The main function is to break down food and absorb nutrients! Without digestion, you could not get energy from food. The digestive system is also important for immunity (gut-associated lymphoid tissue) and produces hormones that regulate digestion. Digestion begins in the mouth (chewing and saliva) and continues in the stomach (acid and enzymes) and small intestine (most absorption). The large intestine (colon) absorbs water and electrolytes and forms feces. The entire digestive process takes about 24-72 hours from ingestion to elimination, depending on the food and individual. Common digestive problems include heartburn (GERD), ulcers, constipation, diarrhea, irritable bowel syndrome (IBS), and food intolerances (lactose intolerance, celiac disease). Maintaining a healthy diet (fiber, water, probiotics) promotes digestive health.

Amylase is the correct answer! Amylase begins the chemical digestion of carbohydrates. Try this experiment: hold a cracker in your mouth for a few minutes – it will start to taste sweet as amylase breaks down the starch into sugar. The mouth also contains lingual lipase (which begins fat digestion, though most fat digestion occurs in the small intestine). Chewing (mastication) increases the surface area of food, allowing enzymes to work more efficiently. Teeth types: incisors (cutting), canines (tearing), premolars and molars (grinding). Humans have 20 baby teeth (deciduous) and 32 permanent teeth (including wisdom teeth). The tongue is a muscular organ that moves food, tastes (taste buds), and helps form the food into a bolus (a lump) for swallowing. The bolus moves into the pharynx (throat) and then into the esophagus.

Pepsin is the correct answer! It was one of the first enzymes discovered (Theodor Schwann, 1836). Pepsin breaks proteins into smaller peptides. The stomach also produces gastric lipase (minor fat digestion) and intrinsic factor (needed for vitamin B12 absorption). The stomach can expand to hold about 1-2 liters of food. Food stays in the stomach for 2-4 hours, depending on the meal (fats take longer). The stomach is protected by a mucus layer; if the mucus layer is damaged, acid can erode the stomach lining, causing gastritis or peptic ulcers. Most ulcers are caused by the bacterium Helicobacter pylori (discovered by Barry Marshall and Robin Warren, Nobel Prize 2005), not by stress or spicy food. Spicy food can worsen symptoms but does not cause ulcers. Chyme (partially digested food) is released into the small intestine through the pyloric sphincter.

The duodenum is the correct answer! The name "duodenum" means "twelve" (about 12 finger-widths long). The pancreas secretes pancreatic juice (containing trypsin, chymotrypsin, amylase, lipase, and bicarbonate) into the duodenum. The liver produces bile (stored in the gallbladder), which is released into the duodenum to emulsify fats (break them into small droplets). The jejunum (about 2.5 m / 8 ft) and ileum (about 3.5 m / 11 ft) are where most absorption occurs. Villi and microvilli create a huge surface area for absorption. Nutrients (amino acids, monosaccharides, fatty acids, vitamins, minerals) pass through the intestinal wall into the bloodstream or lymphatic system. The small intestine also has its own immune system (Peyer's patches). The small intestine is where the majority of digestion (90%) and absorption (95%) occurs.

Bile emulsifies fats! Without bile, fats would float as large globules, making it hard for lipase to break them down. The gallbladder stores bile between meals (about 30-50 mL). When you eat a fatty meal, the gallbladder contracts and releases bile into the duodenum. If the gallbladder is removed (cholecystectomy), bile drips continuously into the small intestine (still works, but fatty meals may cause diarrhea). The liver has over 500 functions! It filters blood from the digestive tract (hepatic portal vein), removing toxins and processing nutrients. The liver converts ammonia (toxic) into urea (excreted by kidneys). It synthesizes clotting factors and plasma proteins (albumin). It also stores vitamins (A, D, E, K, B12) and minerals (iron, copper). Liver damage (cirrhosis) can be caused by alcoholism, hepatitis, or fatty liver disease. The liver can regenerate (grow back) if part is removed (up to 70%).

