General Principles Of Alimentary Tract Secretion

Secretory Functions of the Alimentary Tract General Principles Of Alimentary Tract Secretion • • • Secretory glands subserve two primary functions: Digestive enzymes are secreted (from the mouth to the distal end of the ileum). Mucous glands (from the mouth to the anus) provide mucus for lubrication and protection of all parts of the alimentary tract. Secretory Functions of the Alimentary Tract Types of Alimentary Tract Glands • • • • Single-cell mucous glands (mucous cells or goblet cells) Pits that represent invaginations of the epithelium into the submucosa Deep tubular glands (stomach and upper duodenum) Complex glands (salivary glands, pancreas) Secretory Functions of the Alimentary Tract Basic Mechanisms Of Stimulation Of The Alimentary Tract Glands • Contact of Food with the Epithelium Stimulates Secretions • Local epithelial stimulation also activates the ENS. The types of stimuli that activate this are (1) tactile stimulation, (2) chemical irritation, (3) distention.  Autonomic Stimulation of Secretion • Parasympathetic Stimulation Increases the Glandular Secretion Rate. • Glands in the upper portion innervated by the glossopharyngeal and vagus nerves (salivary glands, esophageal glands, gastric glands, pancreas, and Brunner’s glands in the duodenum). • Glands in the distal portion of the large intestine are innervated by pelvic nerves. • Secretion in the remainder of the small intestine and in the first two thirds of the large intestine occurs in response to local neural and hormonal stimuli. • • • Sympathetic Stimulation Has a Dual Effect on the Glandular Secretion (1) Sympathetic stimulation alone usually slightly increases secretion. (2) If parasympathetic or hormonal stimulation is already causing copious secretion, superimposed sympathetic stimulation usually reduces the secretion, because of vasoconstrictive reduction of the blood supply. Secretory Functions of the Alimentary Tract Lubricating and Protective Properties of Mucus • • • • • • • Mucus is a thick secretion composed mainly of water, electrolytes, and a mixture of several glycoproteins that are composed of large polysaccharides bound with much smaller quantities of protein. Mucus has adherent qualities that make it adhere tightly to the food. Coats the wall of the gut. Mucus has a low resistance for slippage. Causes fecal particles to adhere to one another. Strongly resistant to digestion by the gastrointestinal enzymes. Glycoproteins of mucus have amphoteric properties. Secretory Functions of the Alimentary Tract Secretion of Saliva • • • • Saliva Contains a Serous Secretion and a Mucus Secretion. The principal glands of salivation  Parotid glands  Submandibular glands  Sublingual glands  Buccal glands Daily secretion 800 and 1500 ml. Saliva contains two major types of protein secretion: • Serous secretion that contains ptyalin (α-amylase) • Mucus secretion that contains mucin (lubricating and surface protection) Secretory Functions of the Alimentary Tract Secretion of Saliva • • • • Parotid glands secrete serous type of secretion. Submandibular and sublingual glands secrete both serous secretion and mucus. Buccal glands secrete only mucus. Saliva has a pH between 6.0 and 7.0, (favorable range for the digestive action of ptyalin). Acinus Branching duct system acinar cells epithelium Secretory Functions of the Alimentary Tract Secretion of Ions in Saliva • • • Saliva contains large quantities of potassium and bicarbonate ions. Concentrations of both sodium and chloride ions are several times less in saliva than in plasma. The acini secrete a primary secretion that contains ptyalin and/or mucin in a solution of ions. As the primary secretion flows through the ducts, two major active transport processes take place.  Sodium ions are actively reabsorbed from all the salivary ducts and potassium ions are actively secreted.  Bicarbonate ions are secreted by the ductal epithelium into the lumen of the duct. Secretory Functions of the Alimentary Tract Secretion of Ions in Saliva • • • • Concentrations of sodium and chloride ions in the saliva about one seventh to one tenth their concentrations in plasma. Concentration of potassium ions 7 times as great as in plasma, and the concentration of bicarbonate 2 to 3 times that of plasma. During maximal salivation Sodium chloride concentration is about one half or two thirds that of plasma, and the potassium concentration rises to only 4 times that of plasma. Secretory Functions of the Alimentary Tract Function of Saliva for Oral Hygiene • • • Wash away pathogenic bacteria, as well as food particles. Saliva contains several factors that destroy bacteria.  Thiocyanate ions  Proteolytic enzymes (lysozyme) • Attack the bacteria • Aid the thiocyanate ions in entering the bacteria • Digest food particles Antibodies that can destroy oral bacteria, including some that cause dental caries. Secretory Functions of the Alimentary Tract Nervous Regulation Of Salivary Secretion • Taste and tactile stimuli (especially the sour taste) • Smooth objects • Nervous signals arriving in the salivatory nuclei from higher centers of the CNS. • Reflexes from stomach and upper small intestines. • Sympathetic stimulation A secondary factor that also affects salivary secretion is the blood supply to the glands. Vasodilator effect is caused by kallikrein secreted by the activated salivary cells, which split alpha2-globulin, to form bradykinin. Secretory Functions of the Alimentary Tract Esophageal Secretion • • • • • Esophageal secretions are entirely mucous The main body of the esophagus is lined with many simple mucous glands. At the gastric end and in the initial portion of the esophagus, many compound mucous glands can also be found. The mucus secreted by the compound glands in the upper esophagus prevents mucosal excoriation by newly entering food. The compound glands located near the esophagogastric junction protect the esophageal wall from digestion by acidic gastric juices.