Human Digestive System UPSC & Other Exams

Ragini Chopra (MSc, Biological Sciences, Science Faculty UPSC)

Biology UPSC, Human Digestive System UPSC CSE

Human Digestive System UPSC

Biomacromolecules (carbohydrates, proteins etc.) in food cannot be utilized by our body in their original form. They have to be broken down and converted into simple substances (glucose, amino acids etc.) before utilized by our body.

The human digestive system is responsible for breaking down the food we consume into nutrients that can be absorbed and used by the body.

Digestion is the process by which the human body breaks down the food we consume into smaller, simpler molecules that can be absorbed and used by our cells for energy, growth, and other bodily functions.

1. Stages of Nutrition

Stages of nutrition are a systematic sequence of processes by which living organisms obtain, process, and utilize nutrients from food to support life, growth, and metabolic functions.

Ingestion → Digestion → Absorption → Assimilation → Egestion

Ingestion

• Process of taking food into the mouth
• First step in the digestive process
• Involves physical entry of food into the alimentary canal

Digestion

• Mechanical and chemical breakdown of food
• Converts complex food molecules into simpler, absorbable forms
• Involves physical breakdown (chewing, churning) and chemical breakdown (enzymes)
• Occurs in mouth, stomach, and small intestine
• Different enzymes break down carbohydrates, proteins, and fats

Absorption

• Transfer of digested nutrients from digestive tract into bloodstream
• Primarily occurs in small intestine
• Nutrients pass through intestinal walls into blood and lymph
• Nutrients include glucose, amino acids, fatty acids, vitamins, minerals

Assimilation

• Process of incorporating absorbed nutrients into body cells and tissues
• Occurs in cells throughout the body
• Involves cellular metabolism and utilization of nutrients
• Nutrients are used for: Energy production, Cell repair, Growth, Metabolic processes

Egestion

• Removal of undigested food, waste materials from the body
• Occurs through the anus as feces
• Waste products that cannot be absorbed
• Includes: Indigestible fiber, Dead cells, Bacteria, Water
• Regulated by muscular contractions of large intestine

Note-

• These stages are interconnected
• Occur sequentially but with overlapping processes
• Regulated by digestive enzymes, hormones, and nervous system

2. Organ in Human Digestive System

The human digestive system is a group of organs that work together to digest and absorb nutrients from the food you eat.  It consists of two main components:

• The Alimentary Canal (Digestive Tract) (Gastrointestinal: GI Tract)
  • Mouth
  • Pharynx (Throat)
  • Oesophagus
  • Stomach
  • Small Intestine – Duodenum, Jejunum and Ileum
  • Large Intestine (Colon) – Cecum, Appendix, Colon, Rectum, Anus
• The Accessory Organs
  • Liver
  • Gallbladder
  • Pancreas

3. Alimentary Canal

The GI tract is a long, muscular tube that starts at the mouth and ends at the anus. It includes the following organs:

3.1 Mouth (Buccal Cavity or Oral Cavity)

It Includes teeth, tongue and saliva.

Mouth is the first site of mechanical and chemical digestion.

The process of digestion begins in the mouth, where food is broken down by teeth and mixed with saliva. Saliva contains enzymes that start to break down carbohydrates.

3.1.1 Human Teeth

Majority of mammals including human being forms two sets of teeth during their life, a set of temporary milk or deciduous teeth [milk teeth] replaced by a set of permanent or adult teeth [permanent teeth].

Primary (baby) teeth: 20 teeth, emerge between 6 months to 3 years

Permanent teeth: 32 teeth, replace primary teeth starting around age 6

Wisdom teeth: Last to emerge, usually between 17-25 years old.

The study of teeth is called Odontology.

Teeth are primarily composed of calcium phosphate, with a smaller amount of calcium carbonate.

Humans are heterodont, meaning we have different types of teeth.

An adult human has four different types, namely, incisors (I), canine (C), premolars (PM) & molars (M).

