🫃 Digestive System
Study human alimentary canal, digestive glands and enzymes for CUET Agriculture. Peristalsis, bile secretion, absorption and liver functions.
Alimentary Canal — Overview
The human alimentary canal is a continuous muscular tube extending from the mouth to the anus, measuring approximately 9 metres in length. This tube is responsible for the entire journey of food — from ingestion to digestion, absorption, and finally elimination of waste. Every region of this canal is specialized for a particular function, making it one of the most efficient organ systems in the body.
Mouth → Buccal cavity → Pharynx → Oesophagus → Stomach → Small intestine
(Duodenum → Jejunum → Ileum) → Large intestine (Caecum → Ascending colon →
Transverse colon → Descending colon → Sigmoid colon → Rectum → Anus)
NOTE
The alimentary canal is also called the gastrointestinal (GI) tract. Although it is a single continuous tube, each segment has distinct histological and functional characteristics suited to its role.
Histology of the Alimentary Canal
The wall of the alimentary canal has 4 basic layers (from inside to outside). These layers remain consistent throughout the tract, though their composition changes regionally to match function. For instance, the mucosa in the stomach has gastric glands, while in the small intestine it forms finger-like villi for absorption.
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Alimentary Canal — Overview
The human alimentary canal is a continuous muscular tube extending from the mouth to the anus, measuring approximately 9 metres in length. This tube is responsible for the entire journey of food — from ingestion to digestion, absorption, and finally elimination of waste. Every region of this canal is specialized for a particular function, making it one of the most efficient organ systems in the body.
Mouth → Buccal cavity → Pharynx → Oesophagus → Stomach → Small intestine
(Duodenum → Jejunum → Ileum) → Large intestine (Caecum → Ascending colon →
Transverse colon → Descending colon → Sigmoid colon → Rectum → Anus)
NOTE
The alimentary canal is also called the gastrointestinal (GI) tract. Although it is a single continuous tube, each segment has distinct histological and functional characteristics suited to its role.
Histology of the Alimentary Canal
The wall of the alimentary canal has 4 basic layers (from inside to outside). These layers remain consistent throughout the tract, though their composition changes regionally to match function. For instance, the mucosa in the stomach has gastric glands, while in the small intestine it forms finger-like villi for absorption.
| Layer | Components | Function |
|---|---|---|
| Mucosa (innermost) | Epithelium + lamina propria + muscularis mucosae | Secretion and absorption; epithelium varies by region — stratified squamous in oesophagus (protection), simple columnar in intestine (absorption) |
| Submucosa | Dense connective tissue with blood vessels, lymphatics, Meissner's plexus (submucosal plexus) | Nerve control of mucosal secretions; blood supply to the mucosa |
| Muscularis externa | Inner circular + outer longitudinal smooth muscle; Auerbach's plexus (myenteric plexus) between layers | Peristalsis and segmentation movements; Auerbach's plexus controls motility |
| Serosa (outermost) | Thin connective tissue + mesothelium (visceral peritoneum) | Protection; reduces friction between organs during movement |
TIP
Memory aid for nerve plexuses: SubMucosa → Meissner's plexus (both have M). Muscularis → Auerbach's plexus (alphabetically: A for the muscle layer between which it lies).
Mouth and Buccal Cavity
The buccal cavity is where digestion begins — both mechanical digestion (chewing by teeth) and chemical digestion (salivary amylase acting on starch) start here. The mouth is bounded by the lips, cheeks, hard and soft palate, and the tongue.
Teeth
Human dentition has three important characteristics:
- Diphyodont — two sets of teeth develop in a lifetime. The first set is the milk teeth (20 teeth), which are replaced by permanent teeth (32 teeth) starting around age 6.
- Thecodont — teeth are embedded in bony sockets (alveoli) of the jaws, providing a strong anchorage. This is in contrast to acrodont teeth (fused to bone edges) seen in some reptiles.
