Digestive System Overview
A long tube plus helper organs that turn food into absorbable nutrients.
The digestive (gastrointestinal, or GI) system has two big jobs: break food down into molecules small enough to absorb (digestion), absorb those nutrients into the blood, and then eliminate the waste. It is built from two groups of organs.
Two Groups of Organs
- GI tractalimentary canal
- The continuous tube food travels through: Mouth → pharynx → esophagus → stomach → small intestine → large intestine → rectum → anus
- Accessory organsno food passes through
- Aid digestion but food does not pass through them: Teeth, tongue, salivary glands, liver, gallbladder, and pancreas
Mechanical vs Chemical Digestion
- Mechanical digestion — physically breaks food into smaller pieces (chewing, churning in the stomach). This increases the surface area for enzymes.
- Chemical digestion — enzymes and secretions break the chemical bonds in carbohydrates, proteins, and fats.
The GI Tract Wall & Peristalsis
From the esophagus on, the GI tract wall has four layers: the mucosa (innermost lining), submucosa, muscularis (smooth muscle), and serosa (outer covering). The smooth muscle produces peristalsis — wave-like contractions that push food along the tract.
Almost every part of the GI tract has a matching set of disorders that are heavily tested — GERD, peptic ulcers, gallstones, pancreatitis, liver failure, and bowel obstruction. If you understand what each segment normally does, you can reason out what goes wrong when it fails, and why interventions like NPO status, NG tubes, and diet changes are used.
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Mouth, Esophagus & Swallowing
Where digestion begins — and where aspiration risk lives.
Digestion starts in the mouth. The teeth perform mechanical digestion (mastication), while salivary amylase begins the chemical digestion of carbohydrates. The tongue forms the chewed food into a bolus and pushes it back to be swallowed.
Swallowing & the Airway
- During swallowing, the epiglottis folds down to cover the trachea, directing the bolus into the esophagus and protecting the airway.
- If this protection fails (impaired swallowing, or dysphagia), food or fluid can enter the airway — aspiration.
The Esophagus
The esophagus is a muscular tube that moves the bolus to the stomach by peristalsis (not gravity). At its lower end, the lower esophageal sphincter (LES), also called the cardiac sphincter, opens to let food into the stomach and then closes to keep acidic stomach contents from flowing back up.
A weak or relaxed LES allows stomach acid to back up into the esophagus — this is the basis of GERD (gastroesophageal reflux). And because the airway and food passage cross at the pharynx, clients with dysphagia (after a stroke, for example) are at high risk for aspiration — which is why swallowing is assessed before giving food, fluids, or oral medications.
The Stomach
A muscular bag that stores food, churns it, and starts protein digestion.
The stomach is a J-shaped organ that stores food, mixes it with gastric juice into a thick liquid called chyme, and begins the digestion of proteins. A pyloric sphincter at its base controls how fast chyme empties into the small intestine.
Gastric Juice
- Hydrochloric acid (HCl)
- Creates an acidic environment (about pH 1.5–3.5); activates pepsin and kills many microbes
- Pepsinfrom pepsinogen
- Begins protein digestion
- Intrinsic factor
- Required to absorb vitamin B12 later in the ileum
- Mucus
- Coats and protects the stomach lining from its own acid
The stomach makes intrinsic factor, which is required for vitamin B12 absorption in the ileum. If the stomach is removed (gastrectomy) or its lining is damaged (atrophic gastritis, pernicious anemia), B12 cannot be absorbed and must be given by injection or nasal spray — not oral pills. This is a classic NCLEX connection between the stomach and a blood disorder.
? What does the stomach secrete that is required to absorb vitamin B12?
Small Intestine & Absorption
Where most digestion finishes and almost all absorption happens.
The small intestine is about 20 feet long and is the site of most chemical digestion and nearly all nutrient absorption. It has three parts:
- Duodenumfirst
- Receives bile and pancreatic enzymes; most chemical digestion happens here
- Jejunummiddle
- Most nutrient absorption
- Ileumlast
- Absorbs vitamin B12 and bile salts; ends at the ileocecal valve
Built for Absorption
- The inner wall is folded and covered with villi and microvilli (the "brush border"), which dramatically increase the surface area for absorption.
- Segmentation contractions mix chyme with digestive juices; brush-border enzymes complete digestion.
- Absorbed nutrients pass into the blood and travel to the liver first (via the portal vein) before reaching the rest of the body.
Because almost all nutrients are absorbed here, conditions that damage or shorten the small intestine — Crohn's disease, celiac disease, or surgical resection — cause malabsorption and weight loss. Damage to the ileum specifically threatens B12 and bile-salt absorption.
Accessory Organs: Liver, Gallbladder & Pancreas
Food never enters them, but digestion can't finish without them.
