Anatomy/Digestive System

The digestive system is a topic of the event Anatomy. It scheduled to be a part of the event during the 2012 season with the respiratory 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 to provide 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=


 * Take in food
 * Break down food
 * Absorb digested molecules
 * Provide nutrients
 * Eliminate waste

=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.

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.

Diagram


=In Depth Descriptions=

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
 * 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.



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).



Esophagus
After food bolus passes through the pharynx, it goes through the esophagus. 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.

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

Layers
(From innermost to outermost)
 * 1) mucosa (made of simple columnar epithelium)
 * 2) submucosa
 * 3) oblique muscle (helps churn food)
 * 4) circular muscle (helps churn food)
 * 5) longitudinal muscle (helps churn food)
 * 6) serosa (reduces friction between stomach and neighboring body parts)

Sections
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.

Inner Surface
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.

Stages of Secretion
There are three main stages in which hormones and enzymes are secreted in the stomach.
 * Cephalic Phrase- prepares stomach for food. Stimulated by taste, thought, and smell of food.
 * Gastric Phrase- food enters through the cardiac sphincter, lots of stomach secretions
 * Gastrointestinal Phrase- mainly inhibition of previous gastric secretions

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: 25cm
 * the jejunum: 2.5m
 * the ileum: 3.5m



Layers
(from innermost to outermost)
 * Mucosa
 * Submucosa
 * Muscularis- circular
 * Muscularis- longitudial
 * Serosa

Mucosa
The small intestine mucosa is simple columnar epithelium. There are four major cell types in the mucosa.
 * absorptive cells digest and absorb food
 * goblet cells produce mucus
 * granular cells protect from bacteria
 * endocrine cells produce regulatory hormones

Villi and Microvilli
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.



Movement of Food
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.

Differences Between Duodenum, Jejunum, and Ileum

 * gradual decrease in diameter (from duodenum to ileum)
 * gradual decrease in thickness of intestinal wall
 * gradual decrease in number of circular folds
 * gradual decrease in number of villi
 * only the ileum contains Peyer's patches, which are clusters of lymph nodes

Serous Membranes
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.

=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.

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 there appropriate section below.

Esophageal (Oesophageal) Cancers
There are two major types of cancer occuring 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%)