Every human being on the planet has to get rid of their waste products, usually through defecation, or pooping. There is much more to digestion then just pooping. Most people think that the stomach is the place where digestion occurs, but it's actually the small intestine where most of the digestion happens. The gastrointestinal tract starts at the mouth and ends at the anus. There are many very important organs throughout our body that are used in the digestion process. Absorption and digestion are the main functions of the digestive system. Many enzymes are used to aid the process of digestion, and the body needs to protect itself from the very acidic enzymes. The digestive system acts like a dis-assembly line for the food, and the system uses many factors to help dis-assemble the food. Motility, secretion, digestion, aborption, storage and elimination, and an immune barrier are the functions that are included in the important digestive system. Without the proper function of the digestive system, homeostasis would not be maintained.

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Concept #1: Introduction to the digestive system

Motility, secretion, digestion, absoption, storage and emlination, and an immune barrier are the main functions of the digestive system. Motility means movement of food through the digestive tract, and there are many different ways. Ingestion is the food entering the mouth, and mastication is when you chew the food and mix it all together with saliva. Deglutition is swallowing the food. Peristalsis and segmentation are movements that the intestinal tract uses to transport the food. Segmentation is moving the food in different segments, and peristalsis are wavelike contractions of the intestinal tract in a rhythmic form. There are different forms of secretions, endocrine and exocrine. Water, bicarbonate, hydrochloric acid, and many other enzymes are secreted into the lumen of the gastrointestinal tract by exocrine secretions. Endocrine secretions come from the small intestine and the stomach which secrete many hormones to regulate the digestive tract. Digestion is the breakdown of food into smaller particles so it can be absorbed. Absorption is the passage of digested end products from the digestive system into the blood and lymph. Elimination and storage is the temporary storage of ingestible food molecules. The immune barrier are simple columnar epithelium that line the intestines to protect it from them from foreigners meant to do harm. The gastrointestinal tract, or GI, is very long and includes many organs. The oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine are the organs that are included in the GI tract. Accessory organs aid in the digestion, and include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. Tunics are the four layers of the GI tract. Mucosa which is the major secretory and absorptive layer which lines the lumen in the GI tract. The submucosa is thicker and is a highly vascular layer that servers the mucosa. Muscularis is the layer which is responsible for the segmentation and peristalsis movements of the GI tract. Serosa is the protective and binding layer which consists of a layer of simple squamos epithelium. All of these movements, layers, and organs must work together in order for the proper function of the digestive system.


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Concept #2: The small intestine

The small intestine is actually the largest part of the intestinal tract, between the stomach and the large intestine. The reason why it's called the small intestine is because of the small diameter of the intestine. Duodenum extends from the pyloric sphincter and is approximately 20-30 centimeters long, and is the first part of the small intestine. The next part of small intestine is the jejunum and the last part is called the ileum. The ileum hooks up with the large intestine through the ileocecal valve. Most of the products are digested through the small intestines through the epithelial lining of the intestinal mucosa. Carbohydrates, lipids, amino acids, calcium, and iron absorption mostly happens in the duodenum and the jejunum. Absorption in the ileum includes vitamin, bile salts, water, electrolytes, and vitamin B12. The surface area is increased by folds in the mucosa called villi, and smaller microscopic microvilli. Each villi are covered with columnar epithelial cells. The connective core of each villi is called the lamina propria. The lamina propria contain many lymphocytes, blood capillaries, and a lymphatic vessel called the central lacteal. Amino acids and monosaccharides enter through the blood capillaries, and absorbed fats enter the central lacteals. Epithelial cells are continuously shed and replaced by new cells. The base of the epithelium of the villi go downward and then form pouches that open through pores to the lumen of the intestine. Those structures are called intestinal crypts. Each intestinal crypt contains many stem cells, which divide to replenish themselves. Microvilli are microscopic foldings and look like a brush border on the edge of the columnar epithelial cells. The small intestine also contains many enzymes that are needed for the digestion of food. The brush border enzymes remain attached to the plasma membrane with their active sites exposed to the chyme. Enterokinase, is one brush border enzyme which is needed for the activation of the protein digesting enzyme trypsin. There are two main movements of the small intestine, which include perastalsis and segmentation. Perastalsis is much weaker in the small intestine then in the mouth and esophagus, and the movement of chyme is relatively slow due to high pressures. Segmentation is the main movement of the small intestine and refers to muscular contractions of the lumen. The main job of the segmentation is to move around the chyme. Segmentation happens more often at the proximal end of the intestine, to create the pressure to move the chyme. The smooth muscle contractions occur naturally due to endogenous pacemaker activity. The interstial cells of Cajal produce the slow waves which aid in the contractions. Gap junctions join the interstial cells of Cajal together, and also permit the spread of depolarization from one cell to another. The very long small intestine, which is the main sight of digestion in the body, must have all the functions working together and properly in order for it to work correctly.


