Digestion and Absorption

The Digestive System

The digestive system in humans is responsible for the breakdown of complex food substances into simpler molecules that can be absorbed into the bloodstream and transported to the body's cells. This process involves both mechanical and chemical means.

The human digestive system consists of two main parts:

1. The Alimentary Canal (a long, continuous tube through which food passes).
2. The associated Digestive Glands (that secrete digestive enzymes and juices).

Alimentary Canal

The alimentary canal is a tube measuring about 8-10 metres in length, extending from the mouth to the anus. It is also known as the digestive tract.

Parts of the Alimentary Canal (in order):

1. Mouth: The opening where food is taken into the body. It leads into the buccal cavity or oral cavity.
2. Buccal Cavity (Oral Cavity): Contains the teeth and tongue.
   • Teeth: Used for mastication (chewing) of food. Humans have different types of teeth (incisors, canines, premolars, molars), representing heterodont dentition. We have two sets of teeth during our lifetime (milk teeth followed by permanent teeth), representing diphyodont dentition.
   • Tongue: A muscular organ that helps in mixing food with saliva, swallowing, and tasting food.
3. Pharynx: A short passageway that serves as a common passage for both food and air. The opening to the trachea (windpipe) is covered by a cartilaginous flap called the epiglottis, which prevents food from entering the trachea during swallowing.
4. Oesophagus: A long, muscular tube that passes through the neck, thorax, and diaphragm, leading to the stomach. Food is moved down the oesophagus by peristalsis (wave-like muscular contractions).
5. Stomach: A J-shaped muscular organ located in the upper left portion of the abdominal cavity. It stores food and mixes it with gastric juices. The stomach has four main parts: cardiac (near oesophagus), fundic, body (main central region), and pyloric (opening into small intestine). The opening of the oesophagus into the stomach is guarded by the gastro-oesophageal sphincter. The opening of the stomach into the small intestine (duodenum) is guarded by the pyloric sphincter.
6. Small Intestine: The longest part of the alimentary canal (about 6-7 metres in adults). It is the primary site for the completion of digestion and absorption of nutrients. It is highly coiled and consists of three parts:
   • Duodenum: The first short, U-shaped part. Receives bile from the liver and pancreatic juice from the pancreas.
   • Jejunum: The middle, longer, and more coiled part.
   • Ileum: The final, highly coiled part, which opens into the large intestine.

   The wall of the small intestine is lined with millions of finger-like projections called villi, which increase the surface area for absorption.

7. Large Intestine: Shorter than the small intestine (about 1.5 metres) but wider. It is mainly involved in the absorption of water, some minerals, and vitamins produced by intestinal bacteria. It also stores undigested food material. It consists of three parts:
   • Caecum: A small blind sac at the junction of the small and large intestine. A vestigial organ, the vermiform appendix, is a finger-like projection from the caecum.
   • Colon: The main part of the large intestine, divided into ascending, transverse, descending, and sigmoid colon.
   • Rectum: The final section, which stores faeces temporarily.
8. Anus: The terminal opening of the alimentary canal through which faeces are eliminated from the body (defaecation). It is controlled by anal sphincters.

*(Image shows a diagram of the human digestive tract highlighting the main organs: mouth, pharynx, oesophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (caecum, appendix, colon, rectum), anus, and associated glands like salivary glands, liver, pancreas)*

Histology of the Alimentary Canal Wall:

The wall of the alimentary canal, from the oesophagus to the rectum, generally has four layers:

1. Serosa: The outermost layer, a thin mesothelium (epithelium of visceral organs) with some connective tissue.
2. Muscularis: Formed by smooth muscles, usually arranged into an inner circular layer and an outer longitudinal layer. An oblique muscle layer may be present in the stomach. Muscularis is responsible for peristalsis.
3. Submucosa: Loose connective tissue layer containing nerves, blood vessels, and lymph vessels. Glands may be present in the submucosa of the duodenum (e.g., Brunner's glands).
4. Mucosa: The innermost layer lining the lumen of the alimentary canal. It forms irregular folds in the stomach (rugae) and small finger-like foldings called villi in the small intestine. The mucosal epithelium also has goblet cells, which secrete mucus for lubrication. The mucosa forms glands in the stomach (gastric glands) and crypts between the bases of villi in the intestine (crypts of Lieberkuhn).

