Physiology of Digestive System by Dr

Physiology of Digestive System By Dr. Hassan Y. Hassan 3rd Year/ Lecture 1 Introduction

u Most nutrients we eat cannot be used in existing form u Must be broken down into smaller components before the body can make use of them

u Digestive system—essentially a disassembly line u To break down nutrients into a form that can be used by the body u To absorb them so they can be distributed to the tissues

u Gastroenterology—the study of the digestive tract and the diagnosis and treatment of its disorders Digestive Function

u Digestive system—the organ system that processes food, extracts nutrients from it, and eliminates the residue Digestive Function

u Five stages of digestion u Ingestion: selective intake of food u Digestion: mechanical and chemical breakdown of food into a form usable by the body u Absorption: uptake of nutrient molecules into the epithelial cells of the digestive tract and then into the blood and lymph u Compaction: absorbing water and consolidating the indigestible residue into feces u Defecation: elimination of feces Digestive Function

u Mechanical digestion—the physical breakdown of food into smaller particles u Cutting and grinding action of the teeth u Churning action of stomach and small intestines u Exposes more food surface to the action of digestive enzymes Digestive Function

u Chemical digestion—a series of hydrolysis reactions that breaks dietary macromolecules into their monomers (residues) u Carried out by digestive enzymes produced by salivary glands, stomach, pancreas, and small intestine u Results u Polysaccharides into monosaccharides u Proteins into amino acids u Fats into monoglycerides and fatty acids u Nucleic acids into nucleotides Digestive Function

u Some nutrients are present in a usable form in ingested food u Absorbed without being digested u Vitamins, free amino acids, minerals, cholesterol, and water General Anatomy

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. u Digestive system has two Oral cavity Tongue Parotid gland anatomical subdivisions Teeth Sublingual gland Pharynx

Submandibular u Digestive tract gland Esophagus (alimentary canal) u 30 ft long muscular tube Diaphragm extending from mouth to Liver Stomach anus Pancreas Gallbladder Transverse u Mouth, pharynx, Bile duct colon Ascending colon Descending esophagus, stomach, colon small intestine, and large Small intestine Cecum intestine Appendix Sigmoid colon Rectum u Anal canal Gastrointestinal (GI) tract Anus is the stomach and intestines Figure 25.1 General Anatomy

Oral cavity

Tongue Parotid gland Teeth Cont. Sublingual gland Pharynx Submandibular u Accessory organs gland Esophagus u Teeth, tongue, salivary

glands, liver, Diaphragm

gallbladder, and Liver Stomach pancreas Pancreas Gallbladder Transverse Bile duct colon Ascending colon Descending colon Small intestine

Cecum Appendix Sigmoid colon Rectum Anal canal Anus

Figure 25.1 General Anatomy

u Digestive tract is open to the environment at both ends

u Most material in it has not entered the body tissues u Considered to be external to the body until it is absorbed by the epithelial cells of the alimentary canal

u On a strict sense, defecated food residue was never in the body Tissue Layers of the Digestive Tract

Diaphragm

Esophageal hiatus

Enteric nervous system:

Mucosa: Myenteric plexus Stratified squamous epithelium Submucosal plexus Lamina propria Muscularis mucosae Parasympathetic ganglion of myenteric plexus

Submucosa: Esophageal gland

Lumen

Muscularis externa: Inner circular layer Outer longitudinal layer Blood vessels Serosa

Figure 25.2 General Anatomy

u Enteric nervous system—a nervous network in the esophagus, stomach, and intestines that regulated digestive tract motility, secretion, and blood flow u Thought to have over 100 million neurons u More than the spinal cord u Functions completely independently of the central nervous system u CNS exerts a significant influence on its action

u Enteric nervous system contains sensory neurons that monitor tension in gut wall and conditions in lumen General Anatomy

u Composed of two networks of neurons u Submucosal (Meissner) plexus: in submucosa u Controls glandular secretion of mucosa u Controls movements of muscularis mucosae u Myenteric (Auerbach) plexus: parasympathetic ganglia and nerve fibers between the two layers of the muscularis interna u Controls peristalsis and other contractions of muscularis externa Regulation of the Digestive Tract

u Motility and secretion of the digestive tract are controlled by neural, hormonal, and paracrine mechanisms

u Neural control u Short (myenteric) reflexes: stretch or chemical stimulation acts through myenteric plexus u Stimulates parastaltic contractions of swallowing u Long (vagovagal) reflexes: parasympathetic stimulation of digestive motility and secretion u Hormonal control u Gastrin and secretin The Mouth

u The mouth is known as the oral, or buccal cavity

u Functions u Ingestion (food intake) u Other sensory responses to food: chewing and chemical digestion u Swallowing, speech, and respiration

u Mouth enclosed by cheeks, lips, palate, and tongue Mastication

u Mastication (chewing)—breaks food into smaller pieces to be swallowed and exposes more surface to the action of digestive enzymes u First step in mechanical digestion u Food stimulates oral receptors that trigger an involuntary chewing reflex u Tongue, buccinator, and orbicularis oris manipulate food u Masseter and temporalis elevate the teeth to crush food u Medial and lateral pterygoids, and masseter swing teeth in side-to-side grinding action of molars Saliva and the Salivary Glands

u Saliva u Moisten mouth u Begin starch and fat digestion u Cleanse teeth u Inhibit bacterial growth u Dissolve molecules so they can stimulate the taste buds u Moisten food and bind it together into bolus to aid in swallowing Saliva and the Salivary Glands

u Hypotonic solution of 97.0% to 99.5% water and the following solutes: u Salivary amylase: enzyme that begins starch digestion in the mouth u Lingual lipase: enzyme that is activated by stomach acid and digests fat after the food is swallowed u Mucus: binds and lubricates the mass of food and aids in swallowing u Lysozyme: enzyme that kills bacteria u Immunoglobulin A (IgA): an antibody that inhibits bacterial growth u Electrolytes: Na +, K +, Cl −, phosphate, and bicarbonate

u pH: 6.8 to 7.0 Saliva and the Salivary Glands

u Extrinsic salivary glands—three pairs connected to oral cavity by ducts u Parotid: located beneath the skin anterior to the earlobe u Mumps is an inflammation and swelling of the parotid gland caused by a virus u Submandibular gland: located halfway along the body of the mandible u Its duct empties at the side of the lingual frenulum, near the lower central incisors u Sublingual glands: located in the floor of the mouth u Has multiple ducts that empty posterior to the papilla of the submandibular duct Extrinsic salivary glands secrete about of 1 to 1.5 L of saliva per day The Extrinsic Salivary Glands

Parotid gland Parotid duct

Tongue Sublingual ducts

Figure 25.9

Masseter muscle

Submandibular duct Lingual Submandibular frenulum gland

Sublingual Opening of gland Mandible submandibular duct Histology of Salivary Glands

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. u Compound tubuloacinar Mucous acinus Mucous cells glands Serous cells u Branched ducts ending in acini Salivary duct

Serous u Mucous cells secrete mucus Serous demilune acinus Mixed acinus on mixed (a) acinus

Mucous u cells Serous cells secrete thin fluid rich in amylase and

Serous demilune electrolytes Stroma

Duct

(b) u Mixed acinus has both b: © The McGraw-Hill Companies, Inc./Dennis Strete, photographer Figure 25.10a,b mucous and serous cells Salivation

u Cells of acini filter water and electrolytes from blood and add amylase, mucin, and lysozyme

u Salivary nuclei in the medulla oblongata and pons respond to signals generated by presence of food u Tactile, pressure, and taste receptors u Salivary nuclei receive input from higher brain centers as well u Odor, sight, thought of food stimulates salivation Salivation

