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

2. 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

3. 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

4. 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

5. 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

6. 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)

7. 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

8. 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.

9. 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

10. 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)

11. 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

12. 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

13. 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

15. 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

16. Layers of Digestive Tract Wall

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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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).

25. Autonomous smooth muscle function

26. 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).

27. 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.

28. 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

29. 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.

30. 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.

31. Summary of Pathways Controlling GIT activities

32. 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