Digestion

Digestion is the mechanical and chemical break down of food into small organic fragments. During the process of digestion large complex  proteins, polysaccharides, and lipids molecules are reduced to simple molecules  such as simple sugar, acids acids and fatty acids  before they can be absorbed by the intestinal epithelial cells. 

Main Components of the Digestive System

1. Mouth: Digestion of food  begins in the mouth. The food is chewed and mixed with saliva in the mouth so as to break it down into a form that can be easily swallowed and digested.
2. Esophagus: It is the muscular tube that connects the mouth to the stomach. The food  moves from the mouth through  esophagus by a wave-like muscle contraction process called  peristalsis  .
3. Stomach: The food is further broken down in the stomach  using stomach acids and enzymes.  The food is now turned into a semi-liquid form called chyme.The stomach links the esophagus to the duodenum. The empty stomach is only about the size of  fist, but can stretch to hold as much as 4 liters of food and fluid,  and then return to its resting size when empty. There are four main regions in the stomach: the cardia, fundus, body, and pylorus.
   • The cardia (or cardiac region) : This is the point where the esophagus connects to the stomach and through which food passes into the stomach.
   • Fundus : is located inferior to the diaphragm, above and to the left of the cardia. It  is the dome-shaped .
   • Body : This is  the main part of the stomach and is located below the fundus.
   • Pylorus : This is  funnel-shaped. The  pylorus connects the stomach to the duodenum. The wider end of the funnel is called  the pyloric antrum, This  connects to the body of the stomach. The narrower end is called the pyloric canal, which connects to the duodenum. The pyloric sphincter is located at the bottom of the stomach, between the stomach and the duodenum. It controls stomach emptying. In the absence of food, the stomach deflates inward, and its mucosa and submucosa fall into a large fold called a ruga.
4. Small Intestine: The small intestine includes the the duodenum, jejunum, and ileum. Most of the digestion and absorption of nutrients occurs in the small intestine. It is the longest part of the alimentary canal, the small intestine is about 3.05 meters (10 feet) long in a living person. The diameter of the small intestine is about 2.54 cm (1 in).The folds and projections of the lining of the small intestine gives  an enormous surface area of about  200 m² 
5. Large Intestine: The large intestine runs from the appendix to the anus and includes the cecum, colon, and rectum. The water and salts from the material that has not been digested are absorbed in the large intestine. The large intestine  is responsible for forming and expelling feces. The large intestine frames the small intestine on three sides and has a  diameter more than twice of the small intestine, about 3 inches. The ileocecal valve, located at the opening between the ileum and the large intestine. It controls the flow of chyme from the small intestine to the large intestine. The cecum is a sac-like structure that is suspended inferior to the ileocecal valve. It is about 6 cm (2.4 in) long. It  receives the contents of the ileum, and continues the absorption of water and salts. Attached to the cecum is the appendix (or vermiform appendix) which is a winding tube . The 7.6-cm (3-in) long appendix contains lymphoid tissue, suggesting an immunologic function. The cecum blends seamlessly with the colon. On the right side of the abdomen, the food residue initially passes through the ascending colon. The colon becomes the transverse colon at the inferior surface of the liver, where it bends to form the right colic flexure (hepatic flexure).The final third of the transverse colon marks the start of the area known as the hindgut. After passing through the transverse colon, food waste reaches the left side of the abdomen, where it reaches the left colic flexure (splenic flexure), where the colon angles abruptly just inferior to the spleen. From there, food residue goes through the descending colon, which runs down the left side of the posterior abdominal wall. It becomes the s-shaped sigmoid colon after entering the pelvis .Food residue leaving the sigmoid colon enters the rectum in the pelvis, near the third sacral vertebra. The final 20.3 cm (8 in) of the alimentary canal, the rectum extends anterior to the sacrum and coccyx. 
6. Anus: This is the  final part of the digestive tract. Anus expels feces  from the body. The anal canal is located in the perineum and is  completely outside of the abdominopelvic cavity. This 3.8–5 cm (1.5–2 in) long structure opens to the exterior of the body at the anus. The anal canal has two sphincters namely  the internal anal sphincter  and the external sphincter .The internal sphincter is made of smooth muscle, and its contractions are involuntary. The external anal sphincter is made of skeletal muscle, which is under voluntary control.  Both the sphincters usually remain closed except when defecating.

Accessory Organs of the digestive system

1. Liver: The bile is produced in the liver  which helps digest fats.
2. Gallbladder: The bile produced in the liver is stored and concentrated in the Gall bladder from where it is released  into the small intestine.
3. Pancreas: Produces digestive enzymes and bicarbonate, which are released into the small intestine to aid in digestion.

Phases of Digestion

1. Cephalic Phase: Triggered by the sight, smell, or thought of food, stimulating saliva and digestive juices.
2. Gastric Phase: Begins when food enters the stomach, stimulating the release of gas
3. Intestinal Phase: Starts when chyme enters the small intestine, triggering the release of digestive enzyme and bile

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