Anatomy of the Digestive System

The process of digestion is a fascinating and complex one that turns the food we place in our mouths, into energy. This process takes place in the gastrointestinal tract, a long, connected, tubular, muscular structure that starts with the mouth and ends you know where :P Food is moved along the system, altered by enzymes and hormones into usable molecules which are absorbed and used for all functions in the body. A handful of organs that sit along the GI tract support the digestive process -  the liver, gallbladder, and pancreas. The time it takes for food to travel from entering the mouth to be excreted as waste (poop) is normally around 30 to 40 hours. Average intestinal transit times in healthy individuals (which varies a lot depending on the types of solids, mixed foods and liquids ingested and differs between men and women)(1) -

• Total emptying of the stomach = 4 - 5 hrs
• 50% emptying of the small intestine = 2.5 - 3 hrs
• Transit time through the colon = 40 hrs

The site, smell and even the thought of food begins the digestive process, stimulating saliva which is released by the salivary glands into the mouth. Once in the mouth, food is chewed and broken down into smaller particles that can be more easily broken down by the enzymes in the saliva. The tongue helps by mixing the food as well as shaping the food into a bolus with the roof of the mouth (soft palate), so it can be more easily swallowed to begin it’s GI tract journey at the esophagus.

So right from the beginning, the digestive process is both chemical and mechanical. The reason we are not aware of all this constant movement inside us, as our organs perform their synchronised dance routines, is due to the parietal and visceral membranes

 The pharynx (throat) is the transition area from the mouth to the esophagus. From the pharynx there are two paths that the food bolus can take - 1) the wrong path down the windpipe which causes us to choke, or 2) the right path down to the stomach. When we swallow, a small elastic cartilage “flap” called the epiglottis seals off the trachea, allowing food to pass over the larynx and windpipe - saving our lives every time! - down into the stomach.

Peristalsis - Keeping It all Moving

Via a series of synchronised neuromuscular contractions called peristalsis, the esophagus delivers food to your stomach. Peristalsis, also known as the MMC wave or “Housekeeper Wave” continues throughout the digestive process, helping to move food down the GI tract.

*Fun Fact*

The uniquely human proximity of the trachea and esophagus is risky but reveals a lot about our evolutionary history, especially in terms of our vocal anatomy which may have even made human speech possible! - “...it could be said that this positioning of the two ‘pipes’ is a result of natural selection and evolution that gave our primate ancestors the ability to communicate. This obviously proved to be a competitive edge over other species over an extended period of time.”(1) This topic really excites me but I digress - back to digestion!

 This is the first of many muscular valves along the GI tract. When food approaches, this muscular valve relaxes to allow entry into the stomach. This sphincter has the important function of closing the stomach so no food or stomach acid reenters the esophagus (and thus avoiding heartburn or regurgitation).

Glands that line the stomach organ release gastric acid (potassium chloride, sodium chloride, and hydrochloric acid) and enzymes which continue the breakdown process of the food while the stomach muscles contract periodically, churning food and enhancing digestion. At the end of this process, the food you placed in your mouth has been turned into an unrecognisable “semi-fluid mass of partly digested food” - yum! - called chyme. 

*Fun Facts*

• Several mucosal defence mechanisms protect the stomach against the highly acidic hydrochloric acid and noxious agents. This includes mucus cells which form the mucosal inner lining of the organ and prevent the stomach from digesting itself
• A typical adult human stomach will secrete about 1.5 liters of gastric acid daily
• Gastric acid production is regulated by both the autonomic nervous system and several hormones
• The pH of stomach acid usually ranges from 1 to 3. At its strongest, the pH of stomach acid just below that of battery acid!
• Gastric rugae (gastric folds) are coiled sections of tissue that exist in the mucosal and submucosal layers of the stomach. They provide elasticity by allowing the stomach to expand when a bolus enters. 

 Once the chyme is well-mixed, waves of muscle contractions (peristalsis) propel it through a valve called the pylorus and into the first 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.

In the SI, digested food can now pass into the blood vessels in the wall of the intestine through either diffusion or active transport. The inner wall, or mucosa, of the small intestine is lined with simple epithelial tissue. Structurally, the mucosa is covered in wrinkles or folds called plicae circulares. From the plicae circulares project microscopic finger-like pieces of tissue called villi. The individual epithelial cells also have finger-like projections known as microvilli. The functions of the plicae circulares, the villi, and the microvilli are to increase the amount of surface area available for the absorption of nutrients, and to limit the loss of nutrients to intestinal flora and fauna.

Each villus has its own network of capillaries and fine lymphatic vessels called lacteals close to its surface. The epithelial cells of the villi transport nutrients from the lumen of the intestine into these capillaries (amino acids and carbohydrates) and lacteals (lipids). The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances such as proteins required for specific functions by our body. The material that remains undigested and unabsorbed passes into the large intestine where it feeds the bacterial population.

The digestive tube is full of lymphocytes, macrophages and other cells that participate in immune responses. Aside from all of its other functions, the gastrointestinal tract is a lymphoid organ, and the lymphoid tissue within it is collectively referred to as the gut-associated lymphoid tissue or GALT. Thus, the small intestinal immune cells supports the body's immune system(3). Gut lymphocytes are distributed in three basic populations -

1. Peyer's patches. Similar in many ways to lymph nodes, they are part of the lymphatic system and are located in the mucosa and extending into the submucosa of the small intestine. Located within the ileum of the small intestine, they play a vital part in the digestive tract's local immune system, providing a site for antigens from potentially harmful bacteria or other microorganisms to be sampled, and subsequently presented to the immune system.
2. Lamina propria lymphocytes. These are lymphocytes scattered in the lamina propria of the mucosa. The majority of these cells are IgA-secreting B cells.
3. Intraepithelial lymphocytes. These are lymphocytes that are positioned in the basolateral spaces between lumenal epithelial cells, beneath the tight junctions (they are "inside" the epithelium, but not inside the epithelial cells themselves.