Arrangement of teeth in each half of the upper and lower jaw in the order I, C, PM, M is represented by a dental formula which in human is 2123/2123 [2-I,1-C,2-PM,3-M].

Wisdom teeth, also known as 8th molars, are the last teeth to erupt.

Incisors (8 total)

• 4 in upper jaw, 4 in lower jaw
• Sharp, chisel-shaped edges
• Used for cutting and biting food
• Located at the front of the mouth

Canines (4 total)

• 2 in upper jaw, 2 in lower jaw
• Pointed, sharp teeth
• Help tear and grip food
• Located on the sides of incisors

Premolars (8 total)

• 4 in upper jaw, 4 in lower jaw
• Flat surfaces with ridges
• Used for crushing and grinding food
• Located behind canines

Molars (12 total)

• 6 in upper jaw, 6 in lower jaw
• Largest teeth with broad, flat surfaces
• Primary function is grinding and chewing food
• Located at the back of the mouth

Tooth Structure:

Crown: Visible part above the gum line

Root: Part below the gum line anchored in jawbone

Enamel: Hard, protective outer layer

Dentin: Layer underneath enamel

Pulp: Inner core containing blood vessels and nerves

The hard chewing surface of the teeth, made up of enamel (Enamel is the hardest substance in the human body and contains the highest percentage of minerals), helps in the mastication (chewing) of food.

Enamel is particularly susceptible to damage from phosphorous compounds (Phosphine), when present in acidic environments (like those caused by sugary foods or drinks), can weaken enamel and lead to cavities.

3.1.2 Human Tongue

Muscular organ

Soft, flexible tissue

Located in the mouth cavity

Enables speech, swallowing, and taste perception

Covered with mucous membrane

Contains blood vessels and nerves

Taste Buds:

Work as a Taste Receptors

Taste Buds are Microscopic sensory organs.

Teste Bud Contain 50-100 taste receptor cells

Average human has 2,000-8,000 taste buds

Constantly regenerate every 10-14 days

Send signals to brain via nerve endings

Distributed across tongue surface

Found on sides, back, and front of tongue

Located in small bumps called papillae

Sweet (Tip/Front of the tongue): Detect sugars and sweet compounds

Salty (Front Sides): Recognize sodium and salt

Sour (Back Sides, Behind the salt taste buds): Identify acidic substances

Bitter (Back of the tongue): Detect potentially harmful substances

Umami (Centre of the tongue): Sense savory/protein-rich foods

3.1.3 Saliva

Our mouth has the salivary glands which secrete saliva. Average daily production: 0.5-1.5 liters. Production increases during eating. Decreases during sleep.

There are three major pairs of salivary glands:

Parotid glands (near ears): Located in front of and just below each ear, these are the largest salivary glands in humans.

Submandibular glands (under jaw): Located below the jaw, these glands carry saliva up to the floor of the mouth.

Sublingual glands (under tongue): Located under the tongue on the floor of the mouth.

The saliva secreted into the oral cavity contains water, electrolytes, proteins, and enzymes.

Water (99%)

• Primary component
• Helps dissolve food particles
• Facilitates swallowing

Enzymes

• Salivary Amylase (Ptyalin): This enzyme breaks down starch (carbohydrates) into maltose, which is a disaccharide. Maltose is then further broken down into glucose (monosaccharide) by other enzymes in the small intestine.
• Diastase: This enzyme also breaks down starch (carbohydrates) into maltose, which is a disaccharide. It is a mixture of enzymes rather than a single enzyme.
• Salivary Lipase: Starts initial fat breakdown in the mouth. This enzyme breaks down triglycerides into fatty acids and glycerol. Triglycerides are a type of fat found in food.
• Lysozyme: This enzyme breaks down the cell walls of bacteria, which helps to kill bacteria and prevent infection.

Proteins

• Mucins: These are large glycoproteins that give saliva its viscous properties. They help to lubricate the oral cavity and protect the teeth from damage.

