Intake, elimination & reproduction · Topic 16 of 18

Digestive System

The GI Tract

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A&P · digestive system

The GI Tract

Food moves one way, mouth to anus; it is digested mechanically and chemically, nutrients are absorbed in the small intestine, water in the large.

01 · The pathMouth → esophagus → stomach → intestines

02 · Two kindsMechanical = pieces; chemical = enzymes

03 · AbsorptionSmall intestine absorbs the nutrients

What it is

Small intestine = main site of digestion and nutrient absorption must know

Large intestine absorbs water + electrolytes and forms feces must know

Peristalsis = waves of smooth-muscle contraction that push food along

Sugars + amino acids → blood capillaries; fats → lacteals (lymph)

Colon bacteria make vitamin K (and some B vitamins)

Parts & jobs

Mouth → pharynx → esophagus → stomach → small int. → large int. → anus must know

Mouth: chewing starts mechanical digestion; food becomes a bolus

Swallowing: epiglottis covers the larynx; esophagus carries food down

Sphincters: lower esophageal → pyloric → ileocecal valve

Stomach: churns food with acid into liquid chyme; protein digestion begins

Small intestine: duodenum → jejunum → ileum (B12 + bile salts)

Villi + microvilli = huge surface area for absorption

Cecum → ascending → transverse → descending → sigmoid colon → rectum

The six digestive processes

Ingest → propel → digest (mechanical + chemical) → absorb → defecate

Don't confuse

Mechanical = break food into pieces; chemical = enzymes split molecules must know

Mechanical or chemical digestion?

Commonly confused

	Mechanical	Chemical
What happens	Food is broken into smaller pieces	Molecules are split into building blocks
Changes the molecules?	No	Yes
Done by	Teeth, tongue, muscle contractions	Enzymes (helped by stomach acid)
Examples	Chewing, stomach churning, segmentation	Starch → sugars, protein → amino acids
Where	Mouth, stomach, small intestine	Starts in the mouth, finished in the small intestine

Mix-ups to avoid

Stomach absorbs almost nothing — mainly alcohol and aspirin

More detail

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Segmentation = back-and-forth mixing in the small intestine

Intrinsic factor (stomach) is needed to absorb vitamin B12

Small intestine ≈ 3 m (10 ft) — the longest part of the tract

About 90% of water is absorbed in the small intestine; the colon takes the rest

Wall layers, inside out: mucosa, submucosa, muscularis, serosa

Skeletal muscle at the ends = voluntary swallowing and defecation

Pearl

The small intestine soaks up the nutrients; the large intestine soaks up the leftover water.

Sources · OpenStax Anatomy & Physiology 2e §23.1 Overview of the Digestive System · OpenStax Anatomy & Physiology 2e §23.2 Digestive System Processes and Regulation · OpenStax Anatomy & Physiology 2e §23.3 The Mouth, Pharynx, and Esophagus · OpenStax Anatomy & Physiology 2e §23.4 The Stomach · OpenStax Anatomy & Physiology 2e §23.5 The Small and Large Intestines · OpenStax Anatomy & Physiology 2e §23.7 Chemical Digestion and Absorption: A Closer Look · Union Test Prep — HESI A2 Anatomy & Physiology study guide, page 5 (digestive system) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

Accessory Organs & Digestive Enzymes

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A&P · digestive accessory organs

Accessory Organs & Digestive Enzymes

Salivary glands, liver, gallbladder and pancreas send secretions into the tract; enzymes named for their targets split carbohydrates, proteins and fats.

01 · Match the enzymeAmylase-starch · protease-protein · lipase-fat

02 · BileLiver makes bile; gallbladder stores it

03 · PancreasPancreas: enzymes + bicarbonate

What it is

Accessory organs: salivary glands, liver, gallbladder, pancreas (+ teeth, tongue) must know

Salivary amylase starts starch digestion in the mouth

Stomach acid turns pepsinogen into pepsin, which starts protein digestion

Pancreatic amylase (starch) + trypsin, chymotrypsin (protein)

Pancreatic lipase digests fat → fatty acids + monoglyceride

Brush border: lactase, sucrase, maltase finish sugars; peptidases finish proteins

Liver also processes absorbed nutrients, stores glycogen, detoxifies blood

CCK (fatty food in the duodenum) → gallbladder contracts, pancreas releases enzymes

Parts & jobs

Salivary glands: parotid, submandibular, sublingual

Liver = the largest gland in the body (about 3 lb)

The rules

Liver makes bile; gallbladder stores and concentrates it must know

Pancreatic juice: enzymes for carbs, proteins and fats + bicarbonate must know

Amylase → starch; protease (pepsin, trypsin) → protein; lipase → fat must know

Carbs → monosaccharides; proteins → amino acids; fats → fatty acids

Bile or pancreatic juice?

Commonly confused

	Bile	Pancreatic juice
Made by	Liver (stored in the gallbladder)	Pancreas
Contains enzymes?	No	Yes — amylase, trypsin, chymotrypsin, lipase
What it does to fat	Emulsifies it into small droplets	Lipase digests it chemically
Other key part	Bile salts; bilirubin pigment	Bicarbonate that neutralizes acid chyme
Delivered to	Duodenum	Duodenum

Mix-ups to avoid

Bile emulsifies fat into droplets — it is not an enzyme must know

More detail

Show 7 more facts

Secretin → pancreatic bicarbonate; gastrin → more stomach acid

Bile duct + pancreatic duct share an opening into the duodenum

Pancreas is exocrine (enzymes) and endocrine (islets: see Endocrine)

Bilirubin (from old red cells) gives bile and stool their color

Nucleases (pancreas) digest DNA and RNA

Too little lactase → lactose intolerance (bacteria ferment the sugar)

Saliva also holds mucus, lysozyme and antibodies (about 1–1.5 L a day)

Pearl

Bile breaks fat apart; lipase breaks it down.

Sources · OpenStax Anatomy & Physiology 2e §23.3 The Mouth, Pharynx, and Esophagus · OpenStax Anatomy & Physiology 2e §23.6 Accessory Organs in Digestion: The Liver, Pancreas, and Gallbladder · OpenStax Anatomy & Physiology 2e §23.7 Chemical Digestion and Absorption: A Closer Look · OpenStax Anatomy & Physiology 2e §23.4 The Stomach · OpenStax Anatomy & Physiology 2e §23.5 The Small and Large Intestines · OpenStax Anatomy & Physiology 2e §23.1 Overview of the Digestive System · LibreTexts Human Biology (Wakim & Grewal) §18.6 Accessory Organs of Digestion · OpenStax Anatomy & Physiology 2e §23.2 Digestive System Processes and Regulation · Union Test Prep — HESI A2 Anatomy & Physiology study guide, page 5 (digestive system) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

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