Digestive System Organs

Why This Matters

The digestive system isn't just a tube that food passes through—it's a coordinated assembly line where mechanical breakdown, chemical digestion, and nutrient absorption happen in precise sequence. You're being tested on understanding how each organ contributes to this process, why certain organs secrete specific enzymes, and what happens when food moves from one compartment to the next. The exam loves questions about enzyme specificity, pH optimization, and the division of labor between organs.

Think of digestion as a series of handoffs: each organ receives partially processed material, does its specialized job, and passes the product forward. Master the sequence of digestion, the enzyme-substrate relationships, and the accessory organ contributions, and you'll be ready for anything from multiple choice to complex FRQ scenarios. Don't just memorize organ names—know what each organ breaks down, what it secretes, and why its location in the GI tract matters.

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Mechanical Processing and Initial Breakdown

The digestive process begins with physical manipulation of food—reducing particle size increases surface area for enzymatic action. These early organs prepare food for the chemical digestion that follows.

Mouth

• Mechanical digestion begins here—teeth physically break food into smaller pieces, dramatically increasing surface area for enzyme access
• Salivary amylase initiates carbohydrate digestion by breaking starch into maltose; this is why bread tastes sweeter the longer you chew
• Bolus formation occurs as saliva lubricates food particles, preparing them for safe swallowing and transport

Salivary Glands

• Three paired glands (parotid, submandibular, sublingual) produce 1-1.5 liters of saliva daily containing amylase and lingual lipase
• Lysozyme in saliva provides antimicrobial protection, defending the oral cavity against pathogens entering with food
• Mucins lubricate the bolus and protect oral tissues; saliva's buffering capacity also protects tooth enamel from acid erosion

Esophagus

• Peristalsis—coordinated smooth muscle contractions—moves the bolus toward the stomach; this works even against gravity
• Lower esophageal sphincter (LES) prevents gastric reflux; dysfunction here causes GERD and potential esophageal damage
• No digestion or absorption occurs here—the esophagus is purely a transport organ connecting the pharynx to the stomach

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Chemical Digestion and Protein Processing

The stomach creates a harsh environment optimized for protein denaturation and pathogen destruction. The low pH activates specific enzymes while inactivating salivary amylase.

Stomach

• Gastric juice contains hydrochloric acid (pH 1.5-3.5), which denatures proteins and activates pepsinogen into pepsin for protein digestion
• Chief cells secrete pepsinogen while parietal cells produce HCl and intrinsic factor; intrinsic factor is essential for vitamin $B_{12}$ absorption later in the ileum
• Chyme formation—food mixed with gastric secretions becomes a semi-liquid mass released gradually through the pyloric sphincter into the duodenum

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Nutrient Absorption Hub

The small intestine is where the real nutritional payoff happens. Its structure is specifically designed to maximize contact between digested nutrients and absorptive surfaces.

Small Intestine

• Three regions with distinct functions: duodenum (receives secretions, chemical digestion), jejunum (primary absorption), ileum (absorbs $B_{12}$ and bile salts)
• Villi and microvilli create a surface area of approximately 250 square meters—roughly the size of a tennis court—for maximum nutrient absorption
• Brush border enzymes (maltase, sucrase, lactase, peptidases) complete final digestion at the absorptive surface before nutrients enter enterocytes

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Accessory Organs: The Support Team

These organs don't contact food directly but produce secretions essential for digestion. Their contributions are delivered to the small intestine via ducts.

Liver

• Bile production is the liver's digestive role—bile contains bile salts that emulsify fats, breaking large lipid globules into smaller droplets for lipase access
• Metabolic processing hub for absorbed nutrients: converts glucose to glycogen, synthesizes plasma proteins, and detoxifies harmful substances
• Stores fat-soluble vitamins (A, D, E, K) and releases glucose from glycogen stores when blood sugar drops; the liver processes everything absorbed from the GI tract via the hepatic portal vein

Gallbladder

• Concentrates and stores bile produced by the liver, increasing bile salt concentration up to tenfold between meals
• Cholecystokinin (CCK) triggers gallbladder contraction when fats enter the duodenum, releasing bile through the common bile duct
• Emulsification (not digestion) is bile's function—bile salts don't break chemical bonds but increase surface area for pancreatic lipase

Pancreas

• Exocrine function produces digestive enzymes: pancreatic amylase (carbohydrates), trypsin/chymotrypsin (proteins), and pancreatic lipase (fats)
• Bicarbonate secretion neutralizes acidic chyme, raising pH to 7-8; this protects the duodenum and creates optimal conditions for pancreatic enzymes
• Endocrine function involves insulin and glucagon secretion from islets of Langerhans—connecting digestion to metabolic regulation

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Water Recovery and Waste Processing

The final stages focus on reclaiming water and housing the microbiome. Most nutrient absorption is complete before material reaches these organs.

Large Intestine

• Water and electrolyte absorption recovers approximately 1.5 liters of fluid daily, converting liquid chyme into solid feces
• Gut microbiota ferment undigested fiber, producing short-chain fatty acids (energy for colonocytes) and synthesizing vitamin K and B vitamins
• Mass movements (strong peristaltic waves) propel feces toward the rectum; the large intestine absorbs no macronutrients—only water, electrolytes, and microbial products

Appendix

• Lymphoid tissue concentration suggests immune surveillance function at the junction of small and large intestines
• Microbiome reservoir hypothesis—may serve as a "safe house" for beneficial bacteria to repopulate the gut after illness
• Previously considered vestigial, but current research supports roles in immune function and maintaining healthy gut flora composition

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Quick Reference Table

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Self-Check Questions

1. Which two organs both contribute to carbohydrate digestion through amylase secretion, and why does the first enzyme stop working once food reaches the stomach?

2. Compare the absorptive functions of the small intestine and large intestine—what does each absorb, and how do their structural features support these different roles?

3. If a patient has their gallbladder removed, which macronutrient would be most affected and why? What organ would partially compensate?

4. Explain why the pancreas secretes bicarbonate along with digestive enzymes. What would happen to enzyme function without this pH adjustment?

5. An FRQ asks you to trace a piece of bread from ingestion to absorption. Identify the three locations where carbohydrate digestion occurs and name the specific enzyme active at each site.