Digestive System: Alimentary Canal & Mouth

• The digestive system facilitates food intake, breakdown, nutrient absorption, cellular utilization of nutrients, and waste elimination.
• This system comprises the alimentary canal and accessory organs, each adapted for specific digestive roles.

Alimentary Canal

• This canal is the continuous digestive pathway from mouth to anus.

Mouth (Oral Cavity)

• Functions include ingestion and initial digestion of food.
• Mechanical digestion involves teeth breaking food into smaller pieces, increasing surface area.
• Chemical digestion starts with salivary amylase from salivary glands breaking down starch into maltose.
• Saliva contains mucins for lubrication and antibacterial enzymes for infection prevention.
• The tongue manipulates food, mixes it with saliva, and forms a bolus for swallowing.
• Incisors cut food via their sharp edges.
• Canines, which are pointed, tear and grip meat using their shape.
• Premolars and molars grind and crush food through their broad shape.
• Salivary glands secrete enzymes and mucus to start digestion and lubricate food; ptyalin becomes salivary amylase.
• The tongue is muscular and flexible, which aids in food manipulation and swallowing.
• The hard palate allows tongue movement, while the soft palate prevents food from entering the nasal cavity during swallowing.

Pharynx and Esophagus

• The pharynx directs food into the esophagus and prevents entry into the respiratory tract.
• The epiglottis prevents choking by closing over the trachea during swallowing.
• The esophagus transports food to the stomach via peristalsis (wave-like contractions).
• No digestion happens here, the primary function is transport.
• The epiglottis seals off the trachea during swallowing.
• The esophagus' muscular walls contract to push food downward.
• Mucus-secreting glands lubricate the passage and reduce friction.

Stomach

• It temporarily stores food and releases it gradually into the small intestine.
• Mechanical digestion involves muscular contractions that churn food with gastric juices to form chyme.
• Chemical digestion involves pepsin (activated by stomach acid) breaking down proteins into peptides.
• Hydrochloric acid (HCl) creates an acidic environment (pH 1.5–3.5), killing pathogens and denaturing proteins.
• Intrinsic factor, a glycoprotein, is secreted for vitamin B12 absorption in the small intestine.
• The stomach has three muscle layers (longitudinal, circular, and oblique) which allows strong churning for mechanical digestion.
• Parietal cells secrete HCl for acidification and intrinsic factor.
• Chief cells produce pepsinogen, which is converted to active pepsin in acidic conditions.
• Mucous cells secrete a thick mucus layer that protects the stomach lining from self-digestion.
• The pyloric sphincter regulates food passage into the small intestine.

Small Intestine (Duodenum, Jejunum, Ileum)

• This is the primary site of digestion and absorption.
• The duodenum receives bile (for fat emulsification) and pancreatic enzymes.
• The jejunum and ileum specialize in nutrient absorption.
• Peristalsis moves food and mixes it with digestive enzymes.
• Circular folds (plicae circulares) increase surface area and slow food movement.
• Villi contain capillaries and lacteals for nutrient uptake.
• Microvilli (brush border) are microscopic extensions that house enzymes for final digestion.
• Enterocytes absorb nutrients into the bloodstream and lymph.
• Goblet cells produce mucus for lubrication.
• Crypts of Lieberkühn produce intestinal enzymes (e.g., maltase, sucrase, peptidases).
• A rich blood and lymphatic supply ensures rapid nutrient transport.

Large Intestine (Colon, Rectum, and Anus)

• It absorbs water, salts, and vitamins from undigested material.
• It hosts gut bacteria that produce essential vitamins (B12, K).
• It compacts waste into feces and stores it before egestion.
• A thicker mucosa with goblet cells secretes mucus that facilitates smooth feces movement.
• Slow peristalsis allows water absorption and waste solidification.
• Gut microbiota ferments undigested food and produce beneficial compounds.
• Stretch receptors in the rectum signal when defecation is needed.

Accessory Organs

• These organs support the function of other organs or systems, but are not part of the main organ system itself

Salivary Glands

• They secrete saliva, which contains enzymes, mucus, and antimicrobial agents.
• It begins starch digestion and lubricates food for easy swallowing.
• Three major glands (parotid, submandibular, sublingual) allow efficient saliva production.
• Ducts transport saliva directly to the mouth.