Cont. u Send signals by way of autonomic fibers in the facial and glossopharyngeal nerves to the glands

u Parasympathetics stimulate the glands to produce an abundance of thin, enzyme-rich saliva u Sympathetic stimulation stimulates the glands to produce less, and thicker, saliva with more mucus

u Bolus—mass swallowed as a result of saliva binding food particles into a soft, slippery, easily swallowed mass The Esophagus

u Esophagus—a straight muscular tube 25 to 30 cm long u Begins at level between C6 and the cricoid cartilage u Extends from pharynx to cardiac orifice of stomach passing through esophageal hiatus in diaphragm u Lower esophageal sphincter: food pauses at this point because of this constriction u Prevents stomach contents from regurgitating into the esophagus u Protects esophageal mucosa from erosive effect of the stomach acid u Heartburn—burning sensation produced by acid reflux into the esophagus The Esophagus

Cont. u Nonkeratinized stratified squamous epithelium u Esophageal glands in submucosa secrete mucus u Deeply folded into longitudinal ridges when empty u Skeletal muscle in upper one-third, mixture in middle one-third, and only smooth muscle in the bottom one- third u Meets stomach at level of T7 u Covered with adventitia Swallowing

Soft palate

Uvula

Epiglottis

Relaxation Bolus of food Esophagus Pharynx Tongue 2 Bolus passes into pharynx. Misdirection Epiglottis of bolus is prevented by tongue blocking oral cavity , soft palate blocking nasal cavity, and epiglottis blocking larynx. Glottis

1 Tongue compresses food against palate Trachea to form a bolus.

3 Upper esophageal sphincter constricts and bolus passes downward.

Constriction

Peristaltic 4 Peristalsis drives bolus down wave esophagus. Esophagus constricts above bolus and Bolus dilates and shortens below it. Upper esophagus

Relaxation Peristaltic Figure 25.11a,b Shortening contraction

Bolus of ingested matter passing down esophagus

5 Lower esophageal sphincter relaxes to admit bolus to stomach. (b) Constriction Stomach

Cardiac Lower esophageal orifice sphincter Relaxation

(a)

u Swallowing (deglutition)—a complex action involving over 22 muscles in the mouth, pharynx, and esophagus u Swallowing center: pair of nuclei in medulla oblongata that coordinates swallowing u Communicates with muscles of the pharynx and esophagus by way of trigeminal, facial, glossopharyngeal, and hypoglossal nerves Swallowing

u Swallowing occurs in two phases u Buccal phase: under voluntary control u Tongue collects food, presses it against the palate forming a bolus, and pushes it posteriorly u Food accumulates in oropharynx in front of “blade ” of the epiglottis u Epiglottis tips posteriorly and food bolus slides around it through the laryngeal opening u Bolus enters laryngopharynx and stimulates tactile receptors and activates next phase Swallowing

Cont. u Pharyngoesophageal phase: involuntary u Three actions prevent food and drink from reentering the mouth or entering the nasal cavity or larynx

u Root of the tongue blocks the oral cavity

u Soft palate rises and blocks the nasopharynx

u Infrahyoid muscles pull the larynx up to meet the epiglottis while laryngeal folds close the airway u Food bolus is driven downward by constriction of the upper, then middle, and finally the lower pharyngeal constrictors u Bolus enters esophagus, stretches it, and stimulates peristalsis Swallowing

u Peristalsis—wave of muscular contraction that pushes the bolus ahead of it u Entirely involuntary reflex u When standing or sitting upright, the food and liquid drops through the esophagus by gravity faster than peristalsis can keep up with it u Peristalsis ensures you can swallow regardless of body position u Liquid reaches the stomach in 1 to 2 seconds u Food bolus in 4 to 8 seconds u When it reaches lower end of the esophagus, the lower esophageal sphincter relaxes to let food pass into the stomach Swallowing

Upper esophagus

Peristaltic contraction

Figure 25.11b Bolus of ingested matter passing down esophagus