Chapter 1: Digestive System

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Chapter Overview & SLOs

What is the digestive system? The digestive system is responsible for breaking down food into smaller molecules that can be absorbed into the bloodstream and used by the body for energy, growth, and repair.

How does food move through the alimentary canal? Food is pushed forward by peristalsis, a wave-like muscle contraction where circular muscles contract behind the food and longitudinal muscles relax in front, propelling food through the digestive tract.

What is the role of bile in the digestive process? Bile is produced by the liver and stored in the gall bladder; it is released into the duodenum to emulsify fats (break them into smaller droplets) and neutralize acidic chyme coming from the stomach.

How does digestion occur in the oral cavity? Digestion in the mouth involves:

• Mechanical digestion: Chewing by teeth breaks food into smaller pieces, increasing surface area for enzyme action.
• Chemical digestion: Salivary amylase (ptyalin) begins breaking down starch into maltose (a disaccharide).

What happens in the stomach?

• HCl creates an acidic environment (pH 1.5 to 2) that kills bacteria and activates pepsinogen.
• Pepsinogen is converted to its active form, pepsin, which breaks down proteins into smaller peptides.
• Mucous cells secrete a thick, alkaline mucus to protect the stomach wall from self-digestion by HCl and pepsin.

What are the pancreatic enzymes? The pancreas secretes:

• Amylase: Continues starch digestion.
• Lipase: Breaks down fats into fatty acids and glycerol.
• Trypsin: Continues protein digestion.

How does absorption occur in the small intestine? The small intestine is the primary site for nutrient absorption due to finger-like projections called villi.

• Villi structure: Each villus is covered by a single layer of epithelial cells with microvilli (brush border) to increase surface area.
• Capillary network: Absorbs water-soluble nutrients (amino acids, glucose, vitamins) and carries them to the liver via the hepatic portal vein.
• Lacteal: Lymphatic capillary within each villus that absorbs fats (fatty acids and glycerol are reassembled into triglycerides and transported as chylomicrons).

What are common digestive disorders?

• Diarrhea: Frequent watery stools often caused by infections (bacteria, viruses, parasites). Can lead to dehydration.
• Constipation: Hard, dry stools usually due to a low-fiber diet, dehydration, or lack of physical activity.

Summary of digestive enzymes:

| Enzyme | Source | Substrate | Product | |--------|--------|-----------|---------| | Salivary amylase | Salivary glands | Starch | Maltose | | Pepsin | Stomach (chief cells) | Proteins | Peptides | | Pancreatic amylase | Pancreas | Starch | Maltose | | Lipase | Pancreas | Fats | Fatty acids + glycerol | | Trypsin | Pancreas | Proteins | Peptides |

These notes are strictly aligned with the Student Learning Outcomes (SLOs) for the FBISE 2026 annual examination.

• How does digestion occur in the oral cavity? It involves mechanical digestion through chewing by teeth and chemical digestion where salivary amylase begins breaking down starch into maltose.
• How is the stomach wall protected from self-digestion? Protection is provided by specialized mucous cells that secrete a thick, alkaline mucus and by producing the protease enzyme in its inactive form, pepsinogen, which is activated only when needed.
• What are the structural adaptations of a villus for absorption? Each villus is covered by a single layer of epithelial cells with microvilli (brush border) and contains a capillary network for water-soluble nutrients and a lacteal for fat absorption.
• How do common digestive disorders like diarrhea and constipation differ? Diarrhea involves frequent watery stools often caused by infections, while constipation involves hard stools usually due to a low-fiber diet or dehydration.

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