*(Image shows a cross-section diagram of the gut wall highlighting the four layers: Serosa, Muscularis (circular, longitudinal), Submucosa, and Mucosa (epithelium, lamina propria, muscularis mucosae) with villi/rugae and glands)*

Digestive Glands

Digestive glands secrete enzymes and other substances that aid in the digestion of food. These glands are either associated with the alimentary canal or are part of its wall.

Major Digestive Glands:

1. Salivary Glands: Three pairs of salivary glands are located outside the buccal cavity:
   • Parotid glands: Located near the cheeks.
   • Submaxillary/Submandibular glands: Located below the jaw.
   • Sublingual glands: Located below the tongue.

   They secrete saliva into the buccal cavity. Saliva contains salivary amylase (ptyalin), lysozyme, and electrolytes.

2. Liver: The largest gland in the body. Located in the upper right portion of the abdominal cavity. It secretes bile, which is stored in the gall bladder. Bile is released into the duodenum. Bile does not contain enzymes, but it helps in the emulsification of fats (breaking down large fat globules into smaller ones) and provides an alkaline medium for the action of pancreatic and intestinal enzymes.
3. Pancreas: A mixed gland (both exocrine and endocrine). Located behind the stomach. The exocrine part secretes pancreatic juice, which contains various digestive enzymes (e.g., pancreatic amylase, lipases, nucleases, trypsinogen, chymotrypsinogen). Pancreatic juice is released into the duodenum through the pancreatic duct.

Glands in the Alimentary Canal Wall:

• Gastric glands: Present in the wall of the stomach. They secrete gastric juice, which contains hydrochloric acid (HCl), pepsinogen, mucus, and intrinsic factor.
• Intestinal glands: Present in the wall of the small intestine (crypts of Lieberkuhn and Brunner's glands in the duodenum). They secrete intestinal juice (succus entericus), which contains various enzymes (e.g., disaccharidases, lipases, peptidases) and mucus.

The coordination of muscle contractions (peristalsis) and the secretion of digestive juices are regulated by both nervous and hormonal mechanisms.

Digestion of Food

Digestion is the process of breaking down complex food molecules (carbohydrates, proteins, fats, nucleic acids) into simpler, absorbable molecules. This is achieved through both mechanical and chemical processes.

Digestion in the Mouth (Buccal Cavity):

• Mechanical digestion: Mastication (chewing) by teeth breaks down food into smaller pieces.
• Chemical digestion: Saliva is mixed with food.
  • Salivary amylase (Ptyalin): Digests starch into disaccharides (maltose). Works best at a slightly acidic pH (around 6.8). About 30% of starch is hydrolysed in the mouth.
  • Lysozyme: An antibacterial agent, prevents infections.
• Food is converted into a bolus (mass of chewed food) and swallowed (deglutition) into the pharynx and then the oesophagus.

$ \text{Starch} \xrightarrow{\text{Salivary Amylase, pH 6.8}} \text{Maltose (disaccharide)} + \text{Isomaltose} + \text{Limit dextrins} $

Digestion in the Stomach:

• Food is mixed with gastric juice by the churning movements of the stomach wall.
• Hydrochloric acid (HCl):
  • Provides an acidic pH (1.8-2.0) for the action of pepsin.
  • Kills most bacteria and other pathogens ingested with food.
  • Denatures proteins, making them more accessible to pepsin.
• Pepsinogen: An inactive enzyme precursor (zymogen) secreted by chief cells. In the presence of HCl, pepsinogen is converted into the active enzyme Pepsin.

  $ \text{Pepsinogen} \xrightarrow{HCl} \text{Pepsin (active)} $

• Pepsin: A protein-digesting enzyme (protease). Digests proteins into proteoses and peptones (smaller peptides).

  $ \text{Proteins} \xrightarrow{\text{Pepsin, pH 1.8-2.0}} \text{Proteoses} + \text{Peptones} $

• Mucus: Secreted by goblet cells. Lubricates food and protects the stomach lining from the action of HCl and pepsin.
• Rennin: A proteolytic enzyme found in the gastric juice of infants. Helps in the digestion of milk proteins (casein) by coagulating milk. Absent in adults.
• The partially digested, acidic food in the stomach is called chyme. Chyme is slowly released into the duodenum through the pyloric sphincter.

Digestion in the Small Intestine:

Digestion is completed in the small intestine, particularly in the duodenum, with the help of bile, pancreatic juice, and intestinal juice.

Action of Bile:

• Bile (from liver/gall bladder) is released into the duodenum.
• Contains bile salts (e.g., sodium taurocholate, sodium glycocholate), which emulsify fats into smaller droplets. This increases the surface area for lipase action.
• Bile also makes the medium alkaline (pH around 7.8), which is necessary for the action of pancreatic and intestinal enzymes.

