The Gastrointestinal (G-I) System allows us to obtain water and nutrition for our bodies. It’s like one long tube (called the “Lumen“) that extends from the mouth to the rectum. The tube is made of smooth muscle, which is involuntary, so it moves on its own, pushing food onward. This constant movement of the G-I Tract is called “peristalsis.” Other organs like the liver, gallbladder, and pancreas connect with the lumen, but are not part of it.
The food we chew and swallow goes down the Esophagus to the Stomach, where acids dissolve the food (digestion). The slurry of dissolved food moves on to the Duodenum (first segment of the Small Intestine), where bile (produced in the Liver and stored in the Gallbladder) and enzymes from the Pancreas, digest it more [see diagram below].
Digested food then travels through the Small Intestine, where nutrients are absorbed into the blood stream, and then carried throughout the body. Food that doesn’t get digested and absorbed, like fiber, moves on into the Large Intestine (Colon). There it gets compacted into stool (feces; poop), until it reaches the Rectum, where the body gets rid of it.
The following is a close-up of the Liver, Gallbladder, and Pancreas. It shows in more detail how the ducts from the Liver, Gallbladder, and Pancreas deliver bile or digestive enzymes into the Duodenum, digesting food as the slurry moves through the rest of the Small Intestine.
Various diseases can affect the G-I Tract. Stomach acid, which is supposed to stay in the stomach, can creep up the wrong way into the esophagus, causing Gastro-Esophageal Reflux Disease (“GERD,” aka “heartburn”). If that occurs long enough, it can cause scars which interfere with swallowing. The stomach can develop an Ulcer (called Peptic Ulcer), a sore that can cause bleeding or perforation if it erodes deep enough. Superficial destruction of the stomach is called Gastritis. And plain indigestion, called Dyspepsia, can also occur.
The liver can be infected with various viruses, including Hepatitis A, B, C, D, and E, which all act differently. Other liver diseases are genetic, causing damage by inflammation, copper or iron overload, or even just too much fat (Fatty Liver, considered to be a new epidemic in the making). Many of these diseases, if untreated, gradually lead to Cirrhosis, when the liver is eventually so scarred and damaged as not to work fully (or at all). Toxins can also cause cirrhosis, especially alcohol; other toxins can cause rapid Liver Failure, like poison mushrooms (and also alcohol, if a very large amount is drunk in a short time).
Gallstones can develop in the gallbladder. They’re usually harmless, but if one works its way down a duct and gets stuck, it can cause terrible pain, and even serious infection. If the pancreas becomes inflamed (Pancreatitis), its digestive enzymes can leak out, causing bad pain.
Certain diseases can prevent the G-I Tract from absorbing nutrients.ย For example, Vitamin B12 Deficiency can be caused by an auto-immune disease in the stomach.ย Celiac Disease, another autoimmune condition, prevents absorption of gluten in the small intestine.ย Other types of Malabsorption can cause deficiencies of many nutrients.
Diseases of the small and large intestines cause diarrhea. The most common, Gastroenteritis, is due to a wide variety of infections. Inflammatory diseases like Crohn’s and Ulcerative Colitis cause chronic diarrhea and can permanently damage the bowel. The very common condition Irritable Bowel Syndrome, when the normal peristaltic movement of the bowel becomes irregular, can cause diarrhea or constipation, with abdominal pain, but not actual damage.
Appendicitis, bacterial infection of the appendix, causes abdominal pain, maybe even death if it ruptures. Diverticulitis, a similar infection on the left side of the large intestine mostly in older persons, usually isn’t as serious and rarely needs surgery.
And of course, Cancer can occur in any of the organs shown in the diagrams. Colon cancer (in the large intestine) usually grows slowly enough that it’s easy to find in time, through routine examinations like colonoscopy. But cancers of the other G-I organs unfortunately progress so rapidly that by the time they’re diagnosed, it’s too late for surgery.
The G-I Tract sits in the belly in what’s called the peritoneal cavity. See the diagram below; “in situ” just means “in the body.” The “greater omentum” is the peritoneal membrane covering the organs.
If bacteria leak out of the G-I lumen (from the stomach, small, or large intestine), they can cause a terrible life-threatening infection Peritonitis.ย This can happen from any disease that affects the bowel wall, like appendicitis, cancer, etc.ย In people with severe Cirrhosis, liver scars obstruct normal circulation, causing fluid to leak out of the bloodstream into the peritoneal cavity (Ascites), where it can get infected and cause Peritonitis.
In terms of diagnosis, we use different methods for each organ. Diseases of the liver and pancreas are initially suspected or diagnosed by blood tests. Cancer inside the lumen can only be seen by looking: colonoscopy for the large intestine, endoscopy for the esophagus & stomach (small intestine cancer is complex & quite rare). Cancer outside the lumen is found by CT Scan. CT scan can also diagnose Appendicitis and Diverticulitis. Ultrasound can find Gallbladder Diseases, and sometimes Liver Cancer, but not much else (in children, it’s fine for appendicitis).
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