- Heterodont — four different types of teeth exist, each shaped for a specific function.
| Tooth Type | Number (per half jaw, adult) | Function |
|---|---|---|
| Incisors (I) | 2 | Cutting and biting food |
| Canines (C) | 1 | Tearing food (less prominent in humans than in carnivores) |
| Premolars (PM) | 2 | Crushing and grinding |
| Molars (M) | 3 | Crushing and grinding (largest teeth) |
Dental formula (adult human, per half jaw): I 2/2, C 1/1, PM 2/2, M 3/3 = 32 teeth total
Milk teeth formula: I 2/2, C 1/1, M 2/2 = 20 teeth total (no premolars in the deciduous set)
Tooth Structure
Each tooth has three main regions, with distinct tissue layers:
- Crown — the visible part projecting above the gum line, covered by enamel. Enamel is the hardest substance in the human body, composed of about 96% mineral (hydroxyapatite crystals). Since enamel has no living cells, it cannot regenerate once damaged.
- Neck — the narrow region at the gum line where the crown meets the root.
- Root — the portion embedded in the jawbone (alveolar bone), covered by cementum, a bone-like tissue that anchors the tooth via the periodontal ligament.
- Dentine — forms the bulk of the tooth beneath the enamel and cementum. Dentine is bone-like (~70% mineral) but softer than enamel. It contains microscopic tubules.
- Pulp cavity — the central hollow space containing blood vessels, nerves, and odontoblasts (cells that produce dentine). The pulp is what makes a tooth "alive" and sensitive.
Salivary Glands
Three pairs of salivary glands secrete approximately 1–1.5 litres of saliva per day. Saliva is essential not just for digestion but also for lubricating food into a bolus, maintaining oral hygiene, and beginning starch breakdown.
| Gland | Location | Type of Secretion |
|---|---|---|
| Parotid | Below and in front of ears | Serous (watery, rich in amylase) — the largest salivary gland. Mumps is an infection of this gland. |
| Submandibular / Submaxillary | Below the jaw | Mixed (serous + mucous) — contributes the most saliva (~70% of daily output) |
| Sublingual | Below the tongue | Mainly mucous — the smallest of the three pairs |
Saliva Composition
Saliva is a complex fluid that performs multiple protective and digestive roles:
- pH: 6.8 (slightly acidic to neutral) — this is the optimal pH for salivary amylase activity
- Salivary amylase (ptyalin) — the key digestive enzyme in saliva. It begins the digestion of starch, breaking it down into maltose (a disaccharide). This enzyme works best at pH 6.8 and is inactivated once it reaches the acidic stomach.
- Lysozyme — an antibacterial enzyme that breaks down bacterial cell walls, providing a first line of defense in the mouth
- Mucin — a glycoprotein that lubricates food, making it easier to chew and swallow as a bolus
- Electrolytes — Na⁺, K⁺, Cl⁻, HCO₃⁻ (bicarbonate helps buffer acids in the mouth)
Tongue
The tongue is a muscular organ that plays a central role in mixing food with saliva, tasting, swallowing, and speech. Its upper surface has papillae — small projections that house taste buds:
- Circumvallate (vallate) — the largest papillae, arranged in a V-shape at the back of the tongue. Each is surrounded by a trench containing taste buds.
- Fungiform — mushroom-shaped papillae scattered mainly on the tip and sides. They contain taste buds and are visible as small red dots.
- Filiform — the most numerous papillae but they have no taste buds. They sense touch and texture, giving the tongue its rough surface.
- Foliate — found on the lateral edges of the tongue; contain taste buds (more prominent in children than adults).
NOTE
The old "tongue map" showing distinct taste zones is a myth. All regions of the tongue can detect all five basic tastes — sweet, sour, salty, bitter, and umami.
