Digestion: a monumental effort to nourish the body
Ultimately, digestion is all about transforming the food that we eat into molecular particles that can be absorbed and used to create energy or to form the building blocks used to build new tissue, immune system, endocrine (hormonal) or enzymatic components.
The digestive process begins in the mouth which provides both mechanical and chemical mechanisms of action.
First steps – the mouth, teeth and tongue
The mouth and teeth are the first “organs” of the digestive tract. Their part of the mechanical digestive process is called mastication. This is the process by which food is ground by the teeth and rolled into a bolus by the tongue. The primary purpose of mastication is to break the food down into smaller and smaller pieces. This increases the surface area of the food and allows greater contact with salivary enzymes. In conjunction with the mechanical process of mastication, the taste receptors (papillae) which cover the tongue send messages via the nervous system to the brain, which signals the stomach to prepare for food.
During mastication the food is mixed with saliva produced by the parotid, submandibular and sublingual glands and also by buccal glands in the mucosa in the cheeks and gums. Saliva is made up of water, salts, mucin, serous fluid, lysozyme, IgA, growth factors and the enzyme amylase. Saliva adds moisture and is slightly acidic to support the enzymatic activity of amylase as it begins the breakdown of starches (polysaccharides) to shorter chains.
Whilst in the mouth, especially if it is held there for extended periods, certain small molecular weight substances (such as drug chemicals or supplemental nutrients) may be absorbed directly into the blood stream via a process called sublingual absorption. The tissues beneath the mucous membrane under the tongue and buccal mucosa in the cheeks are rich in capillaries. Small enough molecules that come into contact with these tissues may diffuse through them, into the capillaries, then directly into the blood stream. In contrast, substances swallowed and subsequently absorbed in the intestine are subject to "first pass metabolism" in the liver before entering the general circulation.
The journey begins – the oesophagus
Once the pulpy food mass (bolus) is ready it is moved to the back of the tongue, then swallowed, which propels the bolus into the oesophagus and initiates peristaltic (wave-like) contractions of a dual layer of muscles surrounding the oesophageal tube. The oesophagus delivers the saliva-mixed food from the mouth to the stomach and also serves as an air lock between the outside world and the rest of the gastrointestinal (digestive) tract. It takes only about seven seconds for food to pass through the oesophagus. No additional digestive processes are initiated during that time.
At the bottom of the oesophagus the bolus encounters the lower oesophageal sphincter (LES), the point where the oesophagus joins the top of the stomach. The LES is not actually a structural sphincter, but a specialist oesophageal muscle that, whilst normally closed, relaxes (or opens) as food is moved down the oesophagus. As soon as swallowing stops, the LES contracts (closes) to keep the food in the stomach. A normally functioning LES prevents food and stomach acid from backing up into the oesophagus and ultimately into the trachea or "windpipe".
The gastric furnace – the stomach
The human stomach is a muscular, elastic, pear-shaped bag, lying crosswise in the abdominal cavity, beneath the diaphragm and between the oesophagus and small intestine (duodenum). In humans, the stomach has a relaxed, near empty volume of about 45ml. It is a distensible organ that can expand to hold about 1 litre of food, but will hold as much as 2-3 litres, whereas a newborn baby will only be able to retain 30ml.
Once in the stomach, the food is once again subject to both mechanical and chemical digestion. Various chemicals in the stomach like the digestive enzymes pepsin, rennin and lipase interact to break down the food. In addition, hydrochloric acid kills living organisms that may be present in the food, creates a suitable environment for protein digesting enzymes and assists in digestion. Also, watery mucus provides a protective lining for the muscular walls of the stomach so it will not be digested by the acid or enzymes.
The wall of the stomach has a number of folds or ridges, called rugae, formed by the submucosa and mucosa, which serve to accommodate the filling and expanding of the stomach. Deep in the rugae are gastric pits and four major types of secretory epithelial cells that produce digestive chemicals. These are:
- Mucous cells: secrete an alkaline mucus that protects the epithelium against shear stress and acid
- Parietal cells: secrete hydrochloric acid
- Chief cells: secrete pepsin, a proteolytic enzyme
- G cells: secrete the hormone gastrin
Very little nutrient absorption occurs in the stomach, though some alcohol and water does pass through the stomach lining.
The mechanical action of the muscles in the stomach constrict and relax in a continuous motion blending, whipping and stirring the stomach's contents into chyme, a watery, porridge-like substance that is passed through the pyloric sphincter into the small intestine. In humans, the process thus far has typically taken an hour or two.
Taking the long road – the small intestine
The small intestine is the longest organ of the digestive tract, averaging about 5 meters. It is divided up indiscriminately into three sections: the duodenum, the jejunum and the ilium. Whilst the small intestine is much longer than the large intestine (typically around 3 times longer), it gets its name from its comparatively smaller diameter. The small intestine is where the most intensive digestion takes place and reaches completion, where the acidity of chyme is neutralised and the majority of nutrient and fluid absorption takes place.
Once in the duodenum the acidic, fatty chyme triggers the release of three different digestive liquids:
1. Bile, produced in the liver, stored and released by the gallbladder, emulsifies fats to allow absorption, neutralises the chyme and is used to excrete waste products such as bilin and bile acids.
2. Pancreatic juice contains a variety of enzymes, including trypsinogen, chymotrypsinogen, elastase, carboxypeptidase, pancreatic lipase and amylase. Pancreatic juice is also alkalinising due to the high concentration of bicarbonate ions.
3. Intestinal enzymes of the alkaline mucosal membranes. These enzymes include maltase, lactase and sucrase (all three of which process only sugars), trypsin and chymotrypsin.
