Principles of Gastrointestinal Secretion and Secretion of Saliva

Introduction

Salivary secretion, though limited in digestive action, is vital for mastication, swallowing, speech, and oral hygiene. It is mainly enhanced by parasympathetic activity and reduced by sympathetic influence, with stress-related dryness of mouth reflecting altered neural regulation.

• Gastrointestinal secretions are produced by exocrine glands associated with the digestive tract.
• Major sources include salivary glands, gastric glands, pancreas, liver, and intestinal glands. These secretions contain enzymes and fluids essential for digestion.
• They facilitate chemical breakdown of food in the lumen.
• They enhance nutrient absorption across the intestinal epithelium. They also lubricate contents and promote smooth movement along the tract.

Principles Of Gi Secretion

Phases of GI Secretions

• Gastrointestinal secretion occurs in three coordinated phases: cephalic, gastric, and intestinal. These phases are defined by the location and nature of stimuli. They ensure timely release of digestive fluids for efficient processing of food.

Cephalic Phase

• Cephalic phase begins before food enters the stomach.
• Sensory stimuli such as sight, smell, taste, and thought of food initiate this phase.
• Chewing and emotional states also enhance secretion.
• Neural control is mediated through the vagus nerve, which activates enteric pathways.This phase stimulates salivary, gastric, pancreatic, and bile secretions.
• It contributes about 90 percent of salivary and around 40 percent of gastric and pancreatic secretions. It prepares the digestive system in advance for incoming food.

Physiological Importance of Cephalic Phase

• The physiological significance of the cephalic phase lies in early enzyme availability.
• Digestive processes can begin immediately when food reaches the stomach and intestine.

Gastric Phase

• Gastric phase starts when food enters the stomach.
• Major stimuli include gastric distension, presence of peptides and amino acids, and changes in pH.
• Release of gastrin further enhances secretion.
• Gastric distension activates both local and central reflexes. These reflexes regulate secretion from the stomach, pancreas, liver, and intestine.
• This phase plays a key role in controlling the rate and extent of digestion.

Physiological Importance of Gastric Phase

• The physiological importance of the gastric phase includes regulation of satiety.
• Distension signals fullness and limits further food intake.
• Coordinated secretions optimize digestion and prepare nutrients for absorption.
• Overall, these phases integrate neural and hormonal signals. This coordination ensures efficient digestion, absorption, and progression of contents through the gastrointestinal tract.

Intestinal Phase

• The intestinal phase begins when chyme enters the duodenum.
• Stimuli include fatty acids, peptides, acidic pH, intestinal distension, and osmolarity changes. These factors activate local and central reflexes that modify gastrointestinal secretions.
• Hormones such as cholecystokinin, secretin, glucose-dependent insulinotropic polypeptide, and vasoactive intestinal peptide are released. These hormones regulate pancreatic, biliary, and intestinal secretions for digestion. They ensure efficient breakdown of macromolecules into absorbable forms.

Physiological Importance of Intestinal Phase

• The physiological importance of this phase lies in coordinated enzyme secretion.
• Acidic chyme stimulates release of secretin and cholecystokinin, enhancing pancreatic and bile secretion.
• Intestinal signals also inhibit gastric secretion and motility. This prevents overloading of the intestine and optimizes digestion and absorption.

Principles of Regulation

• Regulation of gastrointestinal secretions is controlled by neural and hormonal mechanisms.
• The dominant mechanism varies with the phase of secretion.
• The cephalic phase is mainly regulated by neural pathways.
• The gastric and intestinal phases involve both neural and hormonal control.
• Different clinical tests assess secretion based on these phases.

General Functions of GI Secretion

• General functions of gastrointestinal secretions are diverse and essential.
• Digestion is facilitated by enzymes such as amylases, proteases, and lipases.
• These enzymes break down carbohydrates, proteins, and lipids into absorbable forms.
• Protection against infection is provided by antimicrobial components.
• Lysozyme, immunoglobulin A, and gastric acid help destroy pathogens.
• Mixing of food with digestive juices is enabled by the fluid component of secretions.
• This ensures uniform exposure of nutrients to enzymes.
• Lubrication is achieved by mucus, which eases the movement of food along the tract.
• It supports smooth propulsion in the aboral direction.
• Protection of mucosa is ensured by mucus and alkaline secretions.
• These factors shield the epithelium from acid, bile salts, and toxins.
• Absorption is enhanced by dissolving nutrients and presenting them to the epithelial surface.
• This facilitates transfer into blood and lymphatic circulation.
• Appetite regulation is influenced by components of gastrointestinal secretions.
• Moderate levels of acid and bile stimulate appetite, while excessive levels may suppress it.
• Overall, gastrointestinal secretions integrate multiple functions to maintain efficient digestion, protection, and nutrient uptake.