Electrolytes

• Sodium
• Potassium
• Calcium
• Magnesium
• Bicarbonate

Functions of Saliva:

Digestive Functions

• Initial chemical digestion of carbohydrates
• Moistens food for easier swallowing
• Helps form food bolus
• Begins breakdown of starches

Protective Functions

• Antibacterial properties
• Neutralizes acids in mouth
• Prevents tooth decay
• Helps heal mouth wounds
• Protects tooth enamel

Sensory Functions

• Dissolves food chemicals
• Enables taste perception
• Helps spread taste molecules across taste buds

Mucus in saliva helps in lubricating and adhering the masticated food particles into a bolus.

The bolus is then conveyed into the pharynx and then into the oesophagus by swallowing or deglutition.

The bolus further passes down through the oesophagus by successive waves of muscular contractions called peristalsis.

3.2 Pharynx (Throat)

The oral cavity leads into a short pharynx which serves as a common passage for food and air.

The oesophagus (food pipe) and the trachea (wind pipe) open into the pharynx.

Pharynx – Connects mouth to oesophagus.

Pharynx – Connects nose to trachea.

Epiglottis: Leaf-shaped flap of cartilage. Located at the base of the tongue. Attached to the top of the larynx (voice box). Prevents food and liquids from entering the trachea. Temporarily covers the windpipe (trachea) during swallowing.

During eating/drinking: Folds down to cover windpipe

During breathing: Moves upward to allow air passage

3.3 Food Pipe (Esophagus)

The esophagus is a muscular tube that transports the food from the mouth to the stomach. It uses peristalsis, a wave-like muscle contraction, to move food down.

3.4 Stomach

The stomach is a J-shaped organ that stores and further breaks down the food. Located in upper left abdomen.

The inner lining of the stomach secretes mucous and digestive juices (gastric juices).

The mucous protects the lining of the stomach. Prevents self-digestion. Neutralizes stomach acid effects. The mucus and bicarbonates present in the gastric juice play an important role in lubrication and protection of the mucosal epithelium from excoriation by the highly concentrated hydrochloric acid.

The digestive juices break down the proteins into simpler substances. Stomach produced Gastrin hormone which stimulates the release of gastric acid and pepsinogen, aiding in protein digestion.

Stomach also produced Ghrelin hormone (hunger hormone) which stimulates appetite and promotes food intake. When your stomach is empty, ghrelin levels rise. The hormone also causes the stomach muscles to contract more intensely. The needle-like pricking sensation you feel in your stomach when you’re hungry is primarily caused by stomach contractions and the hormone ghrelin.

Enzymes in digestive juices:

• Pepsin – Pepsin converts proteins into proteoses and peptones (peptides). Peptic ulcer is a condition where the stomach wall is partially eroded or damaged by the pepsin enzyme.
• Rennin – It is a proteolytic enzyme found in gastric juice of infants which helps in the digestion of milk proteins.
• Gastric Lipase: Begins fat digestion in the stomach
• Gelatinase: Breaks down gelatin and collagen proteins

Digestive juice is acidic. It contains hydrochloric acid. The hydrochloric acid kills many bacteria that enter along with the food and makes the medium in the stomach acidic.

Primary Functions:

Mechanical Digestion

• Breaking down food into smaller particles
• Creating chyme (partially digested food mixture)
• Mechanical contractions mix food with digestive juices

Chemical Digestion

• Produces hydrochloric acid (HCl)
• Creates acidic environment (pH 1.5-3.5)
• Kills harmful bacteria in ingested food
• Breaks down proteins into smaller peptides
• Vitamin B12 absorption

3.5 Small Intestine

The small intestine is the longest part of the digestive tract, measuring about 6-7 meters (20-23 feet) long.

It is a narrow, coiled tube which divided into three sections: the‘C’ shaped duodenum, a long coiled middle portion jejunum and a highly coiled ileum.

Most of the digestion and absorption of nutrients occurs in the small intestine.

