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By the end of the section, you will be able to:
- Identify the organs of the digestive tractfrom proximal to distal, and briefly state their functions
- Identify the accessory digestive organs and briefly state their primary function
- Describe the four fundamental tissue layers of the digestive tract
- Contrast the roles of the enteric and autonomic nervous systems in digestive system functioning
- Describe the structure and function of the peritoneum and mesenteries
The function of the digestive system is to break down the foods you eat by secreting enzymes to mix with food, release their nutrients, and absorb those nutrients into the body. Although the small intestine is the workhorse of the system, where the majority of digestion occurs, and where most of the released nutrients are absorbed into the blood or lymph, each of the digestive system organs makes a vital contribution to this process. Figure \(\PageIndex{1}\)shows all the organs of the digestive tract, a long tube that starts with the oral cavity, to the esophagus, stomach, small intestine, large intestine, and ending at the anus. The picture also shows some accessory digestive organs, such as the salivary glands, liver, gallbladder, and pancreas.
Digestive System Organs
The easiest way to understand the digestive system is to divide its organs into two main categories. The first group is the organs that make up the digestive tract, also known as the alimentary canalor gastrointestinal (GI) tract, the long tube through which food passes. Accessory digestive organs comprise the second group and are critical for orchestrating the breakdown of food and the assimilation of its nutrients into the body by releasing exocrine secretions into the tract. Accessory digestive organs, despite their name, are critical to the function of the digestive system.
Digestive Tract Organs
The digestive tract, gastrointestinal (GI) tract or gut,is a one-way tube about 7.62 meters (25 feet) in length during life and closer to 10.67 meters (35 feet) in length when measured after death, once smooth muscle tone is lost. The main function of the organs of the tract is to nourish the body. This tube begins at the oral cavity and terminates at the anus. Between those two points, the canal is modified as the pharynx, esophagus, stomach, and small and large intestines to fit the functional needs of the body (Figure \(\PageIndex{2}\)). In this system, relative locations closer to the oral cavity are considered “proximal” and further from the oral cavity (closer to the anus) are considered “distal”. Both the oral cavity and anus are open to the external environment; thus, food and wastes within the tract are technically considered to be outside the body. Only through the process of absorption, moving from digestive organs into the blood, do the nutrients in food enter into and nourish the body’s “inner space.”
Accessory Organs
Each accessory digestive organ aids in the breakdown of food (Figure \(\PageIndex{3}\). Within the oral cavity, the teeth and tongue begin mechanical digestion, whereas the salivary glands begin chemical digestion. Once food products enter the small intestine, the gallbladder, liver, and pancreas release secretions—such as bile and enzymes—essential for digestion to continue. Together, these are called accessory organs because they sprout from the lining cells of the developing gut (mucosa) and augment its function; indeed, you could not live without the vital contributions from the liver and pancreas, and many significant diseases result from their malfunction. Even after development is complete, they maintain a connection to the gut by way of ducts.
Digestive Organs Summary
| Tract Organ | Accessory Organ |
| Oral Cavity- Site of mechanical digestion; start of chemical digestion. | Salivary Glands- Produce saliva that empties into oral cavity. |
| Esopuagus- Takes food to stomach. | |
| Stomach- Chemical digestion of protein and mechanical breakdown of food. | |
| Small Intestine- Finish chemical digestion and absorbs food. 3 parts
| Liver- Produces bile that emulsifies fat. Empties into duodenum Pancreas-Produces pancreatic juice which contains enzymes to digest lipids, proteins, and starch. Empties into duodenum |
| Large Intestine - Bacteria ferment soluble fiber. 5 parts
| |
| Rectum- Collects fecal matter. |
Histology of the Digestive Tract Organs
Throughout its length, the digestive tract is composed of the same four tissue layers; the details of their structural arrangements vary to fit the specific functions of each organ or region. Starting from the lumen and moving outwards, these layers are the mucosa, submucosa, muscularis, and serosa (or adventitia). Figure \(\PageIndex{4}\) also shows the blood vessels and nerves sandwiched between the two serosal layers called the mesentery. More details about mesentery are found in upcoming paragraphs in this section.
