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   The liver functions to detoxify chemicals, metabolize drugs, secrete the digestive enzymes in the form of bile, and produces blood clotting factors. In the digestive system, the liver functions to secrete the digestive enzymes in the form of bile which helps to break down fats (lipids). The bile produced in the liver gets stored in the gallbladder which lies directly underneath the liver. The gallbladder's function is to store and concentrate the bile that was made in the liver. Bile can also bypass the gallbladder altogether and get secreted by a different pathway. 

   The destination of the bile is the duodenum of the small intestine. Bile coming from the gallbladder will leave the gallbladder through the cystic duct, then travel through the common bile duct to reach the duodenum. Bile coming from the liver directly, will travel through the hepatic ducts, then the common bile duct, and will end up at the duodenum. Therefore, the cystic duct and the hepatic duct come together to form the common bile duct. 

The Small Intestines

​    The small intestine is the longest part of the alimentary canal. It functions to digest food (chyme) and absorb its nutrients. The small intestine is the part of the intestine is where 90% of the digestion and absorption of food occurs.  Digestive enzymes made by the pancreas travel to the duodenum of the small intestine. The small intestines are also lined with smooth muscle that contracts and relaxes to assist digestion and absorption and to move food through the 3 to 5 meters of small intestine you have in your body. This process takes 3 to 6 hours.

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Subdivisions of the Small Intestine

     The small intestine begins at the pyloric sphincter of the stomach and is divided into three sections:

1. The duodenum
2. The jejunum
3. The ileum.  

​    The jejunum is the segment of the small intestine that comes after the duodenum. Most of the nutrients present in food are absorbed by the jejunum before being passed on to the ileum for further absorption.

    The ileum is the last part of the three part tube that makes up the small intestine. It is where the remaining nutrients are absorbed before moving into the large intestine.

     The stomach transforms your food into a paste-like, partially digested substance called chyme. The chyme leaves the stomach via the pyloric valve, and enters the duodenum which is the first part of the small intestine. The duodenum is the smallest portion of the small intestine. 

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Large Intestine

    The large intestine is the terminal portion of the alimentary canal (commonly known as the gastrointestinal tract). The large intestine function to absorb water and vitamins while converting undigested food into feces. The large intestine borders the small intestine and can be up to 5 feet long.    
     The large intestine is separated into the following areas:

1. Cecum
2. Ascending Colon
3. Transverse Colon
4. Descending Colon
5. Sigmoid Colon
6. Rectum
7. Anal Canal 

    Food (or chyme) travels from the ileum of the small intestine to the cecum of the large intestine via the ileocecal valve. The cecum is the beginning of the large intestine. The cecum functions to provides a space for the mixing of bacteria with partially digested food from the small intestine to form feces. 

ascending colon

transverse colon

descending colon

sigmoid colon

The Appendix

    Extending from the inferior end of the large intestine’s cecum, the human appendix is a narrow outcropping of tissue. whose resemblance to a worm inspired its alternate name, vermiform (worm-like) appendix. The appendix now functions to store immune cells and healthy bacteria (probiotics) that aid digestion. The appendix is sometimes referred to as a vestigial organ, because early humans, the appendix carried enzymes needed to breakdown cellulose. This organ has since lost this function due to evolution.  

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The ileocecal valve and the cecum

Chyme (paste of food with gastric juices) leaves the ileum of the small intestine and enter into the cecum of the large intestine through the ileocecal valve. The ileocecal valve is a sphincter muscle valve that separates the small intestine and the large intestine. It functions to prevent substances flowing back into the ileum once they have reached the cecum. 

Once food has been further broken down in the cecum, it will travel up the ascending colon, across the transverse colon, down the descending colon, then through the sigmoid colon, rectum and finally, out the anus. 

via GIPHY

The main function of the large intestine (also called the colon) is to concentrate and produce waste in the form of feces. 

As the food makes its journey, some water will be absorbed, leaving behind waste products to be eliminated from the body in the form of feces. 

The Mesentery

     You might recall that body cavities are lined with a specialized epithelium formed by the fusion of two layers of serous membranes. The peritoneum is the serous membrane that lines the abdominal cavity and covers the abdominal organs. The abdominal organs are also called the visceral organs. The mesentery is a double layer of peritoneum that extends from the body wall to the digestive organs.
    All intraperitoneal organs, including both the jejunum and ileum, are attached to the posterior wall of the abdomen by the mesentery. The mesenteries function to hold organs in place, store fat, and allows vessels and nerves to reach the organs. For example, in order for the digestive organs to receive their blood supply, blood vessels branch off the aorta and travel through mesenteries to reach the digestive organs. The majority of the mesenteries connect the posterior abdominal wall to the alimentary canal. There in one mesentery that lies in the superior abdominal which is a ventral mesentery that extends from the stomach and liver.

​The mesenteries function to attach the digestive organs to the abdominal wall.

Histology of the Jejunum (small intestine)

In the cross-section of the jejunum, several histological features can be seen. Since the small intestine is essentially a long tube, a cross-section will result in a donut-shaped structure. The central part of the small intestine is filled with liquid called the lumen.

The lumen is lined by the inner-most layer of the jejunum called the mucosa. The mucosa contains finger-like projection called villi which function to increase the surface area. The villi contain simple columnar epithelial cells and goblet cells that secrete mucus. 

 The layers of the jejunum. starting with the innermost layer, is are the

1. MUCOSA
   1. has the villi which are lined with columnar epithelial cells and goblet cells.
2. SUBMICOSA
   1. loose connective tissue underlying the mucosa.
3. MUSCLE LAYERS
   1. consists of the outer and inner layer of muscle fibers that function to move food through the intestines and to make contact with food for absorbtion of nutrients. The action of these muscles is called paristalsis. 
4. SEROUS LAYER
   1. connective tissue covered by peritoneum.​