OVERVIEW OF DIGESTIVE SYSTEM

1. OVERVIEW OF DIGESTIVE SYSTEM Lecture 1 Dr. Zahoor Ali Shaikh

2. DIGESTIVE SYSTEM • The primary function of the Digestive System is to transfer nutrients, water and electrolytes from the food into the body. • Ingested food is source of energy from which cell generate ATP to carry out energy dependent activities e.g. active transport, contraction, secretion.

3. DIGESTIVE SYSTEM • Food we take is digested or broken down into small molecules that can be absorbed from the GIT [Gastro Intestinal Tract] into circulatory system, for distribution to the cells. • Normally 95% of ingested food is available for use by the body. • Food is ingested – digested – absorbed – distributed and used.

4. DIGESTIVE SYSTEM • We will have overview of Digestive System, we will discuss: • Basic Digestive Process • Components of Digestive System • Regulation of Digestive System

5. 1. Basic Digestive Processes • Digestive System performs four basic processes: • Motility • Secretion • Digestion • Absorption We will discuss one by one.

6. Basic Digestive Processes 1. ‘Motility’ • Motility means movements, muscle contraction that mix and move the food forward. • Smooth muscle in GIT has - Tone [constant level of contraction] – tone is important to maintain study pressure on the contents present in the GIT. - Action Potential – which induces contraction

7. 1. ‘Motility’ • There are two types of muscle contraction (i). Propulsive movements (ii). Mixing movements • (i). Propulsive movement • They propel or push or move forward the contents. Rate of propulsion varies in GIT. • Why? Because it depends on the function of the region. E.g. movement of food is fast in esophagus, and slow in small intestine.

8. 1. ‘Motility’ • (ii). Mixing movement • It has two functions: - mixing the food with digestive juices to help in digestion of food - help in absorption of food due to exposing the contents of food to the absorbing surface of GIT.

9. 1. ‘Motility’ • In GIT, where smooth muscle is present, it is functioning by Involuntary mechanism. • We have voluntary control in GIT only on chewing, swallowing, and defecation. Why? Because in these areas skeletal muscles are involved which are under voluntary control.

10. Basic Digestive Processes 2. ‘Secretion’ • By exocrine glands (through ducts). • Digestive secretion consist of water, electrolytes, enzymes, mucus, or bile salts. • Secretion of digestive juices is active process (require energy).

11. Basic Digestive Processes 3. ‘Digestion’ • We take three type of food: i). Carbohydrate ii). Proteins iii). Fats • These are large molecules, therefore, they are digested and then absorbed into the blood or lymph. • Word ‘Digestion’ means breakdown of complex food stuff into smaller, absorbable units by enzymes.

12. Basic Digestive Processes 3. ‘Digestion’ i). CARBOHYDRATES • Carbohydrates are absorbed as monosaccharide e.g. glucose, fructose, and galactose. • Remember • Cellulose [polysaccharide found in plants] can not be digested, therefore, works as bulk or ingestible fiber.

13. Basic Digestive Processes 3. ‘Digestion’ ii). PROTEIN • Protein are absorbed as - Amino acids - Small polypeptides iii). FAT • Fat are absorbed as - Monoglyceride [glycerol with one fatty acid] - Free fatty acid

14. Basic Digestive Processes 4. ‘Absorption’ • In small intestine, digestion is completed and most absorption occurs. • Water, vitamins and electrolytes are also absorbed.

15. GIT ANATOMY

16. GIT FUNCTIONS

17. 2. DIGESTIVE TRACT & ACCESSORY DIGESTIVE ORGANS • Digestive Tract • It is a tube about 4.5 meter [15 feet] in length. • It includes mouth, pharynx [throat], esophagus, stomach, small intestine [duodenum, jejunum and ileum], large intestine [cecum, appendix, ascending, transverse, descending colon, rectum] and anus. • NOTE – they are continuous but there are regional modifications which allow them to specialize in digestive activity.

18. 2. DIGESTIVE TRACT & ACCESSORY DIGESTIVE ORGANS • Digestive Tract • It is continuous from mouth to the anus, the lumen of this tube is continuous with external environment. • Contents in the lumen are technically outside the body. • Only after the substance is absorbed from the lumen then it becomes the part of body. E.g. pH of stomach content is 2 due to presence of HCL, but body fluid pH compatible with life is 6.8 – 8.

19. DIGESTIVE TRACT & ACCESSORY DIGESTIVE ORGANS • ACCESSORY DIGESTIVE ORGANS - Salivary glands - Pancreas [Exocrine] - Biliary system – liver, gallbladder • Exocrine gland means organs lie outside the digestive tract and empty their secretion through ducts into the digestive tract lumen.

