The Digestive System
The digestive system domain tests mechanical and chemical digestion, secretion, absorption and regulation along the gastrointestinal tract. Expect questions on enzyme specificity, pH, villi, transporters and the roles of the liver, pancreas and gallbladder.
Core rules and concepts
- The GI tract has four layers: mucosa, submucosa, muscularis externa and serosa or adventitia. Villi and microvilli increase absorptive surface area.
- Salivary amylase begins starch digestion. Pepsinogen is secreted by chief cells and activated to pepsin by stomach acid. Parietal cells secrete hydrochloric acid and intrinsic factor.
- The pancreas secretes bicarbonate and enzymes, including amylase, lipase and proteases. The gallbladder stores bile, which emulsifies fats but does not digest them.
- Carbohydrates are absorbed as monosaccharides; proteins as amino acids or small peptides; fats as fatty acids and monoglycerides, then reassembled into chylomicrons.
- The enteric nervous system and hormones such as gastrin, secretin and cholecystokinin coordinate secretion and motility.
- The liver processes nutrients, detoxifies, stores glycogen and produces bile. The large intestine absorbs water and electrolytes and houses bacteria.
Worked example
A fatty meal enters the duodenum. Cholecystokinin causes gallbladder contraction and pancreatic lipase release. Bile salts emulsify fat into small droplets, increasing lipase access. Lipase breaks triglycerides into fatty acids and monoglycerides, which enter intestinal cells, are reassembled into triglycerides, packaged into chylomicrons, and enter lacteals rather than blood capillaries. This explains why fat absorption initially bypasses the portal vein.
Traps that cost marks
Do not say bile contains lipase or that bile digests fat; it emulsifies fat. Do not confuse pepsinogen with pepsin. Do not say intrinsic factor absorbs vitamin B12 in the stomach; it is needed for B12 absorption later in the ileum. Remember that most water absorption occurs in the small intestine, although the large intestine absorbs water from remaining material.
Worked example
Removal of the terminal ileum can impair vitamin B12 absorption because intrinsic factor–B12 complexes are absorbed there.