The digestive system is a collection of organs that work together to digest and absorb food.
The gastrointestinal tract consists of a hollow muscular tube from the oral cavity. Food enters the mouth, continuing through the pharynx, esophagus, stomach, and intestines to the rectum and anus, where food is expelled. Waste.
Several accessory organs assist the tract by secreting enzymes to help break down food into its component nutrients.
Therefore, the salivary glands, liver, pancreas, and gallbladder have essential functions in the digestive system.
Food is propelled through the digestive tract by peristaltic movements of the muscle walls.
Monogastric organisms like humans have two digestive processes that occur in the digestive tract: mechanical and chemical digestion.
Once the food is ingested, the digestive process begins in the mouth with mechanical digestion. Here the teeth are used to cut, tear and grind pieces of food into smaller particles.
This chewing process involves the alternating action of the chewing muscles: the superficial and deep masseter, the pterygoids, and the temporal powers.
Chewing is a reflex action stimulated once the food is present in the mouth.
There is an inhibition of the chewing muscles that results in a drooping of the jaw. This causes distension of the chewing muscles, resulting in a reflex contraction of the muscle fibers, thus raising the jaw.
This action causes the apposition of the upper and lower rows of the teeth, crushing the food between them. The cycle repeats until the food particles can be turned into a bolus.
As the tongue turns the chewed food into a bolus, the salivary glands secrete saliva to moisten the bolus to pass smoothly into the stomach.
Some chemical digestion also occurs in the mouth, as saliva contains the enzyme amylase, which breaks down some carbohydrates in the mouth.
Ingestion and physical digestion
Once the bolus is prepared, swallowing will begin. This is another complex reflex arc that involves the action of the afferent and efferent tracts of various cranial nerves that are transmitted to and from the nucleus of the solitary tract and the heart ambiguous of the brainstem.
The nerves send motor signals to the tongue, which moves the bolus against the hard and soft palate, then towards the oropharynx (which is also under the regulation of the brainstem).
The bolus then continues down into the laryngopharynx, and the swallowing reflex begins in the esophagus.
All swallowing actions up to this point were under voluntary control. However, the remainder of the action is carried out by involuntary peristaltic contractions that move craniocaudally.
At the level of the pharynx, superiorly, the bolus is prevented from entering the nasopharynx by the actions of the Passavant crest.
This structure is formed by the joint actions of the palatopharyngeal sphincters, the superior constrictor muscles, the salpingopharyngeal, and the muscles of the soft palate.
The epiglottis closes the larynx to prevent food from entering the airways. The vocal cords are also adduced as an additional protective measure.
At the level of the esophagus, there is relaxation of the cricopharyngeal sphincter, and the bolus enters the proximal esophagus.
The presence of the bolus causes enlargement of the myenteric plexus within the esophagus walls, initiating the primary esophageal peristaltic wave.
The continuous presence of food stimulates secondary peristaltic waves in a craniocaudal direction.
These waves, along with the action of gravity, move the bolus toward the lower esophageal sphincter at a speed of four centimeters per second.
At rest, the sphincter has a high tone. However, the presence of the bolus helps the relaxation of the lower esophageal sphincter, and food can enter the stomach. This is where most of the chemical digestion will take place.
Once the bolus enters the stomach, there is a regulated release of various enzymes that facilitate chemical digestion.
Some enzymes also stimulate the accessory digestive organs to release their enzymes to aid digestion.
The stomach can be functionally divided into proximal and distal motor pumps, which store food content and pump chyme along the duct. Portions of chyme are passed to the pylorus and small intestine.
Once the chyme has entered the first part of the duodenum, it activates the neurohormonal axis that promotes the release of bile (from the liver and gallbladder) and other enzymes from the pancreas.
The peristaltic waves continue to move the chyme along the intestinal tract.
The intricate folding of the intestines facilitates the absorption of nutrients from the chyme. Most of the nutrients are absorbed in the small intestine. The debris is passed through the one-way ileocecal valve into the cecum.
As the peristaltic waves continue into the colon, the chyme moves along the tract. There is increased absorption of electrolytes and water from the remaining chyme, and the chyme is converted into the stool, which is stored in the rectum.
As the rectum stretches, the stretch receptors signal the brain that promotes defecation.
While the internal anal sphincters are under autonomous regulation, the external anal sphincters are under voluntary control. Therefore, the individual can resist the urge to defecate until an appropriate time and place are identified.
