Human Digestive System
The human digestive system is a complex biological process that breaks down insoluble complex food molecules into simple, soluble, and absorbable nutrients.
Total Length: The complete Alimentary Canal in an adult human is about 8 to 10 meters (25 to 30 feet) long, extending from the mouth to the anus.
Main Phases:
- Ingestion (Taking in food)
- Digestion (Breaking down food mechanically and chemically)
- Absorption (Passage of nutrients into the bloodstream)
- Assimilation (Utilization of absorbed nutrients by body cells)
- Egestion / Defecation (Elimination of undigested waste)
Organs of the Alimentary Canal & Their Functions
| Organ | Anatomical & Physiological Facts | |
| Mouth & Oral Cavity | Contains teeth, tongue, and salivary glands. Digestion of carbohydrates starts here. | Salivary Amylase (Ptyalin) breaks starch down into maltose. Saliva pH is ~6.8. |
| Pharynx (Throat) | A short fibromuscular passage connecting the oral cavity to the oesophagus. | Acts as a common pathway for both food and air. The Epiglottis (a flap of tissue) prevents swallowed food from entering the trachea/windpipe. |
| Oesophagus (Food Pipe) | A muscular tube (~25 cm) connecting the pharynx to the stomach. No enzymes are secreted here. | Food moves down by a rhythmic wave-like muscular contraction called Peristalsis. |
| Stomach | J-shaped muscular pouch. Stores food for 4–5 hours and secretes Gastric Juice. | Hydrochloric Acid (HCl) is secreted by Parietal/Oxyntic cells to kill bacteria and activate Pepsin for protein digestion. |
| Small Intestine | Longest segment (~6 meters / 20 feet). Comprises Duodenum, Jejunum, and Ileum. | Primary site of complete digestion and maximum absorption. Finger-like projections called Villi increase surface area. |
| Large Intestine | Wider but shorter (~1.5 meters). Comprises Caecum, Colon, and Rectum. | No digestive enzymes are produced. Main role is absorption of water and minerals from undigested food. |
| Rectum | The terminal 12–15 cm chamber of the large intestine. | Stores semi-solid faeces temporarily until a neural reflex triggers the urge for defecation. |
| Anus | The terminal opening of the alimentary canal, controlled by internal and external anal sphincters. | Responsible for Egestion / Defecation (expulsion of solid body waste out of the body). |
Digestive Glands & Enzymes
A. Major Digestive Glands
- Salivary Glands: Humans have 3 pairs of salivary glands:
- Parotid Glands: Largest (located near the ears).
- Submandibular / Submaxillary Glands: Located under the lower jaw.
- Sublingual Glands: Located under the tongue.
- Liver:
- The largest gland in the human body.
- Produces Bile, which contains no enzymes but emulsifies fats (breaks large fat globules into tiny droplets).
- Bile is stored in the Gallbladder.
- Pancreas:
- A Mixed / Heterocrine Gland (secretes both digestive enzymes and hormones like insulin and glucagon).
- Secretes Pancreatic Juice, which contains enzymes for digesting carbohydrates, proteins, and lipids.
B. Summary of Digestive Enzymes
| Enzyme | Secreted By | Substrate (Food Targeted) | End Product Formed |
| Ptyalin (Salivary Amylase) | Salivary Glands | Starch (Carbohydrate) | Maltose |
| Pepsin | Stomach (Gastric Juice) | Proteins | Peptones & Proteoses |
| Rennin | Stomach (in infants) | Milk Protein (Casein) | Paracasein (Curdles milk) |
| Trypsin | Pancreas | Proteins & Peptones | Peptides & Amino Acids |
| Pancreatic Amylase | Pancreas | Carbohydrates | Maltose |
| Pancreatic Lipase (Steapsin) | Pancreas | Emulsified Fats | Fatty acids & Glycerol |
| Erepsin / Peptidase | Small Intestine | Peptides | Amino Acids |
| Lactase, Sucrase, Maltase | Small Intestine | Disaccharides | Glucose, Fructose, Galactose |
Chemical Reactions in Digestion
- Carbohydrate Digestion:
- Protein Digestion:
- Fat Digestion:
