Anatomy/Digestive System

The digestive system is a topic of the event Anatomy. It scheduled to be a part of the event during the 2013 season with the nervous system and the excretory system.

For a list of secretions in the digestive system, see the Digestive Secretion List.

Overview
The digestive system is the system that breaks down food and returns water and nutrients to the blood stream. It is necessary because it provides energy to the cells in your body. The digestive system is made up of organs such as the esophagus, stomach, liver, and the intestines.

Functions
It provides nutrients by:


 * Taking in food (ingestion)
 * Breaking down food (digestion)
 * Absorbing digested molecules (absorption)
 * Eliminating waste (elimination)

Summary of the Digestive Tract


The digestive tract is about 8 meters, or 28 feet long. It can take from 15 to 72 hours for food to pass from the mouth to the anus.

The mouth is where the digestion of food starts. The mouth is a good example of mechanical digestion and chemical digestion at work. The teeth chomp, chew, and tear food down into smaller pieces (mechanical digestion). The molecular structure of the food does not change in mechanical digestion. Then, the salivary glands secrete saliva into the mouth, breaking down starches into dextrin and maltose (chemical digestion). Saliva is a form of chemical digestion because it changes the molecular structure of the food. These digestive processes create food boluses.

After food boluses are developed in the mouth, they go down the pharynx. The pharynx is a shared connection between the nasal cavities, the mouth, the air tract, and the esophagus. Therefore, there must be a structure to prevent the boluses from entering the trachea (passage to the lungs). This is why we have an epiglottis. The epiglottis is a small flap which can open and close the entrance to the trachea. When food boluses travel through the pharynx, the epiglottis folds down to protect the airway. However, in instances like when you talk and eat, the epiglottis can get confused. In order to talk, the epiglottis must be open. In order to swallow, the epiglottis must be closed. Therefore the epiglottis might malfunction and you could choke. This is why you shouldn't talk with food in your mouth.

After the pharynx, food bolus is transported to the stomach via the esophagus/oesophagus. The esophagus is a muscle-walled tube that goes from the throat to the lower digestive tract. To help food get down, muscular contractions called peristalsis push food down towards the stomach. To demonstrate peristalsis motions, imagine a sphere in a rubber tube. the diameter of the sphere is a little more then the diameter of the tube. If you wanted to get the ball out, you would squeeze the tube from one end, and slowly move your hand towards the ball while squeezing to push it out. The reason peristalsis is important is when we are working against gravity. For example, when you eat upside down, the food bolus is still able to move to the stomach because of peristalsis contractions.

Next in the digestive tract comes the stomach. When we think of digestion, the first thing that comes to mind is the stomach. It acts like a waiting room for food bolus. While the food boluses are waiting, the stomach breaks the food down into a liquid-like mixture by churning the food (mechanical digestion) and secreting gastric acid (chemical digestion). Gastric acid also kills bacteria that may be in the food.

When food boluses are ready to be further digested, they transfer to the duodenum by contractions of the stomach walls. The duodenum is the first part of the small intestine and is about 25 cm in length. The duodenum is stationary, and is fixed behind sheets of connective tissue called peritoneum. Glands in the duodenum secrete a thick alkaline fluid that counteract the acidic nature of the chemicals the food bile has absorbed. The gall bladder also secretes pancreatic juice and bile into the duodenum.

The liver is a very important part of the digestive system, even though food does not directly pass through it. The liver produces bile (a greenish fluid which aids in the digestion of fats), which is stored in the gall bladder. When bile is needed, it joins with secretions from the pancreas and goes through the common bile duct into the duodenum.

After the duodenum comes the two other parts of the small intestine, the jejunum and the ileum. These are what make up the bulk of the small intestine. The jejunum is the first part while the ileum is the second part. Here, food boluses are digested even further, and some nutrients are absorbed through the walls.

After passing through the duodenum, jejunum, and the ileum (the small intestine), food bolus makes its way through the large intestine. The large intestine is actually shorter then the small intestine, but larger in diameter. Water and electrolytes are removed in the large intestine. Also, microbes such as bacteria aid in further digestion. Finally, food bolus comes out of the anus as feces.