20. DIGESTIVE TRACT WALL • Digestive Tract Wall has four layers: 1. Mucosa 2. Sub-mucosa 3. Muscular layer 4. Serosa

23. DIGESTIVE TRACT WALL • 1. Mucosa • It lines the luminal surface. • It is divided into three layers: - Mucus membrane - Lamina propria - Muscularis mucosa • Mucus membrane – Inner epithelial layer, protective and specialized for absorption. It has exocrine gland cells for secretion of digestive juices. It has also endocrine gland cell for secretion of GIT hormones.

24. DIGESTIVE TRACT WALL • 1. Mucosa [cont] • Lamina propria – Middle layer of connective tissue. It has Gut associated lymphoid tissue [GALT] important in defense against intestinal bacteria. • Muscularis mucosa – Smooth muscle. • Mucosal surface is highly folded which increases the surface area for absorption.

25. DIGESTIVE TRACT WALL • 2. Sub-mucosa [under the mucosa] • It is thick layer of connective tissue which provides distensibilityand elasticity. • It has large blood vessels and lymph. • It has nerves – known as SUB-MUCOSAL PLEXUS or MEISSNERS PLEXUS.

26. DIGESTIVE TRACT WALL • 3. Muscular layer • Smooth muscle coat of digestive tube which surrounds sub-mucosa. • It has two layers: - Inner circular muscle layer - Outer longitudinal muscle layer • Contraction of inner circular layer decreases the diameter of lumen at the point of contraction. • Contraction of outer longitudinal layer shortens the tube.

27. DIGESTIVE TRACT WALL • 3. Muscular layer [cont] • Contraction of both layers [circular and longitudinal] causes propulsive and mixing movements. • Another nerve network called ‘Myentricplexus’ lies between the circular and longitudinal muscle layer. • IMPORTANT – Gut Local activity is regulated by sub-mucosal [meissners plexus] and Myentric plexus and also by hormones.

28. DIGESTIVE TRACT WALL • 4. Serosa • It is outer connective tissue covering of digestive tract. • It secretes watery, slippery fluid that lubricates and prevents friction between digestive organs and surrounding viscera. • Serosa is continuous with the mesentery, which is attached to inner wall of abdominal cavity.

29. 3. Regulation of Digestive System • Four factors regulate the digestive functions: • 1. Smooth muscle [autonomous] function • 2. Intrinsic nerve plexus [sub-mucosal and myentric] • 3. Extrinsic nerves • 4. GIT hormones

30. 3. Regulation of Digestive System • 1. Smooth muscle [autonomous] function • Some smooth muscle cells are specialized and work as pace-maker cells called interstitial cells of CAJAL [like cardiac muscle cell]. • WHY? Because they have automatic changes in membrane potential. They are non-contractile cells. They lie between circular and longitudinal smooth muscle layers.

31. 3. Regulation of Digestive System • 1. Smooth muscle [autonomous] function • Self-induced electrical activity in smooth muscle is slow wave potential or Basic Electrical Rhythm [BER]. • Slow wave are below threshold level, if they reach threshold level, action potential will occur and lead to muscle contraction.

32. 3. Regulation of Digestive System • 2. Intrinsic nerve plexus [sub-mucosal and myentric] • They run throughout the GIT and have about 100 million neurons and do the self-regulation of intestine.They are called ‘ENTERIC NERVOUS SYSTEM’. • ENTERIC NERVOUS SYSTEM – it has sensory neurons which respond to local stimuli. It has motor neuron which supply smooth muscle, exocrine and endocrine cells, therefore, affect the motility, secretion, and GIT hormones.

33. 3. Regulation of Digestive System • 3. Extrinsic nerves • These are ANS [Autonomic Nervous System]. • It originates outside the digestive tract and supplies the digestive organs. • ANS – sympathetic and para-sympathetic ANS affects motility, secretion by acting directly on smooth muscle or affecting the intrinsic plexus [sub-mucosal and myentric]. • Para-sympathetic nerve supply to GIT arise via VAGUS NERVE. It increases motility and secretion.

34. 3. Regulation of Digestive System • 4. GIT hormones • In Mucosa of some regions of Digestive tract are located endocrine gland cells. • GIT hormones are carried by blood to other areas of GIT and cause excitation or inhibition of smooth muscle of GIT.

36. RECEPTORS PRESENT IN GIT • Three types of sensory receptors are present which respond to local changes. • (i). Chemoreceptors – sensitive to chemical changes in lumen. • (ii). Mechano receptors [pressure receptors] – sensitive to stretch on the wall. • (iii). Osmo receptors – sensitive to Osmolarity of luminal contents.

37. RECEPTORS PRESENT IN GIT • Stimulation of these receptors causes - Neural reflexes or Secretion of hormones which effect motility, secretion of digestive juices. • In GIT, two type of reflexes occur: 1. Short reflexes – local reflex in wall of digestive tract 2.Long reflexes – reflex between CNS and Digestive system

38. Thank you