Pharynx and Oesophagus
The pharynx serves as a common passage for both food and air. This creates a potential risk — food could enter the airway. To prevent this, during swallowing the epiglottis (a flap of elastic cartilage) folds down to cover the glottis (opening of the larynx), directing food into the oesophagus instead of the trachea.
The oesophagus is a muscular tube approximately 25 cm long that connects the pharynx to the stomach. Food does not simply fall through it by gravity — instead, it is propelled by peristalsis, coordinated waves of muscular contraction and relaxation that push the bolus downward. The oesophagus has three distinct muscular regions:
- Upper third: skeletal muscle (voluntary control at the start of swallowing)
- Middle third: mixed skeletal and smooth muscle
- Lower third: smooth muscle (involuntary peristalsis)
The gastro-oesophageal sphincter (also called the cardiac sphincter) sits at the junction of the oesophagus and stomach. This ring of muscle prevents the acidic stomach contents from refluxing back into the oesophagus. When this sphincter is weak, it leads to GERD (gastro-oesophageal reflux disease).
Stomach
The stomach is a J-shaped muscular organ with a capacity of approximately 1–1.5 litres. It acts as a temporary storage reservoir and mixing chamber where food is churned with gastric juice to produce a semi-liquid mixture called chyme.
The stomach is divided into four regions:
- Cardiac region — the entrance from the oesophagus, named for its proximity to the heart
- Fundus — the dome-shaped upper portion that often contains trapped gas
- Body — the main central region where most churning occurs
- Pyloric region — the lower portion that connects to the duodenum via the pyloric sphincter, which carefully regulates the release of small amounts of chyme into the small intestine
Gastric Glands
Located in the mucosa of the stomach, there are approximately 35 million gastric glands. These glands contain several specialized cell types, each with a distinct secretory product:
| Cell Type | Secretion | Function |
|---|---|---|
| Parietal / Oxyntic cells | HCl and Intrinsic factor | HCl creates the highly acidic environment (pH 1.5–3.5) that kills ingested bacteria, denatures proteins (unfolding them for enzymatic attack), and activates pepsinogen. Intrinsic factor is a glycoprotein essential for vitamin B₁₂ absorption later in the ileum. |
| Chief / Zymogenic / Peptic cells | Pepsinogen (inactive proenzyme) | Pepsinogen is activated to pepsin by HCl. Pepsin is a powerful protease that digests proteins into smaller peptides (peptones and proteoses). It is secreted in inactive form to prevent self-digestion. |
| Mucous neck cells | Mucus | Secretes a thick alkaline mucus layer that coats and protects the stomach lining from the corrosive HCl and pepsin — without this barrier, the stomach would digest itself. |
| G-cells | Gastrin (hormone) | Gastrin is a hormone that stimulates parietal cells to secrete more HCl and chief cells to release pepsinogen, creating a positive feedback loop during a meal. |
Digestion in Stomach
- Food is mixed with gastric juice by powerful churning contractions of the three muscle layers → forms chyme (a creamy, acidic paste)
- Pepsin (from pepsinogen + HCl activation): breaks down proteins → smaller peptides. Pepsin works optimally at pH 2.
- Gastric lipase: performs a small amount of fat digestion, particularly effective on short-chain fatty acids found in milk
- Rennin (in infants): converts casein (milk protein) → paracasein, which coagulates milk so it stays in the stomach longer for better digestion. Rennin is absent or negligible in adults.
- Food stays in the stomach for approximately 3–5 hours depending on the meal composition — fatty meals take longer
IMPORTANT
The stomach has three muscle layers (oblique, circular, longitudinal) rather than the usual two. This extra oblique layer enables the powerful churning needed to mix food into chyme.