Liver

• Produces bile, which emulsifies fats for better enzyme digestion.
• It detoxifies blood (e.g., processing alcohol, drugs).
• It stores glucose as glycogen as releases it when necessary.
• Lobular structure maximizes processing efficiency.
• Hepatocytes detoxify and produce bile.
• An extensive blood supply ensures efficient metabolic activity.

Gallbladder

• It stores and concentrates bile before releasing it into the small intestine.
• Expandable walls allow bile storage.
• A duct system allows efficient bile release into the duodenum.

Pancreas

• Secretes digestive enzymes (amylase, lipase, proteases).
• Produces bicarbonate to neutralize stomach acid.
• Acinar cells specialize in enzyme production.
• It acts as both a digestive gland and an endocrine organ (insulin, glucagon).

Digestive Enzymes

• Amylase, from salivary glands and the pancreas, breaks down starch into maltose.
• Maltase from the Small Intestine breaks down Maltose into Glucose.
• Pepsin from the stomach breaks down proteins into peptides.
• Trypsin from the Pancreas breaks down peptides into amino acids.
• Lipase from the pancreas breaks down lipids (fats) into fatty acids and glycerol.

Carbohydrates

• These are consumed as polysaccharides (starch, glycogen) and disaccharides (sucrose, lactose, maltose).
• They must be broken down into monosaccharides (glucose, fructose, galactose) for absorption.

Mouth

• Salivary amylase (ptyalin) from salivary glands starts breaking down starch into maltose and dextrins.
• Activity is limited because salivary amylase is inactivated in the stomach’s acidic environment.

Small Intestine

• Pancreatic amylase (from pancreas) continues starch digestion, breaking it into maltose and oligosaccharides.
• Maltase breaks maltose into glucose + glucose.
• Sucrase breaks sucrose into glucose + fructose.
• Lactase breaks lactose into glucose + galactose.
• Monosaccharides (glucose, fructose, galactose) are absorbed into the bloodstream through active and facilitated transport.

Proteins

• Proteins are made up of amino acids linked by peptide bonds.
• They must be broken into individual amino acids, dipeptides, or tripeptides for absorption.

Stomach

• Pepsin (activated by HCl from pepsinogen) starts breaking down proteins into polypeptides and peptides.

Small Intestine

• Trypsin & Chymotrypsin break large polypeptides into smaller peptides.
• Carboxypeptidase cleaves amino acids from the ends of peptide chains.
• Aminopeptidases & Dipeptidases break small peptides into amino acids for absorption.
• Amino acids, dipeptides, and tripeptides are absorbed into the bloodstream via active transport.

Lipids (Fats and Oils)

• Lipids are hydrophobic, thus lipid digestion needs emulsification prior to enzyme breakdown.

Mouth and Stomach

• Lingual lipase (secreted by tongue glands) starts breaking down triglycerides into fatty acids and glycerol.
• Gastric lipase in the stomach slightly digests fats, especially in infants.

Small Intestine

• Bile salts from the liver break large fat droplets into smaller ones, increasing surface area.
• Pancreatic lipase hydrolyzes triglycerides into monoglycerides + free fatty acids.
• Monoglycerides + fatty acids combine with bile salts to form micelles, aiding absorption.
• Inside intestinal cells, triglycerides are reassembled and packaged into chylomicrons, which enter the lymphatic system before reaching the bloodstream.
• Monoglycerides and free fatty acids are absorbed via passive diffusion.

Nucleic Acids

• Nucleic acids from food (e.g., fish, plants) must be broken down into their nucleotide components.

Small Intestine

• Pancreatic nucleases (deoxyribonuclease DNase and ribonuclease RNase) break DNA & RNA into nucleotides.
• Brush border enzymes (nucleotidases & nucleosidases) further break nucleotides into nitrogenous bases, pentose sugars, and phosphate groups.
• Nucleotides and nitrogenous bases are absorbed into the bloodstream.

