DIGESTIVE SYSTEM: PARTS, FUNCTIONS, DISEASES - ANATOMY AND PHYSIOLOGY - 2024

The digestive system is made up of all the organs involved in the process of acquisition, processing, digestion and absorption of nutrients in food, in addition to mediating the release of waste substances.

The main structures that make up the digestive system are the components of the mouth, salivary glands, esophagus, stomach, pancreas, liver, gallbladder, small intestine, large intestine, and anus.

By User: Arcadian (This file was derived from: Illu dige tract.jpg), via Wikimedia Commons

These hollow organs are connected in the form of a chain, mediating the passage of food, which undergoes various modifications as it progresses through the digestive system.

In general, the digestive tract is a structure flanked by two openings to the outside and formed by sphincters that modulate the entry and exit of materials. At the beginning of the digestive process, the ingested food comes into contact with mechanical, chemical and bacterial forces.

After the first stage of treatment, the nutritive material passes through the canal and is mixed with the enzymes of the digestive juice. With proper processing, food can be absorbed and nutrients are carried into the circulatory system. Waste products are eliminated in a controlled manner in the phenomenon of defecation.

The digestive system varies widely depending on the animal group and its trophic habits.

Feeding types

In the animal kingdom, the way of obtaining food and assimilating it is extremely varied. There are organisms - such as aquatic invertebrates, protozoa, and parasites - that can absorb food through the surface of your body, without the help of specific organs. The process consists of capturing the nutrients that are in their environment.

The absorption of nutritive molecules through the surface of the body can be carried out by endocytosis, where the cell takes up solid (phagocytosis) or liquid (pinocytosis) molecules. During this process, the cell encompasses the particle and forms a vesicle.

There are aquatic animals that can feed by filtration, capturing the food that is diluted in the aqueous environment. They generally consume phytoplankton or zooplankton. These animal lineages include sea sponges, brachiopods, tunicates or sea squirts, among others.

As animal complexity increases, specialized structures emerge for uptake and digestion of food.

Some have a liquid diet and focus on absorbing nutrients. Among these groups are hematophages (animals that feed on blood), some worms, arthropods and some chordates such as lampreys, hagfish and some bats.

Types of digestive systems

Physiologically, digestive systems can fall into three categories: batch reactors, where there are cavities that capture food and remove waste before the next “batch” of food arrives. In this type of tube, there is a single opening that allows the entry and expulsion of the material.

Another group are the ideal continuous flow stirred tank reactors, which work in the following way: the system receives the feed and, contrary to the previous case, it can do so continuously. The food is transformed into a mass that after being processed is eliminated when the cavity is already full.

Finally, there are the flow bolus reactors, where the "bolus" refers to a discrete portion of food that is processed and digested as it progresses through the digestive tract. In vertebrates, the small intestine works in this way.

The types of digestive systems are not mutually exclusive. There are animals that combine more than one strategy in their organs.

Parts of the digestive system (organs)

The term "digestion" can refer to intracellular digestion, which is carried out by digestive enzymes, or extracellular digestion, where the process is carried out by true organs dedicated to the assimilation and absorption of nutrients.

One of the most outstanding characteristics of the digestive tract is its ability to contract, called motility.

This property of movement occurs due to the presence of musculature. Thanks to these movements, the ingested matter can move throughout the tube, while it is mechanically crushed and mixed with gastric juices.

From the functional and structural point of view, the digestive tubes can be divided into four regions: cephalic, anterior, middle and posterior tracts, where the phenomena of food reception, storage, digestion and absorption of nutrients and water occur.

In general, the organs involved in the digestion of vertebrates are the following:

Cephalic tract

Mouth

This area is located in the skull of individuals and is responsible for receiving food. It consists of an opening to the outside through which the nutritive material enters. It is made up of specific pieces that can capture food, call it the mouth, oral cavity, pharynx, teeth, tongue and salivary glands.

If there is a common route through which the food enters, and gas exchange also occurs, there must be structures that function as valves to direct the ingested food and air to the appropriate channels.

Language

The tongue is a muscular and voluminous organ that participates in the process of swallowing food, previously chewed. In it there are a series of chemical receptors - the taste buds - that actively participate in the taste system and react before the flavors of food.

Salivary glands

The salivary glands are responsible for the secretion of saliva, a substance that helps lubricate the passage of food. Saliva also contains digestive enzymes that contribute to the fractionation and processing of consumed material.

Among these enzymes is α-amylase, involved in the degradation of carbohydrates and lipases that participate in the digestion of lipids. In addition, saliva is rich in immunoglobulins and lysozymes.

Anterior tract: the esophagus

The main functions of the anterior tract are the conduction, storage and digestion process of food. It is made up of two structures: the esophagus and the stomach.

The function of the esophagus is the conduction of food - now called a food bolus - from the cephalic region to the stomach. In certain animals it may have storage functions.

The esophagus is about 25 centimeters long and has a sphincter that connects with the stomach and allows food to pass through. This contractile structure prevents stomach contents from returning to the esophagus.

