Digestive

What are the main functions of the digestive system?

Process food, extract nutrients, and eliminate waste residue.

What are the two stages of digestion?

Mechanical digestion and chemical digestion.

What is mechanical digestion?

Physical breakdown of food into smaller particles by chewing and churning.

What structures perform mechanical digestion?

Teeth and the churning actions of the stomach and intestines.

What is chemical digestion?

A series of hydrolysis reactions that break macromolecules into monomers.

What is the end product of polysaccharide digestion?

Monosaccharides.

What is the end product of protein digestion?

Amino acids.

What is the end product of fat digestion?

Glycerol and fatty acids.

Which organs provide digestive enzymes?

Salivary glands, stomach, pancreas, and intestines.

What is the enteric nervous system (ENS)?

The "brain of the gut" that can function independently of the CNS.

What are the two nerve networks of the ENS?

Myenteric plexus and submucosal plexus.

Where is the myenteric plexus located?

Between the longitudinal and circular smooth muscle layers of the muscularis externa.

What is the function of the myenteric plexus?

Controls peristalsis and contractions of the muscularis externa.

Where is the submucosal plexus located?

Within the submucosa.

What is the function of the submucosal plexus?

Controls glandular secretion and contraction of the muscularis mucosa.

What nerve provides most parasympathetic innervation to the GI tract?

The vagus nerve.

How does parasympathetic stimulation affect digestion?

Increases secretion and motility.

How do sympathetic nerves affect digestion?

Decrease secretions and motility by inhibiting ENS neurons.

What are the major functions of saliva?

Moisten food, begin starch and fat digestion, cleanse teeth, inhibit bacteria, and form a bolus.

What enzyme begins starch digestion in the mouth?

Salivary amylase.

What enzyme begins fat digestion in the mouth?

Lingual lipase.

When is lingual lipase activated?

By stomach acid.

What is the function of mucus in saliva?

Lubricates food and aids swallowing.

What is the function of lysozyme in saliva?

Kills bacteria.

What is the function of Immunoglobulin A (IgA) in saliva?

Inhibits bacterial growth.

What are intrinsic salivary glands?

Small glands that continuously secrete saliva containing lingual lipase and lysozyme.

What are extrinsic salivary glands?

The major salivary glands: parotid, submandibular, and sublingual.

Where does the parotid gland empty?

Adjacent to the second upper molar.

Where does the submandibular gland empty?

Through Wharton's duct on either side of the lingual frenulum.

Where does the sublingual gland empty?

Into the floor of the oral cavity.

How is salivation stimulated?

Food stimulates receptors that signal salivatory nuclei in the medulla and pons.

What type of saliva does parasympathetic stimulation produce?

Thin, watery saliva rich in enzymes.

What type of saliva does sympathetic stimulation produce?

Thicker, mucus-rich saliva in smaller amounts.

What can stimulate salivation before food enters the mouth?

Sight, smell, taste, or thought of food.

What is the function of gastric mucus?

Protects the stomach lining from acid and pepsin.

What stimulates increased mucus production in the stomach?

Irritation of the stomach lining.

What are the functions of hydrochloric acid (HCl)?

Kills bacteria, stops carbohydrate digestion, denatures proteins, activates pepsin and lingual lipase, breaks down tissue, and converts ferric to ferrous iron.

Why is ferrous iron important?

It is required for hemoglobin synthesis.

What is intrinsic factor?

A protein that binds vitamin B12 and allows absorption in the small intestine.

What is the function of gastric lipase?

Digests 10-15% of dietary fat in the stomach.

What is pepsinogen?

The inactive precursor of pepsin.

How is pepsinogen activated?

By hydrochloric acid.

What is the function of pepsin?

Digests proteins.

What is the autocatalytic effect of pepsin?

Newly formed pepsin converts additional pepsinogen into pepsin.

What occurs during the cephalic phase of gastric activity?

Sight, smell, taste, or thought of food stimulates gastric secretion and motility through the vagus nerve.

What activates the gastric phase?

The presence of food or partially digested protein in the stomach.

What percentage of gastric secretion occurs during the gastric phase?

About two-thirds.

What hormone is released by G cells during protein digestion?

