The digestive system consists of all the organs that share the common function of getting nutrients into the body. It includes a series of hollow organs extending from the mouth to the anus: the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum and anus. These organs form a hollow tube called the gastrointestinal (GI) tract. The space within this special, hollow tube - the area through which food and liques travel - is called the lumen. The digestive system also includes four accessory organs - the salivary glands, liver, gallbladder, and pancreas. From the esophagus to the anus, the walls of the GI tract share common structural features. The walls of the GI tract consist of four layers of tissue:
Some of the organs of the GI tract are separated from each other by thick rings of circular smooth muscle called sphincters. When these sphincters contract they can close off the passageway between organs. In practical terms, the digestive system is a huge, honkin’ assembly line that starts with huge chunks of raw material (food) and takes them apart so that the nutrients in food can be absorbed into the body. The digestive system accomplishes this task with five basic processes: chewing through mechanical processing and movement; fluid secretion of fluids, digestive enzymes, acid, alkali, bile and mucus; digestion in which the contents of the lumen are broken down into smaller particles; absorption in which nutrient molecules pass across the mucosal layer of the GI tract into the blood or lymph and elimination, in which undigested material is eliminated from the body via the anus. The smooth muscle of the GI tract produces two kinds of motility, called peristalsis and segmentation. Peristalsis propels food forward through a peristaltic wave of contraction ripples through the organs of the GI tract, mixing the contents of the stomach and pushing things forward. Segmentation mixes food while short sections of smooth muscle contract and relax in seemingly random fashion. The result is a back and forth mixing of the contents of the lumen. Food particles are pressed against the mucosa, enabling the body to absorb their nutrients. Segmentation occurs primarily in the small intestine as food is digested and absorbed. Let's go on a journey through the GI tract, shall we? The process begins with our teeth. We chew food into pieces small enough to swallow. During this process food remains between the teeth and our mouths contain large numbers of bacteria that flourish on the leftover food. During their metabolism these bacteria release acids that can dissolve enamel and create cavities. Huzza. Three pairs of salivary glands in our mouth produce a watery fluid called saliva. The parotid gland lies near the back of the jaw, and the smaller submandibular and sublingual glands are located just below the lower jaw and below the tongue. All three glands connect to the oral cavity via ducts. Saliva moistens food making it easier to chew and swallow. Saliva contains four ingredients, each with an important function. One is mucin, a mucus like protein that holds food particles together so they can be swallowed more easily. An enzyme called salivary amylase begins the process of digesting carbohydrates. Bicarbonate in saliva maintains the pH of the mouth between 6.5 and 7.5, the range over which salivary amylase is most effective. Salivary bicarbonate may also protect your teeth against those pesky acid-producing bacteria. Saliva also contains small amounts of an enzyme called lysozyme that inhibits bacterial growth. After chewing our food and mixing it with saliva, the tongue pushes it into the pharynx or throat for swallowing. Just beyond the pharynx is the esophagus, a muscular tube consisting of both skeletal and smooth muscle that connects the pharynx to the stomach. The lining of the esophagus produces lubricating mucus that helps food slide easily. The presence of a bolus of food in the esophagus initiates peristaltic contractions of the smooth muscle that push things to the stomach. Gravity also assists peristalsis in propelling food. However, peristaltic contractions enable the esophagus to transport food even against gravity, such as when we are lying down. The lower esophageal sphincter, located at the base of the esophagus, opens briefly as food arrives and closes after it passes into the stomach. The sphincter prevents reflux of the stomach's’ contents back into the esophagus. The stomach is a muscular, expandable sac that performs three important functions:
Typically, the stomach produces 1-2 liters of gastric juice per day, most of it immediately after meals. The pepsin and acid in gastric juice dissolve the connective tissue in food and digest proteins and peptides into amino acids so they can be absorbed by the small intestine. The watery mixture of partially digested food and gastric juice that is delivered to the small intestine is called chyme. A protective barrier of mucus lines the stomach and protects it from the gastric juices. It takes two to six hours for the stomach to empty completely after a meal. Most forceful when the stomach is full, peristalsis declines as the stomach empties. Chyme with a high acid or fat content stimulates the release of hormones that slow stomach peristalsis, giving the small intestines time to absorb the nutrients. Between the stomach and the small intestine is the pyloric sphincter, which regulates the rate of transport of chyme into the small intestine. Neat fact: the stomach does not absorb nutrients because it lacks the required cellular transporting mechanisms and its inner lining is coated with mucus. Exceptions to this rule are aspirin and alcohol, both of which are small lipid-soluble substances that can cross the mucus barrier and be absorbed into the bloodstream directly from the stomach. When the stomach contains food, alcohol is absorbed more slowly. The small intestine has two major functions: digestion and absorption! The stomach partially digests proteins and this process continues in the small intestines. Digestion of proteins, carbohydrates, and lipids in the small intestines involves neutralizing the highly acidic gastric juice