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 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.
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Digestive Tract:
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Accessory Digestive Organs:
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Mouth
Pharynx
Esophagus
Stomach
Small intestine
- Duodenum
- Jejunum
- Ileum
Large intestine
- Cecum
- Appendix
- Colon
- Rectum
Anus
 Salivary glands
 Exocrine pancreas
 Biliary system
- Liver
- Gall bladder
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 Digestive tract is 4.5 m (15 feet) in normal contractile state.
 Lumen is continuous from mouth to anus and hence is continuous with external environment.
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Primary Function:
 Transfer nutrients, water, and electrolytes from ingested food into body’s internal environment.
 Food is ingested – digested – absorbed – distributed and used.
The Digestive System Performs Four Functions:
1- Motility
2- Secretion
3- Digestion
4- Absorption
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 Muscular contractions that m ix and m ove forward the contents of the digestive tract.
 Two Types of Digestive Motility:
 Propulsive (peristaltic) movements
 Mixing (segmenting) movements
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1- Propulsive (= Peristaltic) Movements:
 P ush contents forward through the digestive tract.
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Velocity with which contents are moved forward (rate of P ropulsion) varies in different regions of GIT, depending on functions of that region. For example:
* Rapid movements in esophagus.
* S low movements in S mall intestine.
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Movements of contents through most of digestive tract is accomplished by contraction of smooth muscles (involuntary component) except:
1- Mouth (chewing).
2- Early part of esophagus (swallowing).
3- External anal sphincter (defecation).
 In these regions, motility involves skeletal muscle
(voluntary component).
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Serve Two Functions:
1Mixing food with digestive juices & hence promotes digestion of foods.
2- Facilitates absorption by exposing all parts of intestinal contents to absorbing surfaces of digestive tract.
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Segmentation
Gut law: Distension of gut leads to peristaltic wave starting at the point of distension & proceeds anal wards.
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 Digestive juices are secreted in to GIT lumen by exocrine glands (through ducts).
 Digestive secretions consists of:
1- Water.
2- Electrolytes.
3- Specific organic constituents (enzymes, bile salts, or mucus) important in digestive process.
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 Secretions are released into the digestive tract lumen on appropriate neural or hormonal stimulation.
 Normally absorbed in one form or another into blood after their participation in digestion.
 Total quantity of fluid that is secreted from digestive glands into GIT lumen equals 7 liters / day.
 Failure of absorption of digestive juices, as in diarrhea & vomiting results in loss of fluid (dehydration).
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3- Digestion:
 Biochemical breakdown of structurally complex foodstuffs into smaller, absorbable units by enzymes produced within GIT.
 Complex food stuffs: i). Carbohydrate ii). Proteins iii). Fats
These are large molecules, therefore, they are digested and then absorbed into the blood or lymph.
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i). CARBOHYDRATES
 Carbohydrates are absorbed as monosaccharides e.g.
glucose, fructose, and galactose.
 Cellulose is a polysaccharide found in plants. It can not be digested, therefore, works as bulk or ingestible fibers.
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 ii). PROTEIN
Proteins are absorbed as
- Amino acids.
- Small polypeptides.
iii). FAT
Fat are absorbed as
- Monoglyceride [glycerol with one fatty acid].
- Free fatty acid.
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4- Absorption:
 In the small intestine , digestion is completed & most absorption occurs.
The small intestine, with its epithelial folds, villi, and microvilli, has an internal surface area of 200 m 2 .
 Through process of digestion, small absorbable units resulting from digestion, along with water, vitamins, and electrolytes are transferred from digestive tract lumen into blood or lymph.
 Total quantity of fluid that must be absorbed: 2 L (ingested)
+ 7 L (secreted) = 9 liters / day.
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 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.
 Musculosa (Muscularis externa).
 Serosa ( Outer layer).
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Layers of Digestive Tract Wall
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MUCOSA
 Lines luminal surface of digestive tract.
 Highly folded surface greatly increases absorptive area.
 Of Three layers:
1- Mucous membrane
2- Lamina propria
3- Muscularis mucosa
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MUCOSA
 Inner epithelial layer serves as a protective surface.
 Modified in particular areas for secretion and absorption.
Contains:
 E xocrine gland cells secrete digestive juices.
 E ndocrine gland cells secrete blood-borne gastrointestinal hormones.
 E pithelial cells specialized for absorbing digestive nutrients.
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MUCOSA
2- Lamina Propria:
 Middle layer of connective tissue on which epithelium rests.
 Houses gut-associated lymphoid tissue (GALT)
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Important in defense against disease-causing intestinal bacteria.
3- Muscularis Mucosa:
 S parse layer of smooth muscle, contraction modifies the pattern of surface folding.
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 Thick layer of connective tissue.
 Provides digestive tract with distensibility and elasticity.
 Contains larger blood and lymph vessels.
 Contains S ubmucosal nerve plexus or
Meissners plexus that regulates GIT
S ecretion.
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 Major smooth muscle coat of digestive tube. In most areas, it consists of two layers:
1- Circular layer (Inner layer):
 Contraction decreases diameter of lumen.
2- Longitudinal layer (Outer layer):
 Contraction shortens the tube.
 Together, contractile activity of these layers produces propulsive and mixing movements.
 M yenteric nerve plexus lies between the two muscle layers that controls M ovements of GIT.
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 Outer connective tissue covering of GIT.
 Secretes serous fluid (watery & slippery fluid) that
Lubricates and prevents friction between digestive organs and surrounding viscera.
 Continuous with mesentery throughout much of the tract. This Attachment provides relative fixation and supports digestive organs in proper place while still allowing them freedom for mixing and propulsive movements.
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 Digestive motility and secretion are carefully regulated to optimize the digestion.
 Four factors are involved in regulating digestive system function:
1- Autonomous smooth muscle function.
2- Intrinsic local nerve plexuses.
3- Extrinsic autonomic nerves.
4- Gastrointestinal hormones.
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 In the wall of GIT, some specialized smooth muscle cells are pacemakers cells known as interstitial cells of Cajal.
 These cells lie in between circular & longitudinal layer of smooth muscles.
 These are self-excitable cells that display rhythmic spontaneous variations in membrane potential known as slow wave potential or basic electrical rhythm (BER).
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Autonomous Smooth Muscle Function
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 If slow wave potential (slow depolarization) reaches threshold, action potentials are triggered resulting in rhythmic cycles of contraction .
 Reaching threshold depends on mechanical, neural and hormonal factors that influence starting point of slow wave potential (e.g. presence of food bolus in
GIT).
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 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, the higher the cytosolic calcium, the stronger the contraction.
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 Submucosal plexus and myentric plexus, together are often termed as
Enteric nervous system.
 Primarily coordinate local activity in GIT.
 Intrinsic nerve plexuses can affect all functions of digestive tract, i.e. motility, secretion of digestive juices and gastrointestinal hormones.
 Intrinsic nerve plexuses activity can be influenced by endocrine, paracrine and nerve signals.
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 Both branches of ANS influence GIT motility
& secretion either by:
 Modifying activity of intrinsic nerve plexuses.
 Altering level of GIT hormones secretion.
 Directly acting on smooth muscle and glands.
 Sympathetic inhibits the motility and secretion and parasympathetic increases both.
 Extrinsic nervous system coordinates activity between different regions of GIT.
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Parasympathetic
(Dominant)
Sympathetic
* Vagus supplies from the esophagus till 1st half of large intestine.
* Greater splanchnic nerve: from LHCs of lower 8 Thoracic segments.
* Sacral (S2, 3, 4) segments supply the rest of GIT till anal region.
* Lesser splanchnic nerve: from LHCs of upper 2 Lumbar segments.
1.
Contraction of GIT wall &
Relaxation of sphincters.
1.
Relaxation of GIT wall &
Contraction of sphincters.
2.
Increase GIT secretions.
3.
VD.
2.
3.
Decrease GIT secretions.
VC.
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 Endocrine gland cells are tucked within mucosa of certain regions of GIT that release hormones into blood on appropriate stimulation.
 These hormones acts on other areas of GIT and exert either stimulatory or inhibitory influences on smooth muscle and exocrine cells.
 E.g. CCK, Secretin, Gastrin ……… ……etc
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Pathways Controlling GIT activities
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GIT RECEPTORS & REFLEXES
(i) Chemoreceptors: sensitive to chemical changes in lumen.
(ii) Mechanoreceptors: sensitive to stretch on the wall.
(iii) Osmoreceptors: sensitive to osmolarity of luminal contents.
 Stimulation of these receptors causes neural reflexes or secretion of hormones which effect motility and secretion of digestive juices.
 In GIT, two types of reflexes occur:
1. Short reflexes: local enteric reflex in wall of digestive tract.
2. Long reflexes: between CNS and Digestive system.
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 Human Physiology, Lauralee Sherwood, seventh edition.
 Text book Physiology by Guyton &Hall,11 th edition.
 Text book of Physiology by Linda S. Contanzo, third edition.
 Physiology by Berne and Levy, sixth edition.
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