1. PHYSIOLOGY: Digestion, Absorption & Defecation

2. Reference
Guyton A., and Hall, J. Textbook of Medical Physiology. 12th ed.
Seeley, R., Stephens, T., and Tate, P., Anatomy & Physiology. 8th ed. McGraw Hill Company Inc., (2008)

3. Functions of the Digestive System
Motility Functions
Propulsion
Mechanical Digestion
Secretory Functions
Chemical Digestion
Regulation
Absorptive Functions

4. Six basic processes are involved: ingestion, secretion, propulsion, digestion, absorption and defecation.
Ingestion is taking food into the mouth.
Secretion is the act of expelling a liquid. The cells lining the GI tract secrete about 9 liters (9.5 quarts) of water, acid, buffers, and enzymes each day to lubricate the canal and aid in the process of digestion.

5. Propulsion consists of alternating contraction and relaxation of smooth muscle in the walls of the GI tract to squeeze food downwards.

6. Digestion has two parts, mechanical and chemical.
Mechanical digestion is chewing up the food and your stomach and smooth intestine churning the food
Chemical digestion is the work the enzymes do when breaking large carbohydrate, lipid, protein and nucleic acid molecules down into their subcomponents -these and others are the nutrients.

7. Absorption occurs in the digestive system when the nutrients move from the gastrointestinal tract to the blood or lymph.
Defecation the act or process by which solid or semisolid waste material (feces) from the digestive tract are eliminated via the anus.

8. Digestion in the Mouth Mechanical Digestion Mastication reflex
Food enters mouth
Relaxation of muscles of mastication
Stretch reflex causes contraction

9. Digestion in the Mouth Chemical Digestion by Saliva
Serous secretion for digestion of starches: amylase
Mucous secretion for lubrication

10. Swallowing
Oral
Pharyngeal
Esophageal

11. DEGLUTITION (SWALLOWING) accomplishes the propulsion of ingested food the mouth to the stomach.
Three stages:
Buccal
Pharyngeal
Esophageal

12. BUCCAL STAGE
tongue moves upward and backward to push the food toward the pharynx.
PHARYNGEAL STAGE
coordinated involuntary actions which direct food to the esophagus closing the airway passages

13. ESOPHAGEAL STAGE
peristaltic waves propel food from the esophagus to the stomach

14. NO Digestion in the Esophagus
No actual digestion
Propulsion via peristalsis
Mucous secretion for lubrication

15. Digestion in the Stomach
Food storage in the fundus
LES prevent reflux of stomach contents
Mechanical Digestion via segmental contractions
No absorption

17. Digestion in the Stomach
Chemical digestion by Oxyntic cell secretion
CELL
SECRETION
FUNCTION
Mucous Neck Cells
Mucus
Lubrication & protection
Peptic Chief cells
Pepsinogen
+ HCl to produce a proteolytic enzyme Pepsin for protein digestion
Parietal Cells
HCl
Intrinsic Factor
B12 Absorption

18. Chemical Digestion by Pancreas
Acini cells secrete pancreatic digestive enzymes into the hepatopancreatic duct to empty into the small intestine
Enzymes are not activated until they are mixed with acidic chyme
Enzymes secreted by the pancreas can digest the organ itself, that is why the pancreas secrete the inactive form.

19. Chemical Digestion by the Pancreas
Trypsin
Split proteins into peptides
Chymotrypsin
Carboxypolypeptidase
Split proteins into AA
Pancreatic Amylase
Carbohydrate breakdown
Pancreatic Lipase
Fat digestion
Cholesterolesterase
Phospholipase

20. Biliary Secretions by the Liver & Gall Bladder
For fat digestion & absorption
Emulsify large particles for more efficient lipase action
Ease of absorption
Means of excretion of waste
bilirubin

21. Biliary Secretions by the Liver & Gall Bladder
Liver constantly produces bile and is concentrated in the gall bladder
Biliary secretions include HCO3- to neutralize the acidic chyme

23. Digestion & Absorption in the Small Intestine
Dominating chemical digestion via
Pancreatic secretions
Biliary secretions
Intestinal secretions

24. Digestion & Absorption in the Small Intestine
Mucus
Protection & Lubrication
Peptidase
Split peptides into individual amino acids
Maltase
Split maltose  glucose + glucose
Lactase
Split lactose  glucose + galactose
Sucrase
Split sucrose  fructose + glucose
Lipase
Split fats into  glycerol + free fatty acids

26. Digestion & Absorption in the Small Intestine
Absorption via transport processes
Water: diffusion through tight junctions
Carbohydrates: Na-Glucose cotransport; fructose via facilitated diffusion

27. Digestion & Absorption in the Small Intestine
Absorption via transport processes
Lipids: diffusion with bile salts
Proteins: Na-Glucose cotransport or endocytosis

