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Large Intestine

Cecum

  • The cecum is located in the lower abdominal cavity at the very beginning of the large intestine and receives undigested food material from the ileum of the small intestine. It closely resembles a large pouch. 
  • Tissue: The ascending colon is composed of four tissue layers- the mucosa, the submucosa, the muscularis, and the serosa.
    • Mucosa: innermost layer made of smooth mucus membrane and goblet cells which secrete mucus for lubrication. Absorption of nutrients is carried out by the epithelial cells of this mucus membrane.
    • Submucosa: surrounds the mucosa and contains blood vessels and nerves which support surrounding tissues.
    • Muscularis: has bands of smooth muscle tissue arranged in a perpendicular fashion to contract the cecum walls. This contraction enables the chyme to mix with the bacteria properly and also propels the chyme upward into the ascending colon.
    • Serosa: outermost layer composed of simple squamous epithelial tissue. This layer produces a slick fluid lubricating the exterior of the cecum to prevent friction from the surrounding abdominal tissues.
  • Digestion: One of the main duties of the cecum is to absorb fluids and salts left over from small intestinal digestion as well as to mix its contents with the mucus as a lubricating substance. This mucus aids in the absorption of salts and water. The cecum also provides space for the digested food to mix with bacteria and form feces.
  • Under the thick mucus membrane of the cecum lies a deep layer of muscle tissue which helps kneading and churning motions to move the semi-digested food up to the ascending colon and help mix the chyme with the bacteria even more.


http://medicalpicturesinfo.com/wp-content/uploads/2011/08/Cecum-Picture-6.jpg

COMPARISONS:
 Rat Shark Human
 thin walled pouch shaped like a comma. It is very long in rats and allows for fermentation where cellulose fibers from wood are digested. Sharks do no have the cecum portion of the large intestine. 


In humans, the cecum is simply a dead-end pouch at the beginning of the large intestine. The vestigial appendix organ attached to the human cecum.







Ascending Colon

  • The ascending colon is a hollow tube with small pouches along its length to increase the surface area for absorption. The ascending colon ascends (as the name suggests) up along the side of the right side of the abdominal cavity from the cecum to the transverse colon. 
  • Tissue: The ascending colon is composed of multiple tissue layers...
    • Mucosa: innermost layer made of smooth mucus membrane and goblet cells which secrete mucus for lubrication. Absorption of vitamins and water from the feces is carried out by the epithelial cells of this mucus membrane and transferred to blood in capillaries close by. 
    • Submucosa: surrounds the mucosa and contains blood vessels and nerves which support surrounding tissues.
    • Muscularis: has layered bands of smooth muscle tissue arranged in a perpendicular fashion which can create peristalsis contractions. The pouches of the colon are a result of the contraction of smooth muscle here.
    • Outermost layer is covered with the peritoneum on the anterior side and with areolar connective tissue on the posterior side. Both of these tissues hold the colon in place and provide it with blood vessels as well as protect it against friction from surrounding abdominal organs
  • Digestion: In the ascending colon, bacteria digest the waste that  we cannot and make available vitamins K, B1, B2, and B12. These vitamins are absorbed by the walls of the colon, as well as water in order to prevent dehydration and maintain water homeostasis. 


http://www.elu.sgul.ac.uk/rehash/guest/scorm/229/package/content/images/ascending.gif

COMPARISONS:
 Rat Shark Human
 Food travels upward from the cecum into the ascending colon and bacterial digestion continues. Sharks do not have an ascending colon due to the fact that their colons only descend. Food travels upward from peristalsis contractions in the cecum area and bacterial digestion continues.




Transverse Colon

  • The transverse colon is the largest segment of the colon and gets its name due to the fact that it crosses the abdominal cavity from the right side to the left. Many pouches called haustra are formed here by the contraction of smooth muscle tissue.
  • Tissue: composed of four tissue layers- the mucosa, the submucosa, the muscularis, and the serosa.
    • Mucosa: innermost layer made of smooth mucus membrane and goblet cells which secrete mucus for lubrication. Absorption of nutrients is carried out by the epithelial cells of this mucus membrane.
    • Submucosa: surrounds the mucosa and contains blood vessels and nerves which support surrounding tissues.
    • Muscularis: has bands of smooth muscle tissue arranged in a perpendicular fashion to contract the intestinal walls. 
    • Serosa: outermost layer composed of simple squamous epithelial tissue. This layer produces a slick fluid lubricating the exterior of the cecum to prevent friction from the surrounding abdominal tissues.
  • Digestion: The transverse colon's job is to mix feces through segmentation. Segmentation is the contraction of small regions in the intestinal wall. While the feces are being mixed, bacteria further digest the material and release vitamins and nutrients to be absorbed back into the bloodstream along with water. Peristalsis contractions then move the feces further along on its path through the GI tract. 


http://static1.squarespace.com/static/5133c92be4b03c8ebed848b4/t/53e6325ce4b065351cded2d5/1407595101668/?format=300w

COMPARISONS:
 Rat Shark Human
 After ascending, the food travels across the transverse colon where it's bacterial digestion process continues. Sharks do not have a transverse colon because their colons only go downward and do not lay across their abdominal cavities. The chyme travels up the ascending colon and across the transverse colon and continues to be digested by bacteria and pass on the nutrients, vitamins, and water into the bloodstream through absorption.




