Exam 4 Digestive System

What are the three functions of the digestive system?

  Process food, extract nutrients, and eliminate residue.

 What is the enteric plexus?

 A nervous network in the esophagus, stomach, and intestines.

What is the enteric plexus subdivisions and functions?

Submucosal plexus (in submucosa): regulates glandular secretions.

Myenteric plexus (between muscle layers): controls motility.

 What hormones, paracrines, and reflexes regulate digestive motility and secretion?

Hormones: gastrin, secretin (long-distance effects); Paracrines: histamine, prostaglandins (local effects);

Reflexes: Short (myenteric): local stretch/chemical triggers contraction; Long (vagovagal): brain-gut communication via vagus nerve.

 What are the functions of saliva?

Moistens, cleans mouth, inhibits bacteria, aids taste, digests starch/fat, eases swallowing.

What is the composition and pH of saliva?

97–99.5% water, pH 6.8–7.0, plus mucus, enzymes, lysozyme, IgA, electrolytes.

 What are the functions of the stomach?

Stores food, liquefies it, digests proteins/fats, produces chyme.

What is the gastric mucosa and it’s structure?

inner lining of the stomach that has depressions called gastric pits lined w. Columnar epithelium

the five cells

Mucous cells: secrete mucus.; Stem cells: regenerate lining. ;Parietal cells: secrete HCl, intrinsic factor, ghrelin.; Chief cells: secrete pepsinogen, lipase.; Enteroendocrine cells: secrete hormones/paracrines.

gastric pits

depressions along the gastric mucosa lined with columnar epithelium 

Gastric mucosa glands

2-3 tubular glands that open into the gastric pit and diverge into lamina propria; Cardiac and pyloric secrete mucus; Gastric glands secrete acid and enzymes 

mucous cells

 secrete mucus; located in cardiac and pyloric glands; “mucous neck cells” in gastric glands

regular stem cells

located at the base of pit and neck of gland, divide and create new cells fast; migrate to gastric surface or down into glands to replace dead cells

Parietal cells

located in upper half of gastric glands; secrete HCL, intrinsic factor, and ghrelin

chief cells

most abundant; secrete gastric lipase and pepsinogen; located in lower half of gastric glands

Enteroendocrine cells

in lower end of glands; secrete hormones and paracrine messengers to regulate digestion; abundant in gastric and pyloric glands; 8 types

What is the composition of gastric juice?

High in HCl, very acidic (pH ~0.8)

How do parietal cells secrete HCl and what does it do?

 Use H⁺/K⁺ ATPase pump; HCl activates enzymes, denatures proteins, kills microbes.

What is the difference between a zymogen and an active enzyme?

Zymogens are inactive precursors; become active when specific amino acids are removed.

 How is pepsin formed and what is its function?

 Chief cells secrete pepsinogen; HCl activates it to pepsin, which digests proteins.

What is gastric lipase and its function?

Secreted by chief cells; digests ~10–15% of fat in stomach.

 What is intrinsic factor, what does it do, and what happens if it’s deficient?

Secreted by parietal cells; protein that helps absorb vitamin B₁₂. Deficiency causes pernicious anemia; treated with B₁₂ injections or oral supplements with intrinsic factor

What are the three phases of gastric control?

Cephalic- brain triggers stomach via vagus nerve

Gastric- food stretches stomach, triggers local and vagovagal reflexes

intestinal- chyme in duodenum inhibits stomach via enterogastrix reflex, CCK, and secretin

What is the composition of bile?

 Made of bile acids, cholesterol, phospholipids, pigments

What is the function of bile?

Emulsifies fats for digestion.

What are the digestive functions of the pancreas?

Secretes enzymes and bicarbonate; releases juice via main and accessory ducts into duodenum.

What is the composition and function of pancreatic juice?

Contains water, enzymes (amylase, lipase, nucleases), zymogens, and bicarbonate to neutralize acid.

 Name pancreatic zymogens and their functions.

Trypsinogen → trypsin: activates others.

Chymotrypsinogen, procarboxypeptidase: digest proteins.

What hormones regulate bile and pancreatic secretion?

ACh: stimulates enzymes.

CCK: triggers enzyme release, bile discharge.

Secretin: stimulates bicarbonate secretion.

 How is the small intestine protected from stomach acid?

Bicarobonate-rich mucus neutralizes acid.

Why is the surface area important in the small intestine?

Maximizes digestion and absorption.

What are the brush border enzymes and their functions?

Dextranase, glucoamylase: digest oligosaccharides.
Sucrase, lactase: digest disaccharides.

What are the two types of intestinal motility?

Segmentation: mixes contents.

Peristalsis: moves chyme toward colon.

What are the steps of carbohydrate digestion?

• In the mouth, salivary amylase starts breaking down starch into smaller pieces called oligosaccharides.

• When the food reaches the stomach, amylase stops working because the acid (pH ~4.5), pepsin, and stomach movements denature the enzyme.

• If the meal is large, amylase has more time to work before being inactivated.

• Once inactive, amylase itself gets digested by pepsin, just like other proteins.

• About 50% of the starch you eat is already broken down before it even reaches the small intestine.

• In the small intestine, enzymes like dextrinase and glucoamylase continue breaking down oligosaccharides.

• Maltase breaks down maltose (a sugar made of two glucose units).

• The final result is glucose, which is then absorbed into the bloodstream.

How are monosaccharides absorbed?

• Glucose and galactose use SGLT-1, a sodium-dependent co-transporter.

• Fructose uses GLUT5, a facilitated diffusion transporter.

• All three exit the cell into the bloodstream via GLUT2 on the basolateral membrane.

 What are the steps of protein digestion?

Stomach: pepsin begins digestion.

SI: trypsin, chymotrypsin, carboxypeptidase continue.

Brush border: peptidases finish digestion.

How are amino acids absorbed?

Na⁺ or H⁺-dependent co-transporters on apical side; exit via facilitated diffusion.

What are the steps of fat digestion?

Stomach: lingual/gastric lipase begin digestion.
SI: bile emulsifies fats; pancreatic lipase completes digestion.

Why is emulsification important for fat digestion?

Increases surface area for enzyme action.

 How are lipids absorbed and transported?

Micelles deliver lipids to enterocytes.

Repackaged as chylomicrons, enter lacteals, then lymph.

What are emulsification droplets, micelles, and chylomicrons?

Droplets: large fat broken into small ones by bile.

Micelles: transport lipids to enterocytes.

Chylomicrons: transport fats via lymph.

How are vitamins absorbed?

Fat-soluble: with dietary fat in micelles.

Water-soluble: via diffusion (except B12, which needs intrinsic factor).

What are the functions of the large intestine?

 Absorbs water/salts; compacts feces for defecation.

What are the intrinsic and parasympathetic defecation reflexes?

Intrinsic (myenteric): local stretch triggers peristalsis and sphincter relaxation.

Parasympathetic: spinal reflex enhances peristalsis and relaxation.

What is the Valsalva maneuver and its role in defecation?

Forced exhalation against a closed airway increases abdominal pressure, triggering rectal emptying.