Digestive system

Trigger and initiation

The function of the digestive system truly begins within the brain. Whenever the body’s energy stores (i.e. blood glucose, protein, or fat stores) fall below a set point, the hunger centres of the hypothalamus are activated. These centres regulate satiety (fullness) and appetite in order to maintain energy homeostasis. This signals to the brain that there is a need to obtain food. Keep in mind, however, that this is not the only source of hunger stimulation, as there are elements of pleasure and comfort that can be derived from ingesting food.

Mastication

Monogastric organisms like humans have two kinds of digestive processes occurring in the digestive tract – mechanical and chemical digestion. Once the desired food is obtained, the digestive process begins in the mouth with mechanical digestion. Here the teeth are used to cut, tear, and grind chunks of food into smaller particles. This process of mastication involves the alternating action of the muscles of mastication (namely, the superficial and deep masseter, the pterygoids, and the temporalis muscles).

Mastication is actually a reflex action that is stimulated once food is present in the mouth. At that point, there is inhibition of the muscles of mastication which results in a fall of the mandible. This causes distention of the muscles of mastication, resulting in reflex contraction of the muscle fibres; thus raising the mandible. That action causes apposition of the upper and lower rows of teeth, crushing the food that is between them. The cycle is repeated until the food particles can be rolled into a bolus.

As the tongue rolls the chewed food into a bolus, the salivary glands secrete saliva to moisten the bolus in order for it to pass smoothly to the stomach. Also note that some amount of chemical digestion also occurs in the mouth, as saliva contains the enzyme amylase, which breaks down some carbohydrates in the mouth.

Swallowing and physical digestion

Once the bolus is prepared, then swallowing will be initiated. This is another intricate reflex arc that involves the action of afferent and efferent tracts of several cranial nerves being relayed to and from the nucleus tractus solitarius and nucleus ambiguus of the brainstem.

The end result is that these nerves send motor signals to the tongue, which moves the bolus against the hard and soft palate, then into the oropharynx (which is also under brainstem regulation). The bolus then continues inferiorly towards the laryngopharynx and the swallowing reflex is initiated at the esophagus. All the actions of swallowing up to this point were under voluntary control; however, the rest of the action is carried out by involuntary peristaltic contractions that travel in a craniocaudal fashion. At the level of the pharynx:

• Superiorly, the bolus is prevented from entering the nasopharynx by the actions of Passavant’s ridge. This structure is formed by the joint actions of the palatopharyngeal sphincters, the superior constrictor muscles, salpingopharyngeus and the muscles of the soft palate.
• The epiglottis closes off the larynx to prevent food from entering the airway. The vocal cords are also adducted 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 distention of the myenteric plexus within the walls of the esophagus, initiating the primary esophageal peristaltic wave.
• The continued presence of the food stimulates the 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 rate of 4 cm/s. At rest, the sphincter has a high tone. However, the presence of the bolus aids relaxation of the lower esophageal sphincter, and food is able to enter the stomach. Here, the majority of the chemical digestion will take place.

Chemical digestion

Once the bolus enters the stomach, there is regulated release of a variety of enzymes that facilitate chemical digestion. Some of these enzymes also stimulate the accessory digestive organs to release their enzymes to aid in digestion. In addition to chemical digestion (particularly of proteins), the stomach also functions as:

• A storage point, which gradually releases its contents into the small intestines, to allow adequate time for further digestion and absorption.
• A mixer: the mode of contraction and arrangement of the stomach mucosa results in further mixing of the food contents to form chyme.
• A conduit: essentially passing food from the esophagus to the small intestines.
• Immunological defence: the acidic pH of the stomach aids in dissolving invading pathogens before they are able to cause an infection.
• Absorption of micronutrients: the absorption of some nutrients such as iron (Fe), vitamin B12 and folate is heavily regulated by the stomach.

The stomach can be functionally divided into proximal and distal motor pumps, which stores food content and pumps chyme along the conduit, respectively. Portions of chyme are passed into the pylorus and into the small intestines.

