Digestive Secretions and Processes

Digestive Secretions of the GI Tract

• Overview of Digestive Secretions

  • Digestion and absorption require the aqueous environment provided by digestive secretions.

  • Synthesis and secretion are regulated by:

  • Endocrine events

  • Paracrine events

  • Neural events

  • The total daily volume of digestive secretions exceeds the volume of fluids ingested.

  • Digestive secretions are rich in electrolytes.

  • Absorption of these secretions is critical for maintaining homeostasis.

Saliva

• Function and Composition of Saliva

  • Facilitates molding of food into boluses for easier swallowing.

  • May have functions such as:

  • Antibacterial properties

  • Digestive functions

  • Evaporative cooling (varies by species)

• Antibacterial Activity

  • Resulting from antibodies and antimicrobial enzymes known as lysozymes.

• Digestive Enzymes in Saliva

  • Omnivorous animals (e.g. rats, pigs) possess:

  • Starch-digesting enzyme: salivary amylase (absent in carnivorous animals)

  • Lingual lipase is present in milk-fed young (e.g. calves) but disappears with maturity.

• Digestive Action of Salivary Enzymes

  • Major effects occur in the proximal stomach.

  • Slightly basic pH activates amylase; low pH of the distal stomach inactivates it.

• Salivary Gland Structure

  • Typical acinar gland with:

  • Collection ducts ending in cellular evaginations (acini).

  • Initial composition: water, electrolytes, enzymes, mucus.

  • Final saliva product: hypotonic with lower sodium concentration compared to extracellular fluid.

• Salivary Gland Types

  • Most mammals have three pairs:

  • Parotid glands: located under the ears.

  • Mandibular glands: found in the intramandibular space.

  • Lingual glands: situated at the base of the tongue.

  • Each gland drains into a main duct with a single opening into the mouth.

  • Minor salivary glands located in the tongue and buccal mucosa.

• Nerve Regulation of Salivation

  • Parasympathetic fibers (facial and glossopharyngeal nerves) stimulate secretory cells through cholinergic receptors.

  • Afferent stimuli for salivation include:

  • Chewing

  • Stimulation of taste buds

  • Anticipation of eating

  • Salivary secretory cells also have beta-adrenergic receptors influenced by sympathetic stimulation or circulating catecholamines.

  • Unique among digestive glands, salivary glands lack endocrine regulatory components.

• Ruminant Saliva

  • Differences in composition from monogastric saliva:

  • Isotonic with high concentrations of bicarbonate, phosphate, and high pH.

  • Necessary for neutralizing acids from rumen fermentation.

  • Cows secrete 100–200 liters of saliva daily, requiring rapid reabsorption and recirculation of water and electrolytes.

Gastric Secretions

• Gastric Mucosa Variability

  • Most domestic monogastric animals possess only glandular mucosa in the stomach.

  • Horses and rats have non-glandular stratified squamous epithelium in the proximal stomach region.

• Glandular Regions of Stomach

  • Divided into three regions:

  • Cardiac mucosa: secretes mucus

  • Parietal mucosa: secretes hydrochloric acid (HCl)

  • Pyloric mucosa: secretes pepsinogen

  • Gastric pits (invaginations) are covered by surface mucous cells and gastric glands, which produce a protective thick mucus.

• Regulation of Gastric Acid Secretion

  • Stimulated by anticipation of eating and presence of undigested food in the stomach.

Pancreatic Secretions

• Structure and Function of the Pancreas

  • Composed of two distinct types of glandular tissue:

  • Endocrine pancreas: secretes hormones into the bloodstream from islets within the parenchyma.

  • Exocrine pancreas: produces digestive secretions delivered into the intestinal lumen, resembling the acinar structure of salivary glands.

• Digestive Enzyme Production

  • Acinar cells produce enzymes in inactive forms, which are activated in the duodenal lumen.

• Nerve Stimulation of Pancreatic Secretion

  • Neurons from the ENS promote secretion and respond to:

  • Sight and smell of food (vagal responses).

  • Distention of the stomach (vagovagal reflex).

  • Distention of the duodenum.

• Bicarbonate Secretion

  • Acidic conditions in the duodenum stimulate pancreatic bicarbonate secretion, alkalizing the ingesta.

• Adaptations to Digestive Process

  • Pancreatic secretion decreases to low basal rates as food is digested, absorbed, and acid is neutralized.

Bile Secretion

• Function of the Liver in Digestion

  • The liver serves as a secretory gland, producing bile acids for fat digestion and emulsification of dietary lipids.

  • Bile contains bile pigments, primarily bilirubin, the breakdown product of hemoglobin turnover.

  • Bilirubin imparts the characteristic green color to bile and is converted to secondary products responsible for feces' brown color in non-herbivorous animals.

  • Bile pigments facilitate waste excretion but have no useful digestive function.

• Gallbladder Functionality

  • In animals with gallbladders, bile acids are stored and concentrated between feedings.

  • In species without gallbladders (e.g., horses, rats), bile is secreted continuously.

  • In response to fat-rich food in the duodenum, cholecystokinin is secreted by GI endocrine cells, initiating gallbladder contraction.

• Bile Acids and Absorption

  • Bile acids aid in fat digestion and absorption in the jejunum but are not absorbed until reaching the ileum.

  • After absorption, bile acids travel via the hepatic portal vein back to the liver.

  • The enterohepatic circulation describes bile acids' continuous cycle: liver → intestine → portal blood → liver → back to intestine.

  • A positive feedback system is established with gallbladder contraction but inhibited post-fat digestion.