The digested food passes into the blood vessels in the wall of the intestine. This process is called absorption.

The inner walls of the small intestine have thousands of finger-like outgrowths. These are called villi (singular villus). The villi increase the surface area for absorption of the digested food.

Villi are supplied with a network of capillaries and a large lymph (a colourless fluid containing white blood cells) vessel called the lacteal.

The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build required substances such as the proteins required by the body. This is called assimilation.

In the cells, glucose breaks down with the help of oxygen into carbon dioxide and water, and energy is released.

The food that remains undigested and unabsorbed then enters into the large intestine.

Sections of Small Intestine:

Duodenum > Jejunum > Ileum

Duodenum (shortest and first part of the small intestine, approximately 25 cm long)

• Shortest segment
• Receives digestive enzymes from pancreas
• Receives digestive enzymes from liver and gallbladder (Bile)
• Releases hormones controlling digestive processes.
  • Secretin: Originates from S cells in the duodenum. It stimulates the pancreas to release bicarbonate to neutralizing stomach acid in the duodenum. Bicarbonate is a base that helps the body maintain a normal acid-base balance, or pH. It’s also known as sodium bicarbonate, or baking soda.
  • Cholecystokinin (CCK): Originates from I cells in the duodenum. It stimulates the gallbladder to release bile, aiding in fat digestion, and also stimulates the pancreas to release pancreatic enzymes.
  • Enterogastrone: Enterogastrone is a hormone primarily secreted by the duodenum in response to the presence of acidic or fatty foods. When fat and acidic chyme (partially digested food) enter the duodenum, enterogastrone is released into the bloodstream. Its main role is to inhibit gastric motility and gastric acid secretion. This helps slow down the movement of food from the stomach to the small intestine, allowing more time for digestion and absorption of nutrients. Enterogastrone inhibits gastric secretions and movements by blocking the production or activity of gastrin hormone.

Jejunum (Middle Section, approximately 200 cm long)

• Primary site of nutrient absorption
• Extensive blood vessel network
• Produces additional digestive enzymes
  • Maltase: Breaks down maltose into glucose
  • Sucrase: Breaks down sucrose into glucose and fructose
  • Lactase: Breaks down lactose into glucose and galactose
  • Peptidases: Further break down peptides into amino acids
  • Enterokinase: Activates trypsinogen into trypsin

Ileum (longest and last part of the small intestine, approximately 300 cm long)

• Absorbs remaining nutrients
• Connects to large intestine
• Absorbs vitamin B12

Length wise increasing order:

Duodenum > Jejunum > Ileum

3.6 Large Intestine (Colon)

The large intestine, consisting of the cecum, colon, rectum, and anus, is responsible for the final stages of digestion and the elimination of waste from the body.

Absorbs water, minerals and electrolytes.

Forms and stores feces.

Houses beneficial gut bacteria.

Malfunctioning of large intestine can cause diarrhoea disease.

Parts of Large Intestine:

• Colon: The colon, absorbs water and electrolytes from undigested food, forming solid waste. It also stores waste until it is eliminated from the body.
• Cecum: A large pouch-like structure in the right iliac fossa that’s part of the large intestine. The cecum’s main functions are to absorb fluids and salts that remain after digestion and to mix its contents with mucus.
• Appendix: A small, hollow tube that’s attached to the cecum at one end and closed at the other. Once thought to help digest cellulose, but is now considered vestigial. But recent research suggests it may help restore healthy bacteria to the intestines after an infection or when antibiotics are used.
• Rectum: The rectum is the final part of the large intestine, where stool is stored before it is eliminated through the anus. Bilirubin from the liver gives stool its yellow colour. It also prevents stool from decomposing. When bilirubin spreads throughout the body instead of being confined to the bile, jaundice occurs. In constipation, wounds develop on the rectal wall, which leads to hemorrhoids (piles) disease. To prevent the rectum from having dry stool, mucus is added.
• Anus: The anus is the opening at the end of the digestive tract through which stool is expelled.

