The Digestive System
Arnie Shapiro, M.D.
July – Aug 2025 • Vol 4, No 17
Our population of body cells run, for the most part, on consistent supplies of oxygen and glucose. The oxygen comes from the Earth’s atmosphere via our respiratory system and circulatory system. (Read up on both these systems HERE.) The glucose comes from the ingestion of carbohydrates, which are produced by the plant kingdom. The other nutrients our cells utilize are proteins (amino acids), fats, vitamins, and minerals/salts.
We can easily observe that all complex members of the animal kingdom have a front-end orifice, designed for the intake of liquids and foodstuffs, and a long, interior, alimentary canal—the digestive tract, which breaks down foodstuffs, as a food processor would, into a soupy pulp. The tract gradually absorbs the foodstuffs into the bloodstream for transport to all body cells. In adults, the digestive system has a tubular length of 20 to 25 feet! If flattened out, it has an interior surface area of approximately 90 square feet.
After chewing and swallowing, movement of the foodstuffs through the tract is guided by peristalsis. This refers to coordinated, sequential muscular contractions from throat to rectum. These contractions are controlled by the Autonomic Nervous System (ANS).
The stomach is the large (½ liter) initial holding chamber. It produces hydrochloric acid and digestive enzymes, and has muscular walls for slow churning.
The neurologic interplay between stomach and nervous system assesses fullness, and determines the timing and amount of emptying into the small intestine, through the pyloric valve. The small intestine is where most digestion and absorption take place. Strong enzymes from the pancreas and liver break down foodstuffs into molecules that can pass through the intestinal lining cells and enter the bloodstream.
All functions of the digestive system are coordinated by the Autonomic Nervous System (ANS). The gut-related parts of the ANS are called the Enteric Nervous System (ENS) (see next page). There is significant, two-way communication between the ENS and the brain, via the vagus nerve (a key part of the ANS).
The number of neurons (nerve cells) in the digestive system is a large fraction of the number of neurons in the brain, and its total body-nerve connections. As far as neurotransmitters go, there is more serotonin produced in the gut than in the brain! The digestive system has been called by many researchers the “second brain.”
With this great brain-gut connection, dysfunctions of one will likely affect the other. Many mental illnesses, including depression and anxiety disorders, cause digestive-system disruptions. Correspondingly, many digestive-system illnesses, like IBS, gastritis and colitis, heighten anxiety and depression.
Another strong player in the brain-gut connection is our gut microbiome. There is an astonishingly high number of friendly bacteria that reside in our colon. These are called “normal flora.” It is life-long, symbiotic mutualism. The gut microbiome can be disrupted by pathogens, viral, bacterial, or parasitic infections, that enter, and trigger an immune system reaction. Other disruptions of the gut microbiome and bowel function in general may arise from emotional disorders, either acute or chronic. Further, it has been found that a high diversity of gut microbiome species is related to overall wellness. Low diversity of gut microbiome species is linked with many mental-emotional disorders, as well as physical ailments.
How can we take good care of our amazing digestive system?
- Eat at regular times, day to day. Don’t skip breakfast.
- Choose a variety of foods: fruits, vegetables, whole grains, lean meats, nuts, peas, beans, olive oil, and low-fat dairy. Do your best to avoid fast foods, and highly-processed foods. Include some probiotic foods like yogurt, sauerkraut, kimchi, and kombucha.
- Hydrate well. Drink 6–8 glasses of clean water every day. Maintain moderation in alcohol and caffeine.
- Eat slowly and chew adequately. Don’t mix stress with digestion. Apply a re-laxation response before starting a meal. Be grateful for the food—savor it, eat mindfully.
- Maintain good oral health. Brush your teeth twice daily. Use dental floss daily. Visit your dentist every six months, including professional cleanings.
- Get regular, aerobic exercise. Wait an hour after meals before starting exercise.
In summary, our digestive system is a marvel of evolutionary and biological design. It provides the body’s organs, systems, and cells with the nutrients and liquids that are essential for well-being. The nerves within it are so extensive,
it acts as our “second brain.” It has a giant population of friendly microbes. If we give it love, appreciation, gratitude, wise food choices, and good hydration, it will serve us well through all our decades.
— Namaste!
Dr. Shapiro is a career clinic physician who has utilized relaxation techniques with many of his patients. His YouTube channel is: “Arnie Shapiro, MD • Breathing Easy.” Email Dr. Shapiro at: alloydshapiro@gmail.com.