Anterior tract: the stomach

The stomach, along with the middle tract, is the physical region where most of the digestion takes place. In this organ, the enzymatic secretion of pepsinogen and hydrochloric acid occurs, creating an environment due to acidic pH that generates the activation of pepsin.

Likewise, the stomach can mechanically contract and mix food. There are different types of stomach, depending on the diet of the animal. When food reaches the stomach it is transformed into chyme (previously called bolus).

In humans, the stomach is located in the abdominal region towards the left side, under the diaphragm. It consists of four parts: the cardia is the region that joins the esophagus, follows the upper portion called the fundus and the central region called the body. The antrum is the lower region and finally there is the pylorus, which communicates with the duodenum.

Thin mid-instinctum tract

The middle tract is made up of the small intestine, which is divided into three parts: the duodenum, the jejunum, and the ileum.

The first portion is a relatively short area and is responsible for secreting fluid and mucus, as well as receiving secretions from the ducts from the liver and pancreas.

Liver cells produce bile salts, which are responsible for emulsifying fats and neutralizing acidity derived from the stomach.

The pancreas produces pancreatic juice, rich in enzymes such as lipases and carbohydrases that are essential for proper digestion and, like bile, helps the neutralization process.

The jejunum participates in the processes of digestion and absorption and also secretes fluids. The last section, the ileum, is primarily responsible for the absorption of nutrients.

The intestine is an area that favors symbiotic relationships with different types of organisms, such as protozoa, fungi and bacteria, which contribute to the processing and digestion of ingested material. In addition, many of these organisms have an important role in the synthesis of vitamins.

The structure of the intestinal epithelium contributes to the amplification of the surface that will absorb nutrients.

Posterior tract-Thick instinct

The posterior tract is responsible for the absorption of ions and water for their return to the blood, in addition to directing the processes of storage and elimination of waste. It is made up of the large intestine or colon and, as its name indicates, it has a larger diameter than the thin one.

This region plays an important role in bacterial digestion, harboring a large number of microorganisms, particularly in mammals with herbivorous feeding.

The number of bacteria is particularly abundant in the first half of the structure. The colon behaves like a modified flow bolus reactor.

Rectum and anus

The final part of the colon is wider and is called the rectum, this area serves as a reservoir for fecal matter. The process ends with the voluntary act of defecation, through the anus, which functions as a valve.

Features

All organisms require energy to be able to maintain their complex and highly ordered structure. This energy must be extracted from the chemical bonds in food.

The digestive system is made up of a series of organs directly related to the process of digestion of food and absorption of nutrients, such as carbohydrates, proteins and lipids.

Two main functions of the digestive system can be mentioned: the transformation of food into substances that are easily absorbed by the body and the taking of these nutritional products, which are transported to different tissues of the body.

To fulfill these functions, the digestive system requires the nervous impulse, the presence of digestive enzymes and the secretion of substances such as bile salts, peptides, amines, among others.

The digestive system of many animals is a region inhabited by microscopic organisms that contribute to the digestive process.

Finally, the digestive system is responsible for eliminating from the body the substances that were not absorbed in the digestion process and the waste generated in the oxidation of food, through the formation and expulsion of fecal matter.

How does it work? (Digestive process)

Swallowing and transport to the stomach

The digestive process begins with the reception of food and the swallowing of it through the parts of the mouth, adequately lubricated thanks to the secretions of the salivary glands.

The food is mechanically crushed by the teeth and its manipulation in the mouth is assisted by the tongue.

The chemical digestion process - particularly the breakdown of carbohydrates - occurs thanks to the presence of enzymes in saliva. When food is swallowed, these enzymes can continue to act until they are denatured by the acidic pH of the stomach.

After the food is swallowed, the tongue pushes it into the pharynx, where the nasal cavity is closed thanks to the soft palate. Upon reaching the esophagus, peristaltic movements direct the material to the stomach. The act of swallowing is voluntary due to the presence of muscles in the initial regions of the esophagus.

The early stages of digestion occur in the stomach, where food is stored and mixed with digestive juices.

Digestion in the stomach

The material enters the stomach through the cardiac sphincter, where peristaltic movements allow filling, approximately every three minutes in humans.

This “J” shaped organ has glands that secrete about two liters of gastric juice a day. The secretions are mucus, pepsinogen, and hydrochloric acid, produced by goblet cells, main cells, and parietal cells, respectively.

Pepsinogen is a zymogen, which means that it is a precursor to an enzyme and is not yet ready to catalyze. Pepsinogen gives rise to pepsin - an enzyme capable of hydrolyzing proteins into small polypeptides - when the environment is acidic.

Accompanying pepsin, there are a series of enzymes capable of contributing to the degradation of proteins found in food.

There is a small volume of gastric juices that are secreted continuously, however the presence of food (either by visual or olfactory stimuli) increases secretion.

The intestinal mucosa is not digested by the acids that it produces, thanks to the secretion of mucous substances that protect it from chemical and mechanical destruction.

