestinal enzymes are secreted into the lumen of the small intestine including peptidase, maltase, lactase, sucrase, lipase and nucleosidases.
So the soup mixture of food (chyme) sloshes back and forth between segments of the small intestine allowing for physical digestion.
Additional hormones are secreted into the small intestine to help with digestion including Secretin, which stimulates the pancreatic secretion of sodium bicarbonate, halts the release of gastric juices in the small intestine and stimulates the growth and repair of the small intestine and pancreas.
The enzyme cholecystokinin (CCK) is also secreted. This hormone inhibits stomach movements and secretions, increases pancreatic secretions of digestive enzymes and increases gall bladder contractions.
The small intestine contains ridges that increase its surface area. Each ridge has small bumps called villi. Nutrient poor blood enters the villi and leaves nutrient rich. But most lipids are not allowed to pass into the blood stream, so they are absorbed into a special structure called a lacteal. The lymphatic system takes these lipids and safely delivers them back to the circulatory system when needed.
The Large intestine is 3 meters in length and has a larger diameter than the small intestine. Digestion is complete before contents enter the large intestine. The large intestine stores and eliminates solid wastes.
There are 3 Distinct parts of the large intestine: The Transverse Colon, the Ascending Colon and the Descending Colon. Materials move slowly through these colons, allowing time for excess water in the faeces to be removed.
The large intestine secretes sodium bicarbonate which neutralizes the acidic products from bacterial activity. (There are >1000 varieties of bacteria that are useful for decomposition of solid wastes and vitamin production)
It also secretes mucus which allows solids to slide through the intestinal lining.
So basically the 4 main functions of the large intestine are to:
So I hope you enjoyed reading my 'basic' summary of the digestive system. It's always nice to be informed of exactly what goes on inside your body. And maybe next time you eat, you'll remember the amazing processes that take place inside you.