The Oral Cavity
The oral cavity is another name for the mouth. It is the first part of the digestive tract.
 * The lips, formed by the orbicularis oris muscles, help in food intake and determining temperature of food.
 * The buccinator muscles assist in the process of mastication, or chewing. Mastication is the first process of digestion
 * The tongue, a large muscle, helps move the food around the oral cavity. It also is necessary in swallowing.
 * The surface that forms the top of the oral cavity is called the palate. It is split into two parts.
 * The hard palate is the most anterior section. It contains bone. This is often referred to as the "roof of the mouth".
 * The soft palate is more posterior. It separates the oral cavity from the nasal cavity. It helps by preventing food from entering the nasal cavity.
 * The tonsils are part of the lymphatic system. They are tissues to the right and left of your mouth.
 * The uvula is also part of the lymphatic system. It is an extension of the soft palate, and is hanging down from it. The uvula is often wrongly referred to as the tonsil.

Teeth
The teeth chew and break down the food mechanically. An child has 20 deciduous teeth while an adult has 32 permanent teeth. There are four types of teeth: Incisors, canines, premolars, and molars.

Teeth consist of the crown, the neck, and the root.
 * The crown is the upper surface of the tooth. Bumps on the crown are called cusps.
 * The neck is the region under the crown, but above the alveolar bone.
 * The root is the region inside the alveolar bone.



Salivary Glands
The salivary glands produce saliva, which is secreted into the oral cavity. There are three pairs of salivary glands: the parotid, submandibular, and sublingual.
 * The parotid glands are the largest. They are located slightly posterior to the cheeks.
 * The submandibular glands are below the mandible bone. They procuce more serous saliva.
 * The sublingual glands are the smallest. They are below the floor of the oral cavity. They produce mostly mucus.

The Pharynx
The pharynx is often referred to as the back of the throat. Food pieces, now called bolus, travel from the oral cavity to the pharynx in the process of deglutition, or swallowing. The pharynx is also a shared passageway, connecting the nasal cavity, oral cavity, air passageway, and epiglottis. There are certain structures inside the pharynx to conduct the movement of air and food.
 * During deglutition, the soft palate elevates to prevent food from entering the nasal cavity.
 * Another structure, the epiglottis, bends down to prevent food from entering the air passageway.
 * If the epiglottis fails to function, food bolus may be lodged in the air passageway (choking).

The Esophagus
After food bolus passes through the pharynx, it goes through the esophagus, or oesophagus. The esophagus is directly under the pharynx. There is no defining structure that separates the pharynx from the esophagus. The esophagus is about 25 cm long in adults.

Muscular contractions called peristaltic waves propel food down the esophagus. This is why food can still travel through the esophagus if you are lying down or even upside-down.

The Stomach


The stomach is the first part of the gastrointestinal tract. Its main purpose is to store and churn food.


 * The layers, from innermost to outermost, are the mucosa (made of simple columnar epithelium), submucosa, oblique muscle (helps churn food), circular muscle (helps churn food), longitudinal muscle (helps churn food), and serosa (reduces friction between stomach and neighboring body parts).
 * There are three main sections to the stomach: the fundus, the body, and the pylorus. The fundus is the top part of the stomach, or the roof. The body is the bulk of the stomach. The leftmost region is the pylorus.
 * The texture of the inner surface of the stomach largely depends on how much food bolus it contains. If it does not contain much food bolus, the stomach is small and the inner surface has lots of folds. These folds are called rugae. If the stomach does contain lots of food bolus (like after a meal), the stomach is bigger and rugae are not present.

The Small Intestine


The small intestine is the next part of the digestive tract. The small intestine is about 6 meters long, and so it is the longest part of the digestive tract. Its main purpose is the digestion of food and absorption of nutrients. The small intestine consists of three sections: The duodenum (25 cm), the jejunum (2.5 m), and the ileum (3.5 m).

The layers are, from innermost to outermost, the mucosa, submucosa, muscularis - circular, muscularis- longitudinal, and serosa.

