The digestion of the food we consume is a set of processes that transform food into nutrients that can be absorbed by the body. This transformation is generally studied in two main types, mechanical digestion and chemical digestion.
o Mechanical digestion It is the part of digestion that does not produce changes in the chemical composition of food. It involves only mechanical processes. Its purpose is to facilitate the passage of food through the digestive tract and the action of chemical digestion.
Among the mechanical digestion processes are crushing, transporting and mixing. Nutrient absorption can also be considered a part of mechanical digestion.
Chemical digestion, in turn, encompasses the processes that chemically transform food into substances that are absorbable and usable by the body.
Stages of mechanical digestion
Mechanical digestion occurs throughout the digestive tract simultaneously with chemical digestion.
Mechanical digestion has three main functions: crushing food, mixing with different digestive secretions, and transporting the food bolus along the digestive tract.
The most important steps of mechanical digestion are mastication, swallowing and peristalsis.
chewing It is a part of mechanical digestion that consists of grinding food.
Chewing takes place in the oral cavity and is done by the teeth, especially the molars, in coordination with the tongue and cheeks. Food is broken down into smaller, more digestible fragments.
Crushing increases the surface-to-volume ratio, leaving much more surface area exposed for enzymatic action later on.
During mastication, a reflex action produces an increase in salivation which lubricates the bolus which facilitates crushing and subsequent swallowing.
Saliva also contains enzymes that initiate the chemical digestion of certain nutrients, especially carbohydrates.
Swallowing is the process by which the bolus of food passes from the mouth to the pharynx and then into the esophagus while closing the epiglottis. In mechanical digestion, it involves transporting the bolus formed in the mouth to the stomach.
Swallowing occurs in three phases, the oral phase, the pharyngeal phase and the esophageal phase, each controlled by a different but coordinated neuromuscular mechanism.oral phase: This phase is voluntary. After chewing, the person pushes the bolus towards the pharynx with the help of the tongue. The medial temporal lobes and the limbic system are involved. Pharyngeal phase: It is coordinated by the swallowing center of the medulla oblongata. In this phase, the epiglottis rises by reflex action and closes the passage to the larynx so that the contents of the food bolus do not pass into the respiratory system. The soft palate also moves and closes the nasopharynx. Esophageal phase: The esophagus contracts and relaxes forming a wave-like movement that advances the food bolus to the stomach.
Peristalsis and segmentation
The movements that occur along the digestive tract during digestion are called peristalsis and it is essential for the bolus to move forward.
As mentioned above, peristaltic movements in the esophagus help the food bolus advance into the stomach.
In the stomach, peristaltic movements contribute to the mixing of the food bolus with gastric juices. This mixture is chyme.
Once the chyme is formed, it passes into the intestine, where again peristaltic movements make it move forward and mix with digestive secretions.
Two muscle groups are involved in bowel peristalsis: circular muscles and longitudinal muscles. They alternately contract and relax, causing the contents of the intestine to move forward.
In the intestine there are also movements called segmentation movements. While peristalsis moves the food bolus forward, segmentation movements move it back and forth, with the main objective of mixing the contents with the digestive secretions and absorbing the nutrients.
Differences between mechanical and chemical digestion
Unlike mechanical digestion, chemical digestion includes the processes that occur during digestion and that transform food on a chemical level.
Chemical digestion affects both structure and chemical composition and aims to convert the contents of food into substances that the body can absorb and utilize.
Chemical digestion and mechanical digestion can occur simultaneously in many parts of the digestive system.
Chemical digestion begins in the mouth by the action of enzymes that contain saliva. In the stomach, chemical digestion also occurs by enzymatic action and, above all, by the action of hydrochloric acid (HCl).
The combination of enzymes and low pH provided by hydrochloric acid also helps to kill bacteria and germs in food.
The chewed and salivated food that enters the stomach is called bolus and is called chyme when it leaves the stomach and passes into the intestine transformed by the action of hydrochloric acid and stomach enzymes.
In the intestine, the chyme receives three enzymatic secretions that will continue the chemical digestion. These secretions are bile, pancreatic juice, and enzymes secreted by the lining of the small intestine.
The chyme leaves the small intestine and passes into the large intestine, where most nutrients and water are absorbed. Waste is finally expelled through the anus in the form of feces.