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CHAPTER 65
Secretory Functions of the Alimentary Tract

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Mucus Secretion: Mucous glands produce mucus to lubricate and protect the lining of the digestive tract, from the
mouth to the anus
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Secretion Responds to Food
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The body secretes just the right amount of digestive juices needed for the type and amount of food present
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Crypts of Lieberkühn: Located in the small intestine; they are pits that contain cells for secretion
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Complex Glands: Includes the salivary glands, pancreas, and liver, which are located outside the digestive tract
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g
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Direct contact: Stimulates mucus release
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Enteric Nervous System: Local nerve reflexes triggered by:

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▪

Tactile (touch) stimulation
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▪

Stretching of the gut wall
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Parasympathetic Nervous System:
Boosts secretion rates, especially in glands in the upper digestive tract (e
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, salivary glands, stomach

▪

glands)
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▪

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Hormones: Certain hormones, released in response to food, regulate secretion (e
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, gastric juice increases when
food enters the stomach)
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Mechanism of Secretion by Gland Cells
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How Secretions Are Made:
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Water and Electrolytes: These are secreted along with enzymes to help flush them out of the glands and into the




























digestive system
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Saliva Secretion
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Types of Saliva:
◦

Serous Secretion: Contains ptyalin (amylase) to digest starch
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Glands Producing Saliva:
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Parotid Glands: Produce mostly serous saliva
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◦

Buccal Glands: Produce mucus only
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Low in sodium (Na⁺) and chloride (Cl⁻)
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g
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Activated by:

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▪

Taste (especially sour foods) and touch stimuli in the mouth
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Reflexes from the stomach when irritation occurs (e
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, nausea)
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Sympathetic Stimulation:

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◦

Causes less saliva production compared to parasympathetic signals
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Pepsinogen (inactive enzyme) for protein digestion
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Oxyntic (Gastric) Glands: Found in the stomach body and fundus (upper 80%)
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They secrete:

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Mucus for protection
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▪

Gastrin, a hormone that helps regulate stomach activity
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8)
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▪

HCO₃⁻ (bicarbonate) is released into the blood, making gastric venous blood more alkaline
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Main Mechanism:
◦ The parietal cells in the stomach use a hydrogen-potassium pump (H⁺-K⁺ ATPase) to secrete hydrochloric





































acid (HCl)
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2
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◦ H⁺ Secretion: The H⁺ ions are actively pumped into the stomach lumen in exchange for K⁺ ions using the
H⁺-K⁺ ATPase pump
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▪ Na⁺ is pumped out of the cell by another pump (Na⁺-K⁺ ATPase) to maintain the cell’s balance
...

▪ This reaction is catalyzed by the enzyme carbonic anhydrase
...

▪ Cl⁻ then moves into the stomach lumen, combining with H⁺ to form HCl
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Final Composition:














































◦ The secretion contains:

▪
▪
▪
▪

HCl (150-160 mEq/L),
KCl (15 mEq/L),
NaCl (small amounts),
Water (osmosis)
...
Protection of the Stomach:
◦ The stomach protects itself from its own acid by producing alkaline mucus and having tight junctions
between cells
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g
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Factors That Stimulate Gastric Secretion

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◦ Gastrin: Hormone that mainly stimulates HCl secretion by parietal cells
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Pepsinogen and Pepsin Activation

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◦ It is inactive when secreted
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Activation of Pepsinogen:
◦ When exposed to HCl, pepsinogen converts into pepsin, the active enzyme
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8–3
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Intrinsic Factor

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g
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▪ Pernicious anemia: No B12 absorption, causing defective red blood cell production
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Structure and Function:
◦ Located in the stomach’s lower region
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▪ Gastrin: Controls HCl secretion
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Role:
◦ Secrete thick, alkaline mucus to protect the stomach lining from acid and pepsin
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Response to Irritation:


































◦ Even minor irritation or food contact triggers these cells to secrete more mucus
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Parietal Cells:
◦ The only cells that secrete HCl
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◦ Secrete histamine, which directly stimulates HCl secretion
...
Role of Gastrin:
◦ Gastrin (from G cells in the pyloric glands) stimulates ECL cells to release histamine, which enhances HCl
secretion
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Dual Role:





































◦ Secretes digestive enzymes into the small intestine
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◦ Sodium bicarbonate (HCO₃⁻): Neutralizes stomach acid in the duodenum
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Protein Digestion:
◦ Trypsin and chymotrypsin: Break proteins into smaller peptides
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◦ Cholesterol esterase: Breaks cholesterol esters
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Prevention of Pancreatic Self-Digestion

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◦ Prevents the pancreas from digesting itself
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Acute Pancreatitis:
◦ If the pancreas is damaged or ducts are blocked, trypsin inhibitor fails
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Secretion of Bicarbonate Ions (HCO₃⁻)

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▪ H₂CO₃ breaks down into HCO₃⁻ and H⁺
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◦ Water Movement:
▪ Movement of Na⁺ and HCO₃⁻ creates an osmotic gradient, pulling water into the ducts
...
Final Secretion:
◦ Pancreatic juice becomes an isosmotic bicarbonate solution
...
Acetylcholine: Released by vagus and enteric nervous system, stimulates acinar cells to secrete digestive enzymes
...
Cholecystokinin (CCK): Secreted by the duodenum and upper jejunum in response to food, primarily proteins
and fats
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Secretin: Stimulated by acidic chyme (pH < 4
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Phases of Secretion
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Cephalic Phase: Neural signals trigger enzyme production (≈20% of total secretion)
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Intestinal Phase: Dominated by secretin and CCK, leading to bicarbonate secretion and enzyme release,
respectively
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0)
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Bile Secretion by the Liver
Functions

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◦ Forms micelles to facilitate lipid absorption
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Excretion of Waste Products: Includes bilirubin and excess cholesterol
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Hepatocytes produce bile containing bile acids and organic compounds
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Epithelial cells of bile ducts secrete a bicarbonate-rich fluid (stimulated by secretin)
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Concentrates bile up to 20-fold
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Active transport of Na+ drives water reabsorption
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Stimulus for Emptying: Triggered by CCK in response to fatty foods
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Micelle Formation: Facilitates absorption of lipids and cholesterol
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Small Intestine Secretion


































Secrete alkaline mucus to protect against acidic chyme
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Brunner's Glands

Inhibited by sympathetic stimulation, increasing ulcer risk
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•

Secretions:
◦

Watery fluid with Cl−, HCO3−, and Na+ to aid absorption
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g
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Rapid cell turnover (≈5 days) ensures mucosal repair and enzyme renewal
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g
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Large Intestine Secretion




























Stimulation:




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Mucus protects the mucosa and facilitates fecal passage
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◦

Parasympathetic stimulation (e
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, pelvic nerves)
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Electrolyte Content













Contains moderate HCO3− to neutralize acidic waste products

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