Human Digestive System: Structure and Function

The human digestive system is essential for breaking down food into small, absorbable components for energy, growth, and cell repair. This system includes the gastrointestinal (GI) tract, which extends from the mouth to the anus, and accessory digestive organs like the liver, pancreas, and gallbladder. The process of digestion involves several steps: ingestion, secretion, peristalsis, digestion, absorption, and defecation. These coordinated actions ensure that nutrients are effectively utilized by the body and waste products are expelled.

Understanding the Human Digestive System: Structure, Function, and Processes

Functions of the Human Digestive System

The human digestive system breaks down food 

• Proteins are broken down into amino acids.
• Fats are broken down into fatty acids and glycerol.
• Carbohydrates are broken down into simple sugars.
• Absorption and Utilization of Food: These broken-down components are small enough for absorption and utilization by the body.
• Functions in the Body: They serve various functions such as providing energy, supporting growth, and aiding in cell repair.
• Expulsion of Waste Products: Waste products that are not absorbed are expelled from the body.

• Components of the Digestive System: The digestive system is made up of: 
  • Gastrointestinal Tract (also called the GI tract/digestive tract)
  • Accessory Digestive Organs (including liver, pancreas, gall bladder etc.) 
• Processes Involved in Digestion: include ingestion, secretion (of enzymes, digestive juices, buffers etc.), peristalsis (the involuntary constriction and relaxation of the muscles of the GI tract, creating wave-like movements that push the contents of the canal forward), digestion (break down of food into absorbable molecules), absorption (of digested constituents by the epithelial cells lining the GI tract), and defecation (expulsion of waste products).

Gastrointestinal Tract

Structure of the Gastrointestinal Tract: The gastrointestinal (GI) tract, or alimentary canal, is a continuous tube that extends from the mouth to the anus. 

• Its walls, from the lower oesophagus to the anus, have the same basic, four-layered arrangement of tissues (from deep to superficial)- the mucosa, submucosa, muscularis propria, and the serosa. 
• The hollow organs that make up the GI tract are the mouth, oesophagus, stomach, small intestine, large intestine, and anus.
• Mouth: It is also known as the ‘Oral Cavity’ or ‘Buccal Cavity’ and performs two major functions, mastication of food and facilitation of swallowing. 
  • The saliva secreted by salivary glands in the mouth contains electrolytes and enzymes- salivary amylase (digestion initiating carbohydrate splitting enzyme) and lysozyme (that lyses many bacteria and prevents the overgrowth of oral microbial populations).
• Oesophagus: It secretes mucus and transports food into the stomach (not involved in digestion or absorption).
• Stomach: The stomach stores the food for 4-5 hours; here, food mixes thoroughly with the acidic gastric juice by the churning movements of its muscular walls and is called the chyme. 
  • Activation of Pepsinogen: The proenzyme pepsinogen, on exposure to hydrochloric acid, gets converted into the active enzyme pepsin, the proteolytic enzyme (enzymes that break down protein) of the stomach. 
  • Function of Pepsin: Pepsin converts proteins into proteoses and peptones (peptides). 
  • Role of Mucus and Bicarbonates in Gastric Juice: The mucus and bicarbonates present in the gastric juice play an important role in the lubrication and protection of the mucosal epithelium from excoriation (abrasion) by highly concentrated hydrochloric acid. 
  • Optimal pH for Pepsins: HCl provides the pH 1.8 optimal for pepsins. 
  • Function of Rennin: Rennin is a proteolytic enzyme found in the gastric juice of infants, which helps in the digestion of milk proteins.
• Small Intestine: Here, various types of movements generated by the muscularis layer help in a thorough mixing up of the food with various secretions (bile, pancreatic juice and intestinal juice), facilitating digestion.
• Composition of Pancreatic Juice and Bile: Pancreatic juice and bile are released through the hepato-pancreatic duct. 
  • The pancreatic juice contains inactive enzymes- trypsinogen, chymotrypsinogen, procarboxypeptidase, amylases, lipases and nucleases. 
• Functions of Bile: The bile released into the duodenum contains bile pigments (bilirubin and biliverdin), bile salts, cholesterol and phospholipids but no enzymes and helps in emulsification of fats (breaking down of the fats into very small micelles) and activation of lipases. 
• Composition and Functions of Intestinal Juice: The secretions of the brush border cells of the mucosa, along with the secretions of the goblet cells, constitute the intestinal juice or succus entericus. 
  • This juice contains a variety of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc. 
  • The mucus, along with the bicarbonates from the pancreas, protects the intestinal mucosa from acid as well as provides an alkaline medium (pH 7.8) for enzymatic activities. 
• Digestion and Absorption in the Small Intestine: The breakdown of biomacromolecules occurs in the duodenum region of the small intestine, and the simple substances thus formed are absorbed in the jejunum and ileum regions of the small intestine.
• Large Intestine: The undigested and unabsorbed substances are passed on to the large intestine, whose functions include:
  • Absorption of some water, minerals and certain drugs.
  • Secretion of mucus, which helps in adhering waste particles and lubricating them.
  • The undigested, unabsorbed substances called faeces enter into the caecum of the large intestine through the ileocecal valve, which prevents the backflow of the faecal matter. 
  • Faecum is temporarily stored in the rectum till defecation through the anus.

Accessory Digestive Organs

• Components Accessory Digestive Organs: The accessory digestive organs include the teeth, tongue, salivary glands, liver, gallbladder, pancreas etc., which perform various functions in the course of digestion.

• Liver: It is the largest gland in the human body (1.2-1.5 kg), situated in the abdominal cavity, just below the diaphragm and has two lobes. 
  • Structure and Function of Hepatic Lobules: The hepatic lobules are the structural and functional units of the liver containing hepatic cells arranged in the form of cords. 
  • Bile Secretion and Storage: The bile secreted by the hepatic cells passes through the hepatic ducts and is stored and concentrated in a thin muscular sac called the gallbladder. 
  • Role of Gallbladder in Digestion: The gallbladder sends this stored bile into the small intestine to aid in the digestion of food. 
  • Formation of Common Bile Duct: The duct of the gallbladder (cystic duct), along with the hepatic duct from the liver, forms the common bile duct. 
  • Liver’s Detoxification Function: The liver is vital for the detoxification and destruction of endogenous and exogenous substances that are harmful to the body.
• Pancreas: The pancreas is an organ located in the abdomen that has two main functions: 
  • Exocrine Function: The pancreas contains exocrine glands that produce enzymes trypsin and chymotrypsin to digest proteins, amylase for the digestion of carbohydrates, and lipase to break down fats.
  • Endocrine Function: The endocrine component of the pancreas consists of islet cells (islets of Langerhans) that create and release important hormones, insulin (lowers blood sugar) and glucagon (raises blood sugar) directly into the bloodstream regulating sugar levels. 

Conclusion

• The digestive system is essential for breaking down food, absorbing nutrients, and expelling waste. The GI tract and accessory organs work together to perform these functions seamlessly. 
• Key organs like the stomach, small intestine, liver, and pancreas have specialized roles in this complex process. Understanding how these components interact helps us appreciate the importance of maintaining a healthy digestive system. 
• The proper function of this system is vital for overall health and well-being, highlighting the need for a balanced diet and healthy lifestyle.