Once chyme has entered the first part of the duodenum, it activates the neurohormonal axis which 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 absorption of nutrients from the chyme. Majority of the nutrients are absorbed within the small intestines. The remnants are passed through the unidirectional ileocecal valve into the cecum.

As the peristaltic waves continue into the colon, the chyme continues to move along the tract. Further absorption of electrolytes and water from the remaining chyme occurs and the chyme is then converted into stool, which is stored in the rectum. As the rectum becomes distended, the stretch receptors signal to the brain that promotes defecation. While the internal anal sphincters are under autonomic regulation, the external anal sphincters are under voluntary control. Therefore the individual may resist the urge to defecate until an appropriate time and place is identified.

Buccal cavity

The buccal cavity marks the opening of the digestive tract. It is made up by the oral vestibule (space between the inner cheeks and the teeth) and the oral cavity proper (behind the teeth). The buccal cavity also contains the teeth and tongue. It is limited anteriorly by the lips and teeth and posterolaterally by the palatopharyngeal arches. Posteriorly the buccal cavity opens into the oropharynx. The roof is formed by the hard palate (at the anterior two-thirds) and the soft palate (at the posterior third), and the floor of the oral cavity also contains the tongue.

The teeth can be subdivided into incisors (designed for cutting), canines (designed for tearing), premolars, and molars (designed for grinding). The molars and premolars have complementary surfaces that also aid in grinding. The tongue consists of intrinsic and extrinsic muscles. It is also populated  with taste buds that facilitate gustatory sensation.

Pharynx

Posterior to the oral cavity proper is the oropharynx. This is the middle part of the pharynx that communicates superiorly with the nasopharynx and inferiorly with the laryngopharynx. The walls of the oropharynx are formed by the superior and middle pharyngeal constrictor muscles. Anterolaterally, the palatopharyngeal folds form a demarcation between the oral cavity proper and the oropharynx.

The base of the tongue also serves as another landmark in the anteroinferior aspect of the oropharynx. The mucosa of the walls also contains numerous mucosa associated lymphatic tissue (MALT). It can be separated from the nasopharynx by the muscles of Passavant’s ridge and the supporting structures of the soft palate during deglutition.  

Esophagus

The tubular conduit responsible for transferring food from the oropharynx to the stomach is the esophagus. It can be divided into three parts:

• cervical
• thoracic
• abdominal components

The total length of the muscular tube is 25 cm, commencing at the cricopharyngeus at the lower border of the cricoid cartilage at the sixth cervical vertebrae (C6). It journeys posteriorly to the trachea in the neck along its caudal course. It then travels through the superior, then posterior mediastinum alongside the thoracic vertebrae. It pierces the diaphragm at the tenth thoracic vertebra (T10). The remaining 2.5 cm of the esophagus is the abdominal part. It transitions into the stomach at the gastroesophageal junction, where the physiological lower esophageal sphincter exists.

Stomach

Within the abdominal cavity, the esophagus enters the stomach. This is a dilated area of the alimentary canal that participates in both mechanical and chemical digestion. It is divided into four main parts, namely the:

• fundus
• body
• antrum
• pylorus

Laterally, there is a greater curvature, and medially there is a lesser curvature. There are two distinct notches on the stomach. The first is the cardiac incisura formed at the lateral border of the cardioesophageal junction. The other is the less pronounced incisura angularis found distally at the caudal end of the lesser curvature (at the pyloric antrum). The pylorus is the distal aspect of the stomach that is thickened. It acts as a physiological sphincter that regulates the passage of chyme from the stomach to the beginning of the small intestines.

Duodenum

The duodenum marks the beginning of the small intestines. It is roughly 20 – 25 cm long, extending from the pylorus to the ligament of Treitz. Not only is the duodenum the shortest part of the small intestines, but it is also the widest. It can be subdivided into four parts based on its geometrical orientation. The first part is known as pars superioris (the superior part); it is roughly 2 – 3 cm long and travels above the head of the pancreas. The second part is pars descendens which commences behind the neck of the gallbladder. It travels about 8 – 10 cm lateral to the head to the pancreas.