The Liver
The liver is one of the body's busiest organs. Its digestion-related and metabolic jobs include:
- Produces bile, which emulsifies fats
- Metabolizes carbohydrates, proteins, and fats; stores glycogen and vitamins
- Detoxifies drugs, alcohol, and ammonia (converting ammonia to urea)
- Makes plasma proteins, including albumin and most clotting factors
- Processes bilirubin from the breakdown of old red blood cells
The Gallbladder
- Stores and concentrates bile made by the liver
- When fatty food reaches the small intestine, the hormone CCK (cholecystokinin) signals the gallbladder to release bile into the duodenum
The Pancreas (Exocrine Role)
- Secretes digestive enzymes — amylase (carbs), lipase (fats), and proteases such as trypsin (proteins)
- Secretes bicarbonate, which neutralizes acidic chyme as it enters the duodenum
- (Its endocrine role — insulin and glucagon — is covered in the Endocrine System guide.)
If the liver can't do its jobs: it makes fewer clotting factors → bleeding risk; less albumin → edema and ascites; can't clear ammonia → high ammonia and confusion (hepatic encephalopathy); and can't process bilirubin → jaundice. And with no bile reaching the gut, fats aren't absorbed — causing fatty, clay-colored stools and deficiency of fat-soluble vitamins (A, D, E, K).
Large Intestine & Elimination
Reabsorbs water, houses gut flora, and forms stool.
The large intestine (colon) runs from the cecum through the ascending, transverse, descending, and sigmoid colon to the rectum and anus. Little digestion happens here — its main jobs are:
- Reabsorb water and electrolytes, turning liquid chyme into solid stool
- House normal gut flora (bacteria) that produce vitamin K and some B vitamins
- Store and eliminate feces through the defecation reflex
Because the colon reabsorbs water, diarrhea (fast transit) causes fluid and electrolyte loss and dehydration, while constipation reflects too much water reabsorption from slow transit. Broad-spectrum antibiotics can wipe out normal flora — reducing vitamin K production and allowing C. difficile overgrowth and diarrhea.
Digestion & Absorption of Nutrients
Matching each nutrient to its enzymes and where it's handled.
Who Digests What
- Carbohydrates
- Salivary amylase (mouth) + pancreatic amylase (small intestine) → Simple sugars (e.g., glucose)
- Proteins
- Pepsin (stomach) + trypsin/pancreatic proteases (small intestine) → Amino acids
- Fats
- Bile emulsifies (from liver/gallbladder) + pancreatic lipase (small intestine) → Fatty acids + glycerol
Where Things Are Absorbed
- Most nutrients (sugars, amino acids, fats, most vitamins/minerals) — small intestine
- Vitamin B12 and bile salts — ileum (needs intrinsic factor for B12)
- Most water and electrolytes — large intestine
Remember the enzyme-to-nutrient pairs: amylase → carbs, protease/pepsin → protein, lipase → fat, with bile as the fat "detergent" (it emulsifies but is not an enzyme). If a question describes poor fat digestion, think bile or lipase; poor B12, think intrinsic factor or ileum.
? Which organ produces the enzyme that begins carbohydrate digestion?
Clinical Applications & Key Labs
Connecting the anatomy and physiology to the bedside.
A&P at the Bedside
- NPO & NG tubes — rest the GI tract, decompress the stomach, or deliver feeding when swallowing isn't safe.
- Bowel sounds — reflect peristalsis; absent sounds may signal ileus or obstruction.
- Swallow assessment — done before oral intake in at-risk clients to prevent aspiration.
- First-pass metabolism — oral drugs are absorbed from the gut and pass through the liver first, where some are broken down before reaching the bloodstream.
Key GI & Liver Lab Values
- ALT & AST
- ~7–56 U/L (ALT); ~10–40 U/L (AST) — Liver cell injury (rise with liver damage)
- Total bilirubin
- 0.3–1.0 mg/dL — Liver/biliary function (high = jaundice)
- Albumin
- 3.5–5.0 g/dL — Liver protein synthesis (low in liver disease/malnutrition)
- Ammonia
- ~15–45 mcg/dL — Cleared by the liver (high in liver failure → confusion)
- Amylase & lipase
- Amylase ~30–110 U/L; lipase ~0–160 U/L — Pancreatic function (rise in pancreatitis)
You don't need to memorize every GI disorder in A&P — you need to know what each part of the tract normally does. Once you know that the stomach makes intrinsic factor, the small intestine absorbs nutrients, and the liver makes clotting factors and clears ammonia, the signs of disease and the reasons for nursing care follow logically. The Med-Surg GI guide builds on this foundation to cover the disorders and nursing care in detail.
Frequently Asked Questions
Common student questions on this topic.
What is the difference between mechanical and chemical digestion?
Where does most digestion and absorption happen?
What does the liver do in digestion?
Why is intrinsic factor important?
What is the role of bile?
What does the large intestine do?
Quick-Reference Cheatsheet
Every key point from this guide on a single printable page.
Digestive System Cheatsheet
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