Mucosa
The mucosa is referred to as a mucous membrane, because mucus production is a characteristic feature of gut epithelium. The membrane consists of epithelium, which is in direct contact with ingested food, and a layer of areolar connective tissue (Figure \(\PageIndex{5}\)). In addition, the mucosa has a thin, smooth muscle layercalled the muscularis mucosa (not to be confused with the muscularis layer, described below).
- Endothelium:In the oral cavity, pharynx, esophagus, and anal canal, the epithelium is primarily a non-keratinized, stratified squamous epithelium. In the stomach and intestines, it is a simple columnar epithelium. Notice that the epithelium is in direct contact with the lumen, the space inside the tract. Interspersed among its epithelial cells are goblet cells, which secrete mucus and fluid into the lumen, and enteroendocrine cells, which secrete hormones into the interstitial spaces between cells. Epithelial cells have a very brief lifespan, averaging from only a couple of days (in the oral cavity) to about a week (in the gut). This process of rapid renewal helps preserve the health of the tract organs,despite the wear and tear resulting from continued contact with foodstuffs.
- Lamina propria:In addition to areolar connective tissue, the lamina propria contains numerous blood and lymphatic vessels that transport nutrients absorbed through the tract to other parts of the body. The lamina propria also serves an immune function by housing clusters of lymphocytes, making up the mucosa-associated lymphoid tissue (MALT). These lymphocyte clusters are particularly substantial in the distal ileum where they are known as Peyer’s patches. When you consider that the digestive tract organs areexposed to food borne bacteria and other foreign matter, it is not hard to appreciate why the immune system has evolved a means of defending against the pathogens encountered within it.
- Muscularis mucosa:This thin layer of smooth muscle is in a constant state of tension, pulling the mucosa of the stomach and small intestine into undulating folds. These folds dramatically increase the surface area available for digestion and absorption.
Submucosa
As its name implies, the submucosa lies immediately beneath the mucosa. A broad layer of areolar connective tissue ofvarying thickness, it lies between the muscularis mucosa and the muscularis (Figure \(\PageIndex{6}\)). It includes blood and lymphatic vessels (which transport absorbed nutrients), and a scattering of submucosal glands that release digestive secretions. Additionally, it serves as a conduit for a dense branching network of nerves, the submucosal plexus, which functions as described below.
Muscularis
The third layer of thedigestive tractis the muscularis (also called the muscularis externa). The muscularisis made up of a double layer of smooth muscle: an inner circular layer, forming a ring around the tube, and an outer longitudinal layer that runs the length of the tube (Figure \(\PageIndex{6}\)). The contractions of these layers promote mechanical digestion, expose more of the food to digestive chemicals, and move the food along the canal. In the most proximal and distal regions of the tract, including the oral cavity, pharynx, superior part of the esophagus, and external anal sphincter, the muscularis is made up of skeletal muscle to giveyou voluntary control over swallowing and defecation. The basic two-layer structureis modified in the stomach and large intestine. The stomach is equipped for its churning function by the addition of a third layer, the deep oblique muscle. While the large intestine has two layers, its longitudinal layer is segregated into three narrow parallel bands, the teniae coli, which make it look like a series of pouches rather than a simple tube.
Serosa or Adventitia
The serosa is the portion of the digestive tractsuperficial to the muscularis. Present only in the region of the digestive tractwithin the abdominal cavity, it consists of a layer of visceral peritoneum overlying a layer of loose connective tissue. Instead of serosa, the oral cavity, pharynx, and esophagus have a dense sheath of collagen fibers called the adventitia. These tissues serve to hold the tract organsin place near the ventral surface of the vertebral column.
Nerve Supply
As soon as food enters the oral cavity, it is detected by receptors that send impulses along the sensory neurons of cranial nerves. Without these nerves, not only would your food be without taste, but you would also be unable to feel either the food or the structures of your oral cavity. Without these nerves, you would could bite yourself as you chew, an action enabled by the motor branches of cranial nerves.