Individual components of the gastrointestinal system
The oral cavity or the mouth is responsible for food intake. It is lined by an oral squamous mucosa layered with keratin covering areas subject to significant abrasions, such as the tongue, hard palate, and palate.
Chewing refers to the mechanical breakdown of food by chewing and chopping the actions of the teeth. The tongue, a muscular, solid organ, manipulates the bolus to contact the teeth.
It is also the sensory organ of the mouth for touch, temperature, and taste using its specialized sensors known as papillae.
Insalivation refers to the mixing of the contents of the oral cavity with the secretions of the salivary glands. The mucin (a glycoprotein) in saliva acts as a lubricant.
The oral cavity also plays a limited role in the digestion of carbohydrates. The enzyme serum amylase, a component of saliva, initiates digesting complex carbohydrates.
The final function of the oral cavity is the absorption of small molecules such as glucose and water through the mucosa. From the mouth, food passes through the pharynx and esophagus through the action of swallowing.
Three pairs of salivary glands communicate with the oral cavity. Each is a complex gland with numerous acini lined by secretory epithelium.
The acini secrete their contents in specialized ducts. Each gland is divided into smaller segments called lobes.
Salivation occurs in response to the taste, smell, or even the appearance of food.
This occurs due to nerve signals that tell the salivary glands to secrete saliva to prepare and moisten the mouth.
Each pair of salivary glands secretes saliva with slightly different compositions.
The parotid glands are large, irregular glands located under the skin on the side of the face. They secrete 25% of the saliva.
They are located below the zygomatic arch (cheekbone) and cover part of the jaw (lower jaw bone). An enlarged parotid gland can be felt more easily when you clench your teeth.
The parotids produce a watery secretion that is also rich in protein.
Immunoglobins are secret to help fight microorganisms, and α-amylase proteins begin to break down complex carbohydrates.
The submandibular glands secrete 70% of the saliva in the mouth. They are found on the floor of the mouth, in a groove along the inner surface of the jaw.
These glands produce a more viscous (thick) secretion, rich in mucin and with less protein. Mucin is a glycoprotein that acts as a lubricant.
The sublingual is the minor salivary glands, covered by a thin layer of tissue on the floor of the mouth. They produce approximately 5% of saliva, and their secretions are very sticky due to the high concentration of mucin.
The main functions are to provide shock absorbers and lubrication.
The esophagus is a muscular tube approximately 25 cm long and 2 cm in diameter. It extends from the pharynx to the stomach after passing through an opening in the diaphragm.
The esophagus wall is composed of circular inner and outer longitudinal layers of muscle supplied by the esophageal nerve plexus. This nerve plexus surrounds the lower portion of the esophagus.
The esophagus functions primarily as a means of transport between compartments.
The stomach is an expanded J-shaped pouch located just to the left of the midline between the esophagus and the small intestine.
It is divided into four central regions and has two edges called major and minor curvatures.
The first section is the cardia surrounding the cardiac orifice where the esophagus enters the stomach.
The fundus is the upper, dilated portion of the stomach that contacts the left dome of the diaphragm.
The body is the most significant section between the bottom and the curved portion of the J.
This is where most of the gastric glands are found and where most of the food mixing occurs.
Finally, the pylorus is the curved base of the stomach. Gastric contents are expelled into the proximal duodenum through the pyloric sphincter.
The inner surface of the stomach contracts into numerous longitudinal folds called rugae.
These allow the stomach to stretch and expand when food enters. The stomach can hold up to 1.5 liters of material.
In addition to chemical digestion (particularly of proteins), the stomach also functions as:
- A storage point gradually releases its contents into the small intestine to allow adequate time for further digestion and absorption.
- A mixer for the mode of contraction and arrangement of the stomach mucosa results in further mixing of the food contents to form chyme.
- A tube because it essentially passes food from the esophagus to the small intestine.
- In immune defense, the acidic pH of the stomach helps dissolve invading pathogens before they can cause an infection.
- Other micronutrients such as iron (Fe), vitamin B12, and folate absorption are strongly regulated by the stomach.
Most of these functions are accomplished through the secretion of stomach juices by the gastric glands in the body and fundus.
Some cells are responsible for secreting acid, and others secrete enzymes to break down proteins.
The small intestine comprises the duodenum, jejunum, and ileum. It averages about 6 m in length, extending from the stomach’s pyloric sphincter to the ileocecal valve that separates the ileum from the cecum.
The small intestine is compressed into numerous folds and occupies a large proportion of the abdominal cavity.
