Mechanical and Chemical Digestion
The Digestive System's Biological Function
The digestive system is of vital biological importance to the body. Without the ability to process foods, extract nutrients, and eliminate waste, every part of our body would cease to function. Even very small problems with our digestive system can result in nutrient deficiencies, fatigue, and significant discomfort. Every part must work together.
Food must be processed in three ways: digestion, absorption, and elimination, and the digestive system is responsible for the accomplishment of all three of these biological functions. All food ingested must be digested, nutrients absorbed, and unnecessary or harmful agents eliminated from the body.
Digestive System: The Food Journey
The journey of the food we eat through the digestive system is more complicated than one might expect.
Our Mouth: Believe it or not, the mouth will prepare for digestion before food even enters. Just the smell, sight, or sound of food is enough to trigger saliva glands into action. This is important because the chemicals in saliva work with the mechanism of the tongue and teeth to break down food in a way that both prepares it for the next steps and ensure optimal absorption of nutrients.
The Pharynx: Once food has been put through the chemical and mechanical digestion occurring in the mouth, it's time to make its way down to the stomach. To get there, the tongue and soft palate work together to push food back, closing off our trachea and passing it through our throat (otherwise known at the pharynx) - another good reason to make sure food is broken down!
The Esophagus: After passing through the pharynx, food enters the esophagus and is pushed through a series of involuntary contractions (called "peristalsis") toward the lower esophageal sphincter otherwise known as LES. It is this sphincter that malfunctions in conditions like GERD (Gastroesophageal Reflux Disease) or heartburn.
The Stomach: Finally, the food has reached the stomach. With strong muscular walls the stomach acts as mixer and grinder, mechanically digesting our food while its acids and enzymes work to chemically digest it. In the end, our food is reduced to nothing more than nutritious liquid and small solid remnants.
The Small Intestine: After passing through the stomach, it's on to the small intestine where the nutritious liquid and small solid remnant are exposed to even more chemical digestion via enzymes from the pancreas and bile from the liver as peristalsis (the same involuntary contractions which occur in the esophagus) forces the food along its way. It is at this point that our body finally receives the nutrients from our food via the bloodstream, while the remainder makes it way to the colon.
The Colon: The final stop in our food’s journey through our digestive system, the colon’s job is to remove liquid from the non-nutritive food waste until it becomes solid and ready for excretion. By the time our food is ready for the toilet, it’s been approximately 36 hours since it originally entered our mouth.
What is Mechanical Digestion?
Mechanical digestion is simply the aspects of digestion achieved through a mechanism or movement. There are two basic types of mechanical digestion.
- Mastication: The first step when it comes to digestion actually begins as soon as food enters the mouth. Mastication (chewing) begins the process of breaking down food into nutrients. As a type mechanical digestion, chewing our food is an important part of the digestive process because smaller pieces are more readily digested through chemical digestion.
- Peristalsis: Mechanical digestion also involves the process known as peristalsis. Peristalsis is simply the involuntary contractions responsible for the movement of food through the esophagus and intestinal tracts.
What is Chemical Digestion?
Chemical digestion is much like it sounds – those aspects of digestion achieved with the application of chemicals to our food.
Digestive enzymes and water are responsible for the breakdown of complex molecules such as fats, proteins, and carbohydrates into smaller molecules. These smaller molecules can then be absorbed for use by cells.
The presence of these digestive enzymes accelerates the digestion process, where absence of these enzymes slows overall reaction speed. Currently, there exist eight digestive enzymes mainly responsible for chemical digestion. (The following are direct quotes from the online medical dictionary.)
- Nuclease: Any of a group of enzymes that split nucleic acids into nucleotides and other products.
- Protease: Any of various enzymes, including the proteinases and peptidases, that catalyze the hydrolytic breakdown of proteins.
- Collagenase: Any of various enzymes that catalyze the hydrolysis of collagen and gelatin.
- Lipase: Any of a group of lipolytic enzymes that cleave a fatty acid residue from the glycerol residue in a neutral fat or a phospholipid.
- Amylase: Any of a group of enzymes that catalyze the hydrolysis of starch to sugar to produce carbohydrate derivatives.
- Elastase: An enzyme capable of catalyzing the digestion of elastic tissue.
- Trypsin: A proteolytic digestive enzyme produced by the exocrine pancreas that catalyzes in the small intestine the breakdown of dietary proteins to peptones, peptides, and amino acids.
- Chymotrypsin: A proteolytic enzyme produced by the pancreas that catalyzes the hydrolysis of casein and gelatin.
The digestive system is also associated with many accessory organs responsible for producing an array of chemical enzymes: salivary glands, pancreas, liver, gallbladder.
How Long Does it Take to Digest Food?
The amount of time necessary to process food through mechanical and chemical digestion varies by individual circumstances. In healthy adults, this process can range from 24-72 hours but the average is about 36. Typically, after ingestion, food remains in the stomach and small intestine from 6-8 hours. The large intestine is capable of holding undigested food waste for days.
A Necessary Equilibrium: Mechanical and Chemical Digestion
There exists a natural equilibrium in your body for mechanical and chemical digestion. Mechanical digestion preps food for chemical digestion as smaller pieces are more readily broken down and absorbed. The enzymes necessary for proper chemical digestion are also in equilibrium. The accessory organs of the digestive system necessary for the production of these enzymes must all work in a harmonious fashion to accomplish proper digestion.