Parts and membranes of the human digestive system

                   
gafacom image of the parts of the digestive system and their functions

The Mouth

The first part of the digestive system is the mouth, the entry point of food. The smell and sight of food, stimulates your salivary glands to secrete fluid called saliva, consisting of water and enzymes. The purpose of the saliva is to lubricate the food for swallowing, dissolve water soluble food particles, and start chemical digestion of carbohydrates (starch) into smaller molecules. Amylase is the enzyme secreted. The pH of the mouth is 7. 

Salivary glands – are glands in the mouth that produces saliva to begin the chemical digestion of food. 
 
Saliva – is a watery secretion in the mouth that begins the digestive process. 
 
Mechanical digestion also begins here. The teeth and tongue are responsible for the breaking of large food particles into smaller food particles, while increasing surface area for faster chemical digestion. 

Each parotid gland and sub-mandibular gland release saliva into the mouth through the parotid duct and the sub-mandibular duct. The sublingual glands release saliva into the mouth through many smaller ducts.                    

The Esophagus  

The esophagus is a tube connecting the mouth to the stomach running through the Thoracic cavity. Your teeth and tongue move food around in the mouth turning the food into a mushy bolus. The tongue moves the bolus to the back of the throat for swallowing. The bolus enters the esophagus.  
 
The esophagus lies behind your windpipe (Trachea).  The trachea has as an epiglottis which prevents food from entering the windpipe, moving the food to the esophagus while swallowing. This prevents food from entering the windpipe and choking. Food travels down the esophagus, through a series of rhythmic contractions (wave-like) called peristalsis. The lining of the esophagus secretes mucus, lubricating to support the movement of food. 
 
When the bolus reaches the stomach, it must pass through a muscular ringed valve called the esophageal sphincter (Cardiac Sphincter). The role of the sphincter is to prevent stomach acids from back flowing into the esophagus creating a burning feeling known as heart burn. 





Peristalsis moves food through the esophagus by means of muscular contractions. When you vomit, or ‘throw up’ your stomach contents, the contractions of the esophagus are reversed. Similarly, small amounts of acidic liquid can escape from the stomach and move up the esophagus into your throat. This id experienced as a burning sensation in the throat or chest, commonly called heartburn or acid reflux.


The stomach is a muscular J-shaped organ found in the abdominal cavity. Food is temporarily stored in the stomach. The stomach is a muscular organ (three layers of muscle fibres) which performs mechanical digestion by churning the bolus and mixing it with the gastric juices (HCl, salts, enzymes, water and mucus) secreted by the lining of the stomach. The bolus is now called Chyme. 
 
Gastric Juices – are a mixture of hydrochloric acid, salts, enzymes, water and mucus that is produced by glands in the stomach to help digest food. 
 
The environment of the stomach is very acidic. HCl is secreted to kill any microbes that are found in the bolus, creating a pH of 2. Mucus prevents the stomach from digesting itself. 
 
Pepsin is also secreted. This enzyme is responsible for initiating the breakdown of proteins found in the food. Pepsin hydrolyzes proteins to yield polypeptides. Since the pH is 2, the enzyme from the salivary glands stops breaking down carbohydrates. 

Folds in the stomach wall allow it to expand and contract as it fills with food and then empties its contents into the small intestine.  
  
The stomach does not digest itself because of three protective mechanisms. First the stomach only secretes small amounts of gastric juices until food is present. Second the secretion of mucus coats the lining of the stomach protecting it from the gastric juices. The third mechanism is the digestive enzyme pepsin is secreted in an inactive protein called pepsinogen. Pepsinogen is converted to pepsin in the increased presence of hydrochloric acid (pH 1). 
  
The chyme moves from the stomach to the small intestine. It passes through a muscular ringed sphincter called the pyloric sphincter. 
 
Chyme – is a thick liquid produced in the stomach and made of digested food combined with gastric juice. 
 
Pepsin – is an enzyme in gastric juice that helps break down proteins into polypeptides. 

Accessory organs

The accessory organs that support the digestive system but are not part of the digestive tract are; the liver, gall bladder and the pancreas. These organs secrete fluids into the digestive tract, and are connect by ducts.

The liver is the largest of these organs, about the size of a football and a mass of about 1.5 kg. The liver produces bile, agreenishyellow pigment made up bile pigments and bile salts, as it breaks down old red blood cells.

The bile is secreted into a storage sac called the gall bladder. The bile salts are stored here between meals. The bile salts are secreted into the small intestine to digest fats. The bile causes the emulsification (increases surface area) of fat blobs found in the chyme from the stomach.
The pancreas secretes a number of different enzymes into the small intestine to digest carbohydrates, lipids and proteins completely. The pancreas also secretes bicarbonate ions which neutralize the HCl from the stomach and change the pH of the small intestine to a pH of 8. The pancreas will secrete about 1.0 L. of pancreatic fluids per day.

