Digestive System - MCAT Review

Table of digestive tract
Organ	Digestive Activities
Mouth	Mechanical digestion: chewing Chemical digestion: saliva contains amylase and lipase
Stomach	Mechanical digestion: churning Chemical digestion: protease (pepsin)
Small intestine	Chemical digestion: amylase, protease, lipase (assisted by bile from liver/gall bladder), nuclease (all enzymes predominantly from the pancreas) Nutrient and water absorption
Large intestine	Water absorption

Ingestion

saliva as lubrication and source of enzymes
- saliva dissolves food.
- saliva contains mucin, a protein that lubricates the bolus (chewed up food ball).
- saliva contains amylase, which breaks down polysaccharides (starch and glycogen).
- saliva also contains antibodies and lysozyme that kill pathogens.
epiglottal action
- epiglottis = flap of cartilage that closes off airway when you're swallowing.
pharynx (function in swallowing)
- pharynx = throat = between mouth and esophagus.
- muscular tube that squeezes and routes food to the esophagus when swallowing (closes off pathways to nasal cavity and airway).
esophagus (transport function)
- muscular tube that propels bolus (food) to the stomach by peristalsis.
- peristalsis = squeezing stuff through a tube (esophagus/gut) by smooth muscle.

storage and churning of food
- storage = the stomach is a muscular bag that is elastic and can stretch to store food.
- churning = mechanical digestion = mixing food.
low pH, gastric juice, protection by mucus against self-destruction
- Parietal cells secrete HCl that causes the pH to be very acidic.
- Gastric juice = HCl + pepsin + hormones = secreted by the stomach (parietal and chief cells, and enteroendocrine cells)
- Pepsin = protease that works best in acidic environment.
- Goblet cells secrete mucus lining that protect the stomach from the acid and self-digestion.
production of digestive enzymes, site of digestion
- Chemical digestion: Stomach produces pepsin, which digests proteins (secreted in an inactive form, gets activated in acidic environment)
- Pepsin is special in that it works best at very acid pH.
- Mechanical digestion: Stomach churns food.
structure (gross)
- banana shaped bag that can stretch.
- inner membrane densely folded (rugae), so can accommodate stretching.
- sealed off on the top by the cardiac (gastroesophageal) sphinctor.
- sealed off on the bottom by the pyloric sphinctor.

production of bile: liver makes bile from cholesterol, stores it in gall bladder.
role in nutrient metabolism, vitamin storage
- Makes and stores glycogen from glucose.
- Gluconeogenesis from glycerol and amino acids (deamination).
- Breaks down fats, makes cholesterol, makes lipoproteins used to transport fats.
- Stores vitamins (A, D and B12) and iron.
- Detox: metabolize alcohol, remove ammonia in blood.
role in blood glucose regulation, detoxification
- Blood glucose regulation by liver:
  - Blood sugar too low: gluconeogenesis.
  - Blood sugar too high: glycogeneis.
- Detoxification: metabolize alcohol (alcohol dehydrogenase), remove blood ammonia, inactivate various other drugs/toxins.
structure (gross): largest gland in body, spans both sides of the abdomen (though right side much larger). Ducts draining to duodenum and gall bladder.

storage in gall bladder
- Gall bladder stores excess, unused bile, and concentrates it. Secretes it when needed.
function: bile is an emulsifying agent (not an enzyme). Bile breaks down large fat droplets into smaller microscopic droplets by forming micelles. This increases the total surface area of the fat for lipase action.

production of enzymes, bicarbonate
- Pancreas is the major source for all the digestive enzymes.
  - Amylase - digests starch.
  - Various proteases.
  - Lipase - digests fat.
  - Ribonuclease - digests nucleic acids.
- Pancreas makes HCO₃^- to neutralize the HCl from the stomach.
transport of enzymes to small intestine
- Digestive enzymes of pancreas = exocrine = flows into small intestine via duct.
structure (gross): tadpole-shaped gland with duct leading to duodenum.

absorption of food molecules and water
- Small intestine is the major place for digestion and absorption.
- Folds, villi, and microvilli increases the surface area for absorption.
- Absorbs digested food into circulation (fats into lacteals, all others into capillaries).
- Active transport occurs to absorb against the concentration gradient.
  - Intestinal lumen (less glucose) -> enterocyte (more glucose): Secondary active transport by Na⁺-K⁺ pump + Na⁺-Glucose symport.
- Passive/facilitated diffusion occurs to absorb down the concentration gradient.
  - Enterocyte (more glucose) -> extracellular fluid (less glucose): Facilitated diffusion (then the glucose will go from the extracellular fluid to blood).
function and structure of villi
- Villi = finger-like protrusions inside small intestine.
- Microvilli = same as villi but on the surface of a single absorptive cell.
production of enzymes, site of digestion
- The small intestine is the major place for digestion and absorption.
- Pancreas is the major source for enzymes. However, the small intestine does make some of its own enzymes, including protease and amylase.
neutralization of stomach acid
- The pancreas makes bicarbonate ion to neutralize the HCl from the stomach.
- This neutralization facilitates enzymes in the small intestine, which would be denatured by stomach pH.
structure (anatomic subdivisions)
1. Duodenum.
2. Jejunum.
3. Ileum.

anatomic subdivisions (old topic)
1. Cecum: blind pocket containing appendix.
2. Ascending colon
3. Transverse colon
4. Descending colon
5. Sigmoid colon
6. Rectum: stores feces.
absorption of water: The large intestine absorbs any remaining water that is not absorbed by small intestine.
bacterial flora
- Ferment undigested nutrients, make gas.
- Produce vitamin K (important for clotting).
structure (gross): lobes/pockets along its length due to muscle tone. Unlike small intestine, the large intestine has no folds or villi.

sphincter muscle
- Cardiac sphincter (gastroesophageal sphincter): sphincter between esophagus and stomach. Prevents back flow of food.
- Pyloric sphincter: between stomach and small intestine. Releases food into the small intestine a small amount at a time.
- Anal sphincter: at the end of rectum. ties the end of the rectum.
peristalsis: involuntary movement of smooth muscles, squeezes food along the digestive tract.