Digestive System Basics

What does the parasympathetic nervous system stimulate in digestion?

Stimulates flow of saliva and vagus-nerve-driven peristalsis and secretion.

What is the difference between the GI tract and accessory organs?

The GI tract is the continuous tube from mouth to anus; accessory organs secrete substances into the GI tract but food does not enter them.

What are the three phases of digestion?

Cephalic phase, gastric phase, and intestinal phase.

What characterizes the cephalic phase of digestion?

Anticipation, chewing, and swallowing; neural parasympathetic responses.

What characterizes the gastric phase of digestion?

Events related to the stomach.

What characterizes the intestinal phase of digestion?

Events related to the small intestines.

What mediates cephalic-phase responses?

The parasympathetic nervous system.

What is the function of the cephalic phase?

To optimize digestion, absorption, and nutrient use.

How does COVID-19 impact feeding behavior?

It negatively affects feeding behavior.

Why is saliva important for taste?

Saliva is essential for taste perception.

What gives saliva its viscosity for forming a bolus?

Water and glycoproteins.

What does amylase do?

Breaks starches into simple sugars.

What does lipase do?

Breaks triglycerides into fatty acids and glycerol.

What antibacterial enzyme is found in saliva?

Lysozyme, which lyses bacteria.

What is nursing caries (nursing bottle syndrome)?

Tooth decay due to reduced salivary flow during sleep and prolonged sugar exposure.

How does dental plaque form?

Sticky biofilm containing bacteria, food particles, and saliva.

What is mastication?

Alternating, rhythmic, coordinated chewing movements between both dental arches.

Why does mastication require the whole brain?

It involves complex coordination of motor patterns.

What is the purpose of chewing in digestion?

Increases surface area/volume ratio to enhance enzymatic digestion.

What is a bolus?

Chewed food mixed with saliva.

What initiates swallowing?

Compression of the bolus against the hard palate.

What pushes the bolus into the oropharynx?

Retraction of the tongue, which also elevates the soft palate to seal the nasopharynx.

What happens when the bolus enters the oropharynx?

Reflex responses begin to move the bolus toward the stomach.

What structure directs the bolus away from the trachea?

The epiglottis, which folds over the glottis.

What forces the bolus through the entrance of the esophagus?

Contraction of pharyngeal muscles.

What moves the bolus through the esophagus?

A peristaltic wave.

What is peristalsis?

Involuntary waves of muscle contraction and relaxation that propel contents forward.

What triggers the opening of the lower esophageal sphincter?

The approach of the bolus.

Where does the bolus go after the lower esophageal sphincter opens?

Into the stomach.

What triggers the cephalic phase of stomach activity?

Sight, smell, taste, or thoughts of food.

What nerve stimulates the stomach during the cephalic phase?

The vagus nerve (parasympathetic).

What substances are released in the cephalic phase?

Mucus, HCl, pepsinogen, ghrelin, and gastrin.

What begins the gastric phase?

The arrival of food in the stomach.

What activates stretch receptors in the gastric phase?

Stomach distension.

What does gastrin release lead to?

Protein digestion, mixing waves, and increased gastric motility.

What enzyme in the stomach digests fats?

Gastric lipase.

What are the functions of HCl in the stomach?

Provides acidic environment for pepsinogen activation; kills microorganisms; denatures proteins; inactivates enzymes in food; breaks down plant cell walls and connective tissue in meat.

How does stomach volume affect gastric emptying?

↑ volume → ↑ gastric emptying rate.

Why does increased stomach volume speed gastric emptying?

Stretch receptors increase activity → ↑ intragastric pressure → ↑ gastric emptying.

How does osmotic pressure affect gastric emptying?

↑ osmolality → ↓ gastric emptying rate.

Why do hypertonic stomach contents slow gastric emptying?

They ↑ gastric secretions → dilute contents → ↓ gastric emptying.

How does nutrient density affect gastric emptying?

↑ nutrient density → ↓ gastric emptying rate.

Which nutrient slows gastric emptying via duodenal osmoreceptors?

Carbohydrates.

Which nutrients slow gastric emptying via CCK release?

Fats and proteins.

How does particle size affect gastric emptying?

↑ particle size → ↓ gastric emptying rate.

What is the stomach's "sieving mechanism"?

Heavier particles collect in the lower stomach and empty more slowly.

How does viscosity affect gastric emptying?

↑ viscosity → ↓ gastric emptying rate.

Why do soluble fibers slow gastric emptying?

They require more water and activate appetite suppressants like CCK.

What is dietary fiber?

A type of carbohydrate the body cannot digest.

How does dietary fiber help with constipation?

Acts as a stool softener and increases stool bulk.

How does dietary fiber affect gastric emptying and satiety?

Increases viscosity → delays gastric emptying → increases satiety → may support weight loss.

How does dietary fiber affect cholesterol levels?

Binds bile → reduces bile acid reabsorption → lowers circulating cholesterol.

Where are insoluble fibers typically found?

In outer plant skins and cell walls of plants and cereal grains.

What is a key property of insoluble fibers?

They swell in water and promote peristalsis to reduce colon transit time.

Where are soluble fibers typically found?

Inner fleshy parts of fruits, and in endosperm of grains, legumes, nuts, and seeds.

How do soluble fibers treat constipation?

They increase stool bulk by thickening in water.

Which type of fiber promotes peristalsis and reduces colon transit time?

Insoluble fiber.

What triggers vomiting?

Irritation of the upper GI tract that signals the vomiting center in the brain.

What happens to the pyloric sphincter during vomiting?

It relaxes.

What happens to the duodenal contents during vomiting?

They are discharged back into the stomach by reversed peristalsis.

What happens to the gastroesophageal sphincter (LES) during vomiting?

It relaxes.

What are consequences of habitual purging?

Esophageal lesions, scar tissue formation, impaired peristalsis, tooth decay, cardiac irregularities, and death.

What causes gastric ulcers?

Stomach acid damaging the digestive lining.

What are common causes of gastric ulcers?

Aspirin and other anti-inflammatory drugs, and Helicobacter pylori.

How are gastric ulcers treated?

Antibiotics.

When does the intestinal phase begin?

When chyme enters the duodenum.

What does duodenal distension do to gastrin production?

It inhibits gastrin production.

What does duodenal distension do to gastric contractions?

It inhibits gastric contractions.

What does duodenal distension do to the pyloric sphincter?

It stimulates contraction to prevent further chyme entry.

What triggers alkaline mucus secretion in the intestinal phase?

Vagal stimulation (parasympathetic activation).

What triggers CCK release?

Proteins and fats in chyme.

What triggers GIP release?

Carbohydrates in chyme.

What triggers secretin release?

Low pH in the duodenum.

What does GIP trigger?

Insulin release and increased fat synthesis/storage.

What does CCK respond to?

Lipids and proteins in chyme.

What does CCK cause the liver to do?

Produce more bile for fat breakdown.

What does CCK cause the gallbladder to do?

Release bile.

What does CCK cause the pancreas to release?

Digestive enzymes (proteases, amylase, lipase) plus bicarbonate and water.

How does CCK affect the pyloric sphincter?

It causes constriction.

How does CCK affect gastric secretions?

It inhibits gastric secretions.