Enzymes in Large Intestine:

Bacterial enzymes: Help break down undigested material and produce some vitamins

4. Accessory Organs

The accessory organs are organs that help with digestion but are not part of the GI tract. They include:

4.1 Liver

The liver is a reddish brown gland situated in the upper part of the abdomen on the right side.

The liver is the largest internal organ and the largest gland in the human body.

The liver is about 22 cm long and 15 cm wide and weighing around 1.5 kg.

It has two lobes. The hepatic lobules are the structural and functional units of liver containing hepatic cells.

The liver is the organ with the greatest regenerative capacity in the body.

The bile juice secreted by the hepatic cells passes through the hepatic ducts and is stored and concentrated in a thin muscular sac called the gall bladder. The bile plays an important role in the digestion of fats.

The bile duct and the pancreatic duct open together into the duodenum as the common hepato-pancreatic duct.

Other functions:

• Breaks down and processes fats. Produces cholesterol. Synthesizes triglycerides.
• Regulates blood sugar levels. Stores excess glucose as glycogen. Converts glycogen back to glucose when energy is needed. Glycogen is the principal storage form of glucose or energy. In humans, it is stored in liver and muscle cells.
• Converts toxic substances ammonia (produced from protein metabolism) into harmless compounds urea for elimination.
• It prevents alcohol from accumulating and causing cellular damage.
• Alcohol dehydrogenase enzyme converts ethanol to acetaldehyde. Aldehyde dehydrogenase further converts acetaldehyde to acetate. Acetate is then broken down into water and carbon dioxide.
• It processes nutrients from food, such as carbohydrates, proteins, and fats, making them usable for the body.

4.2 Gallbladder

The gallbladder stores and releases bile produced by the liver into the small intestine to help digest fats.

Bile is a greenish-yellow fluid produced by the liver and stored in the gallbladder.

Bile aids in the digestion of fats. Bile contains two salts – sodium glycocholate and sodium taurocholate. Through these salts, fats are broken down in the duodenum. This process is known as emulsification of fats.

Bile is unique in that it contains no enzymes, yet it plays a crucial role in digestion.

4.3 Pancreas

The pancreas produces digestive enzymes that break down carbohydrates, proteins, and fats. It also produces insulin, which helps to regulate blood sugar levels. The pancreas is the second largest gland of the human body.

It produces digestive enzymes. Enzymes in pancreatic juices:

• Pancreatic Amylase: Continues carbohydrate breakdown
• Pancreatic Lipase: Breaks down triglycerides into fatty acids and glycerol
• Trypsin: Breaks down proteins into smaller protein peptides
• Chymotrypsin: Further breaks down of protein peptides into amino acids
• Carboxypeptidase: Breaks protein peptides into amino acids
• Elastase: Breaks down elastin proteins
• Nucleases: Break down nucleic acids (DNA and RNA)

Pancreatic juices contains enzymes for digesting carbohydrates, fats and proteins hence it is called the Complete Digestive Juice.

Insulin (Hormone) Production

• Produced by beta cells in pancreatic Islets of Langerhans
• Regulates blood glucose levels
• Enables cells to absorb and use glucose
• Prevents blood sugar from becoming too high

The pancreas is a organ that functions as both an endocrine and an exocrine gland.

Exocrine Function: Digestive Enzyme (Trypsin, Lipase, Amylase) are released into the small intestine through the pancreatic duct.

Endocrine Function: Produces hormones (Insulin by beta cells, glucagon by alpha cells) released directly into the bloodstream

5. Digestive glands

Digestive glands are part of the digestive system and secrete digestive juices that help break down food into digestible components. Some examples of digestive glands include:

Salivary glands

Exocrine glands that produce saliva in the mouth, which contains the enzyme amylase that breaks down starch into maltose. The parotid gland is the largest salivary gland and is located in front of and below the ears.

Gastric glands

Tubular glands in the stomach that secrete enzymes that are necessary for digestion.