Vitamins and Minerals

• Vitamins are absorbed without digestion, either passively or via carriers.
• Fat-soluble vitamins (A, D, E, K) are absorbed with dietary fats in micelles.
• Water-soluble vitamins (B-complex, C) are absorbed via facilitated or active transport.
• Minerals (iron, calcium, sodium, potassium) are absorbed in the small intestine through active transport or ion channels.

Nutrient Absorption Summary

• Carbohydrates need salivary amylase, pancreatic amylase, maltase, sucrase and lactase.
• Carbohydrates output monosaccharides (glucose, fructose, and galactose).
• Carbohydrates are absorbed in the small intestine capillaries
• Proteins need pepsin, trypsin, chymotrypsin, and peptidase.
• Proteins output amino acids, dipeptides, and tripeptides.
• Proteins are absorbed in the small intestine capillaries
• Lipids need lingual lipase, gastric lipase, and pancreatic lipase.
• Lipids output monoglycerides and fatty acids
• Lipids are absorbed in the small intestine lacteals.
• Nucleic acids need DNase, RNase, and nucleotidases.
• Nucleic acids output nucleotides and nitrogenous bases.
• Nucleic acids are absorbed in the small intestine capillaries.
• Vitamins & Minerals require no enzymes.
• Site of absorption is the Small Intestine.

Absorption of Nutrients

• The primary site of absorption is the small intestine, with the large intestine handling water and some minerals.
• Structural adaptations of the small intestine for absorption include plicae circulares (folds that slow chyme), villi (containing capillaries and lacteals), and microvilli (brush border).
• The thin epithelial lining, rich blood supply, peristalsis, and segmentation also aid absorption.
• Simple diffusion allows passive movement from high to low concentration(e.g., small lipids, fat-soluble vitamins).
• Facilitated diffusion has carrier proteins to help transport molecules (e.g., fructose).
• Active transport uses ATP to move nutrients against a gradient (e.g., glucose, amino acids).
• Endocytosis uses vesicles to engulf large molecules (e.g., immunoglobulins from breast milk).

Nutrient Absorption

• Carbohydrates are absorbed as monosaccharides (glucose, fructose, galactose).
• Glucose & galactose enter via sodium-glucose co-transporters (active transport).
• Fructose is absorbed via facilitated diffusion.
• Once inside, sugars enter capillaries and travel to the liver via the hepatic portal vein.
• Proteins are absorbed as amino acids, dipeptides, or tripeptides.
• Amino acids are actively transported using sodium-dependent transporters.
• Dipeptides & tripeptides are absorbed through peptide transporters.
• Amino acids enter capillaries and are transported to tissues for protein synthesis.
• Bile salts emulsify fats, allowing digestion by pancreatic lipase.
• Monoglycerides & free fatty acids form micelles.
• Inside intestinal cells, lipids are reassembled into triglycerides and packaged into chylomicrons.
• Chylomicrons enter lacteals (lymphatic vessels) and travel through the lymph.
• Nucleic acids are broken down into nitrogenous bases, pentose sugars, and phosphate groups which are then Absorbed via active transport into the bloodstream.
• Water-soluble vitamins (B-complex, C) are absorbed directly into capillaries via facilitated diffusion or active transport.
• Fat-soluble vitamins (A, D, E, K) are absorbed along with lipids in micelles and enter the lymphatic system.
• Sodium is actively transported alongside glucose and amino acids.
• Calcium absorption is regulated by vitamin D and parathyroid hormone.
• Iron is absorbed in the duodenum and transported via ferritin (storage) or transferrin (blood transport).
• Potassium & chloride passively diffuse into the bloodstream.

Large Intestine

• It extracts remaining water and minerals.
• It absorbs electrolytes like sodium and chloride.
• Gut bacteria produce vitamins (biotin, vitamin K), which are absorbed here.

Common Digestive Disorders

• Acid reflux is the backflow of stomach acid into the esophagus.
• Ulcers are open sores in the stomach lining caused by excess acid or H. pylori bacteria.
• Constipation is a difficulty in bowel movement due to lack of fiber or dehydration.
• Diarrhea is an excessive water loss due to infections or digestive issues.