Gastrointestinal Tract Anatomy
The digestive system is made up of the gastrointestinal (GI) tract and the liver, pancreas, and gallbladder. The GI tract is a series of hollow organs joined in a long, twisting tube from the mouth to the anus. The hollow organs that make up the GI tract are the mouth, esophagus, stomach, small intestine, large intestine (which includes the rectum) and anus. Food enters the mouth and passes to the anus through the hollow organs of the GI tract. The liver, pancreas, and gallbladder are the solid organs of the digestive system. The digestive system helps the body digest food. The large, hollow organs of the GI tract contain a layer of muscle that enables their walls to move. The movement of organ walls, known as peristalsis, propels food and liquid through the GI tract and mixes the contents within each organ. Peristalsis looks like an ocean wave travelling through the muscle as it contracts and relaxes.
Path to the Stomach
Saliva lubricates the bolus or ball-shaped chewed up food. Saliva also contains amylase, which breaks down polysaccharides. Antibodies and lysozyme that kill pathogens are also present. When a person swallows, food is pushed into the esophagus and the muscular tube carries the food and liquids from the mouth to the stomach. Once swallowing begins, it becomes involuntary and proceeds under the control of the esophagus and brain. The lower esophageal sphincter, a ring-like muscle at the junction of the esophagus and stomach, controls the passage of food and liquid between the esophagus and stomach. As food approaches the closed sphincter, the muscle relaxes and lets food pass through to the stomach. The stomach is a J-shaped/banana-shaped sac with a densely folded inner membrane (rugae), that can accommodate stretching. The muscle of the upper part of the stomach relaxes to accept large volumes of swallowed material from the esophagus. The muscle of the lower part of the stomach mixes the food and liquid with digestive juice. The stomach stores swallowed food and liquid, mixes the food and liquid with digestive juice (such as pepsin) it produces, and slowly empties its contents, called chyme, into the small intestine. Goblet cells secrete mucus lining that protect the stomach from the acid and self-digestion.
Liver and Gallbladder
The liver is also a part of the digestive system. It makes bile from cholesterol and stores it in the gallbladder. The liver facilitates nutrient metabolism and vitamin storage It makes and stores glycogen from glucose and breaks down fat. It detoxifies the body. An example is the removal of ammonia from blood. The liver is the largest gland in the body, spanning both sides of the abdomen. It has ducts that drain to the duodenum and gall bladder. The gall bladder stores excess and unused bile. It concentrates the bile and secretes it when needed. Bile is an emulsifying agent that breaks down large fat droplets into smaller droplets by forming micelles. This increases the total surface area of the fat for lipase action.
The pancreas is involved with the production of enzymes and bicarbonate production. It is a tadpole-shaped gland with duct leading to duodenum. The pancreas is the major source for all the digestive enzymes such as amylase, various proteases, lipase and ribonuclease. The pancreas makes HCO3– to neutralize the HCl from the stomach and the digestive enzymes of pancreas flows into the small intestine through ducts.
The muscles of the small intestine mix food with digestive juices from the pancreas, liver, and intestine and push the mixture forward to help with further digestion. The small intestine is divided up into the duodenum, jejunum and the ileum. The walls of the small intestine absorb the digested nutrients into the bloodstream through the presence of villi and microvilli. Villi are finger-like protrusions lining the small intestine. Microvilli is same as villi but it is found on the surface of a single absorptive cell. They help to increase the surface area of the small instestine. The small intestine is the major place for digestion and absorption.
The site of major digestion is the duodenum and the site of major absorption is the jejunum. Vitamin B12 is primarily absorbed in the ileum as well as anything left behind by the jejunum.
Bicarbonate ions from the pancreas neutralizes the HCl from the stomach, which facilitates the activity of enzymes in the small intestine, which would be denatured by stomach pH.
The blood delivers the nutrients to the rest of the body. The waste products of the digestive process include undigested parts of food and older cells from the GI tract lining. Muscles push these waste products into the large intestine. The large intestine absorbs water and any remaining nutrients and changes the waste from liquid into stool. Unlike the small intestine, the large intestine has no folds or villi. The rectum stores stool until it pushes stool out of the body during a bowel movement. The small intestine absorbs most digested food molecules, as well as water and minerals, and passes them on to other parts of the body for storage or further chemical change. Specialized cells help absorbed materials cross the intestinal lining into the bloodstream. The bloodstream carries simple sugars, amino acids, glycerol, and some vitamins and salts to the liver. The lymphatic system, a network of vessels that carry white blood cells and a fluid called lymph throughout the body, absorbs fatty acids and vitamins. The cardiac sphincter prevents back flow of food while the pyloric sphincter releases food into the small intestine in a small amount at a time.
Important Digestive Enzymes