Digestion Lecture Notes

General Structure of the Digestive System

• The digestive system is responsible for processing nutrients after consumption.
• Key processes include mechanical processing, motility, secretion, digestion, absorption, and excretion.
• Digestion involves enzymatic breakdown, either by the body's enzymes (enzymatic digestion) or by microbial enzymes (fermentative digestion).
• The gut is a hollow tube where digestion and absorption occur.
• The gut lining is selective in absorption and secretion, unlike skin.

Layers of the Gut

• The gut wall consists of multiple layers, including the mucosa, submucosa, muscularis externa, and serosa.
• Mucosa: Innermost layer with endocrine and exocrine glands.
• Submucosa: Contains nerve plexus and major blood and lymph vessels.
• Muscularis Externa: Circular and longitudinal muscles for motility.
• Serosa: Connects the gut to the rest of the body.
• The mucosa has folds to increase surface area for absorption.
• Exocrine glands secrete digestive enzymes and gastric juices into the gut.
• Endocrine cells produce hormones that regulate digestion based on the composition of digesta.

Regulation of Digestion

• Digestion is tightly regulated by muscles, nerves, hormones, and enzymes.
• The enteric nervous system allows the gut to function independently of the central nervous system.
• The gut contains a rich array of neurotransmitters, including serotonin.

The Mouth

• Saliva lubricates food for swallowing; secretion is under nervous control and can be stimulated by sight, taste, smell, or thought of food.
• Pavlov's experiments demonstrated the conditional reflex related to salivation.
• Saliva contains amylase, which digests starches, and proteins that bind and inactivate toxins.
• Chewing reduces food particle size, increasing the surface area for enzyme action.

The Esophagus

• The esophagus is a smooth tube that transports food from the mouth to the stomach.

The Stomach

• The stomach acts as a short-term reservoir for food, mixing and grinding it into a liquid.
• The rate of food release into the small intestine is controlled by the stomach.
• Some protein digestion and a small amount of fat digestion occur in the stomach.
• The stomach does not absorb nutrients but can absorb ethanol and aspirin.
• Gastric pits contain acid-secreting cells (hydrochloric acid) and enzyme-secreting cells (zymogens like pepsinogen).
• Zymogens are inactive enzyme forms that are activated only when needed to prevent self-digestion of the stomach.
• Pepsinogen converts to pepsin in the presence of hydrochloric acid, digesting proteins.

Digestion in the Stomach

• Substrates: Protein and fat.
• Gastric Enzymes: Pepsin and lipase.
• Products: Peptides and fatty acids (not absorbed in the stomach).
• The stomach is protected by a mucus layer, preventing gastric juices from attacking the stomach lining.
• Stomach ulcers are often caused by Helicobacter pylori bacteria, treatable with antibiotics.
• Barry Marshall won the Nobel Prize in 2005 for discovering the role of Helicobacter pylori in stomach ulcers.

Regulation of Gastric Digestion

• Gastric digestion is under regulated control; the stomach emptying rate is influenced by fat content.
• Fatty meals slow down stomach emptying for optimal energy absorption in the small intestine.
• Gastrin stimulates hydrochloric acid release and pepsin activation, followed by negative feedback to stop further gastrin release.

The Small Intestine

• The small intestine is the primary site of digestion and absorption.
• It consists of the duodenum, jejunum, and ileum.
• The small intestine has villi and microvilli to increase surface area for absorption.
• Proteins, carbohydrates, and fats are predominantly digested here.
• Enzyme production adapts to dietary habits (e.g., vegetarians produce more enzymes for plant-based nutrients).

Digestion in the Small Intestine

• Substrates: Sugars, starches, protein, and fat.
• Enzymes: Sucrases, maltases, amylases (carbohydrates); proteases (protein); lipases (fats).
• Products: Glucose, amino acids, and fatty acids, all of which are absorbed.

Undigested Material

• Undigested substances may reach the large intestine, leading to bloating and gas.
• FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols) can cause bloating when fermented by bacteria in the large intestine.
• Lactose intolerance results from insufficient lactase production, leading to undigested lactose in the large intestine.
• The drug Xenical inhibits lipase production, reducing fat digestion and absorption, but its side effects often deter fat consumption.

The Large Intestine

• The large intestine has less surface area compared to the small intestine.
• The cecum (with the appendix) absorbs electrolytes and salt.
• The colon includes the ascending, transverse, and descending sections, ending in the rectum.
• The human large intestine contains a large amount of bacteria, comprising about 50% of the body's cells, totaling around two kilograms.
• The microbiome in the colon affects immune function, diabetes, obesity, and mental health.
• Digestion in the large intestine is fermentative, carried out by bacterial enzymes.

Digestion in the Large Intestine

• Substrates: Fiber, resistant starch, proteins, fats, and FODMAPs.
• Enzymes: Produced by microbes.
• Products: Short-chain fatty acids, gases (hydrogen and methane), and bacterial cells.
• Only short-chain fatty acids are absorbed and used by colon cells.

Inflammatory Bowel Disease (IBD)

• Conditions like Crohn's disease and ulcerative colitis are autoimmune diseases affecting the gastrointestinal tract.
• Intestinal parasites like worms have been observed to reduce the incidence of Crohn's disease.
• Deliberate infection with pig whipworm has resolved Crohn's symptoms in some cases.
• Worms reduce the amount of inflammation-causing bacteria in the gut.

First Pass Metabolism

• After absorption, nutrients primarily go to the liver via the hepatic portal vein before being transported throughout the body.
• The liver modifies nutrients and toxins through first-pass metabolism.
• The liver synthesizes fatty acids, cholesterol, and packages them for transport.
• Enzymes in the liver metabolize toxins into water-soluble products for excretion in urine.
• Enzyme variations exist between individuals and species, affecting the metabolism of different substances.
• Chemicals absorbed from the rectum bypass the liver and enter directly into systemic circulation.
• Alcohol enemas can be fatal due to the absence of first-pass metabolism.

The appendix lacks significant function in humans.

Summary Diagram

• A summary diagram is provided to illustrate what is digested by which enzymes in different parts of the gastrointestinal tract.