Action of Pancreatic Juice:

• Pancreatic juice is released into the duodenum and contains several enzymes in inactive forms (zymogens) or active forms.
• Trypsinogen: Inactive precursor. Activated by the enzyme Enterokinase (secreted by the intestinal mucosa) into active Trypsin.

  $ \text{Trypsinogen} \xrightarrow{\text{Enterokinase}} \text{Trypsin} $

• Trypsin then activates other zymogens in the pancreatic juice:
  • Chymotrypsinogen $\rightarrow$ Chymotrypsin
  • Procarboxypeptidase $\rightarrow$ Carboxypeptidase
• Pancreatic Amylase: Continues the digestion of starch into disaccharides.
• Pancreatic Lipase: Digests emulsified fats into diglycerides and monoglycerides.

  $ \text{Fats} \xrightarrow{\text{Lipases}} \text{Diglycerides} + \text{Monoglycerides} $

• Nucleases: Digest nucleic acids (DNA, RNA) into nucleotides.
• Protein digestion by pancreatic enzymes: Trypsin, Chymotrypsin, and Carboxypeptidase digest proteins and partially digested proteins (proteoses, peptones) into smaller peptides.

  $ \text{Proteins, Peptones, Proteoses} \xrightarrow{\text{Trypsin/Chymotrypsin/Carboxypeptidase}} \text{Peptides} $

Action of Intestinal Juice (Succus entericus):

• Intestinal juice is secreted by the intestinal glands and acts on the remaining food particles. It contains various enzymes.
• Disaccharidases: Digest disaccharides into monosaccharides.
  • Maltose $\xrightarrow{\text{Maltase}}$ Glucose + Glucose
  • Lactose $\xrightarrow{\text{Lactase}}$ Glucose + Galactose
  • Sucrose $\xrightarrow{\text{Sucrase}}$ Glucose + Fructose
• Peptidases: Digest peptides into amino acids (final products of protein digestion). Includes aminopeptidases and dipeptidases.
• Intestinal Lipase: Digests remaining diglycerides and monoglycerides into fatty acids and glycerol.

  $ \text{Diglycerides/Monoglycerides} \xrightarrow{\text{Lipases}} \text{Fatty acids} + \text{Glycerol} $

• Nucleotidases and Nucleosidases: Digest nucleotides into nucleosides and then into sugars and bases.

The final breakdown products of digestion are absorbed in the small intestine. These include monosaccharides (glucose, fructose, galactose), amino acids, fatty acids, glycerol, nitrogenous bases, and simple sugars (from nucleic acids), water, vitamins, and minerals.

Absorption of Digested Products

Absorption is the process by which the final products of digestion are transported from the lumen of the alimentary canal across the intestinal mucosa into the blood or lymph.

Site of Absorption:

The small intestine is the principal organ for the absorption of nutrients. The structure of the small intestine is highly adapted for absorption, with the presence of villi (finger-like projections on the mucosal surface) and microvilli (brush border on the surface of epithelial cells), which collectively increase the surface area for absorption by several hundred times.

Some absorption also occurs in the mouth (certain drugs), stomach (water, simple sugars, alcohol), and large intestine (water, some minerals, vitamins).

Mechanisms of Absorption:

Absorption occurs through various mechanisms:

• Passive Absorption (Simple Diffusion): Movement of substances along a concentration gradient, without the help of carrier proteins or energy.

  Example: Small monosaccharides (like glucose, some amino acids), electrolytes (like $Cl^-$ ions), and some fatty acids and glycerol.

• Facilitated Transport: Movement of substances along a concentration gradient with the help of carrier proteins in the membrane, but without the direct expenditure of energy.

  Example: Fructose, some amino acids.

• Active Transport: Movement of substances against a concentration gradient, requiring carrier proteins and energy (ATP).

  Example: Glucose, many amino acids, electrolytes (like $Na^+$ ions).

• Osmosis: Movement of water along its osmotic gradient.