Intrinsic (within) innervation of much of the digestive tract is provided by the enteric nervous system, which runs from the esophagus to the anus, and contains approximately 100 million motor, sensory, and interneurons (unique to this system compared to all other parts of the peripheral nervous system). The enteric nervous system is the largest component of the peripheral nervoussystem and because it has interneurons is frequently called the "second brain"! These enteric neurons are grouped into two plexuses. The myenteric plexus (plexus of Auerbach) lies in the muscularis layerand is responsible for motility, especially the rhythm and force of the contractions of the muscularis. The submucosal plexus (plexus of Meissner) lies in the submucosal layer and is responsible for regulating digestive secretions and reacting to the presence of food (Figure \(\PageIndex{7}\).
Extrinsic innervations of the digestive tractare provided by the autonomic nervous system communicating with the enteric nervous system. In general, sympathetic activation (the fight-or-flight response) restricts the activity of enteric neurons, thereby decreasing GI secretion and motility. In contrast, parasympathetic activation (the rest-and-digest response) increases GI secretion and motility by stimulating neurons of the enteric nervous system.
Blood Supply
The blood vessels serving the digestive system have two functions. They transport the protein and carbohydrate nutrients absorbed by mucosal cells after food is digested in the lumen. Lipids are absorbed via lacteals, tiny structures of the lymphatic system. The blood vessels’ second function is to supply the organs of the digestive tractwith the nutrients and oxygen needed to drive their cellular processes.
Specifically, the more anterior parts of the digestive tract are supplied with blood by arteries branching off the aortic arch and thoracic aorta. Below this point, the digestive tractis supplied with blood by arteries branching from the abdominal aorta. The celiac trunk services the liver, stomach, and duodenum, whereas the superior and inferior mesenteric arteries supply blood to the remaining small and large intestines (Figure \(\PageIndex{8}\)).
The veins that collect nutrient-rich blood from the small intestine (where most absorption occurs) and the spleen, empty into the hepatic portal system. This venous network takes the blood into the liver where the nutrients are either processed or stored for later use. Only then does the blood drained from the digestive tract organsand the spleen (not a digestive organ) circulate back to the heart. To appreciate just how demanding the digestive process is on the cardiovascular system, consider that while you are “resting and digesting,” about one-fourth of the blood pumped with each heartbeat enters arteries serving the intestines.
Peritoneum
The digestive organs within the abdominal cavity are held in place by the peritoneum, a broad serous membrane. It is composed of two different regions: the parietal peritoneum, which lines the abdominal wall (red in Figure \(\PageIndex{9}\) below), and the visceral peritoneum, which envelopes the abdominal organs (blue in Figure \(\PageIndex{9}\)). The peritoneal cavity is the space bounded by the visceral and parietal peritoneal surfaces, filled with afew milliliters of watery fluid acting as a lubricant to minimize friction between the surfaces of the peritoneum.
Thevisceral peritoneum is the serosa of the organs within the peritoneal cavity. The visceral peritoneum includes multiple large folds, also called mesenteries, that connect various abdominal organs, holding them to the dorsal surface of the body (trunk) wall and in some cases, each other. General functions of the peritoneal folds are to provide routes for vessels and nerves to reach intraperitoneal (within the peritoneum) organs, hold these organs to a relative location and in some cases insulate and protect other nearby organs.The largest peritoneal fold is the greater omentum,a sheet of the membrane containing adipose that lies anterior to the intestines. The greater omentum provides a small amount of protection (cushioning) in the region not protected by the skeleton.
Note that during fetal development, certain digestive structures, including the first portion of the small intestine (called the duodenum), the pancreas, and portions of the large intestine (the ascending and descending colon, and the rectum) remain completely or partially posterior to the peritoneum. Thus, the location of these organs is described as retroperitoneal.
DISORDERS OF THE...