The inferior duodenal flexure (where pars descendens begins to turn) marks the transition of the second part of the duodenum to the third part – pars horizontalis. It travels for roughly 10 cm before it begins to curve upwards into the final segment of the duodenum, the pars ascendens (which is only 2.5 cm long).

Jejunum

The transition from the duodenum to jejunum occurs at the ligament of Treitz. The difference in the luminal diameter of the jejunum and duodenum is an important distinguishing feature. The duodenum is significantly wider than the jejunum.

However, it may be more challenging to distinguish between the jejunum and ileum as there are no external anatomical landmarks to guide. The key distinguishing features are as follows:

• The external diameter of the jejunum (4 cm) is greater than that of the ileum (3.5 cm).
• The internal diameter of the ileum (2 cm) is also smaller than that of the jejunum (2.5 cm).
• The walls of the jejunum appear thicker than that of the ileum.
• Additionally, the jejunum appears more hyperaemic than the ileum because it has a more extensive vascular supply.
• The luminal surface of the jejunum is significantly folded into plicae circulares that are more numerous and appear deeper than anywhere else within the digestive tract. Furthermore, the plicae circulares become less abundant distally within the ileum.
• Finally, the luminal mucosa of the ileum has more prominent mucosa-associated lymphoid tissue (MALT) than the jejunum.

The ileum terminates at the ileocecal valve; which marks the transition from the small intestines to the large intestines. The ileocecal valve is a one way structure that prevents reflux of the bolus from the large intestines to the small intestines.

Colon

The proximal end of the large intestines – also known as the colon – is formed by a dilated cul-de-sac known as the cecum. There is also a vermiform appendix attached at variable parts of the cecum. The colon is divided into:

• ascending
• transverse
• descending
• sigmoid parts

Externally, the colon has a segmented appearance due to the haustrations that are present on the luminal surface of the conduit. The muscular layers of the colon are concentrated into three muscular bands known as taenia coli; which travel along the length of the colon. The three taenia are:

• Taenia libera is the free taenia that is found at the antimesenteric surface of the colon; which is located on the anterior surface of the colon.
• Taenia omentalis is located posterolaterally and is attached to the omentum of the large intestines.
• Taenia mesocolic is found at the midpoint between the taenia libera and the mesenteric attachment on the colon.

Importantly, the cecum does not have the prominent haustrations seen on the rest of the colon. As the ascending colon travels from the right iliac fossa superiorly, it transitions to the transverse colon at the hepatic (right colic) flexure. The transverse colon travels across from the right to the left hypochondriac regions. It turns caudally at the splenic (left colic) flexure to form the descending colon. As the descending colon travels from the left hypochondrium to the left iliac fossa, it transitions into the sigmoid colon. This distal segment enters the pelvic inlet and terminates at the rectosigmoid junction at the level of the third sacral vertebra (S3).

Rectum

Unlike the preceding colon, the rectum is circumferentially walled by smooth muscles. It does not have haustrations and is devoid of taenia coli. This distal continuation of the large intestines functions of a reservoir for stool, prior to excretion. It terminates at the level of the sacrococcygeal curvature. It passes over the pelvic diaphragm to form the anorectal junction.

Bowel mnemonic

The portion of the alimentary tract between the stomach and the anal canal is known as bowel and is formed of the following components:

• Duodenum
• Jejunum
• Ileum
• Appendix
• Colon
• Sigmoid
• Rectum

You can remember this easily if you use the mnemonic Dow Jones Industrial Average Closing Stock Report, where the first letter of each word corresponds to the first letter of a bowel component!