Small Intestine
The small intestine is the longest part of the alimentary canal at ~6 metres. It is the principal site of both digestion and absorption. Despite being called "small," this name refers to its narrower diameter compared to the large intestine, not its length.
| Part | Length | Features |
|---|---|---|
| Duodenum | ~25 cm (C-shaped) | Receives bile (from the liver via the common bile duct) and pancreatic juice (via the pancreatic duct). The duodenum is the major site of chemical digestion — virtually all macromolecules are broken down here. |
| Jejunum | ~2.5 m | The major site of nutrient absorption. It is highly vascularised with prominent circular folds (plicae circulares) and long villi, giving it a thick, red appearance. |
| Ileum | ~3.5 m | Continues absorption (especially vitamin B₁₂ and bile salts). Contains Peyer's patches — aggregated lymphoid tissue that monitors intestinal bacteria and provides immune surveillance. Ends at the ileocaecal valve, which prevents backflow from the large intestine. |
Intestinal Adaptations for Absorption
The small intestine has evolved multiple structural features that dramatically increase its surface area for absorption:
- Great length (~6 m) — provides a long transit time for thorough absorption
- Circular folds (plicae circulares) — permanent folds of mucosa + submucosa that force chyme to spiral as it passes, increasing contact time
- Villi — finger-like projections of the mucosa (~0.5–1 mm tall), approximately 4 million villi in total
- Microvilli — microscopic projections on the apical surface of each epithelial cell, forming the brush border. Each cell has ~600 microvilli, which collectively increase the surface area approximately 600 times
- The total absorptive surface area reaches an astonishing ~250 m² — roughly the size of a tennis court
Each villus contains a rich internal network for absorbing different nutrients:
- Capillary network — absorbs water-soluble nutrients: amino acids, monosaccharides (glucose, fructose, galactose), and water-soluble vitamins (B and C)
- Lacteal (a central lymphatic vessel) — absorbs fats in the form of chylomicrons and fat-soluble vitamins (A, D, E, K). Fats enter the lymphatic system rather than the blood because chylomicrons are too large for blood capillaries.
Liver
The liver is the largest gland in the body, weighing approximately 1.5 kg. Located in the upper right abdomen beneath the diaphragm, it performs over 500 functions including detoxification, protein synthesis, bile production, and metabolic regulation.
- Has 2 main lobes: the right lobe (larger, about 6 times the size of the left) and the left lobe
- The structural and functional unit is the hepatic lobule — a hexagonal structure with a central vein in the middle, surrounded by radiating cords of hepatocytes (liver cells) arranged like spokes of a wheel
- At each corner of the hexagon lies a portal triad: a branch of the hepatic artery (oxygenated blood), a branch of the portal vein (nutrient-rich blood from the gut), and a bile duct (carries bile away)
Why does the liver receive a dual blood supply?
The liver receives blood from two sources: the **hepatic artery** (25%, oxygen-rich) and the **portal vein** (75%, nutrient-rich blood from the intestines, stomach, and spleen). This dual supply ensures the liver can both receive oxygen for its own metabolism and process all absorbed nutrients before they enter the general circulation — a process called **first-pass metabolism**.Bile
Bile is a complex fluid essential for fat digestion, though it contains no digestive enzymes itself:
- Produced by hepatocytes at a rate of 500–1000 ml/day
- Stored and concentrated (up to 10×) in the gallbladder, which has a capacity of ~50 ml
- Bile composition: water, bile salts (sodium glycocholate, sodium taurocholate — the active digestive components), bile pigments (bilirubin — yellow, biliverdin — green, both waste products from haemoglobin breakdown), cholesterol, and phospholipids
- Bile salts play a crucial role — they emulsify fats, breaking large fat droplets into thousands of tiny droplets (micelles). This dramatically increases the surface area available for pancreatic lipase to act on.
- Bile is alkaline (pH 7.6–8.6) — this helps neutralize the acidic chyme arriving from the stomach, creating the slightly alkaline environment that intestinal enzymes require
WARNING
If bile pigments accumulate in the blood (due to liver disease, bile duct blockage, or excessive RBC breakdown), it leads to jaundice — the characteristic yellowing of skin and eyes.