Digestive System: Peritonitis
Inflammation of the peritoneum is called peritonitis. Chemical peritonitis can develop any time the wall of the digestive tractis breached, allowing the contents of the lumen entry into the peritoneal cavity. For example, when an ulcer perforates the stomach wall, gastric juices spill into the peritoneal cavity. Hemorrhagic peritonitis occurs after a ruptured tubal pregnancy or traumatic injury to the liver or spleen fills the peritoneal cavity with blood. Even more severe peritonitis is associated with bacterial infections seen with appendicitis, colonic diverticulitis, and pelvic inflammatory disease (infection of uterine tubes, usually by sexually transmitted bacteria). Peritonitis is life threatening and often results in emergency surgery to correct the underlying problem and intensive antibiotic therapy. When your great grandparents and even your parents were young, the mortality from peritonitis was high. Aggressive surgery, improvements in anesthesia safety, the advance of critical care expertise, and antibiotics have greatly improved the mortality rate from this condition. Even so, the mortality rate for peritonitis still ranges from 30 to 40 percent.
Concept Review
The digestive system includes the organs of the digestive tractand accessory structures. The digestive tract forms a continuous tube that is open to the outside environment at both ends.
The organs of the tractare the
- oral cavity,
- pharynx,
- esophagus,
- stomach,
- small intestine
- duodenum
- jejunum
- ileum
- large intestine (colon)
- cecum
- appendix
- ascending colon
- transverse colon
- descending colon
- sigmoid colon
- rectum
- anus
The accessory digestive structures include the
- teeth,
- tongue,
- salivary glands,
- parotid
- subliungual
- submandibular
- liver,
- pancreas,
- gallbladder.
The wall of the digestive tract is composed of four basic tissue layers: mucosa, submucosa, muscularis, and serosa (or adventitia).
The enteric nervous system provides intrinsic innervation, and the autonomic nervous system provides extrinsic innervation. Organs of the digestive system receive oxygenated blood from the abdominal aorta and send deoxygenated, nutrient rich blood, to the liver through the hepatic portal vein. The peritoneum, a serous membrane, functions to anchor abdominal organs and provides routes for vessels and nerves along with insulation in specialized regions called peritoneal folds (mesenteries).
Review Questions
Q. Which of these organs is not considered an accessory digestive structure?
A. oral cavity
B. salivary glands
C. pancreas
D. liver
- Answer
-
Answer: A
Q. Which of the following organs is supported by a layer of adventitia rather than serosa?
A. esophagus
B. stomach
C. small intestine
D. large intestine
- Answer
-
Answer: A
Q. Which of the following membranes covers the stomach?
A. falciform ligament
B. mesocolon
C. parietal peritoneum
D. visceral peritoneum
- Answer
-
Answer: D
Critical Thinking Questions
Q. Explain how the enteric nervous system supports the digestive system. What might occur that could result in the autonomic nervous system having a negative impact on digestion?
- Answer
-
A. The enteric nervous system helps regulate alimentary canal motility and the secretion of digestive juices, thus facilitating digestion. If a person becomes overly anxious, sympathetic innervation of the alimentary canal is stimulated, which can result in a slowing of digestive activity.
Q. What layer of the digestive tract wall is capable of helping to protect the body against disease, and through what mechanism?
- Answer
-
A. The lamina propria of the mucosa contains lymphoid tissue that makes up the MALT and responds to pathogens encountered in the digestive tract.
Glossary
- accessory digestive organ
- includes teeth, tongue, salivary glands, gallbladder, liver, and pancreas
- motility
- movement of food through the GI tract
- mucosa
- innermost lining of the alimentary canal
- muscularis
- muscle (skeletal or smooth) layer of the alimentary canal wall
- myenteric plexus
- (plexus of Auerbach) major nerve supply to alimentary canal wall; controls motility
- retroperitoneal
- located posterior to the peritoneum
- serosa
- outermost layer of the alimentary canal wall present in regions within the abdominal cavity
- submucosa
- layer of dense connective tissue in the alimentary canal wall that binds the overlying mucosa to the underlying muscularis
- submucosal plexus
- (plexus of Meissner) nerve supply that regulates activity of glands and smooth muscle
Contributors and Attributions
OpenStax Anatomy & Physiology (CC BY 4.0). Access for free athttps://openstax.org/books/anatomy-and-physiology
