Digestive System


  • The digestive system is a complex network of organs responsible for processing food and extracting nutrients. 
  • The human digestive system comprises the alimentary canal, which is a long tube running from the mouth to the anus, and various associated glands. 
  • It begins with the intake of food through the mouth and ends with the elimination of waste. 
  • Organs of the Digestive System: Mouth,esophagus, stomach, small intestine, large intestine, rectum and anus. 
  • Digestive processes are mechanical digestion, chemical digestion, absorption and elimination. 
  • Role of Enzymes:

- Enzymes are biological catalysts that speed up chemical reactions in the body.

- Digestive enzymes such as amylase, protease, and lipase break down carbohydrates, proteins, and fats respectively into absorbable forms. 

  • Importance of Digestive Health:

- Proper functioning of the digestive system is essential for overall health and well-being.

- Digestive problems can lead to discomfort, nutrient deficiencies, and other health issues.




Alimentary Canal




Mouth and Oral Cavity:




  • The journey begins with the mouth, the anterior opening of the alimentary canal. 
  • The mouth leads to the oral cavity or buccal cavity, which houses teeth and the tongue. 
  • Teeth are embedded in sockets of the jawbone, a type of attachment called thecodont. 
  • Humans, like most mammals, undergo two sets of teeth: deciduous (milk) teeth replaced by permanent teeth, known as diphyodont dentition. 
  • Adults have 32 permanent teeth, including incisors (I), canines (C), premolars (PM), and molars (M), arranged in a specific order represented by a dental formula. 
  • Dental formula of human teeth - 2123/2123 
  • This indicates that in one-half of the upper and lower jaws, there are:

2 incisors

1 canine

2 premolars

3 molars




  • Enamel, the hard surface of teeth, aids in chewing or mastication.









  • The tongue is a muscular organ attached to the floor of the oral cavity by the frenulum. 
  • The tongue, a versatile muscle, assists in moving food around the mouth. 
  • Its upper surface contains papillae, some of which house taste buds responsible for detecting flavors.






  • The oral cavity leads to the pharynx, a short passage for both food and air. 
  • The pharynx serves as a common pathway for the esophagus and trachea (windpipe), with a cartilaginous flap called the epiglottis preventing food from entering the windpipe during swallowing. 
  • From there, the esophagus takes over, gently guiding swallowed food towards the stomach using rhythmic contractions.



  • The oesophagus, a thin and elongated tube, extends from the throat, passing through the neck, thorax, and diaphragm. 
  • It connects the throat to the stomach and transports swallowed food using muscular contractions. 
  • A muscular sphincter, the gastro-oesophageal sphincter, controls the opening between the oesophagus and stomach, preventing reflux of stomach contents.





  • Located in the upper left abdomen, the stomach is a 'J' shaped organ with three main regions: the cardiac portion, fundic region, and pyloric portion. 
  • The cardiac portion receives food from the oesophagus, while the pyloric portion connects to the first part of the small intestine. 
  • Gastric glands in the stomach lining secrete digestive enzymes and acids to break down food further.


Small Intestine:

  • The small intestine consists of three distinct regions: the duodenum, jejunum, and ileum. 
  • The duodenum is shaped like a 'C' and receives partially digested food from the stomach. 
  • The jejunum is a long, coiled portion responsible for further digestion and nutrient absorption. 
  • The highly coiled ileum connects to the large intestine and completes the absorption process.




Large Intestine:

  • The large intestine includes the caecum, colon, and rectum. 
  • The caecum, a small blind sac, houses symbiotic microorganisms that aid in digestion. 
  • The vermiform appendix, a vestigial organ, protrudes from the caecum. 
  • The colon is divided into ascending, transverse, and descending parts, with the descending part connecting to the rectum. 
  • The rectum serves as a temporary storage site for feces before elimination through the anus.




Layers of the Alimentary Canal:




  • Serosa:

- The outermost layer of the alimentary canal is the serosa, composed of a thin mesothelium and connective tissues. 

- It provides protection and structural support to the digestive tract.