I pass through the small intestine

The intestines are specialized structures for the digestion of food and for the absorption of nutrients. It consists of tubes whose length can exceed up to eight times the length of the organism that possesses them.

They have a series of villi, which in turn have microvilli, which contribute to an increase in the absorption surface of molecules. These projections increase the absorption area about a thousand times, compared to the area of ​​a simple cylinder.

The intestine is impervious to polysaccharides, so the absorption of carbohydrates occurs mainly as monosaccharides (call it glucose, fructose, galactose, among others). Likewise, proteins are absorbed in the form of amino acids, although absorption of small peptides may also occur.

Absorption is a process mediated, for the most part, by active transporters anchored in the epithelial cells, which are responsible for transporting nutrients to the bloodstream. In contrast, fats are emulsified by bile salts and then digested by pancreatic lipases.

Triglycerides are split into smaller components, such as fatty acids and monoglycerides, which, when in contact with salts, become micelles that can be absorbed by simple diffusion.

Bile and pancreatic juices

Food enters the small intestine through the pyloric sphincter. In the initial segment of this intestine, food mixes with the secretions of the pancreas and with bile. These secretions are high in sodium bicarbonate, which manages to increase the pH from 1.5 to 7.

The change in pH is necessary, since the optimal pH at which intestinal enzymes work is neutral or slightly alkaline.

The liver secretes bile through the bile duct, which are essential for the digestion of fats. The typical color of bile is greenish yellow and is the product of the breakdown of hemoglobin. Similarly, the pigments produced in the bile are responsible for the color of the stool.

Pancreatic juices are rich in different enzymes, such as trypsin and chymotrypsin, which are capable of cleaving proteins at specific sites.

It also has: carboxypeptidases that can remove amino acids from the carboxyl terminal; pancreatic lipases that participate in the hydrolysis of lipids; pancreatic amylase that hydrolyzes starch and nucleases that degrade nucleic acids into their structural components, nucleotides.

Passage through the large intestine

In the large intestine the remnants of digestion are located and the reabsorption of water occurs, to form a solid or semi-solid substance that will be expelled from the body in the form of feces.

The colon is the habitat of an immense number of bacteria that contribute to the process of digestion. In fact, in humans more than a third of the dry weight of feces corresponds to bacteria.

Digestive tract and its layers

In the digestive tract it is made up of four layers: mucosa, submucosa, muscular and serous. The outer layer is called serous and is the same tissue that makes up the visceral organs located in the abdomen.

The serous layer is superimposed on an inner layer of circular smooth muscle, in turn an epithelial layer of connective fibrous tissue and mucous membrane forms the submucosa and mucosa, respectively. The mucous layer is in direct contact with food.

Towards the interior of the tube are a significant number of circular folds, called Kerckring folds, which increase the surface area and delay the passage of food through the intestine, thus increasing the time it spends in the digestive tract.

At a more detailed anatomical level, we find the villi that are located on the edge of the folds and one of them has invaginations called Lieberkühn crypts.

The villi have blood vessels, arterioles, capillaries, venules, and lymphatic vessels. When nutrients pass through the intestine, they are transferred to this system to be transported to other tissues in the body.

The apical surface of each absorptive cell has structures called microvilli that form the so-called “brush border”.

Common diseases

Pathologies related to the digestive system have a high frequency in the human population. They can be annoyances that do not lead to serious risks, such as flatulence, which according to surveys is present in up to 30% of the healthy population.

Likewise, gastroesophageal reflux is also quite common and more than a third of the population has reported this condition at least once a month, and 5 to 7% present it daily.

The rest of the diseases related to the digestive tract have a varied prevalence, from 0.1% for celiac disease, to 10-80% for lactose intolerant.

Celiac Disease

Celiac disease is a disorder that involves the digestive system and the immune system. It lies in an intolerance to gluten (small proteins present in cereals) and the symptoms are widely variable.

Lactose intolerance

With regard to lactose intolerance, it is a pathology where the body does not have lactase, an enzyme necessary for the processing of the sugar present in milk.

Symptoms include bloating, flatulence, and diarrhea. Therefore, people who suffer from it should avoid dairy consumption.

Gastritis

Gastritis is another common pathology that consists of inflammation of the gastric mucosa, caused by infections (generally Helicobacter pylori), excessive consumption of alcohol, certain foods or drugs.

Cancer

The organs that make up the digestive system are prone to the development of different types of cancer, including cancer of the colon, esophagus, stomach, pancreas and liver. Causes range from infections and genetic predisposition to inappropriate lifestyles.

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. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology. McGraw-Hill.
4. Hill, RW, Wyse, GA, Anderson, M., & Anderson, M. (2004). Animal physiology. Sinauer Associates.
5. Randall, D., Burggren, WW, Burggren, W., French, K., & Eckert, R. (2002). Eckert animal physiology. Macmillan.
6. Rodríguez, MH, & Gallego, AS (1999). Nutrition treaty. Díaz de Santos editions.