Introduction: Overview of the Gastrointestinal (GI) System

Presented by Shanti Eswaran, MD

Shanti Eswaran, MD
Dr. Swapna Eswaran
Presentation Overview

Our digestive system is made up of a series of organs that allows our bodies to absorb nutrients and water from the food we eat.  As food travels through the digestive system it is broken down, sorted, and reprocessed before being circulated around the body to nourish and replace cells and supply energy to our muscles. 

A fresh-baked apple pie has just come out of the oven. Just the sight and smell of it are enough to make you start salivating. So even before you take a bite, your digestive system has swung into action. After the first morsel enters your mouth, the many organs of your digestive tract kick into high gear. Digestion starts in the mouth where chewing and saliva breaks down food so it is more easily processed by your body. 

Esophagus: The esophagus is a muscular tube that connects the throat to the stomach. The esophagus contracts as it moves food into the stomach. Muscles in the wall of your esophagus create synchronized waves. In this process, called peristalsis, muscles behind the bolus of pie contract, squeezing it forward, while muscles ahead of it relax, allowing it to advance without resistance. When the bolus reaches the lower end of your esophagus, pressure from the food signals a muscular valve — the lower esophageal sphincter or LES— to relax and let the food enter your stomach.  This valve opens to let food pass into the stomach from the esophagus and it prevents food from moving back up into the esophagus from the stomach.  Common disorders of the esophagus include gastroesophageal reflux disease, strictures, Barrett’s esophagus, and motility disorders.

Stomach: With its powerful muscular walls, the stomach begins churning and mixing the food into smaller and smaller pieces. Your digestive glands in your stomach lining produce stomach acid and enzymes, which mix with the food to form a murky semifluid mass or paste called chyme. Once the chyme is well mixed, waves of muscle contractions propel it through a valve called the pylorus and into the upper section of your small intestine (duodenum). The pylorus might release about an eighth of an ounce (about 4 milliliters) of chyme at a time. The rest is held back for more mixing. Common disorders of the stomach include ulcer disease, gastroparesis, and functional dyspepsia.

Small Intestine (Small Bowel): Almost 20 feet long, the small intestine is the workhorse of the digestive system. It will continue to break down food with enzymes released by the pancreas and bile released from the liver. It is made up of three segments, the duodenum, which continues the breakdown of food; and the jejunum and ileum, which are mainly responsible for the absorption of nutrients. As bile and pancreatic digestive juices mix with other juices secreted by the wall of your small intestine, digestion continues. Here it’s further broken down into smaller molecules of nutrients that can be absorbed. Then it moves into the final and longest portion of your small intestine — the ileum — where virtually all of the remaining nutrients are absorbed through the lining of the ileum’s wall. What remains of the food when it reaches the end of the ileum is a combination of water, electrolytes and waste products, such as plant fibers that can’t be broken down.  Common conditions of the small bowel include celiac disease, small bowel overgrowth, Crohn’s disease, and irritable bowel syndrome.

Pancreas: Your pancreas is located behind your stomach and is attached to both your gall bladder and your small intestine. Among other functions, the pancreas aids in digestion by producing digestive enzymes and secreting them into the duodenum.  These enzymes break down protein, fats, and carbohydrates.  Disorders of the pancreas include pancreatitis, pancreatic enzyme insufficiency, and pancreatic cysts/cancer.

Liver: An organ with many functions, your liver’s two main responsibilities in the process of digestion are to make and secrete bile and to process and purify the blood containing newly absorbed nutrients that are coming from the small intestine. Disorders of the liver include fatty liver disease and cirrhosis. 

Gallbladder:  A pear-shaped reservoir located just under the liver that receives and stores bile made in the liver.  The gallbladder sends this stored bile into the small intestine to aid in the digestion of food.  As fatty food enters the upper portion of your small intestine (the duodenum), the gallbladder squeezes bile into the small intestine through the bile ducts. If the bile isn’t needed immediately, it is stored in the gallbladder. Disorders of the gall bladder include gallstones and infection (cholecystitis).

Colon:  As this residue passes through the colon, nearly all of the water is absorbed, leaving a usually soft but formed substance called stool. Muscles in the wall of your colon separate the waste into small segments that are pushed into your lower colon and rectum. It is made up of the ascending (right) colon, the transverse (across) colon, the descending (left) colon and the sigmoid colon, which connects to the rectum.  Common disorders of the colon may cause diarrhea or constipation.

Rectum and Anus:  The rectum is an 8-inch chamber that connects the colon to the anus.  The rectum receives stool from the colon, sends signals to the brain if there is stool to be evacuated, and holds stool until evacuation can happen. As the rectal walls are stretched, they signal the need for a bowel movement.

Anus:  Made of of pelvic floor muscles and two anal sphincters (internal and external).  Together their jobs are to detect rectal contents, and then control when stool should and shouldn’t be excreted from your body.  When the sphincter muscles in your anus relax, the rectal walls contract to increase pressure. These coordinated muscle contractions expel the stool. Disorders of the anus and rectum are very common, including anal fissures, constipation, hemorrhoids, fecal incontinence, and rectal prolapse.

As we stated earlier, the vast majority of the time, your GI tract is working under the radar to perform its many functions.  When this doesn’t happen, there can be many reasons for this– including disorders of the microbiome, disorders of the brain-gut interaction, and nutritional factors.

About Dr. Eswaran

Shanti Eswaran MD 
Associate Professor
Internal Medicine
Division of Gastroenterology
University of Michigan
Ann Arbor, MI

A native of California, Dr. Shanti Eswaran completed both her medical degree and internal medicine residency at the University of Michigan.  She went on to pursue her fellowship in gastroenterology at Columbia University Medical Center.  She served on the faculty at Northwestern University in Chicago and was thrilled to return to Ann Arbor as faculty in 2009.  She has a research interest in the role of diet and food in functional bowel diseases such as irritable bowel syndrome. Dr. Eswaran’s research interests are complemented by her clinical interests in celiac disease and functional GI diseases.  She has been named a Rome Foundation Fellow, serves on several national society committees, and has several grants supporting her research. She has written numerous publications pertaining to the role of diet and GI symptoms, and is excited to be a part of this conference.

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