The duodenum is the proximal C-shaped section that curves around the head of the pancreas.
The duodenum has a mixing function, combining the digestive secretions of the pancreas and liver with the contents expelled from the stomach.
The beginning of the jejunum is marked by a closed curve, the duodenal jejunal flexion.
It is in the jejunum where most of the digestion and absorption occur.
The final portion, the ileum, is the most extended segment and empties into the cecum at the ileocecal junction.
The small intestine performs most of the digestion and absorption of nutrients.
Partially digested food from the stomach is further broken down by enzymes from the pancreas and bile salts from the liver and gallbladder.
These secretions enter the duodenum at the ampulla of Vater. After further digestion, food components like protein, fat, and carbohydrates are broken down into small building blocks and absorbed into the body’s bloodstream.
The small intestine lining is made up of many permanent folds called circular folds.
Each plica has numerous villi (mucosal folds), and each villus is covered by epithelium with projecting microvilli (brush border). This increases the surface area for absorption by a factor of several hundred.
The mucosa of the small intestine contains several specialized cells. Some are responsible for absorption, while others secrete mucosal and digestive enzymes to protect the intestinal lining from digestive actions.
The large intestine is shaped like a horseshoe and extends around the small intestine like a frame. It consists of the appendix, the cecum, the ascending, transverse, descending, and sigmoid colon, and the rectum.
It is approximately 1.5 m long and 7.5 cm wide. The cecum is the expanded bag that receives material from the ileum and begins to compress food products into fecal material.
The food then travels through the colon. The colon wall is made up of several bags (haustra) that are held under tension by three thick bands of muscle (taenia coli).
The rectum comprises the last 15 cm of the large intestine. It expands to contain stool before passing through the anorectal canal into the anus. Thick muscle bands, known as sphincters, control the passage of stool.
The mucosa of the large intestine is devoid of villi that are seen in the small intestine. The surface of the mucosa is flat with several deep intestinal glands.
Numerous goblet cells line the mucous-secreting glands to lubricate the stool as it solidifies. The functions of the large intestine can be summarized as follows:
- The accumulation of unabsorbed material forms feces.
- Some digestion by bacteria. Bacteria are responsible for the formation of intestinal gas.
- Reabsorption of water, salts, sugar, and vitamins.
The liver is an organ located in the upper right quadrant of the abdomen. It is surrounded by a vital capsule divided into four lobes, namely the right, left, caudate, and square lobes.
The liver has several essential functions. It acts as a mechanical filter by filtering the blood that travels from the intestinal system.
Detoxifies various metabolites, including the breakdown of bilirubin and estrogen. In addition, the liver has synthetic functions, producing albumin and blood clotting factors.
However, its main functions in indigestion are the production of bile and the metabolism of nutrients.
All nutrients absorbed by the intestines pass through the liver and are processed before traveling to the rest of the body.
The bile produced by liver cells enters the intestines in the duodenum. Here, bile salts break down lipids into smaller particles, giving digestive enzymes more surface area to work.
The gallbladder is a hollow, pear-shaped organ that sits in a depression on the posterior surface of the right lobe of the liver.
It consists of a bottom, body, and neck. It empties through the cystic duct into the bile duct system.
The main functions of the gallbladder are the storage and concentration of bile. Bile is a thick fluid that contains enzymes to help dissolve fat in the intestines.
Bile is produced by the liver but is stored in the gallbladder until needed. Bile is released from the gallbladder by the contraction of its muscle walls in response to hormonal signals from the duodenum in the presence of food.
Finally, the pancreas is a pinkish-gray lobular organ behind the stomach. Its head communicates with the duodenum, and its tail extends to the spleen.
The organ is approximately 15 cm long with a long, thin body connecting the head and tail segments. The pancreas has exocrine and endocrine functions. Endocrine refers to the production of hormones that occur in Langerhans’ islets.
The islets produce insulin, glucagon, and other substances, which are the damaged areas in diabetes mellitus. The exocrine (secretory) portion constitutes 80 to 85% of the pancreas and is the relevant area for the gastrointestinal tract.
It comprises numerous acini (small glands) that secrete contents in the ducts that eventually lead to the duodenum.
The pancreas secretes fluid rich in carbohydrates and inactive enzymes. The secretion is caused by hormones released by the duodenum in the presence of food.
Pancreatic enzymes include carbohydrases, lipases, nucleases, and proteolytic enzymes that can break down different components of food.