  • Muscularis:

- The muscularis layer consists of smooth muscles arranged into inner circular and outer longitudinal layers. 

- These muscles facilitate the movement of food along the digestive tract through contractions. 

- In some regions, an additional oblique muscle layer may be present, aiding in digestion.


  • Submucosa:

- The submucosa layer contains loose connective tissues, nerves, blood vessels, and lymph vessels. 

- Glands may be present in the submucosa, such as in the duodenum, contributing to digestive processes.


  • Mucosa:

- The innermost layer lining the lumen of the alimentary canal is the mucosa, consisting of several specialized structures.

 - In the stomach, it forms irregular folds called rugae, while in the small intestine, it contains finger-like projections called villi. 

- Villi are covered with microvilli, increasing the surface area for nutrient absorption. 

- Villi contain capillaries and a lacteal, a large lymph vessel, for nutrient transport. 

- Goblet cells within the mucosal epithelium secrete mucus, aiding in lubrication and protection. 

- Mucosa also houses glands, such as gastric glands in the stomach and crypts of Lieberkuhn in the intestine, contributing to digestion and secretion.




Digestive Glands 

Digestive glands are essential organs associated with the alimentary canal, aiding in the digestion and processing of food.


Salivary Glands:

  • Saliva, essential for digestion, is primarily produced by three pairs of salivary glands located near the buccal cavity. 
  • The parotid glands, situated in the cheeks, the sub-maxillary (or sub-mandibular) glands beneath the lower jaw, and the sub-lingual glands beneath the tongue, all contribute to saliva production. 
  • Salivary glands secrete saliva into the buccal cavity, where it begins the process of digestion by moistening and lubricating food, and containing enzymes like amylase to break down carbohydrates.



  • The liver holds the title of being the largest gland in the human body, weighing approximately 1.2 to 1.5 kilograms in adults. 
  • Positioned in the abdominal cavity just below the diaphragm, the liver boasts two distinct lobes. 
  • Hepatic lobules serve as both the structural and functional units of the liver, composed of hepatic cells arranged in cord-like formations. 
  • Each lobule is enveloped by a delicate connective tissue sheath known as Glisson's capsule, providing structural support. 
  • The liver's primary function includes the production of bile, a greenish-yellow fluid crucial for fat digestion. 
  • Bile, secreted by hepatic cells, traverses through hepatic ducts before accumulating and concentrating in the gallbladder, a thin muscular sac. 
  • The bile ducts, including those from the liver and gallbladder, merge to form the common bile duct. 
  • Concurrently, the pancreatic duct joins the common bile duct, collectively referred to as the common hepato-pancreatic duct. 
  • Together, they transport bile and pancreatic juices to the duodenum, the initial segment of the small intestine, where digestion continues. 
  • Guarding the entrance of the common hepato-pancreatic duct into the duodenum is the sphincter of Oddi, a circular muscle regulating the flow of bile and pancreatic juices.



  • The pancreas, a vital organ with dual functions, is situated amidst the curvature of the 'U' shaped duodenum. 
  • It boasts both exocrine and endocrine glands, each serving distinct roles in bodily functions. 
  • Exocrine Function:

- The exocrine portion of the pancreas secretes pancreatic juice, an alkaline fluid containing digestive enzymes. 

- These enzymes, including amylase, protease, and lipase, aid in the breakdown of carbohydrates, proteins, and fats in the small intestine.


  • Endocrine Function:

- In addition to its exocrine role, the pancreas functions as an endocrine gland, secreting hormones directly into the bloodstream. 

- Two crucial hormones produced by the endocrine portion of the pancreas are insulin and glucagon. 

- Insulin helps regulate blood sugar levels by facilitating the uptake of glucose by cells, while glucagon aids in raising blood sugar levels when they are too low.


  • Dual Role:

- The pancreas showcases its versatility by seamlessly balancing both exocrine and endocrine functions. 

- Its ability to produce digestive enzymes for nutrient breakdown and hormones for blood sugar regulation highlights its pivotal role in metabolic processes.