Digestive System: Key Processes, Structures, and Enzymes

Digestion

The breaking down of larger food molecules into smaller molecules.

Absorption

The passage of smaller molecules into the blood and lymph.

Digestive System

The collective organs that perform digestion and absorption.

Ingestion

The first of six basic processes: taking food in.

Secretion

The release of water, acid, buffers, and enzymes into the lumen of the GI tract.

Mixing and Propulsion

The movement of food through the digestive tract.

Mechanical Digestion

Movements of the gastrointestinal tract that physically break down food to aid chemical digestion.

Chemical Digestion

A series of catabolic (hydrolysis) reactions that break down carbohydrates, lipids, and proteins into smaller usable molecules.

Defecation

The elimination of feces from the GI tract.

Alimentary Canal

The continuous tube extending from the mouth to the anus.

Accessory Structures

Organs that aid in digestion but are not part of the GI tract, including teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Mucosa

The inner lining of the tract supplied with blood vessels, nerve fibers, and endocrine cells.

Submucosa

Connective tissue just below the mucosa containing blood vessels and nerves.

Muscularis

Contains two muscle layers important for peristalsis and mechanical breakdown.

Serosa

The outermost covering composed of visceral peritoneum.

Mesentery

A large double fold of peritoneal tissue that anchors the digestive tract to the posterior abdominal wall.

Mastication

The act of chewing.

Hard Palate

The anterior roof of the mouth.

Soft Palate

The posterior roof of the mouth.

Cheeks

The lateral walls of the oral cavity.

Uvula

A muscular flap that closes off the nasal cavity during swallowing.

Deciduous Teeth

Baby or milk teeth, totaling 20.

Permanent Teeth

Adult teeth, totaling 32.

Crown

The visible upper portion of a tooth made of calcium phosphate.

Enamel

The very hard substance covering the crown.

Dentin

The bulk of the tooth, similar to bone but without cells.

Gingiva

The gums.

Salivary Amylase

An alkaline secretion in saliva that digests starch.

Deglutition

The act of swallowing.

Bolus

The ball of food formed for swallowing.

Epiglottis

The flap covering the opening to the windpipe during swallowing.

Cardiac Sphincter

The valve connecting the esophagus to the stomach.

Pyloric Sphincter

The valve connecting the stomach to the small intestine.

Rugae

Folds in the stomach that disappear when it is distended with food.

Chyme

The semi-fluid mixture of food and gastric juices.

Pepsinogen

The inactive form of pepsin that begins protein digestion.

Parietal Cells

Stomach cells that secrete hydrochloric acid (HCl) and intrinsic factor.

Intrinsic Factor

A substance essential for the absorption of vitamin B12.

Gastrin

A hormone that stimulates gastric glands to produce gastric juice.

Duodenum

The first 10 inches of the small intestine; receives chyme, bile, and pancreatic juice.

Jejunum

The 8-foot middle section where bulk chemical digestion and absorption occur.

Ileum

The final 12-foot section ending at the ileocecal valve.

Villi

Finger-like projections that increase the surface area for absorption 600 times.

Lacteal

A lymph vessel in the villi that absorbs fatty acids.

Bile

Produced by the liver and stored in the gallbladder; acts as an emulsifier.

Emulsification

The process of breaking fat into tiny droplets to increase surface area for enzymes.

Secretin

A hormone that stimulates the pancreas to release bicarbonate to neutralize chyme.

Cholecystokinin (CCK)

A hormone that triggers the release of bile and pancreatic enzymes.

Bilirubin

A breakdown product of hemoglobin that gives bile and feces color.

Cecum

The first part of the large intestine where the appendix is located.

Colon

The largest part of the large intestine, divided into ascending, transverse, descending, and sigmoid sections.

Feces

The solid waste of the large intestine composed of undigested fiber and bacteria.

Vitamin K

A vitamin synthesized by bacteria in the large intestine.

First steps of digestion in the mouth

Digestion begins with ingestion (taking in food) and mastication (chewing) to divide food into smaller pieces.

Mouth's contribution to chemical digestion

Salivary glands secrete saliva containing salivary amylase, an enzyme that initiates the breakdown of carbohydrates (starch).

