The human digestive system has a unique design that uses mechanical and chemical processes that turn the food a person eats into energy and nutrients the body can use for energy, growth, and cell repair. The process of digestion has many stages in which several organs, tissues and body systems work together to achieve an end goal – in this case, breaking down large, complex molecules into smaller pieces for distribution and use throughout the body.
Digestive system organs fall into two main groups: the alimentary canal and accessory organs. Measuring up to 30 feet in length in the average adult, the alimentary canal is a series of hollow organs joined in a long, twisting tube that runs uninterrupted from the mouth to the anus. The liver, pancreas, and gallbladder, which are solid organs, act as accessories to the alimentary canal by adding secretions that aid digestion.
Other body systems participate in the digestive process. The autonomic nervous system largely controls digestion in that it stimulates a variety of functions, including salivary secretions, peristalsis that pushes food along the digestive tract, gastric acid production in the stomach, the opening and closing of sphincters, the release of hormones that aid digestion, and the storage of energy in the form of fat.
The circulatory system absorbs and distributes the nutrients produced by the digestive tract. Blood also carries chemical signals created by the endocrine system, which controls the speed of digestion.
The gastrointestinal tract (GI tract) contains bacteria, also known as gut microbiota, which help with digestion. Bacteria in the gut perform a variety of useful functions, including digestion of unutilized energy substrates, stimulation of cell growth, suppressing pathogen growth, training the immune system to respond only to harmful pathogens, and defending against certain diseases.
Together, this combination performs the complex task of turning food and liquid into energy and useful nutrients.
The human body draws nutrients from food through four main processes: ingestion, digestion, absorption, and assimilation.
Ingestion introduces food into the system. The term ‘ingestion’ includes a group of processes that take place in the mouth. These processes include chewing and grinding using the teeth, the release of saliva to lubricate and add chemicals to food, and swallowing of the food to send it to the next part of the digestive tract.
The processes of ingestion begin the breakdown of food. Chewing breaks down food through mechanical forces. Saliva contains two key enzymes, amylase and lipase, which break down food chemically. Amylase catalyzes the first step in the digestion of starch while lipase initiates fat digestion.
Digestion further breaks down food into particles small enough to be readily absorbed into the bloodstream. The body breaks down nutrients into macronutrients and micronutrients.
The process of taking fluid and nutrients into the bloodstream, known as absorption, occurs primarily in the small intestine. This section of the alimentary canal is largely responsible for the absorption of water and soluble substances, such as simple sugars, salt, and alcohol.
The lining of the ileum has a thin epithelium that allows for the easy diffusion of nutrients and the sheer size of the small intestine provides a significant surface area on which absorption can take place. A human small intestine measures about 6 meters, or 20 feet, and its absorptive area measures 250 square meters (almost 2,700 feet), which is about the size of a tennis court. The mucosal folds on the inner surface create even more surface area for absorption.
Each villus has a network of capillaries and fine lymphatic vessels, known as lacteals, close to its surface. Amino acids and carbohydrates move into capillaries while lipids move into lacteals. “Villi” is Latin for “shaggy hair.” These villi are in constant motion, absorbing nutrients as food moves through the ileum.
The villi and microvilli of the ileum contain a large number of capillaries that absorb fatty acids and glycerol. The ileum also plays an essential role in the absorption of vitamin B12 and other bile salts. Additionally, the ileum secretes protease and carbohydrate enzymes that complete the breakdown of protein and carbohydrates. Specialized cells in the ileum absorb materials into the bloodstream or lymphatic systems, which transport the molecules to the liver, kidneys and other parts of the body.
Undigested food, older cells from the GI tract lining and other waste products move through the large intestine, which absorbs water, vitamin K and the remaining B vitamins, and nutrients. The absorption of water turns liquid waste into stool.
Assimilation is the uptake of nutrients into body cells and tissues, and the reassembly of absorbed nutrients into complex substances. The small intestine absorbs most digested food molecules, as well as water and minerals, during assimilation those molecules are passed on to other parts of the body for storage or further chemical change. Simple sugars, glycerol, amino acids, and some vitamins and salts move through the bloodstream to the liver. The lymphatic system absorbs fatty acids and vitamins.
Bacteria are essential to the operation of the human digestive system. Friendly bacteria in the digestive system, known collectively as the microbiota, live primarily in the colon and ileum. The esophagus, stomach, and duodenum also house bacteria, but generally in lower numbers due to the strong stomach acid and digestive enzymes that are at work here.
Bacteria enjoy a symbiotic relationship with their host. Bacteria have a safe place to live and provide a number of benefits in return. Friendly flora help maintain an optimal pH level, stimulate the immune system, get rid of hormones no longer needed by the body, and reduce potential toxic and carcinogenic substances. One of the most significant contributions of bacteria in the gut is that they provide protection against harmful pathogens that try to invade the cells lining the internal intestinal wall.
Bacteria can also help the host body metabolize food. The body easily breaks down simple carbohydrates into energy-rich adenosine triphosphate (ATP), which stores and releases energy as required. It takes more effort to use complex carbohydrates, as the body needs to break down complex carbohydrates into their component simple sugars before it can process them.
Bacteria use carbohydrate-active enzymes (CAZymes) to break down complex carbohydrates. There are several different kinds of CAZymes and each is specialized in the complex carbohydrate molecules it breaks down. The human body does not make the enzymes that degrade many plant structures so, without beneficial bacteria, it would waste much of the nutritional value of vegetables. Some bacteria species living in the digestive tract can make vitamin K and some B vitamins. The human body cannot make these vitamins and has trouble absorbing them from food. An imbalance in the gut flora, or dysbiosis, is associated with the pathogenesis of many intestinal and extra-intestinal disorders. Dietary supplementation may restore balance to the gut flora.
The human digestive system is a complex process that turns whole foods into molecules tiny enough for absorption into the cells of the body and gets rid of the waste. It is truly a marvel that, given the right intake, will serve to nourish, sustain, and protect the host for decades. Of course, even the most well-designed system will break down every so often. As hosts, it is our responsibility to maintain the health of our digestive system – after all, we cannot live without it! A healthy gut requires a healthy, balanced diet that contains the raw nutrients, vitamins and minerals necessary for life.
The balance of bacteria in the system also needs to be maintained. Back in the days before processed foods, chemicals, GMOs and a nutritionally sparse diet, this was a little easier. For generations, regular consumption of fermented foods was sufficient to maintain digestive health – however, that is rarely enough nowadays, and most of us should consider some sort of supplemental probiotic as part of our daily diet to ensure a healthy and diverse gut microbiota.