Pancreas
The pancreas is a unique mixed gland — it serves both endocrine and exocrine functions simultaneously. Located behind the stomach in the C-curve of the duodenum, it is sometimes called the "dual-function organ."
- Exocrine part (~99% of the gland): consists of acinar cells that secrete pancreatic juice via the pancreatic duct into the duodenum. This juice contains a powerful cocktail of enzymes capable of digesting all major nutrient classes.
- Endocrine part (~1%): the Islets of Langerhans — clusters of hormone-secreting cells. Beta (β) cells produce insulin (lowers blood glucose), and alpha (α) cells produce glucagon (raises blood glucose).
Pancreatic Juice
Pancreatic juice is one of the most important digestive secretions in the body:
- Volume: ~1200–1500 ml/day
- pH: 7.5–8.5 (alkaline) — the high bicarbonate content neutralizes acidic chyme from the stomach, protecting the intestinal mucosa and creating the optimal pH for enzyme activity
- Contains bicarbonate ions (HCO₃⁻) secreted by duct cells for buffering
| Enzyme (Inactive → Active) | Substrate | Products |
|---|---|---|
| Trypsinogen → Trypsin (activated by enterokinase) | Proteins | Peptides |
| Chymotrypsinogen → Chymotrypsin (activated by trypsin) | Proteins | Peptides |
| Procarboxypeptidase → Carboxypeptidase (activated by trypsin) | Peptides | Amino acids |
| Proelastase → Elastase | Elastin protein | Peptides |
| Pancreatic lipase | Fats (emulsified by bile) | Fatty acids + glycerol |
| Pancreatic amylase | Starch | Maltose |
| Nucleases (DNase, RNase) | DNA, RNA | Nucleotides |
IMPORTANT
Most pancreatic proteases are secreted as inactive zymogens (proenzymes) to prevent the pancreas from digesting itself. The activation cascade begins when enterokinase (an enzyme on the brush border of duodenal cells) converts trypsinogen to trypsin. Trypsin then activates all the other zymogens — making enterokinase the "master switch" of protein digestion.
Intestinal Juice (Succus Entericus)
Secreted by the crypts of Lieberkuhn (intestinal glands) located in the intestinal mucosa, intestinal juice is produced at a volume of approximately ~1800 ml/day. These enzymes complete the final stages of digestion, breaking intermediate products into their simplest absorbable forms.
| Enzyme | Substrate | Products |
|---|---|---|
| Maltase | Maltose | Glucose + Glucose |
| Sucrase | Sucrose | Glucose + Fructose |
| Lactase | Lactose | Glucose + Galactose |
| Aminopeptidase | Peptides (from N-terminus) | Amino acids |
| Dipeptidase | Dipeptides | Amino acids |
| Nucleotidase | Nucleotides | Nucleosides + phosphate |
| Nucleosidase | Nucleosides | Bases + sugars |
| Enterokinase (Enteropeptidase) | Trypsinogen | Trypsin (activation, not digestion) |
| Intestinal lipase | Remaining fats | Fatty acids + glycerol |
Why does lactose intolerance occur?