The mucosa has four major cell types:
 * Absorptive cells digest and absorb food
 * Goblet cells produce mucus
 * Granular cells protect from bacteria
 * Endocrine cells produce regulatory hormones

Since digestion only occurs at the surface, there must be a lot of surface area for effective digestion. To increase surface area,the small intestine has circular folds, villi, and microvilli. Villi are tiny finger-like projections. Microvilli are even smaller finger-like projections that are on the villi.

Chyme moves through the small intestine with two types of contractions.
 * Peristaltic contractions propel chyme forward. They are similar to the contractions in the esophagus.
 * Segmental contractions mix chyme by squeezing in outward motions.

The differences between the duodenum, jejunum, and ileum are as follows:
 * A gradual decrease in diameter (from duodenum to ileum)
 * a gradual decrease in thickness of intestinal wall
 * a gradual decrease in number of circular folds
 * a gradual decrease in number of villi.

Only the ileum contains Peyer's patches, which are clusters of lymph nodes.

Accessory Organs
The liver and gall bladder, along with the pancreas, are classified as accessory organs. However, they are not simply fashion garments. The accessory organs of the digestive tract are vital to the processes of the digestive system.

The Liver and Gall Bladder
The liver weighs about three pounds and is on the right-hand side of your body, right beneath the diaphragm. It consists of two major lobes, which are divided by the falciform ligament. The liver is attached to the underside of the diaphragm by the coronary ligament.

The liver is a part of more than one anatomical system. As well as functions associated with the digestive system, the liver also stores nutrients, cleans blood, and makes blood proteins. However, because this is the digestive page, we will be focusing on the digestive functions.

The main function of the liver is the production of bile. The liver produces and secretes about 700 mL of bile every day. The groups of cells that produce bile are called hepatocytes. Bile helps make fat digestion easier by braking fat particles into smaller, more manageable ones. Bile is stored in the gall bladder until it is needed. Then, it is transported to the duodenum. Note that bile is NOT a digestive enzyme.

The Pancreas
The pancreas is an accessory organ that secretes enzymes into the duodenum. It is located underneath and a little to the left to the liver. It is made up of endocrine and exocrine tissue. The endocrine tissue produce hormones, and the exocrine tissue produce digestive enzymes.

The digestive enzymes are created by acinar cells in the pancreas. Secretions of the pancreas travel through the pancreatic duct and join the common bile duct. They enter the duodenum through the duodenal papilla. (Check the Digestive Secretion List for a list of pancreatic secretions.)

The pancreas also produces hydrogen carbonate (HCO3-) which helps counter the acidic quality of chyme that enters the duodenum from the stomach. This stops pepsin digestion, but makes the other secretions of the pancreas more effective.

The Large Intestine
The large intestine is the final part of the digestive tract. It is about 5 feet long, and consists of the cecum, colon, rectum, and anal canal. Note that it is shorter than the small intestine. The reason it is called the large intestine is that it's wider thank the small intestine. The large intestine is mobile, and undergoes several muscular contractions called mass movements. Mass movements propels the colon towards the anus to help move food along with peristalsis. Note that the large intestine generally does not absorb nutrients. Although some nutrients such as vitamin K are absorbed, the main site of nutrient absorption is the small intestine. The large intestine mainly only absorbs water and salt.

The Cecum
The cecum, or caecum, is the first part of the large intestine. Food from the small intestine enters the cecum through the ileocecal valve.

A small pouch-like attachment to the cecum is the appendix. The appendix has no real function, as it was probably lost through human evolution. Organs that have no real function like the appendix are referred to as vestigial organs. Scientists believe the appendix was used do digest a diet with a lot of cellulose (plant fiber). Today, the appendix may store helpful bacteria, although this is not an important function.

The Colon
The colon makes up the bulk of the large intestine. There are three parts to the colon- the ascending colon, the transverse colon, the descending coon, and the sigmoid colon. The ascending colon is the first part, being right after the cecum. Food travels upwards through it. Next is the transverse colon. Food travels from left to right. Then, food goes down through the descending colon and sigmoid colon.

