Take the digestive system, for example. The main function and purpose of the digestive system is to break food into small molecules and then assist in the absorption of nutrients throughout the body. It is a complex system which is anatomically comprised of the mouth, esophagus, stomach, intestines, rectum, anus, liver, pancreas, and gallbladder. Even though there are many anatomical components, the entire digestive process takes place in just four phases: ingestion (eating), digestion, absorption, and egestion (excretion). During these phases, nutrients (proteins, fats, carbohydrates, vitamins, minerals, and water) from food are broken down to molecules small enough for the body to absorb and use for energy, growth, and cell repair. Proteins are fragmented into amino acids, fats are dissolved into fatty acids and glycerol, and carbohydrates break down into simple sugars (National Institute of Diabetes and Digestive and Kidney Diseases, 2017).
The digestion process has two components - mechanical and chemical.
- Mechanical digestion is the physical breakdown of food and begins in the mouth, where the teeth grind food into smaller bits to aid in swallowing. Saliva mixes with the food and also aids in swallowing by lubricating or moistening the food so it can easily pass through the esophagus. During the process of peristalsis, the muscles of the tongue and esophagus begin to make squeezing movements which push food downward toward the stomach. The sphincter muscle at the base of the esophagus tightens and prevents the food from returning up the throat. While the food is in the stomach, it continues to break down and is mixed with acids and creates a fluid called chyme (more about the chemical components later). The pyloric valve at the bottom of the stomach controls the movement of the chyme from the stomach to the small intestine where it gradually moves to the large intestine; muscles continue to squeeze and undulate as they push the food further through the digestive tract, through the intestines (Sizer & Whitney, 2017). The large intestine, also known as the colon, absorbs minerals and excess water from the chyme and leaves indigestible material such as fiber; this indigestible material leaves the body through the rectum as feces. The entire mechanical process of digesting a meal can take anywhere from one to three days (Sizer & Whitney, 2017).
- Chemical digestion is the chemical breakdown of food and relies on digestive enzymes to turn large food molecules into smaller ones which can be absorbed by the body. Chemical digestion also begins in the mouth. Saliva contains enzymes which begin breaking down starches and fat in food before it has even been swallowed; the chemical digestion of protein takes place in the stomach when gastric juices, a combination of water, enzymes, and hydrochloric acid, begin to break down the protein structure of food. A mucus lining protects the digestive tract from being damaged or digested along with the food. Once the food has sufficiently been broken down, it migrates to the intestines where, the gallbladder begins to excrete bile to emulsify fat and oils. At the same time, the pancreas begins to excrete pancreatic juice, which contains both digestive enzymes and protective neutralizing sodium bicarbonate; the alkaline pH of bicarbonate, also the active ingredient in baking soda, helps to counteract the acidic pH of the other digestive juices (Sizer & Whitney, 2017).
Like I said, the human body is amazing!
References:
National Institute of Diabetes and Digestive and Kidney Diseases. (2017). Your Digestive System & How it Works. Retrieved from https://www.niddk.nih.gov/health-information/digestive-diseases/digestive-system-how-it-works
Sizer, F. & Whitney, E. (2017). Nutrition: Concepts and controversies (14th ed.). Mason, OH: Cengage Learning.
TTS International Schools. (n.d.). Digestion [image]. Retrieved from https://www.tts-international.com/digestion-poster/1008602.html
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