Study guide
4.3 The Digestive System
ATI TEAS 7 Nursing Admission
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1 Introduction to the ATI TEAS 7
1.1 About the ATI TEAS 7 Exam 1.2 Content Area Breakdown 1.3 Scoring and Program Requirements 1.4 Building Your TEAS Study Plan
2 Reading
2.1 Identifying the Main Idea 2.2 Making Inferences and Drawing Conclusions 2.3 Summarizing and Paraphrasing 2.4 Understanding Text Structure 2.5 Author's Purpose, Tone, and Point of View 2.6 Evaluating Arguments, Evidence, and Credibility 2.7 Integrating Information and Interpreting Graphics 2.8 TEAS Reading Strategies, Pacing, and Practice
3 Mathematics
3.1 Whole Numbers and Basic Operations 3.2 Fractions 3.3 Decimals and Percentages 3.4 Ratios and Proportions 3.5 Algebra and Equations 3.6 Measurement and Unit Conversions 3.7 Statistics and Data Interpretation 3.8 Geometry Basics
4 Science
4.1 The Cardiovascular System 4.2 The Respiratory System 4.3 The Digestive System 4.4 The Nervous System 4.5 The Musculoskeletal System 4.6 The Urinary and Reproductive Systems 4.7 Cell Biology and Genetics 4.8 Chemistry Basics 4.9 Scientific Reasoning and the Scientific Method
5 English & Language Usage
5.1 Grammar and the Parts of Speech 5.2 Sentence Structure 5.3 Punctuation 5.4 Spelling and Word Usage 5.5 Vocabulary in Context and Word Parts 5.6 Paragraph Organization and the Writing Process 5.7 Formal vs. Informal Language, Audience, and Tone 5.8 TEAS English Section Strategies and Pacing
4.3 The Digestive System
Key Takeaways Explain This Topic AI
The GI tract runs mouth → esophagus → stomach → small intestine → large intestine; accessory organs are the liver, gallbladder, and pancreas
Mechanical digestion physically breaks food apart; chemical digestion uses enzymes to break chemical bonds
The liver produces bile, the gallbladder stores it, and bile emulsifies fats into smaller droplets
Most nutrient absorption occurs in the small intestine, whose villi and microvilli vastly increase surface area
- The large intestine mainly reabsorbs water and houses bacteria that produce vitamin K and some B vitamins
- Last updated: October 2026
The digestive system converts the food you eat into molecules small enough for your cells to absorb and use. The TEAS tests this system from two angles: the anatomy (which organ does what, in what order) and the biochemistry (which enzyme breaks down which nutrient into which product). A clean mental model separates the system into two groups: the GI (gastrointestinal) tract — the continuous tube from mouth to anus that food actually travels through — and the accessory organs — the liver, gallbladder, and pancreas, which sit outside the tube but secrete substances into it.
Food never passes through the accessory organs; it only receives their secretions.
The GI Tract: An Assembly Line
The alimentary canal is one long tube about 9 meters in an adult. Each segment performs a specialized job before passing material along.
- Organ Primary Job Key Process
- Mouth Begins mechanical and chemical digestion Chewing; salivary amylase breaks down starch
- Esophagus Transport Peristalsis pushes food to the stomach
- Stomach Chemical digestion and storage HCl and pepsin break down protein
- Small intestine Main digestion and absorption Enzymes finish digestion; nutrients absorbed
- Large intestine Water absorption, waste formation Reabsorbs water; forms and stores feces
- Rectum / anus Elimination Stores and expels waste
Food moves along by peristalsis , the wave-like contraction of smooth muscle in the tube walls. It is involuntary, which is why you can swallow even while upside down.
Mechanical vs. Chemical Digestion
The exam expects you to distinguish two complementary processes that occur simultaneously.
- Type What It Does Examples
- Mechanical Physically breaks food into smaller pieces Chewing, stomach churning, segmentation
- Chemical Breaks chemical bonds using enzymes Amylase, pepsin, lipase action
Mechanical digestion increases surface area so chemical digestion can work faster — the two are partners, not rivals. Bile's emulsification of fat is a useful borderline case: it physically breaks large fat globules into tiny droplets (mechanical in effect) but is not an enzyme, so it does not chemically digest the fat itself.
Accessory Organs
- Organ Secretion Function
- Salivary glands Saliva (amylase) Begin starch digestion; lubricate food
- Liver Bile Emulsifies fats into smaller droplets
- Gallbladder Stores bile Releases bile into the small intestine when fat arrives
- Pancreas Enzymes + bicarbonate Digests all nutrient types; neutralizes stomach acid
A common trap: the liver makes bile, the gallbladder stores it . If a question asks where bile is produced , the answer is the liver, not the gallbladder.
Digestive Enzymes
Enzymes are named for the nutrient they break down, usually ending in -ase . This pattern is your shortcut: amylase acts on starch (amyl-), lipase acts on fats (lip-), and protease acts on protein.
- Enzyme Source Acts On Produces
- Salivary amylase Salivary glands Starch Maltose
- Pepsin Stomach Proteins Peptides
- Pancreatic amylase Pancreas Starch Maltose
- Trypsin Pancreas Proteins Peptides
- Lipase Pancreas Fats Fatty acids + glycerol
- Lactase Small intestine Lactose Glucose + galactose
- Sucrase Small intestine Sucrose Glucose + fructose
Note that pepsin works only in the acidic stomach (created by hydrochloric acid), while the pancreatic enzymes need the alkaline environment that pancreatic bicarbonate creates in the small intestine.
Absorption: The Job of the Small Intestine
The small intestine — divided into the duodenum , jejunum , and ileum — is where the vast majority of nutrients enter the bloodstream. Its inner wall is folded and covered with finger-like villi , and each villus cell is covered with tiny microvilli (the brush border). These two structures multiply the surface area enormously, turning a tube only a few centimeters wide into an absorptive surface the size of a small room.
- Nutrient Primary Absorption Site
- Carbohydrates (as simple sugars) Small intestine
- Proteins (as amino acids) Small intestine
- Fats (as fatty acids) Small intestine
- Water Large intestine (mainly)
- Vitamin B12 Ileum (last part of small intestine)
- The Large Intestine and Nutrients
The large intestine (colon) receives the watery residue the small intestine leaves behind. Its main jobs are to reabsorb water, compact the remaining material into feces, and host beneficial bacteria that synthesize vitamin K and several B vitamins . It performs little nutrient digestion of its own.
Finally, the TEAS expects familiarity with the six macronutrients and micronutrients : carbohydrates (quick energy), proteins (build and repair tissue, made of amino acids), lipids/fats (energy storage and cell membranes), vitamins and minerals (regulate body processes), and water (the solvent for nearly every reaction).
Worked Example: A person drinks a glass of milk but experiences bloating and cramps because they lack the enzyme lactase . Trace what goes wrong. Lactase normally breaks lactose (milk sugar) into glucose and galactose in the small intestine. Without it, undigested lactose passes into the large intestine, where bacteria ferment it, producing gas and drawing in water — the cause of the symptoms. This shows the rule “enzyme-name = substrate + ase” in action: no lactase means no lactose digestion.