The Small Intestine 

The small intestine is responsible for the complete digestion of all macromolecules and the absorption of their component molecules (glucose, glycerol, fatty acids, amino acids and nucleotides). The process of absorption allows the component molecules to be diffused into the surrounding intestinal cells and then into the circulatory system for transport to all the cells in the body.  
 
  
 
The function of the digestive system is to digest and absorb food. It consists of  a tubular gastrointestinal tract and accessory organs that aid in digestion and absorption.
All organisms require food to sustain life. The cells of the body require nutrients for the chemical reactions of enzyme synthesis, cell division, growth and repair and also for the production of heat energy. Most of the food we eat requires considerable processing before it can be used by the cells. It must be broken down mechanically and chemically before it is transported by the blood to the cells.
The activities that are performed by the digestive system include the following activities:
1.    Ingestion: the taking of food into the mouth
2.    Mastication: chewing food which pulverizes it and mixes it with saliva
3.    Deglutination: Swallowing; moving food from the mouth to the pharynx and into the esophagus.
4.    Digestion: The mechanical and chemical breakdown of food to prepare it for absorption.
5.Absorption: the passage molecules of food through the mucous membrane of the small intestine and into the blood and lymph for distribution to the cells.
6.Peristalsis: the rhythmic wavelike contractions of the smooth muscle of the intestines that move food through the GI tract.
7. Defecation: the discharge of indigestible wastes (feces) from the GI tract.
Anatomically and functionally the digestive system can be divided into a tubular gastrointestinal (GI) tract and accessory digestive organs.  The GI tract which extends from the mouth to the anus is a continuous tube approximately 30 feet (9m) long. It goes through the thoracic cavity and enters the abdominal cavity through the diaphragm.



The organs of the digestive system include the oral cavity (mouth), pharynx, esophagus, stomach, small intestine and large intestine. The accessory organs include teeth, salivary glands, liver, gall bladder and pancreas.
It usually takes about 24-48 hours for food to travel the length of the GI tract. Food travels in an assembly line manner through the tract where it is broken down to the molecular level and transported to the cells. Each region of the GI tract has a specific function in the process 

 Membranes of the Abdominal Cavity

Most of the digestive organs are located in the abdominal cavity. These organs are covered by serous membranes that line the cavity and cover the organs within. Serous membranes secrete a lubricating serous fluid that continuously moistens the organs. The parietal membrane lines the wall of the abdominal cavity and the visceral membrane covers the internal organs. 
The membrane that lines the wall of the abdominal cavity is called the  parietal peritoneum. It comes together to form a double layered peritoneal fold called the mesentery

The mesentery supports the GI tract and at the same time allows the small intestine freedom for peristaltic contractions. It also provides a structure for the passage of blood vessels and nerves. The peritoneal membrane continues around the intestinal organs as the visceral peritoneum. The peritoneal cavity is the space between the parietal and visceral portions of the peritoneum. Certain organs lie posterior to the peritoneal cavity and are said to be retroperitoneal. These organs include  most of the pancreas, the kidneys, adrenal glands and portions of the duode num and colon as well as the abdominal aorta


Peritonitis is an inflammation of the peritoneum usually caused by an infection. This can occur due to trauma, rupture of an organ, an ectopic pregnancy or post operative infection. It is a serious life threatening situation. Treatment usually involves massive doses of antibiotics as well as insertion of a tube to drain excess fluid which accumulates.

Extensions of the parietal peritoneum serve to suspend or anchor organs within the peritoneal cavity. The falciform ligament attaches the  diaphragm and the anterior abdominal wall to  the liver. The greater omentum extends from the stomach to the transverse colon forming an apron like covering over most of the  small intestine. 

Function of the omentum includes storage of fat, cushioning visceral organs, supporting lymph nodes and protection against infection. in cases of infection such as appendicitis the greater omentum may actually compartmentalize the infection, sealing it off from the rest of the peritoneal cavity.  The lesser omentum passes from the lesser curve of the stomach and the upper duodenum to the inferior surface of the liver



Reference
https://www.niddk.nih.gov/health-information/digestive-diseases/digestive-system-how-it-works
https://www.medicinenet.com/the_digestion_process_organs_and_functions/article.htm
https://www.toppr.com/guides/biology/digestion-and-absorption/human-digestive-system/
https://my.clevelandclinic.org/health/articles/7041-the-structure-and-function-of-the-digestive-system
https://mcb.berkeley.edu/courses/mcb32/Miller%20notes-%20digestive%20system%20
https://www.webmd.com/digestive-disorders/digestive-system
https://www.diabetes.co.uk/body/digestive-system.html
https://www.verywellhealth.com/tour-the-digestive-system-4020262
https://training.seer.cancer.gov/anatomy/digestive/regions/

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