Salivary Secretion

The secretion from the salivary glands is called salivary secretion. It is secreted from a heterogeneous group of salivary glands located in and around the mouth cavity. Salivary secretion is distinctive for its exclusive neural regulation.

Salivary Glands

Types

• Salivary secretion is produced by glands located in and around the oral cavity. It is primarily regulated by neural mechanisms, with minimal hormonal influence. It supports lubrication, oral hygiene, and initial digestion.
• Salivary glands are classified into major and minor groups.
• Major glands include parotid, submandibular, and sublingual glands.
• Minor glands are distributed throughout the oral mucosa, palate, and pharynx.
• Based on secretion, glands are serous, mucous, or mixed.
• Serous glands produce watery, enzyme-rich fluid.
• Mucous glands secrete viscous mucus for lubrication.
• Mixed glands produce both components.
• The parotid glands are the largest salivary glands. They are purely serous and secrete fluid rich in water and electrolytes. Their secretion enters the mouth through the parotid duct near the upper molar region.
• The sublingual glands lie beneath the tongue. They are predominantly mucous glands and produce thick, viscous saliva. Their secretions drain through multiple small ducts into the oral cavity.
• The submandibular glands are located beneath the mandible. They are mixed glands, producing both serous and mucous secretions. Their ducts open into the floor of the mouth.
• Structurally, salivary glands are composed of acini and ducts. Acini contain secretory cells, while ducts modify the composition of saliva.
• Overall, salivary glands ensure continuous secretion that facilitates mastication, swallowing, and oral protection.

Histology of Salivary Glands

• The basic structural unit of salivary glands is the salivon.
• Each salivon consists of an acinus, intercalated duct, and striated duct.

Acinus

• The acinus is a sac-like structure composed of pyramidal secretory cells.
• Myoepithelial cells surround the acini and assist in expelling secretions.
• Serous cells contain enzyme-rich granules and produce watery secretions.
• Mucous cells contain mucin droplets and secrete viscous mucus.
• The initial secretion formed here is called primary secretion, which resembles plasma in electrolyte composition.

Intercalated and Striated Ducts

• The intercalated and striated ducts are lined by cuboidal epithelial cells. These ducts modify the ionic composition of the primary secretion.
• The final saliva is formed after ductal modification.
• Overall, this structural organization ensures efficient production and regulation of saliva composition.

Innervation of Salivary Glands

• Salivary glands receive autonomic innervation from both parasympathetic and sympathetic systems.
• Parasympathetic activity is the dominant regulator of salivary secretion under normal conditions.

Parasympathetic Innervation

• Parasympathetic innervation originates from nuclei in the medulla.
• Fibers to the parotid gland travel through the ninth cranial nerve and synapse in the otic ganglion.
• Postganglionic fibers then supply the gland and stimulate secretion.
• Fibers to submandibular and sublingual glands arise from the seventh cranial nerve.
• These fibers synapse in the submandibular ganglion before reaching the glands.
• Parasympathetic stimulation produces copious, watery saliva.

Sympathetic Innervation

• Sympathetic innervation arises from upper thoracic spinal segments.
• Fibers synapse in the superior cervical ganglion and then reach the glands. They supply acini, ducts, and blood vessels.
• Sympathetic stimulation produces scanty, viscous saliva and reduces overall secretion.
• Balanced autonomic input ensures proper salivary function and oral homeostasis.

Salivary Blood Flow

• Salivary glands have high metabolic activity and receive abundant blood supply.
• Blood flow increases in proportion to the rate of saliva production.
• Secretion rate can reach about 50 milliliters per minute per 100 grams of tissue.
• Blood flow to these glands is much higher than that of active skeletal muscle.
• Parasympathetic stimulation markedly increases blood flow through vasodilation. This effect is mediated by vasoactive intestinal peptide, bradykinin, and acetylcholine.
• Sympathetic stimulation reduces blood flow by causing vasoconstriction.
• Adequate perfusion is essential for continuous and efficient salivary secretion.

Salivary Secretion

Rate of Secretion

• Salivary secretion is continuous and averages about 1.0 to 1.8 liters per day.
• A small amount is produced even during sleep.
• Secretion increases markedly with stimuli such as sight, smell, taste, and thought of food.
• Reduced secretion leads to dryness of mouth and discomfort.

Composition of Saliva

• Composition of saliva includes about 99.5 percent water and 0.5 percent solids.
• Solids are divided into organic and inorganic components.

Organic Solids

• Organic components include enzymes such as salivary amylase, lingual lipase, and lysozyme.
• Additional substances include immunoglobulin A, kallikrein, and growth factors. These components support digestion, antimicrobial defense, and tissue maintenance.

Inorganic Solids

• Inorganic components consist of ions such as sodium, potassium, calcium, magnesium, chloride, bicarbonate, and phosphate. These ions maintain osmotic balance and buffering capacity.