Liver

The body’s largest gland, located in the upper right side of the belly. The liver secretes bile juice, which is stored in the gallbladder and helps digest fat.

Gallbladder

An accessory digestive organ that stores, concentrates, and releases bile.

Pancreas

An exocrine gland that secretes enzymes that break down fats, carbohydrates, proteins, and acids in the duodenum.

Intestinal glands

Secrete intestinal juices that break down fat molecules and bile salts into simpler substances.

6. Digestive System Disorders

Digestive system disorders are a broad category of health conditions that affect the organs involved in digestion, including the esophagus, stomach, small intestine, large intestine, liver, gallbladder, and pancreas. These disorders can range from mild to severe and can cause a variety of symptoms, including pain, discomfort, and digestive issues.

6.1.1 Types of Digestive System Disorders

1. Acute Conditions

Acute conditions are health problems that develop suddenly and often resolve within a short period. They are characterized by a rapid onset of symptoms and usually require immediate medical attention.

Jaundice: A yellowish discoloration of the skin, mucous membranes, and eyes caused by elevated levels of bilirubin in the blood. This can result from liver dysfunction, gallbladder problems, or certain blood disorders.

Vomiting: The forceful expulsion of stomach contents through the mouth. It can be triggered by various factors, including food poisoning, excessive alcohol intake, motion sickness, and certain medications.

Diarrhea: Frequent passage of loose, watery stools. It can be caused by bacterial or viral infections, food poisoning, inflammatory bowel disease, or certain medications.

Acid Reflux (Heartburn): A condition characterized by the backflow of stomach acid into the esophagus, causing a burning sensation in the chest. It can be exacerbated by spicy foods, alcohol, caffeine, and certain medications.

Constipation: Difficulty in passing stool or infrequent bowel movements. It can be caused by inadequate fiber intake, dehydration, lack of physical activity, certain medications, or underlying medical conditions.

Appendicitis: Inflammation of the appendix, a small, finger-shaped organ located at the junction of the small and large intestines. It requires prompt medical attention to prevent rupture and infection.

Indigestion (Dyspepsia): A general term for discomfort or pain in the upper abdomen, often accompanied by bloating, gas, and nausea. It can be caused by various factors, including overeating, spicy foods, stress, and certain medical conditions.

Chronic Conditions

Chronic conditions are long-term health conditions that develop gradually and can last for years or even a lifetime. They often require ongoing management and treatment.

Inflammatory Bowel Disease (IBD): A group of chronic inflammatory bowel diseases that affect the gastrointestinal tract. Crohn’s disease and ulcerative colitis are the two main types of IBD.

Gallstones: Hardened deposits of digestive fluid that can form in the gallbladder. They can cause pain, inflammation, and, in severe cases, block the bile duct.

Hemorrhoids: Swollen veins in the rectum and anus. They can cause pain, itching, and bleeding.

Irritable Bowel Syndrome (IBS): A functional bowel disorder characterized by abdominal pain, bloating, gas, diarrhea, and constipation. The exact cause of IBS is unknown.

Lactose Intolerance: A digestive disorder caused by the inability to digest lactose, a sugar found in milk and dairy products. It can lead to bloating, gas, and diarrhea.

Stomach Ulcers (Peptic Ulcers): Open sores that develop on the lining of the stomach or duodenum. They are often caused by infection with the bacterium Helicobacter pylori or long-term use of nonsteroidal anti-inflammatory drugs (NSAIDs).

Other useful facts:

Zymase

• Zymase is a complex enzyme system that plays a crucial role in fermentation, particularly in yeast cells.
• Zymase is not a single enzyme, but a complex mixture of enzymes
• It is found in yeast cells
• Catalyzes the breakdown of glucose into ethanol and carbon dioxide
  • C6H12O6 (glucose) → 2 C2H5OH (ethanol) + 2 CO2 (carbon dioxide)
• Essential in the process of alcoholic fermentation
• Converts sugar into alcohol during processes like beer and wine production

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