The small intestine and remainder of the digestive tract use peristalsis (the rhythmic wave-like contraction of smooth muscle) to transport food from the duodenum to the rectum and to allow the food to be mixed with digestive juices and subsequently absorbed. As the pH level changes in the small intestine, gradually becoming alkaline, more enzymes are activated that further chemically break down various nutrients into smaller molecules to allow absorption into the circulatory or lymphatic systems.
The central part of the small intestine – the jejunum, is the site where most of the nutrients from ingested food are absorbed as the digested food is now in a small enough molecular state to pass through of the wall the intestine into the blood vessels via process called diffusion.
As a simple tube, the length and diameter of the small intestine would have a surface area of only about 0.5m2. However the inner wall, or mucosa, of the small intestine is not smooth. It is lined with small, finger-like structures called villi, each of which is covered with even smaller hair-like structures called microvilli which increase the surface area by a factor of 500 to approximately 200m2, or roughly the size of a tennis court. This large surface area improves the volume and speed of absorption of nutrients through the intestinal wall.
Absorption is the movement of molecules across the intestinal wall for transport to the liver and subsequently into the circulatory system. Most of the end-products of digestion, including vitamins, minerals and water, are absorbed through the wall of the intestinal lumen by four mechanisms of absorption: (1) active transport, (2) passive diffusion, (3) endocytosis and (4) facilitative diffusion.
The majority of water and nutrient absorption takes place in the jejunum – the second or central section of the small intestine, with the following notable exceptions:
- Iron is absorbed in the duodenum
- Vitamin B12 and bile salts are absorbed in the terminal ileum
- Water and lipids are absorbed by passive diffusion throughout the small intestine
- Sodium is absorbed by active transport primarily in the large intestine
Blood containing the absorbed nutrients is carried away from the small intestine via the hepatic portal vein and goes to the liver for filtering, removal of toxins and nutrient processing. This is the "first pass metabolism" process mentioned earlier. From the liver 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 the cells, tissues, enzymes, hormones and immune components required by the body. The food that remains undigested and unabsorbed passes into the large intestine.
At the end of its passage through the small intestine, the chyme has been depleted of around 90 percent of its vitamins and minerals and the majority of its other nutrients. In addition, around eight to 10 litres of fluid is also absorbed in the small intestine each day. Complex carbohydrates that resist enzyme degradation, such as fibre and resistant starch, remain, as do a small amount of other food molecules and nutrients that have escaped the digestion process. For example, about 3-5% of ingested protein normally escapes digestion and continues to the large intestine. Transit time through the small intestine typically takes 4 to 6 hours.
The final countdown – the large intestine
The large intestine is the second to last part of the digestive tract and consists of the ascending, transverse, descending and sigmoid colon — the final portion of the digestive tract is the rectum and anus. The large intestine is not designed for absorption, it is particularly specialised to conserve the sodium and water that escape absorption in the small intestine.
The remaining food particles, which at this point are primarily fibre, will spend more time in the large intestine than anywhere else during the digestive process. On average, food travels through the stomach in ½ to 2 hours, continues through the small intestine for 2 to 6 hours and spends 6 to 72 hours in the large intestine before final removal by defecation.
One reason food stays longer in the large intestine may be that the micro-flora ecosystem in the large intestine is capable of generating nutrients from the food. Bacteria make up most of the flora in the colon and 60% of the dry mass of faeces. Research has shown that the relationship between gut flora and humans is not merely commensal (a non-harmful coexistence), but rather a symbiotic relationship. Indeed, the microorganisms, particularly the probiotics (pro-life) organisms, which include the Bifidobacteria and Lactobaccillus genuses, perform a variety of useful functions such as fermenting unused starches and fibres, supporting the immune system, preventing growth of harmful or pathogenic bacteria, regulating the healthy development and maintenance of the intestinal wall, producing vitamins for the host (such as biotin and vitamin K) and producing hormones to direct the host to store fats.
The remaining undigested and unfermented fibre provides bulk for stool excretion and can bind toxins and waste products to facilitate their removal via defecation. Finally, after its long journey, being propelled along by the wave-like peristaltic movements of the muscular gut wall, the remaining mass, called faecal matter, enters the rectum, controlled by an involuntary sphincter and then the anus which allows for controlled, voluntary elimination of stool.
In a nut shell summary:
1. We eat
2. We chew – mastication
3. We swallow
4. The bolus of food travels down the oesophagus to the stomach
5. The stomach mechanically and chemically breaks down the food (especially proteins) into small pieces whilst the highly acidic hydrochloric acid kills potentially infectious micro-organisms in the food. A fluid, porridge-like substance called chyme passes from the stomach into the small intestine
6. Once in the duodenum (the 1st part of the small intestine) digestive enzymes, bile and alkalinising fluids are released by the pancreas, gallbladder and intestinal lining and mixed with the chyme to finish the digestive process – the breaking down of food into absorbable molecular particles
7. Wave-like muscular contractions, called peristalsis, propel the food along the small intestine, where nutrients and water are absorbed, via the liver, into the systemic arterial circulation for distribution around the body
8. Once the remaining food particles reach the large intestine there is little left beyond fibre and a few proteins and nutrients that avoided digestion. This faecal matter travels slowly through the large intestine where it is utilised to feed and stimulate the intestinal micro-flora which provide a variety of beneficial effects for the human host
9. The remaining undigested and unfermented fibre provides bulk for faecal matter (stool) excretion and carries toxins and waste material away from the body
This educational publication was created for your learning pleasure by naturopath and medical herbalist Jacqui Simcock, North Shore City, New Zealand.All content is subject to copyright Health & Herbs International Ltd 2010 and may not be copied or reproduced, in part or full, for commercial purposes.