Another important component of the GI immune system is the M or microfold cell. M cells are a specific cell type in the intestinal epithelium over lymphoid follicles that endocytose (a method of absorbing) varieties of protein and peptide antigens. Instead of digesting these proteins, M cells transport them into the underlying tissue, where they are taken up by local dendritic cells and macrophages.

The presence of gut flora appears to contribute positively to our immune and overall health also - we discuss this in depth on the Healing the Gut + Autoimmunity page. 

Because elimination is love people!

Interesting facts about poop - we all do it - and poop can reveal a lot of information about the microbiota populations which colonise our guts - “The scientists and doctors who study feces have found that it's the byproduct of a diverse community of bacteria in your gut that impacts your health in all sorts of ways. Paying closer attention to your stool can tell you about the condition of these vital bacteria — and your overall health.”(4)

• It's mostly comprised of live, as well as the dead carcasses of, bacteria. Excluding water, 50-80% is made up of bacteria that lived in your intestines and was ejected as food passed through, as well as indigestible plant matter and intestinal tissue cells which were carried through with the poop
• Dead red blood cells and bile make poop appear brown. The colour is the result of a chemical called stercobilin. This chemical is a byproduct of the hemoglobin in broken-down red blood cells, and it also comes from bile, (see above). Ideally, with an optimally-functioning digestive system, the model stool is a deep chocolatey color - like melted chocolate.
• Plant fiber + beneficial bacteria = the healthy poop. "What really makes a good stool is large amounts of the indigestible plant matter that feed gut bacteria." This plant fiber - mostly cellulose - also directly adds bulk to poop, so a plant-heavy diet is critical for nice, solid bowel movements(4).
• Men + women poop differently! Because of anatomical differences, men and women's GI tracts work a tad differently. As we women have extra internal organs in our abdominal cavity (uterus and ovaries) our colons hang a bit lower than men's, and are a bit longer - on average, by as much as ten centimeters. Men have more rigid abdominal walls which help push food through the GI tract more effectively. This could be one factor increasing womens’ likelihood to develop an autoimmune disease!
• Fecal Transplants. These are becoming an increasingly well known about procedure which can solve issues antibiotics can’t. We were considering this as an option for SIBO treatment before we started taking the botanical antibiotics - see Healing SIBO.

Microbial Life in the Digestive Tract

At birth, there are no bacteria in the gastrointestinal tract. During birth, however, bacteria from the mother's colon and vagina are swallowed by the infant, and, within a few weeks or months, they populate the infant's gastrointestinal tract. In most cases, a "mature" microbial flora is established by 3 to 4 weeks of age(5). 

Microbiome + Human Relationship

The relationship between normal (beneficial) intestinal bacteria and the human they inhabit is a complex one. A typical person harbors more than 500 distinct species of bacteria, representing dozens of different lifestyles and responsibilities(5). The composition and distribution of this microbial garden varies with age, state of health and diet. The relationship is symbiotic, so each benefits from each other. The bacteria benefit from the warm, moist environment of the small intestine ideal for growth, as well as the frequent flow of food passing down the gastrointestinal tract that provides a ready source of nutrition. The human host benefits from this relationship in numerous ways - 

• Beneficial bacteria stimulate the growth of the intestinal lining and the immune system of the intestine. 
• “Healthy” bacteria prevent the growth of disease-causing bacteria within the intestine.
• They produce vitamin K, which is absorbed and used by the human host. 
• The bacteria are important even for the muscular activity of the small intestine - without bacteria, reduced muscular activity can result in gut motility dysfunction. Our good friend peristalsis malfunctions! This is a major cause of SIBO. See Gut Dysbiosis/SIBO In Depth.

Intestinal Self-Protection Mechanisms

There is a delicate balance between the gastrointestinal tract microbiome and the human they colonise. The GI tract, particularly the small intestine, contains an extensive immune system. The immune system protects the intestine from disease-causing viruses, bacteria, and parasites. (The effects of the response of the intestinal immune response to disease-causing organisms have been experienced by anyone who has experienced a stomach bug or food poisoning.) The intestine does not attack the normal bacteria within it, only disease-causing bacteria. Somehow, the intestine becomes tolerant of the normal bacteria and does not mount an attack against them. The intestine has other ways that may be important in protecting it from bacteria - both beneficial and pathogenic - 

• Peristalsis keeps the numbers of bacteria within the intestine at a low level, preventing bacteria from settling and colonising the SI.
• Mucus secreted into the intestine coats the intestinal lining and prevents the bacteria from coming into contact with the lining. 
• The intestine secretes antibodies that can block, and sometimes kill bacteria as well as substances that prevent the growth of bacteria.
• The intestinal lining itself can produce receptors for toxic substances produced by bacteria and can prevent the substances from exerting their toxic effects.

“In sharp contrast to the stomach and small intestine, the contents of the colon literally teem with bacteria, predominantly strict anaerobes (bacteria that survive only in environments virtually devoid of oxygen). Between these two extremes is a transitional zone, usually in the ileum, where moderate numbers of both aerobic and anaerobic bacteria are found.”(5)