Ball of chewed food before swallowing

It is called a bolus.

How body prevents food from entering nose or windpipe

The uvula closes off the nasal cavity, and the epiglottis covers the opening to the windpipe during swallowing (deglutition).

Food movement from mouth to stomach

The esophagus uses peristalsis—involuntary muscular contractions—to push the bolus down the 10-inch tube.

Valve controlling entry of food into stomach

The cardiac sphincter.

Primary function of stomach's mechanical action

The stomach's three muscle layers mix and churn food until it becomes a semi-fluid mixture called chyme.

Enzyme that begins protein digestion in stomach

Pepsinogen is released by chief cells and activated into pepsin by hydrochloric acid (HCl) to digest proteins.

Importance of Hydrochloric Acid (HCl)

Secreted by parietal cells, it kills bacteria, unfolds proteins, and activates pepsinogen.

Hormone triggering release of gastric juices

Gastrin is released into the bloodstream when solid food enters the stomach, stimulating gastric glands.

Where bulk of chemical digestion and nutrient absorption occurs

In the small intestine, specifically the duodenum (first 10 inches) and jejunum (8 feet).

Role of liver and gallbladder

The liver produces bile (stored in the gallbladder), which performs emulsification to break large fat globules into tiny droplets for better enzyme access.

How pancreas assists in small intestine

It secretes pancreatic juice containing enzymes like trypsin and lipase, and sodium bicarbonate to neutralize the acidic chyme.

How surface area is increased for absorption

Finger-like projections called villi increase the surface area 600 times.

Specialized vessel that absorbs fatty acids

The lacteal (a lymph vessel) located within the villi.

Main function of large intestine (colon)

It reabsorbs large quantities of water and electrolytes and stores waste; no digestion of food occurs here.

Beneficial role of bacteria in colon

They synthesize Vitamin K and certain B vitamins, which are absorbed into the bloodstream.

What gives feces its color

Bilirubin, a byproduct of the breakdown of hemoglobin.

How waste is removed from the body

Solid waste (feces) is regulated by the rectum and eliminated through the anus during defecation.

degluttition

swallowing

cardiac sphincter

opening from the esophagus to the stomach

pyloric sphincter

Controls passage of food from stomach to small intestine

ileocecal valve

prevents food from moving back into the small intestine once it has entered the large intestine

decidous teeth

baby teeth (20)

permanent teeth

adult teeth 32

Incisors

front teeth

Premolars

The teeth immediately behind the canines; used in chewing, grinding, and shearing food. The bicuspid teeth

Molars

Back teeth that grind food

Pharynx

throat; passageway for food to the esophagus and air to the larynx

Pharynx divisions

nasopharynx- behind the nose

oropharynx- behind the mouth

laryngopharynx- hyoid bone to esophagus

Tonsils

masses of lymphatic tissue in the back of the oropharynx

Salivary glands

parotid, sublingual, and submandibular glands

small intestine function

Absorbs most nutrients; Main absorption organ of the digestive tract

Small intestine enzymes

Lactase, Maltase, & Sucrose

Lipase

Bicarbonate ions

Peptidase

Stomach secretions/enzymes

HCI- acid that breaks down proteins

Pespin- proteins

Intristic factor- binds to b12, protect

Bicarbonate ions

make pancreatic juice alkaline; buffer stomach acid

pancreas function

regulates blood sugar (insulin & glucagon)

Secretes pancreatic juice which breaks down all categories of food

produces digestive enzymes for fats, carbs, and proteins

Pancreas secretions

amylase- carbs

trypsin- proteins

lipase- lipids

bicarbonate ions- neutralizes

large intestine (colon)

Absorbs water and forms feces

Liver

produces bile

Bile

A substance produced by the liver that breaks up fat particles

tooth pulp

consists of nerves, blood vessels, and loose connective tissue

CCK (cholecystokinin)

released by small intestine- stimulates gallbladder, pancreatic juices

Secretin

A hormone secreted by the small intestine (duodenum) in response to low pH (e.g., from stomach acid). It promotes the release of bicarbonate from the pancreas to act as a buffer.