FOOD BOLUS: WHERE AND HOW IT IS FORMED, TOUR - ANATOMY AND PHYSIOLOGY - 2024

The food bolus is the substance that is formed in the digestive process when food is received by the mouthparts, and is crushed by them. In this step, the action of various enzymes in saliva that help the degradation of the consumed matter also stand out.

By grinding the food, the surface-to-volume ratio of the particles increases. By having more surface exposed, it is easier and more efficient for subsequent enzymes to degrade the bolus.

The food bolus is formed in the early stages of digestion. In the image you can see the complete system that orchestrates the passage of food and allows the extraction of nutrients.

Source: Mariana Ruiz (English version); User: Bibi Saint-Pol, Jmarchn (Spanish version, translation by User: AlvaroRG)

As the digestive process progresses, the bolus undergoes various changes in its properties. These changes - caused mainly by chemical and mechanical digestion - are necessary for maximum extraction of nutrients.

When the bolus reaches the stomach and combines with digestive juices, it is called chyme. Likewise, when the chyme mixes with the substance of the duodenum in the small intestine, it becomes chyle.

Where and how is the bolus formed?

One of the most relevant topics in animal physiology is understanding how food is processed by living beings and how they are able to absorb nutrients in the diet. One of the initial steps in the digestion of food is the formation of the food bolus.

In animals, the reception of food occurs through the cephalic tract of the body. This is located in the cranial region of the digestive tract and provides an opening to the outside, allowing the entry of food. In humans, food is received by mouth.

The cephalic tract is a group of organs formed by specialized structures for the capture and swallowing of food. The parts of the mouth or teeth, salivary glands, the oral cavity, tongue, pharynx and other associated structures make up the basic elements of the reception.

When food enters, it is crushed by the teeth and the matter mixes with enzymes that hydrolyze the components. This is how the bolus is formed.

Carnivores and birds

Depending on the animal group studied, the cephalic tract has adaptations that correspond to the diet of the members. For example, large, sharp canines and beaks are adaptations of the cephalic tract in carnivores and birds, respectively.

Saliva

During bolus formation, saliva is a critical component of the process. Therefore, we will delve a little more into its composition and its work.

In mammals - including humans - saliva is secreted by three pairs of salivary glands. These are located in the oral cavity and are classified according to their position into parotid, submaxillary and sublingual. This secretion is rich in enzymes such as amylase and lipase.

The chemistry of saliva depends on the group and diet of the animal. For example, certain animals have toxins or anticoagulants. In blood-feeding animals, these serve to promote fluid flow during the feeding process.

In addition to promoting the digestion of the macromolecules that make up food, saliva works as a lubricant that facilitates the process of swallowing the bolus. Also, the presence of mucus (a substance rich in mucin) provides additional help.

The secretion of saliva is a process coordinated by the consumption of food itself. The senses of taste and smell also play a very important role in this production. The salivary glands produce saliva under stimuli from the sympathetic and parasympathetic systems.

Travel

Once the body has crushed the food with its teeth and the matter has been mixed with saliva, the process of swallowing or swallowing the bolus occurs. In chordates - including humans - this step is assisted by the presence of a language.

Pharynx and esophagus

The pharynx is a tube that connects the oral cavity with the esophagus. When the food bolus passes through this channel, a series of reflex mechanisms are activated that come from the passage of the ground food to the respiratory channel.

The esophagus is the structure that is responsible for conducting the food bolus from the cephalic tract to the posterior areas of the digestive system. In certain animals, this transport is assisted by a series of peristaltic movements from the oral cavity or the pharynx.

Other animals have additional structures that participate in the conduction of food. For example, in birds we find the crop. This consists of a wider sac-like region that is used primarily for food storage.

Stomach

A large number of animals carry out the digestive processes of the food bolus in an organ called the stomach. This structure has the function of storage and enzymatic digestion of food.

In vertebrates, degradation occurs in the stomach thanks to an enzyme called pepsin and hydrochloric acid. This significantly acidic environment is necessary to stop the activity of enzymes.

The stomach also contributes to mechanical digestion, presenting a series of movements that contribute to the mixing of food and gastric preparations.

