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Digestive System Diseases

Digestive System

Anatomy of the Digestive System

The alimentary canal consists of the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anus.

The digestive system consists of the alimentary canal and the accessory digestive organs. The alimentary canal is also known as the gastrointestinal (GI) tract, alimentary tract, or digestive tract. The function of the digestive system is digesting food, extracting nutrients from food, and protecting the human body from certain diseases. The organs of the alimentary canal are the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anus.

Other specialized accessory digestive organs are also part of the digestive system. These accessory digestive organs are the teeth, tongue, salivary glands, gallbladder, liver, and pancreas. The teeth, tongue, three sets of salivary glands (parotid, sublingual, and submandibular), gallbladder, liver, and pancreas are considered part of the digestive system because they help to break down food. The parotid glands are just in front of the ear, the sublingual glands are found under the tongue, and the submandibular glands are lower in the jaw.

Food and waste move through the alimentary canal because of muscle contractions in the esophagus, stomach, intestines, and rectum. This muscle contraction is known as peristalsis and is a special, wavelike form of muscle contraction that pushes food through the alimentary canal. Digestion begins in the mouth, when the mechanical force of chewing and enzymatic digestion from saliva begin to break down food.

The stomach mixes and churns the food. Enzymatic digestion occurs in the stomach, where the acidic pH of stomach acid begins the chemical breakdown of food into proteins and other substances. The low pH of the stomach kills most microbes not adapted to the alimentary canal.

This partially digested food then moves into the small intestine. The small intestine continues the digestive process and absorbs most of the breakdown products from carbohydrates, proteins, lipids, nucleic acids, and other nutrients. Digested food then passes into the large intestine, which completes most of the water reabsorption, absorbs vitamins and electrolytes, and moves waste toward the rectum. Waste that moves to the rectum is excreted through the anus as feces.

Organs of the Human Digestive System

Organs of the alimentary canal include the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anus. Accessory organs of the digestive system include the teeth, tongue, salivary glands, gallbladder, liver, and pancreas.

Digestive System Defenses

Natural defenses include mucus, secretory immunoglobulin A (IgA), fluids with antimicrobial properties, saliva, and the low pH of stomach fluid.

Foreign substances and pathogens are introduced to the body through the digestive system when contaminants in or on foods are ingested. The digestive system provides an important first-line defense role in protecting the body from disease. There are several defenses throughout the alimentary canal, including the physical barrier of fluid covering the linings of the organs, biochemical substances that are secreted, and immunological molecules. Additionally, the alimentary canal is colonized by a highly diverse and stable community of microbes that under normal conditions successfully compete with potential pathogens for space and resources.

One barrier to pathogen colonization is mucus, which can be found lining the alimentary canal. Mucus a thick fluid secreted by specialized epithelial cells including those lining the inside of the mouth, stomach, small intestine, and large intestine. Because mucus is continuously produced and sloughed off, it provides an effective physical barrier to pathogen adhesion to surfaces of the alimentary canal. Mucus also contains several biochemical defense components, including enzymes and salts, which all have antimicrobial properties.

Homeostatic mechanisms regulate the pH of the alimentary canal and maintain different pH levels in different parts of the digestive system. This provides defense against pathogens that are sensitive to changes in pH as they pass through the alimentary canal. Growth of most bacteria is stopped in low-pH environments, and a low pH will denature the surface proteins of many pathogens. The stomach in particular is deadly to many potential pathogens because its pH of between 1.5 and 3.5 is low enough to be antimicrobial.

Secreted enzymes are released specifically for their antimicrobial properties. The salivary glands, for example, secrete lysozyme, which breaks bonds in the bacterial cell wall component peptidoglycan. Bile salts in the small intestine are strongly antimicrobial, causing denaturation of microbial proteins and disruption of plasma membranes.

The immune system is active throughout the alimentary canal. Salivary glands in the mouth and B cells in the intestine release antibodies into the saliva and epithelial mucus layer, respectively. The primary antibodies found throughout the alimentary canal are secretory immunoglobulin A (IgA). Secretory IgA antibodies protect the body against pathogens that could enter via the alimentary canal, and help to maintain homeostasis between the body and commensal microorganisms, or bacteria that normally live in the human alimentary canal without causing disease. Secretory IgA can tether pathogens to the interior surface of the intestines, where antigens on the surface of the pathogen are presented to cells of the immune system. The complex of IgA and pathogen is then removed from the alimentary canal by peristalsis, or contractions of the muscles lining the alimentary canal.

Gut Microbiota

Millions of microorganisms are residents of the alimentary canal, where they provide digestive and immune services including training their host's immune system.

A large quantity and high diversity of microbes normally colonize the alimentary canal. There are approximately as many microbial cells in the average alimentary canal as there are human cells in a human body and there may be many more. Microbes in the digestive tract are made up of around 1,000 different species of bacteria. An individual's microbiome can change over the course of their lifetime based on factors such as diet, illness, and medication.

The collection of typical microorganisms present in an ecosystem is the ecosystem's microbiome. In the human alimentary canal, the microbiome is composed of commensal microbes and is commonly called the gut microbiome. Commensalism is a relationship in which one species benefits but the other neither benefits nor is harmed. Colonization refers to the establishment of microorganisms at a particular site, and niche factors aid in this colonization. A niche factor, or colonization factor, is a feature enabling commensal and beneficial bacteria to colonize a specific area within a host and evade the host's immune system without damaging the host. Niche factors enable bacteria to attach to host body surfaces, survive nonspecific host defenses (bile or stomach acid), and evade specific host immune responses, all without damaging the host.

These commensal microorganisms serve many important roles in the body, both in helping carry out normal bodily functions and helping protect the body from disease. One key role for the commensal microbiota is to aid in digestion. Microbes secrete enzymes that the human body cannot produce to aid in the breakdown of certain foods, such as complex carbohydrates. Other microbes synthesize essential vitamins that humans do not have the genes to synthesize. Commensal microbiota also help protect the body from pathogenic microorganisms. The commensal microbiota can compete for physical space and access to nutrients within the alimentary canal to prevent pathogenic bacteria from colonizing the gut. Commensal microbiota also secrete compounds known as bacteriocins, which are able to kill other bacteria, including pathogens.

Finally, the commensal microbiota, which are present from soon after birth, help train the immune system to only recognize certain microorganisms as harmful while allowing the commensal microorganisms to survive. There is a constant chemical communication between microbes and epithelial cells throughout the alimentary canal. Epithelial cell exposure to common metabolic products provides the immune system a baseline of "safe" antigens so that when unfamiliar compounds are encountered, immune recognition and response can follow. This immune system training has also been shown to help protect against the development of allergies.
Species in the genus Lactobacillus (light microscope, 63x) are gram-positive bacteria commonly found in the alimentary canals of health humans.
Credit: CDC/Dr. Mike Miller
While pathogenic microbes have niche factors that do not harm the host, they also have virulence factors. A virulence factor is a feature enabling a pathogen to cause disease by releasing toxins, invading host cells and tissues, suppressing the host's immune response, or otherwise damaging the host. Commensal microbes do not utilize host-damaging virulence factors.