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.

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.

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.

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 is a smooth tube that transports food from the mouth to 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.

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.

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 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).

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 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 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.

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.

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.

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.

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