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Concept #3: The large intestine

The large intestine is also known as the colon, and extends from the ileocecal valve to the anus. Chyme comes from the ileum into the cecum, which is a pouch only open at one end at the beginning of the large intestine. The food passes through three segments of the large intestine. The first segment that comes off of the small intestine is the ascending colon. Transverse colon is the part of the large intestine which seems to be horizontal. Lastly the descending colon is the last part of the large intestine which directs the food to through the anal canal. Like the smal intestine, the large intestine contains many random lymphocytes and lymphatic nodules. Columnar epithelium covers the colon and goblet cells which secrete mucus. Unlike the small intestine which contain villi, the large intestine contains no villi, therefore it looks flat. Haustra are the pouch like bulges on the outer surface of the colon. Barely if any digestion occurs in the large intestine, mostly reabsorption of water and electrolytes from the remaining chyme. Inside many areas of the GI tract are bacteria, or microorganisms. There are ten times as many microorganisms than cells in the human body. Intestinal microbiota, or microflora, is the collection of all the bacteria. Mostly the bacteria in the colon are anaerobic bacterial species, meaning without oxygen. The bacteria is mutualistic in which both species are benfiting from each other. If the balance between all the bacteria is off in the colon, one might have diarrhea. The fluid that isn't absorbed by the small intestine is usually absorbed by the large intestine. An osmotic gradient which is created by the active transport of ions causes the passive absorption of water in the colon. After the water is reabsorbed by the colon, the food then is on it's way to the anal canal. There are two sphincters, which are small circular like muscles, in the rectum. The sphincter on the internal side is involuntary, and gives you the urge to defecate. If the urge to defecate is denied, the waste will not enter the anal canal by the external sphincter. When the feces do enter the anal canal, which is usually natural, the longitudinal rectal muscles contract to increase the pressure. The internal and external sphincters realx, and the excretion of the feces is aided by contractions of the abdominal and pelvic skeletal muscles. All of these processes happen everyday, and they must work properly in order for homeostasis to be maintained in the body.

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The digestive system usually grosses people out because they just think of feces. There is so much more to the digestive system then feces. There are many common conditions that are caused by the digestive system. Many people have diarrhea, and being able to explain to them that there can be a reason as to why they have diarrhea is important as a nurse. Cholera, lactose intolerant, and celiac sprue are some examples of conditions that cause diarrhea. As a nurse if I am seeing a patient who is having pain in the lower right quadrant I need to know that it may be a serious pain called an appendicitis which is an inflammation of the appendix. There are so many conditions that may happen with the digestive system, and if I am not knowledgable on the digestive system, I won't be able to do my job to the fullest. Each person eats, drinks, and defecates, and to understand how the process of digestion works is not only vital to a nurse, but important as a human being.

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Essential question:

Many organs in the digestive system have characteristics that help that organ complete it's job. For example the esophagus contains many muscles within that aid in the process of digestion. The esophagus is the portion of the GI tract that connects the pharynx to the stomach. Inside the esophagus are nonkeratinized stratified squamos epithelium cells and either skeletal or smooth muscle. The beginning third part of the esophagus contains skeletal muscle, the middle is both smooth and skeletal, and the lowest part of the esophagus contains only smooth muscle. These muscles in the esophagus help to direct the food into the stomach through wave like contractions called peristalsis. The movement of the bolus, or ball or food and saliva, occurs because the circular smooth muscle contracts behind the bolus, and then relaxes in front of it. There is also the shortening of the tube by the longitudinal muscles contraction. All of these different kinds of muscles, and different movement of muscles aid in the digestive process by moving the food to the stomach. The stomach as well as the esophagus has properties of the organ that aid itself in the digestion process. The stomach is the most disensible part of the GI tract, and it between the esophagus and the duodenum. There are different regions on the stomach that contract to mix the food with the chyme. Not only do the contractions mix the food, but they also push the partially digested food to the lower part of the stomach and the first part of the small intestine. There are many folds inside the stomach called rugae, and the opening of these folds are called gastric pits. Also the cells that line the folds secrete various products into the stomach, and they form exocrine gastric glands. Without these properties of these organs, the digestive process would be slowed, if not stopped.

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Essential Question:-------------Carbohydrates-------------------------Proteins---------------------------------------------- Lipids------------------------

Where the nutrient is digested

* begins in mouth *mainly in duodenum

*begins in stomach *mainly in duodenum and jejunum

*small intestine

Enzymes that help hydrolyze the molecule

*salivary amylase *pancreatic amylase

*pepsin *trypsin, chymotrypsin, elastase

*pancreatic lipase *phospholipase A *brush boader of the intestinal epithelium

How the nutrient is aborbed

*across epithelial cells by secondary active transport *facilitative diffusion

*free amino acids into epithlial cells

*chylmoicrons and lipids are secreted into the central lacteals of the intestinal villi *fatty acids

How the body uses the molecule

*energy *stored as gluclose

*catalyzing chemical reactions *facilitate communication between different cells *transport biological molecules

*energy *stored in fat cells as triglyercerides

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