The colon is where chyme is converted to feces. Water and salts are absorbed in the large intestine, and microorganisms such as helpful bacteria help break down components of the feces. The colon stores the feces until defecation, where the feces is transported out of the digestive tract through the anus.

The inner wall of the large intestine is lined with mucus. It also has straight, tubular glands called crypts that have goblet cells to produce the mucus. Longitudinal, smooth muscle does not have its own layer in the large intestine. Instead, it exists in the form of three strips called teniae coli. Attached to the outer wall are small fatty sacs called appendices epiploicae.

The Rectum and Anal Canal
After food travels through the sigmoid colon, it enters the rectum. The rectum is a straight, muscular tube. The muscle layers are very thick to push food into the anal canal. The anal canal is the last 2-3 centimeters of the digestive tract. The anal canal has an even thicker muscle layer.

The peritoneum is a serous membrane that surrounds the abdominal cavity and organs. The inner layer is called the visceral peritoneum, and the outer layer is called the parietal peritoneum. A double layer of peritoneum is called mesentery.

Regulation of Digestive Secretions and Processes
Digestive secretions are regulated by the brain and the hormones.

Digestion, Absorption, and Transport of Nutrients
Digested molecules of food and water, as well as minerals from the diet, are absorbed throughout the small intestine. These molecules pass through the mucosa via absorptive cells. Lipids are absorbed last of the three main food groups, since they must first be broken down by lipase and bile salts. (See also: Mineral Absorption)

Diseases of the Digestive System
There are many different diseases and disorders that affect the digestive system. In the subcategories below describe how the disease or disorder affects the body, from the cellular level up to the entire organism. The first six diseases/disorders are required for all levels, with the remaining 4 being necessary for the national level competition only.

Stomach and Duodenal Ulcers


Ulcers occur when the mucus in the stomach or the intestines deteriorates, leaving the flesh directly exposed to the acid contained in these organs. Ulcers can also occur in the esophagus, but the focus is on the stomach and intestines.

These ulcers can occur for several reasons:
 * 1) (60% of stomach, 90% of duodenal) A bacteria, Helicobacter pylori, inflames the tissue by living in the mucus.
 * 2) An extreme intake of either sour or spicy foods with a high acid content.
 * 3) The use of NSAID's may also play a role in this condition. NSAID's block the body's ability to make more mucus, thus the mucous layer breaks down, giving the gastric acid an exposed portion of the skin.

Ulcers can be either benign or harmful, with most duodenal ulcers being benign.

Ulcers can be treated by using common drugs. Avoid smoking and drinking, as both inhibit healing of the ulcers.

Cancers of the Digestive System
If a tumor affects the digestive system, it is called a neoplasm. These neoplasms can occur in a variety of places in the system, including the esophagus, stomach, small intestines, and colon. There are several variations of cancer in each of these sub-groups, listed in their appropriate section below.

Esophageal (Oesophageal) Cancers
There are two major types of cancer occurring in the esophagus. There are:
 * 1) Squamous cell cancer (approximately 90%-95% of all cases, worldwide)
 * 2) Adenocarcinoma (fastest growing in United States, responsible for the rest of the approximately 5%-10%)

There are several major symptoms of esophageal cancers. They include vomiting blood, regurgitation of food, heartburn, difficulty swallowing, and having chest pain that has nothing to do with eating.

Tests include:
 * Barium swallow (must swallow a drink containing barium, then x-rays are taken to look for any abnormalities in the travel of the liquid)
 * chest MRI (used for determining the progression of the cancer)
 * endoscopic ultrasound (also used for the progression of the disease)
 * esophagogastroduodenoscopy (use of a small camera inserted through your mouth to view the tissue in the throat, stomach, duodenum)
 * biopsy
 * PET scan (used to determine stage of disease/whether surgery is possible to remove cancerous growth).

For these cancers, surgery, chemotherapy, or radiation are the most common treatments. Chemotherapy and radiation are most often used to help reduce the cancer to a point where surgery can be used, or to help reduce the symptoms (the latter case is generally incurable).