Depending on the animal species, the stomach can present in multiple forms, being classified according to the number of compartments into monogastric and digastric. Vertebrates generally have stomachs of the first type, with a single muscular sac. Stomachs with more than one chamber are typical of ruminants.

In some species of birds - and very few of fish - there is an additional structure called the gizzard. This organ is very powerful and muscular in nature.

Individuals ingest rocks or similar elements, and store them in the gizzard in order to facilitate the grinding of food. In other groups of arthropods there is a structure analogous to the gizzard: the proventriculus.

Small intestine

When the passage through the stomach is finished, the processed nutritional material continues its journey through the middle tract of the digestive system. In this section the absorption events of nutrients occur, including proteins, fats and carbohydrates. Upon absorption, they pass into the bloodstream.

Food leaves the stomach through a structure called the pyloric sphincter. Relaxation of the sphincter allows the processed food to enter the first portion of the small intestine, called the duodenum.

In this stage, the pH of the process changes drastically, going from an acid environment to an alkaline one.

Duodenum

The duodenum is a relatively short portion and the epithelium secretes mucus and fluids from the liver and pancreas. The liver is the producer of bile salts that emulsifies fats and increases the pH of processed food.

The pancreas produces pancreatic juices rich in enzymes (lipases and carbohydrases). This secretion also participates in the neutralization of the pH.

Jejunum and ileum

Then, we find the jejunum, which is also attributed secretory functions. In this second portion of the small intestine absorption occurs. The last one, the ileum, is focused on the absorption of nutrients.

Large intestine

Digestive enzyme secretion does not occur in the large intestine. The secretion of substances is mainly focused on the production of mucin.

The colon (a term used to refer to the large intestine) performs a series of movements, where the semi-solid material that comes from the small intestine can mix with these secretions of the colon.

Microorganisms that live in this region also participate (those that survive the extreme conditions of passage through the stomach).

Food can stay in the colon for a significant time, between 3 and 4 hours, on average. This time encourages the fermentation processes by microorganisms. Note how the lack of hydrolytic enzymes in the colon is compensated for by these small inhabitants.

Bacteria not only participate in fermentation processes; they also participate in the production of vitamins for the host organism.

Defecation

After fermentation and the breakdown of other components, the large intestine fills with matter that was not digested. Furthermore, feces are also rich in bacteria and epithelial cells. The characteristic color of the stool is attributed to the pigment urobilin, a derivative of bilirubin.

The accumulation of stool in the rectum stimulates a series of receptors that promote the defecation process. In humans, the pressure in the system should be around 40 mmHg to stimulate the defecation reflex. Finally, the stool comes out of the anal opening. With this last step the route of the food bolus culminates.

Differences with chyme

As the bolus moves down the digestive system, it undergoes a series of physical and chemical changes. Due to these modifications, the name of the partially processed food substance changes its name. As we mentioned, the food bolus comprises the mixture of food with gastric enzymes and mucus.

When the food bolus reaches the stomach, it mixes with more enzymes and the acidic gastric juices of the organ. At this point, the bolus takes on a semi-liquid, paste-like consistency and is called chyme.

Differences with chyle

The chyme follows the path that we relate When it enters the first portion of the small intestine, the duodenum, it mixes with a number of basic chemicals. At this point in digestion, a liquid mixture is formed that we will call chyle.

Note that the terminology of food bolus, chyme and chyle, seeks to describe the passage of food in different stages of digestion and not to different components. It is a temporary differentiation.

References

1. Anta, R. & Marcos, A. (2006). Nutriguía: manual of clinical nutrition in primary care. Editorial Complutense.
2. Arderiu, XF (1998). Clinical biochemistry and molecular pathology. Reverte.
3. Eckert, R., Randall, R., & Augustine, G. (2002). Animal physiology: mechanisms and adaptations. WH Freeman & Co.
4. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology. McGraw-Hill.
5. Hill, RW, Wyse, GA, Anderson, M., & Anderson, M. (2004). Animal physiology. Sinauer Associates.
6. Rastogi, SC (2007). Essentials of animal physiology. New Age International.
7. Rodríguez, MH, & Gallego, AS (1999). Nutrition treaty. Díaz de Santos editions.