Turning food as diverse as animal protein, seeds and leaves into the energy we need to survive, this complex group of organs perform a range of precise, synchronised, specialised functions, using chemicals and enzymes in a process which begins even before we put food into our mouths.
To understand gut health, we must first understand the anatomy behind the GI tract and digestive function. We focus mostly on the small intestine - the action centre.
Top sources include Cleveland Clinic, VIVO Biomedical Science + Medicine Net.
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) -
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.
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.
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.
As the tasty chyme paste reaches the first section of the small intestines, the other digestive organs join the party. Via ducts, the liver, pancreas and gallbladder release their digestive juices containing enzymes into the duodenum.
The gallbladder stores and concentrates bile, a combination of fluids, fat, and cholesterol. Bile helps break down and absorb fat. The gallbladder delivers bile into the duodenum - the first segment of the small intestine, allowing fat-soluble vitamins and nutrients to be more easily absorbed into the bloodstream.
The pancreas secretes digestive enzymes into the duodenum via ducts. These enzymes break down protein, fats, and carbohydrates. The pancreas also makes insulin - the chief hormone for metabolising sugar - secreting it directly into the bloodstream.
The liver has multiple functions, but its main function within the digestive system is to process the nutrients absorbed from the small intestine. Bile ducts carry bile from the liver to the gallbladder for storage, or are secreted directly into the small intestine. They play an important role in digesting fat. The liver also detoxifies potentially harmful chemicals. The liver is the body's chemical "powerhouse" taking raw materials absorbed by the intestine and from them, making all the various chemicals the body needs to function.
The SI or small bowel is an 22-foot long muscular tubular organ in the gastrointestinal tract (GI) where most of the absorption of nutrients and minerals from food takes place. It lies between the stomach and large intestine, and receives bile from the liver and/or gallbladder and pancreatic enzymes through the pancreatic duct to aid in digestion, further breaking down food. It comprises of 3 segments - duodenum, jejunum, and ileum. It propels the chyme along the GI tract via our good friend - peristalsis.
The duodenum is a hollow jointed tube about 25–38 cm long connecting the stomach to the jejunum. It begins at the pylorus of the stomach and ends at the suspensory muscle of duodenum. The duodenum, the shortest part of the SI, is where preparation for absorption through small finger-like protrusions called villi begins.
The duodenum can be divided into four parts; superior, descending, inferior and ascending. Together these parts form a ‘C’ shape, that is around 25cm long, and which wraps around the head of the pancreas.
The jejunum is specialised for the absorption through its lining by enterocytes - small nutrient particles which have been previously digested by enzymes in the duodenum.
The main function of the ileum is to absorb vitamin B12, bile salts, and whatever products of digestion were not absorbed by the jejunum.
A Messy Business - Bacteria, Enzymes + Water
Bacteria in your small intestine make some of the enzymes you need to digest carbohydrates. Your small intestine moves water from your bloodstream into your GI tract to help break down food. Your small intestine also absorbs water with other nutrients.
Contents of the small intestine start out semi-solid, and end in a liquid form after passing through to the large bowel. Water, bile, enzymes, and mucous contribute to the change in consistency. Once the nutrients have been absorbed and the leftover-food residue liquid has passed through the small intestine, it then moves on to the large intestine/colon.
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 -
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.
6-foot long muscular tube that connects the small intestine to the rectum In your large intestine, water moves from your GI tract into your bloodstream. Bacteria in your large intestine help break down remaining nutrients to make vitamin K. The waste products of digestion, including parts of food that are still too large, form poop.
Poop (stool, or waste) left over from the digestive process, is passed through the colon by means of peristalsis, first in a liquid state and ultimately in a solid form. As stool passes through the colon, water is absorbed back into the body. Stool is stored in the sigmoid (S-shaped) colon until a "mass movement" empties it into the rectum once or twice a day. It normally takes about 36 hours for stool to get through the colon.
The poop itself is mostly food debris, dead cells from our body and bacteria. The beneficial or “good” bacteria present perform several useful functions, such as synthesizing various vitamins, processing waste products and food particles, and protecting us against harmful bacteria.
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)
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 -
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 -
“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)
If any part of the tightly scheduled and precise functions, sections, organs or systems of the digestive system or GI tract goes wrong, the consequences for overall health can be dramatic.
Your hormones and nerves work together to help control the digestive processes. Signals flow back and forth from the GI tract via the enteric nervous system (ENS), to your brain(6) - they talk to each other.
Cells lining your stomach and small intestine make and release hormones that control the function of your digestive system. These hormones tell your body when to make the digestive juices and they send signals to your brain that you are hungry or full. Your pancreas also makes hormones that are important to digestion.
The nerves that connect your central nervous system—your brain and spinal cord—to your digestive system and control some digestive functions. For example, when you see or smell food, your brain sends a signal that causes your salivary glands to "make your mouth water" to prepare you to eat.
The enteric nervous system aka the Gut Brain resides within the walls of your GI tract. When food stretches the walls of your GI tract, the nerves of your ENS release many different substances that speed up or delay the movement of food and the production of digestive juices. The nerves send signals to control the actions of your gut muscles to contract and relax to push food through your intestines, thus playing a vital role in peristalsis.