Descending Colon

  • The descending colon transports feces inferiorly from the transverse colon, down the left side into the sigmoid colon. This portion of the large intestine also contains many haustra pouches from the contraction of smooth muscle tissue. Much of the feces are stored in the descending colon before defecation.
  • Tissue: Composed of numerous tissue layers-
    • Mucosa: innermost layer made of smooth mucus membrane and goblet cells which secrete mucus for lubrication. Absorption of nutrients is carried out by the epithelial cells of this mucus membrane.
    • Submucosa: surrounds the mucosa and contains blood vessels and nerves which support surrounding tissues.
    • Muscularis: has bands of smooth muscle tissue arranged in a perpendicular fashion to contract the intestinal walls. Waves of peristalsis contractions during defecation move the feces down into the sigmoid colon which is made possible by the muscularis tissue.
    • Outermost anterior layer is the visceral peritoneum while anteriorly it is the areolar connective tissue (adventitia). These two tissues anchor the descending colon to the body wall provide it with blood, and prevent friction.
  • Digestion: Usually by the time that feces reach the descending colon, the majority of nutrients, vitamins, and water have already been extracted from it, but the walls of the descending colon still perform bacterial fermentation to release a small amount of vitamins and absorb water. However it mostly serves to accumulate and store feces before defecation.


http://www.mhs.net/library/graphics/images/en/10255.jpg

COMPARISONS:
 Rat  Shark  Human
 The section of the large intestine that travels back down toward the rectum. It is where the final stages of water, nutrient, and vitamin absorption occur. You could argue perhaps that sharks have a descending colon because their colons only descend. However, many consider the colons in sharks and other fish to only be short rectums along with a rectal gland unique to the shark that secretes salt to maintain the osmotic balance between the shark and the seawater. The descending colon stores the remains of undigested food until it can be pushed down to the sigmoid colon and then the rectum to be excreted. Like in the rat, the final stages of nutrient, vitamin, and water absorption can happen here to prevent dehydration and maintain water homeostasis inside of the body.


Sigmoid Colon

  • The curved, S-shaped sigmoid colon is the last region of the large intestine.
  • Tissue: The sigmoid colon is composed of four tissue layers- the mucosa, the submucosa, the muscularis, and the serosa.
    • Mucosa: innermost layer made of smooth mucus membrane and goblet cells which secrete mucus for lubrication. Absorption of nutrients is carried out by the epithelial cells of this mucus membrane.
    • Submucosa: surrounds the mucosa and contains blood vessels and nerves which support surrounding tissues. Nerve tissue here monitors the contents of the sigmoid colon to control the defecation reflex.
    • Muscularis: has bands of smooth muscle tissue arranged in a perpendicular fashion to contract and form haustra. 
    • Serosa: outermost layer composed of simple squamous epithelial tissue. This layer produces a slick fluid lubricating the exterior of the cecum to prevent friction from the surrounding abdominal tissues. It extends to form the mesentery. 
  • Digestion: The feces entering the sigmoid colon have really had nearly all of the nutrients, vitamins, and water absorbed from them so the primary purpose of the sigmoid colon is to store feces and further propel them with peristalsis contractions into the rectum. Thus, the sigmoid colon has a large role in controlling the exiting of feces during defecation. Minimal absorption of minerals through additional bacterial fermentation occurs in the sigmoid colon.


http://static.guim.co.uk/sys-images/Guardian/About/General/2011/10/3/1317664151777/The-large-Intestine-007.jpg

COMPARISONS:
 Rat Shark Human
Technically rats do not have a sigmoid colon that crosses the body transversally.  Sharks don't exactly have a sigmoid colon either. The rectal gland mentioned earlier perhaps could be a part of the "sigmoid region", but the shark colon goes straight down and turns into the cloaca, unlike the sigmoid colon which lays slightly across the abdominal cavity. In humans, the sigmoid colon is the part of the large intestine that is closest to the rectum and anus and passes transversally across the front of the sacrum to the right side of the pelvis. Its function is to expel solid and gaseous waste from the body.

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