Gastrin.

How does gastrin affect gastric activity?

Stimulates gastric secretion and motility.

What happens when stomach pH falls below 2?

HCl inhibits parietal cells and G cells through negative feedback.

What activates the intestinal phase?

Chyme entering the duodenum.

What is the enterogastric reflex?

A duodenal reflex that inhibits stomach secretion and motility.

What substances trigger the enterogastric reflex?

Acid and partially digested fats in the duodenum.

What hormones suppress gastric secretion during the intestinal phase?

Secretin, cholecystokinin (CCK), and gastric inhibitory peptide (GIP).

What is the digestive function of the liver?

Production and secretion of bile.

What substances are found in bile?

Bile salts, cholesterol, fats, fat-soluble hormones, and lecithin.

What does the liver remove from portal blood?

Bacteria, debris, nutrients, toxins, bile pigments, and drugs.

What is the digestive function of the gallbladder?

Stores and concentrates bile.

What is bile?

A yellow-green fluid containing bile acids, cholesterol, phospholipids, minerals, and bile pigments.

What is the function of bile?

Aids fat digestion.

Trace the flow of bile from the liver to the duodenum.

Bile canaliculi → bile ductures → hepatic ducts → common hepatic duct → hepatopancreatic ampulla → duodenum.

What are the endocrine functions of the pancreas?

Secretes insulin and glucagon.

What is pancreatic juice?

An alkaline mixture of water, enzymes, zymogens, bicarbonate, and electrolytes.

What do pancreatic acini secrete?

Enzymes and zymogens.

What do pancreatic ducts secrete?

Bicarbonate.

Why does the pancreas secrete bicarbonate?

To neutralize stomach acid entering the duodenum.

What converts trypsinogen into trypsin?

Enterokinase.

What does trypsin digest?

Proteins.

What converts chymotrypsinogen into chymotrypsin?

Trypsin.

What converts procarboxypeptidase into carboxypeptidase?

Trypsin.

What does pancreatic amylase digest?

Starch.

What does pancreatic lipase digest?

Fats.

What do ribonuclease and deoxyribonuclease digest?

RNA and DNA.

What are the three regions of the small intestine?

Duodenum, jejunum, and ileum.

What occurs in the duodenum?

Receives chyme, bile, and pancreatic juice; neutralizes acid and emulsifies fats.

What happens to pepsin in the duodenum?

It becomes inactivated as pH rises.

Which section of the small intestine performs most digestion and absorption?

The jejunum.

What structural feature is prominent in the jejunum?

Large circular folds.

What are Peyer's patches?

Clusters of lymphatic nodules.

Where are Peyer's patches found?

In the ileum.

What valve separates the ileum from the large intestine?

The ileocecal valve.

What are microvilli?

Microscopic projections on absorptive cells that form the brush border.

What is contact digestion?

Final digestion by brush border enzymes when chyme directly contacts microvilli.

Why is segmentation important?

Mixes chyme with digestive juices and increases contact with the mucosa.

What is segmentation?

Random ring-like contractions that mix intestinal contents.

When does peristalsis become more prominent in the small intestine?

After most nutrients have been absorbed.

What is the migrating motor complex?

Overlapping waves of contraction that move residue through the intestine.

What enzyme begins carbohydrate digestion?

Salivary amylase.

What stops salivary amylase activity?

Stomach acid.

What enzyme completes starch digestion in the small intestine?

Pancreatic amylase.

What enzymes complete carbohydrate digestion at the brush border?

Brush border enzymes acting on oligosaccharides.

How are glucose and galactose absorbed?

By sodium-glucose transport proteins (SGLT).

How is fructose absorbed?

Facilitated diffusion.

What happens to fructose after absorption?

It is converted to glucose within the cell.

How does HCl aid protein digestion?

It denatures proteins.

What enzyme begins protein digestion in the stomach?

Pepsin.

Which pancreatic enzymes digest proteins?

Trypsin and chymotrypsin.

What enzymes complete protein digestion?

Brush border peptidases.

What percentage of dietary fat is digested by lingual and gastric lipase?

About 10%.

Which enzyme digests most fats?

Pancreatic lipase.