and adding additional digestive enzymes from the intestine and pancreas. Eventually everything gets broken down to single amino acids, monosaccharides, fatty acids, and glycerol. These are small enough to be transported across mucosal cells into the blood. Approximately 95% of the food you eat is absorbed in the small intestines. The small intestine consists of three different regions. The first region, the duodenum, is where most of the digestion takes place. The products of digestion are absorbed primarily in the other two segments, the jejunum and ileum, which together are about 10 (freakin’) feet long. HUZZZAAA! The structure of the small intestine wall makes it well suited for absorption. The mucosa contains large folds covered with microscopic projections called villi. Each epithelial cell of the villi has dozens of even smaller, cytoplasmic projections called microvilli. The microvilli give the mucosal surface a velvety appearance. These features enlarge the surface area of the small intestine by more than 500 times, increasing its ability to absorb nutrients. Let’s pause on our journey through the digestive system and take a look at the other accessory organs of the digestive tract. We’ve examined how the salivary glands in the mouth, stomach and small intestine function. What are the roles of the liver, pancreas and gallbladder in digestion? The pancreas, an elegant elongated organ that lies just behind the stomach, has both endocrine and exocrine functions. We’ve looked at its endocrine function in a previous post so let’s turn our attention to its exocrine role. This powerhouse organ produces and secretes digestive enzymes and sodium bicarbonate. Two pancreatic ducts deliver these secretions to the duodenum, where they facilitate the process of digestion. Neat! The liver is a large organ located in the upper right abdominal cavity. The liver performs many significant functions, some of which are associated with digestion. In terms of digestion, the liver’s primary function is to facilitate the digestion and absorption of lipids of producing bile. Bile is a luscious, watery mixture containing electrolytes, cholesterol, bile salts (yummy), a phospho-lipid, and pigments derived from the breakdown of hemoglobin. In addition the liver serves a number of other functions that maintain allostasis. The liver:
Because of its central role in so many different functions, liver injury can be particularly dangerous. Overexposure to toxic chemicals, medications or alcohol can damage the liver because it takes up these substances to “detoxify” them, killing some liver cells in the process. Finally, the bile produced by the liver flows through ducts to the gallbladder. The gallbladder concentrates the bile by removing most of the water and stores it until after a meal, when it is secreted into the small intestine via the bile duct, which joins the pancreatic duct. Okay- back to our journey! By the time the contents of the digestive tract reach the large intestine, most nutrients have been absorbed. The large intestine absorbs the last of the remaining water, ions and nutrients and stores the now nearly solid waste material until can be eliminated. This gal is larger in diameter than the small intestine but shorter in length (roughly 5 feet). The large intestine begins at a pouch called the cecum, which receives the cyme from the small intestine. A small fingerlike pouch, the appendix, extends from the cecum. From the cecum, the large intestine continues as the colon which joins the rectum, the passageway through which the waste materials, now called feces, pass as they are eliminated through the anus. Yahoooo! We made it. Okay, but… how are nutrients absorbed? Once food has been digested, how does your body absorb the nutrients? The mechanism depends on the type of nutrient. -Proteins and carbohydrates are absorbed from the lumen of the small intestine by active transport processes, then move by facilitated diffusion into capillaries. The components of lipid digestion are transported to the mucosa in micelles, diffuse into the cells, and recombine into lipids within the cells. Then they are coated with protein to become chylomicrons that enter the lymph. -The digestive system also absorbs water, vitamins, minerals and digestive secretions. Herbal musings
An out-of-balance gut is no fun. Indigestion, gas, heartburn, ulcers, bowel irregularity and deterioration of the gut can be painful, debilitating and at times life threatening. Thankfully, the gut responds quickly and happily to herbal therapies and diet changes that can heal damage and retrain function while resolving symptoms, supporting health and bringing balance to the entire system. One of my favorite categories of herbs are the bitters! Bitters single handedly turn on your GI tract while also benefiting liver function and detoxification. Bitters can help with a lot of issues from modulating weak or excessive appetite and weight to aiding sluggish digestion and elimination, constipation, indigestion, excess fullness, poor nutrient absorption and high cholesterol. Some great bitters include artichoke leaf, dandelion leaf and root, burdock, turmeric, ginger, schizandra, and citrus peel. Additionally there are a number of soothing, gut-healing herbs that help resolve gastric inflammation and other gut issues. My go-to gut soothing herbs are licorice root and marshmallow root. Other gut healing herbs include plantain, calendula, gotu kola and chickweed (best fresh), all of which you can incorporate daily into tea blends, soup broth, pesto or other kinds of food. These plants soothe inflamed tissue and promote wound-healing process. They're better known as topical wound healers but it's the same concept. Gosh, I could go on about herbs for the GI tract so I'll conclude by saying that gas, pain and bloating can be soothed by fennel and peppermint. Final thoughts: watch out for food allergies and... CHEW YOUR FOOD! xoxoxo
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AuthorThe adventures, studies, and musings of a student at the Vermont Center for Integrative Herbalism.
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