13.1 Digestive Physiology
Key Takeaways
- Mechanical digestion physically breaks food (chewing, churning, segmentation); chemical digestion uses enzymes and secretions to split macromolecules into absorbable units
- Amylase digests starch, pepsin digests protein in acid stomach, proteases continue protein digestion in the small intestine, lipase digests fats after bile emulsification increases surface area
- The small intestine is the primary site of nutrient absorption; villi and microvilli maximize surface area for uptake into blood and lymph
- Peristalsis propels contents forward; stomach acid activates pepsinogen and kills many microbes, while intrinsic factor is required for vitamin B12 absorption later in the ileum
- The liver metabolizes nutrients, detoxifies blood, stores glycogen, and produces bile; the gallbladder stores and concentrates bile for fat digestion
13.1 Digestive Physiology
Quick Answer: Digestion has two complementary jobs: mechanical breakdown (chewing, churning, segmentation) and chemical breakdown (enzymes and bile). Know amylase (starch), pepsin (protein in acid), proteases and lipase (small intestine), and bile emulsification of fats. Absorption occurs mainly in the small intestine. Peristalsis moves contents; stomach acid and intrinsic factor matter for protein digestion start and vitamin B12 absorption. The liver handles metabolism, detoxification, glycogen storage, and bile production. Anatomy named the organs; this section explains how they process a meal for NEX Human Physiology.
Chapter 9 mapped the canal and accessory organs. Physiology asks what happens to a sandwich as it travels: how it is broken apart, which enzymes act where, where nutrients enter the blood, and how motility and secretions are timed. Build from mechanical vs chemical digestion → enzyme table → absorption sites → motility and stomach special factors → liver overview.
Mechanical vs Chemical Digestion
| Type | What it does | Examples |
|---|---|---|
| Mechanical digestion | Physically reduces particle size and mixes food with secretions; does not change chemical identity of nutrients | Chewing (mastication), tongue mixing, stomach churning, small-intestine segmentation mixing |
| Chemical digestion | Enzymes and other secretions hydrolyze macromolecules into monomers or smaller units that can be absorbed | Salivary/pancreatic amylase on starch; pepsin and pancreatic proteases on protein; lipase on triglycerides; bile emulsifies fats to aid lipase |
Mechanical work increases surface area so enzymes can contact more substrate. Chemical work creates the absorbable products: monosaccharides, amino acids/small peptides, fatty acids and monoglycerides, plus vitamins, minerals, and water handled by other transporters.
Both processes often run in the same organ at once—for example, the stomach churns (mechanical) while pepsin cleaves proteins (chemical).
Enzyme Roles and Bile Emulsification
Match each enzyme to substrate and primary location. Intro exams love “which enzyme digests X?” stems.
| Secretion / enzyme | Main substrate | Primary site of action | Notes |
|---|---|---|---|
| Salivary amylase | Starch (complex carbohydrate) | Mouth (continues briefly in stomach until acid inactivates it) | Begins carbohydrate digestion |
| Pancreatic amylase | Starch / remaining carbs | Small intestine lumen | Completes most starch breakdown to maltose and related oligosaccharides; brush-border enzymes finish to monosaccharides |
| Pepsin | Protein | Stomach | Secreted as inactive pepsinogen by chief cells; activated by HCl (and by pepsin itself) |
| Pancreatic proteases (e.g., trypsin, chymotrypsin—intro names) | Protein / peptides | Small intestine | Released as inactive zymogens; activated in the duodenum; continue protein digestion after pepsin |
| Pancreatic lipase | Triglycerides (fats) | Small intestine | Needs fat droplets small enough for efficient contact |
| Bile (not an enzyme) | Fats (emulsification) | Small intestine (bile from liver/gallbladder) | Breaks large fat globules into smaller droplets → ↑ surface area for lipase; does not chemically digest fat by itself |
Emulsification story: Dietary fat is hydrophobic and clumps into large globules. Bile salts surround and disperse fat into tiny droplets. Lipase then hydrolyzes triglycerides at the droplet surface. Without bile, fat digestion and absorption of fat-soluble vitamins (A, D, E, K) become inefficient—clinical link to biliary obstruction or ileal disease that interrupts the bile–fat cycle.
Brush-border enzymes on enterocyte microvilli finish digestion of disaccharides and peptides at the membrane before absorption. You do not need every enzyme name for NEX, but know that final carbohydrate and peptide cleavage often happens at the brush border, not only in the lumen.
Absorption: Why the Small Intestine Wins
| Region | Absorption emphasis (intro) |
|---|---|
| Mouth / esophagus | Essentially none for nutrients (absorption is not the job here) |
| Stomach | Limited (some water, alcohol, certain drugs); not the main nutrient gateway |
| Small intestine | Primary site for carbohydrates, proteins, fats, most vitamins and minerals, and much water |
| Large intestine | Water and electrolytes; vitamin K and some B vitamins from bacterial activity; forms feces |
Absorption depends on the huge surface area created by circular folds, villi, and microvilli (anatomy from Chapter 9; physiology uses that surface). Most products of carbohydrate and protein digestion enter blood capillaries in the villus and travel via the hepatic portal vein to the liver. Products of fat digestion are assembled into chylomicrons and largely enter lacteals (lymph) before rejoining blood—intro-level fat path contrast.