Absorption of Specific Nutrients:

• Monosaccharides: Glucose and galactose are absorbed mainly by active transport and some by facilitated diffusion. Fructose is absorbed by facilitated transport.
• Amino acids: Most amino acids are absorbed by active transport (requiring $Na^+$ ions). Some are absorbed by facilitated diffusion or passive diffusion.
• Fatty acids and Glycerol: These are insoluble in water. They cannot be directly absorbed into the blood.
  • Bile salts emulsify fats, which are then broken down by lipases into fatty acids and monoglycerides.
  • These are incorporated into small, water-soluble spherical aggregates called micelles, which are formed with the help of bile salts.
  • Micelles move into the intestinal lumen and reach the mucosal surface. Fatty acids and monoglycerides are released from micelles and diffuse passively into the intestinal epithelial cells.
  • Inside the epithelial cells, fatty acids and monoglycerides are re-synthesised into triglycerides (fats).
  • These triglycerides are then packaged into small protein-coated fat globules called chylomicrons.
  • Chylomicrons are transported into the lymph vessels (lacteals) within the villi, not directly into the blood capillaries.
  • The lymph eventually carries the chylomicrons into the bloodstream.

  *(Image shows a cross-section of an intestinal villus illustrating fatty acids/monoglycerides entering epithelial cell, forming triglycerides and chylomicrons, and entering the lacteal (lymph capillary))*

• Water: Absorbed by osmosis. The movement of water depends on the osmotic gradient created by the absorption of solutes.
• Minerals and Vitamins: Absorbed by various mechanisms (passive, active, facilitated), depending on the specific mineral or vitamin. Water-soluble vitamins are absorbed into the blood; fat-soluble vitamins (A, D, E, K) are absorbed along with fats in micelles and chylomicrons into the lymph.

Absorption of substances occurs throughout the small intestine, but the rate of absorption is highest in the jejunum and ileum. Absorption of water and electrolytes is significant in the large intestine.

Absorption in Different Parts of the Alimentary Canal:

• Mouth: Certain drugs.
• Stomach: Water, simple sugars, alcohol.
• Small Intestine (Duodenum, Jejunum, Ileum): Maximum absorption of digested nutrients (monosaccharides, amino acids, fatty acids, glycerol, vitamins, minerals), and water.
• Large Intestine: Water, some minerals, drugs, and vitamins produced by bacteria.

The absorbed nutrients are transported by blood and lymph to the body's tissues and cells, where they are utilised for energy, growth, and repair.

Disorders of Digestive System

The digestive system is susceptible to various disorders, often caused by infections, inflammation, or functional problems. These disorders can affect digestion, absorption, and elimination.

Common Disorders:

• Jaundice: A condition where the skin and eyes turn yellow due to the deposition of bile pigments. It is caused by the malfunction of the liver, which affects the processing of bilirubin. It is a symptom of liver disease.
• Vomiting: The ejection of stomach contents through the mouth. It is a reflex action controlled by the vomit centre in the medulla oblongata. A feeling of nausea often precedes vomiting.
• Diarrhoea: An abnormal frequency of bowel movement and increased liquidity of the faecal discharge. It reduces the absorption of food. Causes include bacterial or viral infections, food intolerance, certain medications. Severe diarrhoea can lead to dehydration.
• Constipation: Infrequent bowel movements where the faeces are retained within the rectum. It is caused by reduced peristaltic movement of the colon. Factors include insufficient fibre in the diet, inadequate fluid intake, lack of exercise, and ignoring the urge to defecate.
• Indigestion (Dyspepsia): A feeling of fullness, discomfort, or pain in the upper abdomen. It can be caused by inadequate enzyme secretion, anxiety, food poisoning, overeating, or spicy food.
• Protein-Energy Malnutrition (PEM): Dietary deficiencies of proteins and total food calories. This is a widespread problem in many underdeveloped countries, affecting a large section of the population during drought, famine, and political turmoil. PEM affects children to produce:
  • Marasmus: Severe deficiency of both proteins and calories. Occurs in infants less than a year old if breast feeding is replaced too early by diets poor in protein and calories. Leads to emaciation, loss of body fat and muscle, dry and wrinkled skin, stunted growth, impaired brain development.
  • Kwashiorkor: Deficiency of protein, often with adequate calorie intake. Occurs in children over one year of age, typically after weaning when the diet lacks protein. Leads to wasting of muscles, stunted growth, failure of brain development, oedema (swelling due to fluid retention) of legs and face, pot-belly abdomen, thin limbs, and hair changes.
• Other disorders: Appendicitis (inflammation of the appendix), Peptic ulcers (sores in the lining of stomach or duodenum), Inflammatory Bowel Disease (IBD), Irritable Bowel Syndrome (IBS).

Maintaining a balanced diet, adequate hydration, good hygiene, and a healthy lifestyle are important for the proper functioning of the digestive system.