GI04 - GI Secretions

What are the major functional categories of GI secretions?

Protective → buffers (HCO₃⁻), mucins
Digestion → enzymes (lipases, peptidases, etc.)
Regulation → paracrine factors and hormones
Electrolytes and water

What are goblet cells?

Single-cell mucous glands (ubiquitous throughout GI tract) → direct extrusion of mucus.

What are deep tubular glands?

Crypt-type glands (e.g., acid-secreting glands in stomach).

What are complex glands?

Lie outside GI tract → acini empty into ducts → salivary glands, pancreas (and liver).

What local stimuli regulate GI secretion?

Contact with food/bolus → tactile, chemical irritation, distention of gut (via ENS reflexes).

How does the parasympathetic nervous system affect secretion?

Excitatory.

How does the sympathetic nervous system affect secretion?

Generally inhibitory

What role do hormones play in secretion?

Endocrine regulation of gland activity.

How are organic substances (enzymes) secreted?

ATP- and Ca²⁺-dependent exocytosis.

(They need to be transported in vesicles)

How are water and electrolytes secreted?

Chloride-dependent.

What are the two types of salivary gland secretions?

Serous and mucous.

What does serous saliva contain?

H₂O and ions (Na⁺, Cl⁻, HCO₃⁻) + some proteins.

What does mucous saliva contain?

Mucins, glycoproteins, enzymes.

What percentage of saliva is water?

99% water.

What makes up <1% of saliva?

Electrolytes, enzymes, epithelial cells, bacteria.

Which salivary gland produces only serous secretions?

Parotid gland.

Which gland has mostly mucin-secreting cells?

Sublingual gland.

Which gland produces both serous and mucous secretions?

Submandibular gland.

(Sub Mandibular = Serous and Mucus)

What do most minor glands produce?

Mucous secretions

(Except von Ebner’s gland, which is serous)

What salivary proteins aid digestion?

α-Amylase (ptyalin), lingual lipase, glycoproteins.

What salivary proteins aid lubrication?

Mucins, other glycoproteins, statherins.

What salivary proteins are antimicrobial?

Lysozyme
Lactoperoxidase
Lactoferrin
Histatins
Cystatins
Salivary agglutinin (SAG)
Secretory IgA (sIgA)

What salivary proteins contribute to mineralization?

Proline-rich proteins (PRP), statherins, histatins, cystatins.

What salivary proteins provide tissue coating (protection)?

Mucins, PRP, cystatins, statherins.

What buffers saliva?

Bicarbonate + histatins.

What characterizes salivary molecules?

There is reduncency in their functions. Ex: many molecules inhibit precipitation of calcium phosphate salts.

(Strong: PRP and statherin; moderate: histatin/cystatin; weak: mucin and amylase)

What does the esophagus secrete?

Mucus only (no digestive enzymes).

What is the function of esophageal mucus?

Protects lower esophagus from stomach acid and digestive enzymes.

What do Brunner’s glands secrete in the small intestine?

Alkaline mucus (protects against low pH chyme)

Are brush border enzymes secreted in the SI?

No — they are expressed on the luminal membrane

What are the major brush border enzymes?

Peptidases
Sucrase
Maltase
Isomaltase
Lactase

What hormones are secreted by the SI?

• Secretin

• Cholecystokinin (CCK)

• Glucose-dependent insulinotropic peptide (GIP)

Which cells secrete secretin?

S-cells.

What stimulates secretin?

Low pH [H⁺] in duodenum.

Which cells secrete CCK?

I-cells.

What stimulates CCK?

Lipids + protein in duodenum.

Which cells secrete GIP?

K-cells.

What stimulates GIP?

Carbohydrates in duodenum.

What primarily regulates small intestine secretion?

Local stimuli (tactile, irritation, chyme) and chemical chyme composition.

What does CCK do to gallbladder?

Stimulates contraction → relaxes sphincter of Oddi.

What does CCK do to pancreas?

↑ Trypsin output (acinar cells)
↑ Bicarbonate + fluid secretion (ductal cells)
↑ Pancreatic mass

What does CCK do to stomach?

Relaxes proximal stomach
Contracts pylorus → delayed gastric emptying.

What is the net effect of CCK?

Bile flow into duodenum
Digestion of food
Induction of satiety

Where is the pancreas located?

Behind the stomach.

Where do pancreatic secretions enter?

Duodenum via sphincter of Oddi (shared with common bile duct).

What are zymogens?

Inactive proenzymes secreted by the pancreas → activated in duodenum by trypsin.

What are active enzymes?

Enzymes that are secreted by the pancreas in their active form

What is trypsin inhibitor?

Enzyme that prevents premature conversion of zymogens

What is the exocrine pancreas structurally similar to?

Salivary glands.

What are the components of acinar cell secretions?

Active enzymes (e.g., amylase)
Zymogens
Serous secretions (saline and bicarbonate from ductal cells)

Where are pancreatic zymogens activated?

In the duodenum by trypsin.

What is trypsinogen?

A zymogen (inactive precursor of trypsin).

How is trypsinogen converted to active trypsin?

Brush border enzyme on apical surface of duodenum → converts trypsinogen → trypsin.

What happens if the pancreatic duct is blocked?

The pancreas will self-digest within hours

What are the three phases of pancreatic secretion?

Cephalic phase
Gastric phase
Intestinal phase

What percentage of pancreatic secretion occurs during the cephalic phase?

~20%.

What stimulates the cephalic phase of pancreatic secretion?

Sight, taste, smell of food → vagal stimulation.

What percentage of pancreatic secretion occurs during the gastric phase?

5–10%.

What stimulates the gastric phase of pancreatic secretion?

Stomach distention + gastrin.

Is gastrin a strong stimulator of pancreatic secretion?

No — it is a weak agonist of CCK₁ (CCKA) receptors.

What percentage of pancreatic secretion occurs during the intestinal phase?

70–75%.

What stimulates pancreatic enzyme secretion in the intestinal phase?

CCK.

What stimulates pancreatic bicarbonate (HCO₃⁻) secretion in the intestinal phase?

Secretin.

What is cystic fibrosis in relation to the pancreas?

Progressive pancreatic and pulmonary insufficiency.

What causes cystic fibrosis?

Genetic defect in CF gene → reduced function of CFTR (cAMP-dependent Cl⁻ channel).

What happens to acinar secretions in cystic fibrosis?

↑ Viscosity.

What can increased viscosity of acinar secretions cause?

Ductal obstruction → tissue destruction.

What is acute pancreatitis?

Inflammation of pancreas.

What commonly causes acute pancreatitis?

Gallstones or alcohol.

What causes pancreatic damage in acute pancreatitis?

Pathologic processing + activation of pancreatic enzymes → overwhelm protective mechanisms.

Where does the liver secrete bile?

Into the common bile duct.

Where is excess bile stored?

Gallbladder.

What stimulates contraction of the gallbladder?

CCK.

What are bile acids derived from?

Cholesterol.

What are primary bile acids?

Synthesized by hepatocytes.

What are secondary bile acids?

Taken up by hepatocytes from blood (modified by bacteria).

What happens to bile acids to form bile salts?

They are conjugated.

How do bile salts compare to bile acids in solubility?

Bile salts are more water soluble than bile acids.

What does the large intestine secrete?

Mucus.

What is unique about large intestine mucus?

High concentration of HCO₃⁻.

Why does the large intestine secrete HCO₃⁻?

It neutralizes acid formed by colonic bacteria.

What is the primary function of the large intestine (other than mucus producition)?

Absorption of H₂O and electrolytes.