| Nutrient class | Absorbed mainly as | Typical route from gut |
|---|---|---|
| Carbohydrates | Monosaccharides (e.g., glucose) | Capillaries → portal vein → liver |
| Proteins | Amino acids / small peptides | Capillaries → portal vein → liver |
| Fats | Fatty acids & monoglycerides → reassembled → chylomicrons | Lacteals → lymph → blood |
Peristalsis and Motility Patterns
Peristalsis is a wave of circular-muscle contraction behind a bolus with relaxation ahead, propelling contents forward along the tract (esophagus, intestines). It is the main propulsive pattern.
| Motility pattern | Function |
|---|---|
| Peristalsis | Propulsion toward the anus |
| Segmentation (small intestine) | Alternating constrictions that mix chyme with enzymes and bile and press it against the mucosa for absorption—more mixing than net propulsion |
| Stomach churning | Mixes food with acid and pepsin into chyme; pyloric sphincter meters emptying into the duodenum |
Autonomic tone matters: parasympathetic activity generally increases GI motility and secretions (“rest and digest”); sympathetic activity decreases them. Sphincters time emptying so the small intestine is not overwhelmed.
Stomach Acid and Intrinsic Factor
Parietal cells secrete hydrochloric acid (HCl) and intrinsic factor.
| Factor | Physiologic role |
|---|---|
| HCl (stomach acid) | Lowers pH; kills many swallowed microbes; denatures proteins; activates pepsinogen → pepsin; provides the acidic environment pepsin needs |
| Intrinsic factor | Glycoprotein required for vitamin B12 absorption in the ileum; without it, B12 deficiency and pernicious anemia risk rise |
Mucus and bicarbonate from surface cells protect the stomach lining from autodigestion. When protection fails, acid and pepsin can injure mucosa (ulcer concept at awareness level). Acid is not “digestion of fat”—fat digestion waits for bile and lipase downstream.
Gastric emptying control (intro): Liquid and small particles leave faster than large solids; fats and high acidity in the duodenum slow emptying via feedback so digestion stays matched to capacity.
Liver Metabolic Roles (Overview)
The liver is the body’s central metabolic hub receiving nutrient-rich portal blood.
| Liver role | What it means physiologically |
|---|---|
| Bile production | Continuously makes bile for fat emulsification; gallbladder stores/concentrates it between meals |
| Nutrient processing | Interconverts fuels; packages lipids; handles amino acid metabolism and urea formation from nitrogen waste |
| Glycogen storage | Stores glucose as glycogen; releases glucose when blood sugar falls (cooperates with glucagon) |
| Detoxification / biotransformation | Modifies drugs, alcohol, and toxins for excretion |
| Plasma protein synthesis | Makes albumin and clotting factors (intro functional labels) |
| Storage | Stores certain vitamins and iron |
Do not confuse liver production of bile with gallbladder storage. After a meal, gallbladder contraction releases concentrated bile into the duodenum.
Pancreas (exocrine physiology link): Pancreatic juice delivers amylase, lipase, proteases, and bicarbonate into the duodenum. Bicarbonate neutralizes acidic chyme so intestinal enzymes can work at a near-neutral pH—pair this with stomach acid as a pH handoff story.
Putting Digestion Together: One Meal Timeline
- Mouth: Chewing + salivary amylase begin starch work; bolus formed.
- Esophagus: Peristalsis delivers bolus to stomach.
- Stomach: Acid + pepsin start protein digestion; mechanical churning → chyme; intrinsic factor secreted.
- Duodenum: Bile emulsifies fat; pancreatic enzymes digest carbs, proteins, fats; bicarbonate raises pH.
- Jejunum / ileum: Brush-border finish + majority of absorption; B12–intrinsic factor complex absorbed in ileum.
- Large intestine: Water salvage; feces formed; limited nutrient absorption.
- Liver: Processes portal nutrients and maintains metabolic homeostasis.
Clinical and Nursing Anchors
- Enzyme deficiency or pancreatic disease → maldigestion (especially fats → steatorrhea).
- Bile obstruction → poor fat emulsification and fat-soluble vitamin issues; possible jaundice from backup of bilirubin.
- Intrinsic factor loss (e.g., after gastric surgery or autoimmune parietal-cell damage) → B12 malabsorption.
- Ileus or opioid effects slow peristalsis; diarrhea may reflect hurried transit and less water absorption.
- Portal hypertension / liver failure disrupt the metabolic and detox roles listed above.
Exam Traps
- Bile emulsifies; lipase digests — bile is not a fat-digesting enzyme.
- Pepsin needs acid; pancreatic enzymes need a neutralized duodenum.
- Small intestine = primary absorption, not the stomach.
- Amylase → carbs; pepsin/proteases → protein; lipase → fat.
- Intrinsic factor → B12 absorption in ileum, not iron absorption in the duodenum.
- Peristalsis propels; segmentation mixes.
- Liver makes bile; gallbladder stores it.
Study Map for NEX
- Define mechanical vs chemical digestion with one example each.
- Recite the enzyme–substrate–site table for amylase, pepsin, proteases, lipase, and bile.
- State where most nutrient absorption occurs and how fats’ lymph path differs.
- Explain HCl and intrinsic factor in one sentence each.
- List four liver physiologic roles beyond “makes bile.”
With digestion and absorption locked, urinary physiology can explain how the kidneys regulate water, electrolytes, and wastes after nutrients enter the blood—the next section’s job.
Which statement correctly contrasts mechanical and chemical digestion?
Bile aids fat digestion primarily by:
Most digestion products of carbohydrates, proteins, and fats are absorbed in the: