OVERVIEW OF DIGESTIVE SYSTEM بسم الله الرحمن الرحيم OVERVIEW OF DIGESTIVE SYSTEM Dr.Mohammed Sharique Ahmed Quadri Assistant Professor Physiology Almaarefa College

OBJECTIVES To know the components of GIT and their functional significance. Emphasize the functional importance of four layers of GIT Outline four basic digestive processes. Recognize the importance of regulatory factors that controls digestive functions

Digestive System Primary function Transfer nutrients, water, and electrolytes from ingested food into body’s internal environment The Digestive System Performs Four functions Motility Secretion Digestion Absorption

Functions of Digestive System Motility Muscular contractions that mix and move forward the contents of the digestive tract Two types of digestive motility Propulsive movements Mixing movements

Types of Digestive Motility Propulsive movements Push contents forward through the digestive tract Velocity with which contents are moved forward (rate of propulsion) varies in different regions of GIT, depending on functions of that region. Rapid movement in esophagus Slow movement in small intestine

Types of Digestive Motility Propulsive movements Movements of contents through most of digestive tract is accomplished by contraction of smooth muscle except Mouth Early part of esophagus External anal sphincter In these regions motility involves skeletal muscle (voluntary component)

Types of Digestive Motility Mixing movements Serve two functions Mixing food with digestive juices & hence promotes digestion of foods Facilitates absorption by exposing all parts of intestinal contents to absorbing surfaces of digestive tract

Functions of Digestive System Secretions: Digestive juices are secreted in to GIT lumen by exocrine glands. Digestive secretion consist of Water, Electrolytes, and Specific organic constituents(enzymes, bile salts, or mucus) important in digestive process.

Functions of Digestive System Secretions: Secretions are released into digestive tract lumen on appropriate neural or hormonal stimulation Normally reabsorbed in one form or another back into blood after their participation in digestion. Failure of reabsorption of digestive juices , as in diarrhea & vomiting results in loss of fluid

Functions of Digestive System Digestion Biochemical breakdown of structurally complex foodstuffs into smaller, absorbable units by enzyme produce within GIT Complex foodstuffs and their absorbable units Carbohydrates → Monosaccharides (poly &disaccharides) (Glucose,fructose,galactose) Proteins → Amino acids Fats → Glycerol and Fatty acids (triglyceride)

Functions of Digestive System Absorption: In the small intestine, digestion is completed & most absorption occurs. Through process of absorption small absorbable units resulting from digestion, along with water, vitamins, and electrolytes are transferred from digestive tract lumen into blood or lymph

Digestive System Digestive tract Accessory digestive organs Mouth Pharynx Esophagus Stomach Small intestine Duodenum Jejunum Ileum Large intestine Cecum Appendix Colon Rectum Anus Accessory digestive organs Salivary glands Exocrine pancreas Biliary system Liver Gallbladder

Digestive System Digestive tract is 4.5m (15 feet) in normal contractile state. Lumen is continuous from mouth to anus & hence with external environment Conditions that are essential for digestive process can be tolerated in the digestive tract lumen that could not be tolerated in body proper

Digestive Tract GIT wall has same general structure throughout length from esophagus to anus (with some local characteristic variations) Four major tissue layers Mucosa Innermost layer Submucosa Muscularis externa Serosa Outer layer

Layers of Digestive Tract Wall

Mucosa Lines luminal surface of digestive tract Highly folded surface greatly increases absorptive area Three layers Mucous membrane Lamina propria Muscularis mucosa

Layers of Mucosa Mucous membrane Inner epithelial layer serves as protective surface Modified in particular areas for secretion and absorption Contains Exocrine gland cells – secrete digestive juices Endocrine gland cells – secrete blood-borne gastrointestinal hormones Epithelial cells – specialized for absorbing digestive nutrients

Layers of Mucosa Lamina propria Muscularis mucosa Middle layer of connective tissue on which epithelium rest Houses gut-associated lymphoid tissue (GALT) Important in defense against disease-causing intestinal bacteria Muscularis mucosa Sparse layer of smooth muscle- contraction modifies the pattern of surface folding

Submucosa Thick layer of connective tissue Provides digestive tract with distensibility and elasticity Contains larger blood and lymph vessels Contains nerve network known as submucosal plexus

Muscularis Externa Major smooth muscle coat of digestive tube In most areas consists of two layers Circular layer Inner layer Contraction decreases diameter of lumen Longitudinal layer Outer layer Contraction shortens the tube Together contractile activity of these layers produces propulsive and mixing movements Myenteric plexus Nerve plexus lies between the two muscle layers

Serosa Outer connective tissue covering of GIT Secretes serous fluid(watery, slippery fluid) Lubricates and prevents friction between digestive organs and surrounding viscera. Continuous with mesentery throughout much of the tract This Attachment provides relative fixation Supports digestive organs in proper place while still allowing them freedom for mixing and propulsive movements

Regulation of Digestive System Function Digestive motility and secretion are carefully regulated to optimize the digestion. Four factors are involved in regulating digestive system function. Autonomous smooth muscle function Intrinsic nerve plexuses Extrinsic nerves Gastrointestinal hormones

Autonomous smooth muscle function In the wall of GIT some specialized smooth muscle cells are pacemakers cells –known as interstitial cells of Cajal. These cells lie between circular & longitudinal layer of smooth muscles. These are self excitable cell that displays rhythmic spontaneous variations in membrane potential-known as SLOW WAVE POTENTIAL OR basic electrical rhythm(BER).

Autonomous smooth muscle function

Autonomous smooth muscle function If slow wave reaches threshold a volley of action potential is triggered at the peak resulting in rhythmic cycles of contraction. Whether threshold will reach or not depends on various mechanical, neural & hormonal factors that influence starting point of slow wave.(e.g. presence of food bolus in GIT).

Autonomous smooth muscle function The rate of self induced contractile activity depends on inherent rate established by involved pacemaker. The intensity of contractions depends on number of action potentials occurring at peak of slow wave. Greater the number of contraction--higher the cytosolic calcium--stronger the contraction.

Role of Intrinsic nerve plexuses Submucosal plexus & myentric plexus, together often termed as enteric nervous system. Primarily coordinate local activity in GIT. Intrinsic plexus can affect all functions of digestive tract, i.e. motility, secretion of digestive juices and gastrointestinal hormones. Intrinsic nerve activity can be influence by endocrine, paracrine & nerve signals

Role of Extrinsic Nerves Are through both branches of ANS Influence GIT motility secretion either by Modifying activity of intrinsic plexuses. Altering level of GI hormone secretion. Directly acting on smooth muscle & glands. Sympathetic inhibits the motility & secretion and parasympathetic increases both. Extrinsic nervous system coordinate activity between different regions of GIT.

Role of Gastrointestinal Hormones Tucked within mucosa of certain regions of GIT are endocrine gland cells that releases hormone into blood on appropriate stimulation. These hormones acts on other areas of GIT & exert either stimulatory or inhibitory influences on smooth muscle and exocrine cells.

Summary of Pathways Controlling GIT activities

References Human physiology by Lauralee Sherwood, seventh edition Text book physiology by Guyton &Hall,11th edition Text book of physiology by Linda .s contanzo,third edition