Big+Idea+9


 * __Big Idea 9: Digestive System__**

__**Summary**__ The gastrointestinal tract (also called the digestive system), is an organ system that breaks down food to extract nutrients and energy. It also works to expel solid wastes from the body. The processes are called ingestion, digestion, absorption, and defecation. The specifics of the organs involved vary from animal to animal.Some animals' stomachs contain multiple chambers, while others only have one chamber. All information described below are for humans species Homo Sapiens Sapiens.

This section of the GI tract contains the mouth, pharynx, esophagus, and stomach. The mouth is the main method of which food enters the body. It contains the buccal cavity, which contains openings of the salivary glands, the tongue, and the teeth. Each of these components are important in determining which foods enters the body. The salivary glands produce saliva, which contains enzymes that help break down food before it has even entered the stomach.The tongue detects the taste of the material, which produces responses in the body whether the food is agreeable to the body or not. The teeth help break large chunks of food into smaller bits, allowing the digestion process to flow more smoothly.
 * __Upper Gastrointestinal Tract__**

__Mouth__ The mouth, also called the buccal cavity or oral cavity, is the first portion of the upper gastrointestinal tract. The teeth in the mouth physically break large portions of food into smaller bits, and then mixes it with saliva. The saliva contains an enzyme called //amylase//, which helps break down starches in food.By ingesting smaller bits of food, it insures that large food particles will not be lodged in the esophagus, which can be very dangerous.

__Pharynx__ The pharynx is the next portion in the GI tract. The function of the pharynx is to allow food to further travel in the digestive system, while also allowing the flow of air into the lungs. The epiglottis is a flap of tissue that covers each passage, preventing food particles from entering the lungs.

__Esophagus__ The esophagus is a muscular passage that connects the pharynx to the stomach. It pushes food particles down into the stomach were further digestion continues. The esophagus is lined with mucous so that food particles do not get caught, smoothing its journey through the gastrointestinal system.

__Stomach__ The stomach is where the second stage of digestion occurs after chewing (mastication). It lies between the esophagus and the duodenum. The top of the stomach lies against the diaphragm. It is in the left side of the abdominal cavity. Beneath the stomach is the pancreas. Two smooth muscle valves or sphincters contain keep the contents of the stomach contained. The stomach is surrounded by stimulating and inhibiting blood vessel networks that regulate secretions and the activity of the muscles. The stomach when empty holds about 45 mL. However, when holding food, expands to 1 L, although it can hold up to 1.5 L.

There are four sections in the stomach:
 * Cardia: where the contents of the esophagus empties into the stomach.
 * Fundus: the left portion of the stomach. It allows the accumulation of gases produced by chemical reactions. it can also store undigested food for over an hour.
 * Body/Corpus: the main central region of the stomach
 * Pylorus: connects the stomach to the duodenum

This section of the GI tract contains the bowels or intestines. These consists of two main parts: the small intestine and the large intestine. The small intestine is composed of the duodenum, jejunum, and the lleum. The large intestine is composed of cecum, colon, and the rectum. The final section, separate of all others is the anus.
 * __Lower Gastrointestinal Tract__**

__Small Intestine__ The small intestine is the first part of the lower gastrointestinal tract. It is composed of 3 major parts: the duodenum, the jejunum, and the Ileum. The small intestine is the area responsible for absorbing nutrients from digested food passed along from the stomach. All parts of the small intestine absorb nutrients.

__Duodenum__ The duodenum is the first part of the small intestine in the lower gastrointestinal tract. The duodenum is largely responsible for the breakdown of food in the small intestine, albeit being the shortest section.The duodenum breaks down food by excreting enzymes through its wall. The duodenum is also responsible in controlling the flow of food form the stomach into the lower gastrointestinal tract. A complex set of blood vessels insure that the duodenum can continually digest food and control the flow of food properly.

__Jejunum__ The jejunum is the second part of the small intestine in the gastrointestinal tract. The main purpose of the jejunum is to pass food from the duodenum to the next part of the small intestine, the ileum. The jejunum got its name because the greeks noticed that in an autopsy of a dead person, this section was always empty, devoid of any food, liquid, or waste material. The root jejune means "hungry"/

__Ileum__ The ileum is the final section of the small intestine, and is the longest. The ileum is mainly responsible of absorbing B12 vitamins and bile salts. However, the ileum also absorbs any nutrients not absorbed by the duodenum or the jejunum. On the interior surface of the ileum are microscopic finger-like structures called //villi//. The villi causes the interior surface area of the ileum to increase exponentially, thus allowing more efficient absorption of nutrients from food. Small lymph vessels called lacteals are also present in the villi. The lacteals absorb fatty acids and glycerol.

__Large Intestine__ The large intestine is named for its comparatively wider diameter, although it is much shorter than the small intestine. The large intestine, like the small intestine, is composed of 3 major sections, the cecum, the colon, and the rectum.

__Cecum__ The cecum is the first portion of the large intestine in the lower gastrointestinal tract. It is a small pouch that connects the ileum of the small intestine to the colon in the large intestine. The purpose of the cecum is to receive and store waste material form the small intestine until the later parts of the large intestine are ready to process the waste material.

__Colon__ The colon is the second part of the large intestine. The main purpose of the colon is to absorb any water and salt from material passed along from the small intestine before preparing it to be excreted from the body. If something affects the colon, preventing it from effectively absorbing water, diarrhea can occur. The colon is composed of five main components: the ascending, transverse, descending, sigmoid, and redundant colon.
 * Ascending colon: the ascending colon is on the right side of the human body. It is about 25 cm long.
 * Transverse colon: the transverse colon is the highest part of the large intestine, making a turn and going above the small intestine. It is attached to the stomach.
 * Descending colon: the descending colon makes another turn and is on the left side of the body. It goes down towards the rectum.
 * Sigmoid colon: this is usually the last segment of the colon, between the descending and the rectum. The wall of the sigmoid colon are muscular, compacting waste and forcing stool to the rectum
 * Redundant colon: occasionally, the colon forms an extra loop, known as the redundant colon. It usually poses no major health risks.

__Rectum__ The rectum is the final section of the large intestine, before the anus. The purpose of the rectum is for temporary storage of feces before the human can find an appropriate place to defecate. It is responsible for causing the urge to defecate. This is important, as if the human does not excrete said waste before long, extremely compacted feces can result, causing constipation.

__Anus__ The anus is the final section of the lower gastrointestinal tract. It is the opposite end of the mouth, with the purpose of excreting feces, and other indigestible material, such as bones and cellulose.