Tonicity of Saliva

• Saliva is hypotonic compared to plasma due to ductal modification.
• Sodium and chloride concentrations are lower than in plasma.
• Increased flow rate reduces the degree of hypotonicity.

The pH and K+ Content of Saliva

• The pH of saliva is slightly alkaline, usually around 7.5 to 8.0.
• Bicarbonate secretion from duct cells increases pH during flow.
• Higher secretion rates increase bicarbonate concentration and alkalinity.
• Potassium concentration in saliva is higher than in plasma.
• However, potassium levels decrease as flow rate increases.
• Overall, saliva maintains oral lubrication, buffering, antimicrobial protection, and initial digestion.

Functions of Saliva

• Saliva performs essential digestive and protective roles in the oral cavity. It contains salivary amylase, which begins digestion of starch into smaller carbohydrates.
• This enzyme acts optimally near neutral pH and continues brief activity in the stomach before acid inactivation.
• Saliva provides strong antimicrobial protection.
• It contains lysozyme, immunoglobulin A, and lactoferrin, which inhibit microbial growth.
• Continuous flow helps cleanse the oral cavity and reduces infection risk.
• It maintains moisture of the mouth and tongue, which is necessary for clear speech.
• Reduced secretion leads to dryness and difficulty in speaking.
• Saliva enhances taste perception by dissolving food substances.
• Dissolved molecules can effectively stimulate taste receptors.
• It facilitates mastication and swallowing through lubrication.
• Mucin forms a cohesive bolus, easing passage through the pharynx and esophagus.
• Bicarbonate in saliva provides buffering action against acids.
• It helps neutralize acidity and reduces irritation in the upper digestive tract.
• Saliva protects teeth through proteins that strengthen enamel and bind harmful substances.
• It dilutes irritants and protects oral mucosa from injury.
• It also aids in excretion of certain substances such as heavy metals and drugs.
• Overall, saliva supports digestion, protection, sensation, and oral health.

Mechanism of Secretion

Salivary secretion occurs in two stages: acinar secretion and ductal modification.

Secretion in Acinus of Gland

• Acinar secretion forms the primary saliva.
• Secretory cells synthesize enzymes and store them in zymogen granules.
• On stimulation, these granules release contents by exocytosis.
• Electrolytes such as sodium, potassium, chloride, and bicarbonate are secreted.
• The resulting fluid is isotonic and resembles plasma in composition.

Secretion in Ducts

• Ductal modification occurs as fluid passes through intercalated and striated ducts.
• Duct cells reabsorb sodium and chloride from the primary secretion.
• They simultaneously secrete potassium and bicarbonate into the fluid. This process alters ionic composition without significantly changing volume.
• The final saliva becomes hypotonic compared to plasma due to selective ion transport.
• Overall, coordinated acinar and ductal processes ensure appropriate enzyme content and electrolyte balance in saliva.

Stimuli for Salivary Secretion

• Salivary secretion occurs continuously but increases markedly with specific stimuli.
• Important stimuli include anticipation, sight, smell, and thought of food.
• Discussion about food and presence of food in the mouth also enhance secretion.
• Chewing is a strong stimulus that activates salivary flow.

Control of Salivary Secretion

• Control of salivary secretion is primarily neural, with minimal hormonal influence.
• Both parasympathetic and sympathetic systems regulate secretion.

Neural Control

Parasympathetic Stimulation

• Parasympathetic stimulation is the dominant mechanism. It is triggered by sensory inputs such as taste, smell, and mastication.
• Signals are integrated in salivary nuclei located in the medulla. It produces copious, watery saliva rich in enzymes and mucin.
• It increases blood flow through vasodilation mediated by acetylcholine, vasoactive intestinal peptide, and bradykinin.
• It also directly stimulates glandular cells to enhance secretion.

Sympathetic Stimulation

• Sympathetic stimulation produces a different effect. It initially causes a brief increase in secretion due to contraction of myoepithelial cells.
• Subsequently, it reduces secretion by causing vasoconstriction.
• The resulting saliva is thick and viscous.

Reflex Secretion

• Reflex secretion plays an important role.
• Contact of food with oral mucosa produces an unconditioned reflex present from birth.
• Learned stimuli such as sight or thought of food produce conditioned reflexes.
• Salivary secretion occurs mainly during the cephalic phase of digestion. It is minimal during gastric and intestinal phases.
• Overall, neural mechanisms ensure rapid and appropriate adjustment of salivary flow.

Applied Physiology

Xerostomia

Xerostomia is reduced salivary secretion causing dryness of mouth and increased risk of oral infections. It commonly occurs during stress due to sympathetic dominance.

Sialorrhea

Sialorrhea refers to excessive and persistent salivary secretion.

Sialolithiasis

Sialolithiasis is formation of stones in salivary ducts, which may obstruct flow.

Infections

Infections such as viral inflammation of the parotid gland can occur, especially in children.