Your digestive system is responsible for breaking down the foods you eat so that you can absorb vital nutrients. Food is broken down mechanically—through chewing, for example, and through the use of enzymes—into the form of molecules that can be absorbed by and moved through your blood. Your digestive system is made up of the following organs:
MouthEsophagusStomachLiverGallbladderBiliary TractSmall IntestineLarge Intestine
Saliva production through our salivary glands is the first step in digestion. Saliva is predominantly made up of water, but does contain other substances, and can be stimulated by just thinking about or smelling food. Food breakdown begins in our mouths through the process of chewing and through enzymes present in saliva. Saliva both lubricates food for easy passage into the esophagus and coats the food to protect our teeth and the lining of our mouth and esophagus.
After we’ve chewed our food, it’s formed into something called a bolus. Swallowing is a complex procedure in which the bolus is moved into the pharynx as the larynx (the organ connected to our windpipe) is covered and the esophagus opening is widened to accept the bolus. The bolus is then moved down through the esophagus through coordinated muscle movements known as peristalsis.
The esophagus is bound on each end by a sphincter muscle, which is responsible for an opening to allow the bolus to pass through. Heartburn can occur when the lower esophageal sphincter fails to close completely, allowing stomach acid to travel upward and irritate the tissue in the esophagus and throat.
Luckily, our stomachs are lined by a thick layer of mucus to protect us from the acid and the enzyme pepsin that it produces. The mixing motion of the stomach, along with the acid and the breakdown of protein by the pepsin, turns the bolus into a liquid substance called chyme, which in turn is then slowly released into the small intestine. For a full meal, this process takes approximately two to three hours.
As the chyme makes its way into your small intestine, it’s met with juices produced by the liver and the pancreas. The liver produces bile, which is stored in the gallbladder and then released into the small intestine to break down fats, while the pancreas secretes enzymes into the small intestine that break down protein, carbohydrates, and fats. The pancreas also releases a substance called bicarbonate that neutralizes any acid that’s made its way out of the stomach.
The small intestine is where the most important work of digestion takes place, that of further breaking down the food we eat into molecular components that can be absorbed into the bloodstream. You may remember from high school biology that the small intestine has three parts: the duodenum, the jejunum and the ileum. Bile from the gallbladder and digestive enzymes from the pancreas are mixed into the chyme in the duodenum. The final breakdown and absorption of nutrients occur in the second two parts.
Once the small intestine has completed its breakdown of food and absorption of nutrients, it propels the undigested parts of plant food, known as fiber, into the large intestine. Fiber is classified into two general types: soluble, which dissolves in water, and insoluble, which does not. Fiber both softens and bulks up the stool and affects the health of your gut bacteria which play a role in supporting your immune system. Thus dietary fiber is essential for both digestive and overall health.
Your large intestine receives about one quart of liquid a day from the small intestine.
In addition to fiber, the small intestine propels liquid into your large intestine, where it’s absorbed and stools are formed. Drinking adequate amounts of water help to keep your stool soft and therefore contributing to a comfortable bowel movement. When you don’t drink enough water, water is drawn out of the fecal matter within your colon, with the result of hard, difficult-to-pass stools.
The large intestine is made up of the ascending colon, the transverse colon, the descending colon, the sigmoid colon, and the rectum. Although there’s great variation in frequency, the stool is in general moved once or twice a day into the rectum in preparation for a bowel movement.
We are not born with any bacteria in our digestive systems, but develop a significant population within the first month, most of which can be found in our large intestine.
Small intestine bacterial overgrowth (SIBO) is a health condition in which too many bacteria are present in the small intestine. SIBO has been theorized as a possible factor in the development of irritable bowel syndrome (IBS) for some individuals.
The functioning of the digestive system is regulated by the enteric nervous system (ENS), which is made up of a tremendous amount of nerve cells and is regulated by the same neurotransmitters, most notably serotonin, found in the brain. This similarity has earned the ENS the title of the “Second Brain.”
Your brain and digestive system work in close partnership, a phenomenon that you have first-hand knowledge of any time your stomach flips when you think of something anxiety-provoking—or more dramatically if you experience diarrhea when you are stressed. This collaboration is thought to be essential to our survival as a species; although digestion is essential for life, dealing with threats is just as necessary. The body developed the “flight or fight” system to divert resources away from the digestive system to the systems of the body needed to fight off or run away from things that might do us harm. Dysfunction in the brain-gut connection has been theorized to play a role in the development of functional gastrointestinal disorders (FGDs).