Anal canal

The final passageway through which undigested food and exfoliated mucosa will exit the body is called the anal canal. It continues from the anorectal junction and passes through the loop formed by the puborectalis muscle, which swings the anal canal anteriorly. Distally, the mucosa of the anal canal transitions from the columnar epithelium with goblet cells found throughout the colon, to the squamous epithelium of the perianal skin. This point is referred to as the anal verge.  

Salivary glands

The organs of the accessory digestive system have the principal role of synthesizing and secreting digestive enzymes to further break down food into nutrients. The salivary glands are paired structures in the oral cavity that secrete saliva and other enzymes that mix with the masticated food to form the bolus. There are three major salivary glands in the oral cavity:

• Parotid glands
• Submandibular glands
• Sublingual glands

Liver

Another important accessory digestive organ is the liver. It is located in the right upper quadrant of the abdomen, beneath the right hemidiaphragm. The liver has two anatomical lobes, but eight functional segments. In addition to producing bile to digest fats, all of the nutrients absorbed from the small intestines enter the liver via the hepatic portal venous system.

Gallbladder

From the liver, the nutrients are integrated into various catabolic processes and sent throughout the body. Most of the bile made by the liver is stored in the gallbladder. This muscular, sac-like organ that resides on the posterior surface of the liver drains its contents into the extrahepatic biliary tree following a fatty meal.

Pancreas

Finally, the pancreas is a retroabdominal organ that also provides enzymes for digestion. The head of the pancreas is found within the  C-shaped loop of the duodenum. The body extends superolaterally, behind the gastric antrum. The tail of the pancreas terminates at the hilum of the spleen.

Arterial supply

The vast majority of the digestive system is supplied by the abdominal aorta. In the abdominal cavity, the aorta gives off three major branches that, in succession, supply the derivatives of the foregut, midgut and hindgut:

• Celiac trunk, which after a short course further splits three into major branches; left gastric, common hepatic and splenic arteries. Via these branches, the celiac trunk supplies the foregut, including the abdominal part of the esophagus, stomach, upper 1/3 of the duodenum, liver, spleen, and pancreas.
• Superior mesenteric artery, which gives off the inferior pancreaticoduodenal, middle colic, right colic, ileocolic arteries, jejunal and ileal branches, and the marginal artery of Drummond. These branches supply the midgut, which includes the distal 2/3 of the duodenum, jejunum, ileum, cecum, appendix, ascending colon and proximal 1/3 of the transverse colon.
• Inferior mesenteric artery, which gives off the left colic, sigmoid, and superior rectal arteries. These branches provide blood to the hindgut, supplying the distal 1/3 of the transverse colon, descending colon, sigmoid colon, rectum and anus.

Innervation

The digestive system receives both sympathetic and parasympathetic innervation. Sympathetic innervation stems from the abdominopelvic splanchnic nerves. These nerves arise from the sympathetic trunk and include the thoracic and lumbar splanchnic nerves. The splanchnic nerves synapse with prevertebral ganglia (celiac, aorticorenal, superior mesenteric, and inferior mesenteric ganglia) which project postganglionic fibers to innervate the organs of the digestive system. Sympathetic innervation inhibits peristalsis, constricts blood vessels and redirects blood from the digestive system to the skeletal muscles and contracts the anal sphincters.

The parasympathetic supply to the digestive system stems from the vagus nerve (CN X) and pelvic splanchnic nerves. The vagus nerve supplies the digestive tract from the esophagus to the transverse colon, while the pelvic splanchnic nerves supply the descending colon, sigmoid colon and rectum. Parasympathetic innervation increases peristalsis, promotes secretion of digestive juices, relaxes the internal anal sphincter and contracts the rectum.

Venous drainage

There are two venous systems that drain the organs of the digestive tract; portal venous system and the systemic venous system.

In the portal venous system, the nutrient-rich blood from the digestive tract is drained by the hepatic portal vein into the liver to be filtered and detoxified. The hepatic portal vein is formed by the merger of the superior mesenteric vein and the splenic vein.

After being processed by the liver, the blood is carried into the systemic venous system by the hepatic veins, which drain into the inferior vena cava.