Cram sheet
26 points- Heterotroph groups: herbivore (plants only), carnivore (animals), omnivore (both). Humans are omnivores. Watch The Digestive System: CrashCourse Biology #28 on YouTube, from CrashCourse
- Gastrovascular cavity (cnidarians, flatworms) = ONE opening, no specialization. Alimentary canal = separate mouth and anus, one-way flow, so regions can specialize. Watch The Digestive System: CrashCourse Biology #28 on YouTube, from CrashCourse
- Four tissue layers of the GI tract, lumen outward: mucosa → submucosa → muscularis → serosa. Watch Digestive System, Part 1: Crash Course Anatomy & Physiology #33 on YouTube, from CrashCourse
- Saliva contains salivary amylase → starts starch → maltose. Mouth also does mechanical breakdown. Acidic solutions are the strongest salivation stimulus (lemon juice raises rate eightfold). Watch Digestive System, Part 1: Crash Course Anatomy & Physiology #33 on YouTube, from CrashCourse
- Swallowing starts voluntary, then becomes an unstoppable reflex. Soft palate seals the nasal cavity; the larynx rises so the epiglottis covers the glottis and keeps food out of the trachea. Watch Gastrointestinal | Deglutition (Swallowing) on YouTube, from Ninja Nerd
- Peristalsis moves the bolus down the esophagus — which is why you can swallow upside down. Upper third skeletal muscle (voluntary), lower two-thirds smooth muscle. Watch Gastrointestinal | Deglutition (Swallowing) on YouTube, from Ninja Nerd
- Stomach gastric glands: mucous cells (protection), parietal cells (HCl + intrinsic factor), chief cells (pepsinogen). HCl strips 44 amino acids off pepsinogen → active pepsin. Secreting it inactive stops the cells digesting themselves. Watch Digestive System, Part 2: Crash Course Anatomy & Physiology #34 on YouTube, from CrashCourse Watch Parietal Cell: Gastric Acid Production on YouTube, from Robinson Visual Science
- Stomach pH ≈ 2 — about 250,000× more acidic than blood (pH 7.4). In adults only proteins are partially digested in the stomach; no significant carb or fat digestion there. Watch Parietal Cell: Gastric Acid Production on YouTube, from Robinson Visual Science
- Chyme leaves through the pyloric sphincter. Only water, aspirin, and alcohol are absorbed through the stomach wall. Watch Digestive System, Part 2: Crash Course Anatomy & Physiology #34 on YouTube, from CrashCourse
- Small intestine order: duodenum (first 25 cm) → jejunum → ileum. Villi and microvilli (brush border) push surface area to 300 m² — bigger than a tennis court. Watch Small intestine 3: Absorption | Gastrointestinal system physiology | NCLEX-RN | Khan Academy on YouTube, from khanacademymedicine
- Absorption split you must know: glucose and amino acids → blood capillaries → hepatic portal vein → liver first. Fats → resynthesized into triglycerides → chylomicrons → lymphatic capillaries, bypassing the liver. Watch Small intestine 3: Absorption | Gastrointestinal system physiology | NCLEX-RN | Khan Academy on YouTube, from khanacademymedicine
- Glucose enters by cotransport with Na+; fructose by facilitated diffusion; fatty acids and monoglycerides by simple diffusion (they are nonpolar). Watch Small intestine 3: Absorption | Gastrointestinal system physiology | NCLEX-RN | Khan Academy on YouTube, from khanacademymedicine
- Pancreas is both: exocrine (trypsin, chymotrypsin, amylase, lipase, bicarbonate via acini) and endocrine (islets of Langerhans — β cells → insulin, α cells → glucagon). Watch Endocrinology | Pancreas: Overview on YouTube, from Ninja Nerd
- Enzymes are secreted as inactive zymogens. Trypsinogen is cut by the brush border enzyme enterokinase → trypsin, which then activates the others. Watch Endocrinology | Pancreas: Overview on YouTube, from Ninja Nerd
- Bile from the liver, stored in the gallbladder. Bile salts are detergents — they emulsify fat into small droplets, raising surface area for lipase. Bile pigments are just waste from old red blood cells; blocked excretion → jaundice. Watch Bile Acids, Bile Salts — Lipid Emulsification — Liver & Gallbladder — GI Physiology & Biochemistry on YouTube, from Medicosis Perfectionalis
- Liver jobs: detoxify, convert ammonia → urea, inactivate steroid hormones, make plasma proteins (low plasma protein → edema), store glycogen. Watch Liver Physiology on YouTube, from Armando Hasudungan
- Blood glucose: insulin (β cells) lowers it and builds glycogen and fat. Glucagon (α cells) raises it via glycogenolysis, then gluconeogenesis from amino acids and lactic acid during prolonged fasting. Watch Insulin and glucagon | Chemical Processes | MCAT | Khan Academy on YouTube, from khanacademymedicine
- Muscle stores glycogen but cannot release glucose into blood — it lacks the enzyme to remove the phosphate from glucose 6-phosphate. Watch Insulin and glucagon | Chemical Processes | MCAT | Khan Academy on YouTube, from khanacademymedicine
- The three enterogastrones from the duodenum: CCK and GIP respond to FATTY chyme, secretin responds to ACIDIC chyme. All three slow the stomach. CCK → gallbladder contracts + pancreatic enzymes. Secretin → pancreatic bicarbonate. Secretin was the first hormone ever discovered. Watch Gastrointestinal | Gastric Secretion: The Intestinal Phase on YouTube, from Ninja Nerd
- Gastrin comes from the stomach itself, triggered by protein, and drives HCl and pepsinogen — shut off by falling pH (negative feedback). Watch Gastrointestinal | Gastric Secretion: The Intestinal Phase on YouTube, from Ninja Nerd
- Appetite: leptin (fat tissue) and insulin = LONG-term satiety. CCK/GIP = short-term satiety. Ghrelin = short-term hunger signal, rises before meals. In the hypothalamus NPY induces feeding, α-MSH suppresses it.
- Obese humans usually have HIGH leptin — the problem is reduced sensitivity to leptin, not a shortage of it.
- Essential nutrients for humans: 13 vitamins, 9 essential amino acids, essential fatty acids, and minerals. Watch Introduction to vitamins and minerals | Biology foundations | High school biology | Khan Academy on YouTube, from Khan Academy
- Ruminant stomach order: rumen → reticulum → omasum → abomasum. Only the abomasum secretes gastric juice. The rumen (up to 50 gallons) is the fermentation vat. Watch Digestive Physiology of the Ruminant on YouTube, from Mike Hutjens
- Non-ruminant herbivores (horses, rabbits, rodents) ferment in an enlarged cecum. Because the cecum is past the stomach they cannot regurgitate, so rabbits and rodents use coprophagy — eating their feces for a second pass. Watch Digestive Physiology of the Ruminant on YouTube, from Mike Hutjens
- All mammals depend on gut bacteria for vitamin K (blood clotting). Birds lack these bacteria and must eat it. Long antibiotic courses can require vitamin K supplementation. Watch Introduction to vitamins and minerals | Biology foundations | High school biology | Khan Academy on YouTube, from Khan Academy Watch Colon, rectum, and anus | Gastrointestinal system physiology | NCLEX-RN | Khan Academy on YouTube, from khanacademymedicine