Insulin is the correct answer! Diabetes mellitus occurs when the pancreas does not produce enough insulin (type 1) or the body does not respond properly (type 2). Insulin was discovered by Frederick Banting and Charles Best (1921), a breakthrough that saved millions of lives. The pancreas is located behind the stomach. Pancreatic digestive enzymes are released in inactive forms (zymogens) to prevent them from digesting the pancreas itself. Trypsinogen is activated to trypsin in the duodenum. Pancreatitis (inflammation of the pancreas) can be acute (often caused by gallstones or alcohol) or chronic. The pancreas also produces pancreatic polypeptide (regulates appetite and digestion). The exocrine pancreas secretes about 1-2 liters of pancreatic juice per day, containing bicarbonate (to neutralize acidic chyme). Without this neutralization, the small intestine would be damaged by stomach acid. The pH of pancreatic juice is about 8.0-8.3.

Water absorption is the main function! About 1-2 liters of fluid enter the large intestine each day; most is absorbed, leaving about 100-200 mL of water in feces. The appendix (a small pouch attached to the cecum) may have immune functions and serve as a reservoir for beneficial gut bacteria. The gut microbiome (about 100 trillion bacteria) is essential for digestion of fiber, production of vitamins, training the immune system, and preventing pathogen overgrowth. Dysbiosis (imbalance of gut bacteria) is linked to various diseases (obesity, inflammatory bowel disease, allergies). Feces is about 75% water, with the rest consisting of undigested food (fiber, cellulose), bacteria, dead cells, and bile pigments (which give feces its brown color). If feces moves too quickly (diarrhea), not enough water is absorbed. If it moves too slowly (constipation), too much water is absorbed, causing hard, dry stools. The rectum and anal sphincters control elimination. The large intestine has no villi and does not produce enzymes; it relies on bacteria for digestion of remaining material.

Proteases are the correct answer! The word "protease" comes from "protein" + "ase" (enzyme suffix). Different proteases cut at different sites in the protein chain. For example, trypsin cuts after the amino acids lysine and arginine; chymotrypsin cuts after large hydrophobic amino acids (phenylalanine, tyrosine, tryptophan). The end result is individual amino acids, which are absorbed into the bloodstream. Enzymes are not consumed in reactions; they can be used repeatedly. Enzyme activity can be affected by temperature (denatured by high heat) and pH (each enzyme has an optimal pH). Pepsin works best at pH 2 (stomach); trypsin works best at pH 8 (small intestine). Amylase works best at neutral pH (7). Many enzymes require cofactors (metal ions like zinc, magnesium) or coenzymes (derived from vitamins) to function. Without enzymes, digestion would take weeks or months instead of hours.

H. pylori causes most ulcers! Marshall proved this by drinking a culture of H. pylori and developing gastritis. The bacteria weaken the stomach's mucus layer, allowing acid to damage the underlying tissue. Other causes: nonsteroidal anti-inflammatory drugs (NSAIDs like aspirin, ibuprofen), smoking, and excessive alcohol. Ulcers can cause burning pain, bloating, nausea. Treatment includes antibiotics and proton pump inhibitors (acid reducers). GERD (gastroesophageal reflux disease) occurs when stomach acid flows back into the esophagus, causing heartburn. Chronic GERD can lead to Barrett's esophagus (precancerous condition). Constipation is often caused by low fiber, dehydration, or lack of exercise. Diarrhea can be caused by infections (viruses, bacteria, parasites), food intolerances, or medications. IBS is a functional disorder (no structural damage) treated with diet, stress management, and medications. Lactose intolerance affects about 65% of the global population (most common in East Asia, West Africa, and Native American populations). Celiac disease affects about 1% of the population; it requires a strict gluten-free diet.

Mouth → esophagus → stomach → small intestine → large intestine → anus is the correct order! Food travels through these organs in sequence. The esophagus is a muscular tube (about 25 cm / 10 inches long) that moves food by peristalsis (wave-like muscle contractions). The lower esophageal sphincter (LES) prevents stomach acid from backing up into the esophagus (reflux). The pyloric sphincter controls flow from stomach to duodenum. The ileocecal valve separates the small and large intestines. The anal sphincters (internal and external) control defecation. Peristalsis occurs throughout the digestive tract, moving food even if you are upside down (astronauts can swallow in space). The entire digestive process takes 24-72 hours. Transit time: stomach (2-4 hours), small intestine (4-6 hours), large intestine (12-48 hours). The digestive system is sometimes called the "second brain" because it has its own nervous system (enteric nervous system) with over 500 million neurons – more than the spinal cord! The enteric nervous system can operate independently of the brain (though it communicates via the vagus nerve). This is why you can have "gut feelings" or butterflies in your stomach.