Lactose Intolerance
Lactose intolerance is the digestive system's inability to break down a sugar called lactose, hence the name. This most commonly occurs due to the body's lack or insufficient quantities of an enzyme, called lactase, used to break the lactose for energy. As a result, the person with this condition will experience pain because of their body's inability to digest the sugar. Most infants have a proper amount of lactase enzymes inside their bodies in order to break down the lactose found in their mother's milk. However, as they age, the amount of active lactase dramatically decreases in most cases. Lactose intolerance is the human normal, with lactose "tolerance" being a rather new human evolution, especially in those of Northern European descent. People with lactose intolerance should avoid consuming dairy products.

Appendicitis
Sometimes the appendix may get infected. Appendicitis can be tested by touching the McBurney's point on the patient. This is located about 1/3 away from the left pelvic bone to the belly button. If the McBurney's point is extremely sensitive, there is a good chance the patient has appendicitis. Appendicitis can be treated in most cases with an appendectomy, where the appendix is removed.

GERD
GERD (Gastroesophageal Reflux Disease) is a disease in which the contents of the stomach travel back up the esophagus, irritating the lower esophageal area (and sometimes the upper area too) and causing a sensation described as "heartburn" which is worsened by eating or lying down. GERD is caused by a dysfunctional lower esophageal sphincter (LES) which would typically function to close off the esophagus from the lumen of the stomach. Some of the atypical symptoms of GERD include difficulty swallowing, sore throat, and regurgitation of food.

Usually, a diagnosis can be made by a physician with a description of the symptoms which may be followed up by an endoscopy (more specifically, an esophagogastroduodenoscopy, or EGD) in which the physician searches for tissue erosions, thickening of the mucosal layer, and possibly ulcers in the esophagus to help confirm the diagnosis. For a more definite diagnosis, a pH monitor is placed over the LES, and the results (characterized by timely decreases in pH, indicating acid reflux) are compared with the patient's description of symptoms according to the time they occur. This is called "ambulatory pH monitoring."

There is no actual cure for GERD, but the symptoms of GERD can be treated.


 * 1) For heartburn, antacids can be taken after meals but relief will not last very long. For longer relief, proton pump inhibitors (PPIs) can be used to reduce the amount of HCl (hydrochloric acid) produced in the stomach.
 * 2) Acetaminophen can help relieve pain associated with GERD.
 * 3) Problematic foods should be avoided.

Crohn's Disease
Crohn's Disease is a form of IBD (Inflammatory Bowel Disease) which is characterized by chronic inflammation of the GI tract. Although it is usually associated with the large intestine, Crohn's disease can affect any part of the GI tract, as opposed to ulcerative colitis (which only affects the large intestine). It is categorized as an autoimmune disorder. This means that the body's immune system overreacts and starts attacking healthy body cells in the affected areas of the GI tract.

Individuals with Crohn's disease experience chronic pain due to the inflammation which results from the hyper immune response. Other symptoms include: pain with defecating, fever, diarrhea or constipation, abdominal pain, fatigue, bloody stool, and ulcers anywhere in the GI tract (including the mouth).

The exact cause of Crohn's disease is unknown. Heredity and envrionment factors has been shown to affect development of the disease. Risk factors include smoking and a family history of Crohn's disease. Crohn's disease can occur in individuals of any age or sex, but it has been observed to be most frequent in men ages 15-35.

There are several methods for diagnosis: There is no specific cure for Crohn's disease, although the symptoms can be controlled to some degree by medication and changes in diet. In severe cases, surgery (bowel resection) may be required to remove certain affected areas of the GI tract. Since the cause of Crohn's disease is unknown, there really aren't any known methods of prevention.
 * Colonoscopy, sigmoidoscopy, endoscopy, and enteroscopy to check for physical signs such as strictures and inflammation
 * MRI (Magnetic Resonance Imaging) or CT (Computed Tomography) scan
 * Upper GI series (x-rays or the stomach and esophagues) mainly to look for ulcers
 * Physical examination may reveal abdominal masses/lumps and tenderness when certain areas are palpated