**Lactose intolerance** results from a deficiency of the enzyme **lactase** in the brush border of the small intestine. Without enough lactase, lactose (milk sugar) passes undigested into the large intestine, where gut bacteria ferment it, producing gas (CO₂, H₂), bloating, cramps, and diarrhoea. Most mammals naturally lose lactase production after weaning — lactase persistence into adulthood is actually a genetic mutation most common in populations with a long history of dairy farming.Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Alimentary Canal Length | ~9 metres (30 feet) from mouth to anus |
| Four Wall Layers | Serosa (outermost, visceral peritoneum) → Muscularis (outer longitudinal + inner circular smooth muscle; oblique layer added in stomach) → Submucosa (connective tissue with blood vessels, nerves, Meissner's plexus) → Mucosa (innermost, epithelium + lamina propria + muscularis mucosae) |
| Dentition | Thecodont (teeth in sockets), diphyodont (2 sets: milk + permanent), heterodont (4 types: I, C, PM, M) Adult dental formula: I 2/2, C 1/1, PM 2/2, M 3/3 = 32 teeth Milk teeth formula: I 2/2, C 1/1, M 2/2 = 20 teeth (no premolars) |
| Salivary Glands (3 pairs) | Parotid (largest, serous, near ear) — mumps affects this gland Submandibular/submaxillary (mixed, below jaw) Sublingual (smallest, mucous, under tongue) Daily secretion: ~1500 ml |
| Salivary Enzymes | Salivary amylase (ptyalin): starch → maltose (pH 6.8) Lysozyme: antibacterial |
| Tongue Papillae | Circumvallate (largest, V-shaped row, taste) Fungiform (mushroom-shaped, taste + touch) Filiform (most numerous, touch only, no taste buds) Foliate (leaf-like, sides of tongue) |
| Pharynx and Oesophagus | Pharynx = common passage for food and air Epiglottis prevents food entry into trachea Oesophagus: ~25 cm long, peristalsis moves food, no digestion occurs |
| Stomach | J-shaped, regions: cardiac (entrance), fundus (dome), body (main), pyloric (exit with pyloric sphincter) pH: 1.5–2.5 (highly acidic) Capacity: ~1.5 L |
| Stomach Cell Types | Parietal/oxyntic cells: secrete HCl + intrinsic factor (for vitamin B₁₂ absorption) Chief/zymogenic/peptic cells: secrete pepsinogen (activated to pepsin by HCl) Mucous neck cells: secrete mucus (protects stomach lining) G-cells: secrete gastrin hormone |
| Stomach Digestion | Pepsin: proteins → peptides (pH 1.5–2.5) Gastric lipase: fats → fatty acids (minor role) Rennin (infants): casein → paracasein (milk curdling) |
| Small Intestine | Longest part: ~6 m; three regions: duodenum (25 cm, C-shaped, receives bile + pancreatic juice), jejunum (middle 2/5), ileum (lower 3/5) Brunner's glands in duodenum secrete alkaline mucus |
| Absorption Adaptations | Villi: finger-like projections, each contains lacteal (lymph vessel for fat absorption) + blood capillaries Microvilli (brush border): further increase surface area Plicae circulares (circular folds): increase surface area Total absorptive surface: ~250 m² |
| Liver | Largest internal organ (~1.5 kg) Right lobe (larger) + left lobe Functional unit: hepatic lobule Produces ~600–1000 ml bile/day (stored in gallbladder) Bile salts emulsify fats (no enzymes in bile) Bile pigments: bilirubin (yellow-green), biliverdin |
| Pancreas | Mixed gland (both exocrine and endocrine) Exocrine: pancreatic juice (~1500 ml/day, pH 7.1–8.2) Endocrine: Islets of Langerhans (insulin, glucagon) |
| Pancreatic Enzymes | Trypsinogen → trypsin (activated by enterokinase; proteins → peptides) Chymotrypsinogen → chymotrypsin (by trypsin) Procarboxypeptidase → carboxypeptidase (by trypsin) Pancreatic amylase: starch → maltose Pancreatic lipase: fats → fatty acids + glycerol (most important fat-digesting enzyme) DNase/RNase: nucleic acids → nucleotides |
| Intestinal Juice (Succus Entericus) | ~1800 ml/day, secreted by crypts of Lieberkuhn Maltase: maltose → glucose + glucose Sucrase: sucrose → glucose + fructose Lactase: lactose → glucose + galactose Aminopeptidase/Dipeptidase: peptides → amino acids Enterokinase: activates trypsinogen → trypsin |
| Lactose Intolerance | Deficiency of lactase enzyme; undigested lactose fermented by gut bacteria → gas, bloating, diarrhoea |
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