28. Digestion & Absorption in the Small Intestine
Absorption via transport processes
Na: diffusion down a electrical gradient
Cl: diffusion via solvent drag
HCO3-: indirectly by secretion of H+
*Other ions & vitamins are also absorbed

30. Absorption in the Large Intestine
Absorption of electrolytes occur but backflow is prevented by tight junctions
Water follows as result of osmotic gradient
Secretion of mucus for lubrication

31. Mouth -chewing & lubrication (saliva)
Esophagus -conduit
Stomach -digestion takes place
-secretes pepsin + HCl
Small intestine -duodenum
jejunum
 ileum (functional reserve)
Large intestine -stores and concentrates undigested material
- colon: 3 limbs -ascending
transverse
descending

32. Rectum ampulla
temporary storage for waste.
As the rectal walls expand, stretch receptors from the nervous system located in the rectal walls stimulate the desire to defecate.

34. Colon innervation: Intrinsic
Auerbach plexus- segmentation and peristalsis
Plexus of Meissner- secretions and sensation
Extrinsic
Parasympathetic increases and relaxes sphincters
Sympathetic decreases motility and contracts sphincters

35. The muscular wall of the cecum and colon is innervated by: Vagus nerve- cranial nerve X in PNS Superior mesenteric plexus – SNS cord segment T9-L2 The rectum and anal canal innervation: Pelvic splanchnic nerves- parasympathetic S2-4 Hypogastric nerve- sympathetic T11-L2

36. Anal sphincters innervation: Internal anal sphincter- pelvic splanchnic nerves and hypogastric nerves External anal sphincter – pudendal nerve

37. Defecation Reflex Integrated in the sacral region of the spinal cord
Stimulated by distention of the rectal wall
Weak contractions of rectal wall
Relaxation of anal sphincter

38. Defecation Reflex
May be inhibited by voluntary constriction of external anal sphincter

39. If the urge is not acted upon, the material in the rectum is often returned to the colon where more water is absorbed.
If defecation is delayed for a prolonged period the fecal matter may harden, resulting in constipation.

40. During defecation the chest muscles, diaphragm, abdominal wall muscles, and pelvic diaphragm all exert pressure on the digestive tract and ventilation temporarily ceases as the lungs push the chest diaphragm down in order to exert pressure.
Blood pressure rises.
Not good for high- risk cardiac patients.

41. For defecation external anal sphincter must relax.
Sphincter ani externus muscle for anal and urethral are both closely linked by the same nerves

42. Fecal incontinence is the inability to control one's bowels.
When one feels the urge to have a bowel movement, they may not be able to hold it until they can get to a toilet, or stool may leak from the rectum unexpectedly.

43. Fecal incontinence
may be caused by physical injury (such as damage to the anal sphincter that may result from an episiotomy)
intense fright, inflammatory bowel disease, impaired water absorption (diarrhea), and psychological or neurological factors.

45. BEFORE DEFECATION
Fecal material passes to rectum by the longitudinal muscular contraction. When there is distension in the sigmoid colon and rectum, the internal sphincter relaxes at the same time as the external sphincter contracts. This allows some of the fecal contents to enter the anal canal and be sampled by its sensitive epithelium to determine if its solid, liquid or gas.

46. If the individual decides to continue with defecation, the intrarectal pressure increases on straining. Added pressure from abdominal straining or Valsalva maneuver aids defecation. Straining requires intact innervation of the lower thoracic cord T6-T12. The pressure should be sufficient to overcome the external sphincter which relaxes with the pelvic floor that causing the rectum to straighten up reducing the rectal angle.

47. This increase in intraabdominal pressure forces stool into the rectum with relaxation of the extenal anal sphincter allowing defecation.
Fecal matter stimulate the anal canal further relaxing the external anal sphincter.

48. If the individual decides not to continue with defecation, the rectum relaxes further to allow accommodation of the contents which further stimulates the external sphincter to contract.

50. Upper motor neuron lesion- damage above the defecation reflex center in the sacral cord
Results to:
Abnormal storage of feces
Inability to recognize urge and distinguish contents
External sphincter dyssynergia

51. Lower motor neuron lesion – damage within the reflex defecation center
Results to:
Fecal retention
Oozing of stool through the flaccid sphincter
Sensory and motor pathways are disrupted so the patient is unaware of urge to defecate and unable to exercise voluntary external anal sphincter

52. Assessment is through the bulbocavernosus reflex and the anal relfex

53. The bulbocavernosus reflex is a palpable or visible contraction of the anal sphincter when pressure is applied to the glans penis or clitoris. When contraction is present, (+) result. This indicates that the reflex activity of the sacral cord is intact and therefore the SCI is an UMN lesion. Should be tested soon after SCI, before the spinal shock passes.

54. The anal reflex is a visible contraction of the anal sphincter in response to a pinprick. A positive response indicates an UMN lesion.

56. Thank You for Listening!