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What is the Gut? Part 1


The words gut, belly, tummy, stomach, and core mean many things to many people. These words can mean something to exercise, something to dislike about ourselves, something to feed, something that hurts, or something to be awed by. This 2-part article series will examine how the belly is built and some of its unique functional attributes, as well as how manual therapy can optimize many critical functions of the digestive and reproductive systems.

Belly Wall

There are 6+ layers of tissue here depending on whether you start at the front, sides, or back. This means 6+ layers of distinct densities, functions, communication methods, architecture, and vibrations that move, protect, warm, and keep all of the internal organs in place.

The belly wall wraps us up like a package to be shipped with skin (the wrapping paper), a layer of insulating fat (superficial fascia + adipose), muscle layers attaching ribs, spine, and pelvis (the shipping box /rectus abdominis and obliques). Other aspects of the belly wall include various shaped tendinous and fascial attachments (the packing tape / thoracolumbar and abdominal aponeuroses), and an inside bag that contains the organs themselves (parietal peritoneum).


Within the parietal peritoneum are many of the famous organs of digestion, elimination, and reproduction. These are all held together and suspended by a complex network of connective tissues (fascia, visceral ligaments, and mesenteries). Also, the hollow organs like the stomach, small and large intestines, and urinary bladder naturally expand to fill any empty spaces because of the difference in pressure inside the organs vs. outside the container. Think of steam rising and expanding off the stove, or a helium balloon floating in the air. Our belly wall, respiratory and pelvic diaphragms, and skeleton thankfully keep this expansion of hollow organs in check.

Since the hollow organs are constantly expanding, moving, and nudging up against each other, the "skin" of the organs (visceral peritoneum) secretes serous fluid to keep things lubricated. This serous fluid also serves to attract and hold the organ walls close to each other. Think of water between 2 pieces of glass where the water actually holds the 2 layers together when you try to pry them apart. This phenomenon of expansion and fluid attraction allows the organs to move as a "system" with the breath, movement of the spine, and ambulation.

The next time you jump rope, run, or do yoga ask yourself why all of my organs are still in the right places? Why isn't everything laying on my bladder and pelvic floor? How do we hold everything up despite the ubiquity of gravity?

Nervous System

The nervous system that powers the organs and manages complex communications between the organs is called the enteric nervous system (ENS). This nervous system is sometimes referred to as the "second brain," and can work nearly autonomously from the rest of the central and peripheral nervous system. The ENS contains more nerve cell bodies than the entire peripheral nervous system. Only the brain and spinal cord have more discrete nerve cells.*

What this means for us is the workings of the gut are independent of conscious control. We digest our food; secrete enzymes, manage sugar, pH, and water; and move waste products without having to think about how these things go. It would be a very busy day if we had to do all of this work consciously!

Exhaust Systems

The gut has a number of waste elimination pathways. Like many important functions in our body, waste elimination is redundant. Without a redundant "exhaust system," we would quickly poison ourselves.

Arguably, our best known eliminative organ in the belly is the large intestine which eliminates food waste by way of stool. Thankfully, this is not the only one. The stomach kills unwanted bacteria in our food by secreting hydrochloric acid (pH between 1.5 and 3.0) strong enough to burn paint from our car. The liver denatures alcohol, pharmaceuticals, and other toxins via complex biochemical pathways and secretes its waste as bile. The kidney filters blood and eliminates via the bladder as urine. The spleen filters blood, and lymph nodes around the abdominal organs filter lymph. The small and large intestines also contain supportive gut bacteria that help digest food, maintain our intestinal lining, and kill off non-supportive bacteria in our digestive system.


Our supportive gut bacteria are often referred to as probiotics, meaning "pro life." In fact, there are more bacteria cells in the body than actual human cells. The entire spectrum of bacteria contained in our gut, hair, mouth, and skin weighs 2-3 pounds. We are truly a walking symbiotic relationship!

The acquisition of probiotics begins at birth with our trip down the vaginal canal. Breast milk, fermented foods acquired in our diet, and supplementation continue this process. A deficiency of supportive gut bacteria slows transit time through the GI tract, creates gas and bloating; challenges immune function; and exacerbates food allergies, yeast infections, and other unpleasant digestive events.

Muscles - skeletal and smooth

Skeletal muscle attaches bone to bone as the name implies, and helps us move, supports the spine, assists with breathing, and provides much of the container for our organs. These muscles are made of contractile cells, wrapped with membrane-like fascia, supplied with blood, and innervated with sensory and motor nerves.

Smooth muscle wraps the body's tubes, bags, and fascial wrappings. In the gut, smooth muscle is arranged in 2 layers around the entire GI tract (stomach has 3 layers), at specific functional transition areas (sphincters), in the fascia matrix that supports organs, and around all blood and lymph vessels. Because of smooth muscle we can move food through our digestive system and eliminate stool via peristalsis, manage the movement of material from one container to another, and return fluid to the heart against gravity. We would be in big trouble without smooth muscle!


The workings of our belly are truly magnificent! The 2nd installment of this series will discuss how manual therapy can help optimize the function of our organs, fluid dynamics, and nervous system. Stay tuned!


* The Second Brain, Michael D. Gershon, MD. Harper Collins, 1998.

Marty is in private practice in Seattle, WA.  He is the director of Love Your Guts Seminars, and teaches internationally throughout North America and the UK.  Additional contact info:,


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