1. The Large Bowel and Elimination of Faeces

2. Objectives (1st Year)
1. Label a diagram of the large intestine (to indicate the appendix, the caecum, ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, sigmoid colon, rectum and anus)
2. State the main functions of the colon
3. List the 4 layers of the large bowel wall
4. State the function of the mucous secreted in the large bowel
5. Explain how the large bowel absorbs water and electrolytes
6. Explain how the large bowel moves faeces through the tract (gut motility)
7. Briefly explain the Gastro colic reflex
8. Outline the function of gut flora
9. Outline the process of defecation
10. Discuss the sources and functions of fibre in the diet
11. Define the terms constipation and diarrhoea

3. Label the diagram (Objective 1)

4. Main functions of the colon (Objective 2)
Absorption of fluid and electrolytes
Storage of formed faeces
Incubation of bacteria - synthesis (and absorption) of vitamins B and K
Propulsion of faeces towards the anus
Addition of lubricant mucous to the faecal mass

5. Layers of the large bowel wall (Objective 3)
Mucosa
Submucosa
Muscularis
Serosa
Lumen

6. Layers of the large bowel wall (Objective 3)
Mucosa
No villi, mucosa appears flat
Simple columnar epithelium + mucous secreting goblet cells
Submucosa
Loose connective tissue containing blood vessels, lymphatics and nerves (involuntary).

7. Layers of the large bowel wall (Objective 3)
Muscularis
Smooth muscle which responds to stimulation by the ANS and certain Hormones
Exhibits continuous rhythmic and inherent Contractions (modulated by the above stimuli)

8. Layers of the large bowel wall (Objective 3)
Longitudinal muscles shorter than length of intestine  sacculated appearance, forms haustra or small sacs

9. Layers of the large bowel wall (Objective 3)
Muscularis
Between the 2 layers of muscle fibres is a network of nerve fibres called the myenteric plexus
Internal (Involuntary) sphincter at end of rectum
External (Voluntary) sphincter in the anus

10. Layers of the large bowel wall (Objective 3)
Serosa
Serous membrane called the peritoneum (function is support and protection)
The blood supply to the bowel is via mesenteric artery which is a branch of the abdominal aorta.
Important veins are mesenteric veins and hepatic portal vein

11. Function of the mucous (Objective 4)
Viscous mucous, secreted by
goblet cells lubricates the
colon, protects the mucosa
and helps faecal matter
stick together
The alkaline mucous neutralizes the acid formed by bacterial action
Irritation of colon wall  increased mucous secretion

12. Absorption of water and electrolytes (Objective 5)
H2O is absorbed in large amounts in the colon
Approx ml  2000ml of chyme enters caecum / day  90% of fluid reabsorbed 
100 – 200 ml of fluid
eliminated in faeces daily

13. Absorption of water and electrolytes (Objective 5)
Na, Cl, HCO3, glucose and some drugs are absorbed in small amounts
K is secreted into the lumen of the colon. K+ K+ K+ K+ K+

14. Absorption of water and electrolytes (Objective 5)
Epithelial cells of the intestinal mucosa contain Na+ / K+ pumps  active transport of ions
(passive absorption of water due to osmotic gradient created)

15. Absorption of water and electrolytes (Objective 5)
By absorbing water and other soluble compounds, the large bowel:
Maintains fluid balance
Solidifies faeces
Absorbs vitamins and electrolytes
Sends toxic substances to the liver to be detoxified

16. How the large bowel moves faeces through the tract (Gut motility: Objective 6)
Actions of muscularis (muscle layer of the large bowel):
Segmentation (Covered in lecture on small bowel)
Haustral churning: Contents moved from haustrum to haustrum by muscular contractions
Peristalsis: Contents moved along the length of the colon (particularly ascending) by contractions of circular and longitudinal muscles

17. How the large bowel moves faeces through the tract (Gut motility: Objective 6)
Mass movements: times daily transverse and descending colon undergoes several strong peristaltic contractions.
Contents are forced into the sigmoid colon and rectum by strong peristaltic waves.
These are common after meals, particularly breakfast (presence of food in the stomach).

18. Regulation of Intestinal function (Enteric Nervous System)
The movement of contents through the GI tract is controlled by neurones that innervate both the circular and longitudinal smooth muscle layers of the gut
The mass of faeces in the colon acts as a stimulus by stretching the colon wall  integration and efficient functioning of muscle activity

19. Regulation of Intestinal function (Enteric Nervous System)
Generally,
Parasympathetic nervous system increases bowel motility, increases secretions and brings about relaxation of gut sphincters.
Sympathetic nervous system will reduce blood flow to the gut, reduce secretions, reduce motility and bring about contraction of gut sphincters

20. Gastro colic reflex (Objective 7)
Gastric stimulation  activates Gastro colic reflex: food entering the stomach will stimulate peristaltic movements in the large intestine and movement of faeces into the rectum

21. Gut flora (Objective 8)
Bacterial activity breaks down (ferments) undigested carbohydrates etc into products that can be expelled in the faeces or absorbed and detoxified by the liver

22. Gut flora (Objective 8)
Fermentation produces fatty acids that are used for energy by the epithelial cells of the colon.
This energy is used to assist absorption of Na+ etc in the colon.
Certain B and K vitamins synthesized and absorbed

23. Normal gut flora reduce likelihood of invasion by pathogenic organisms
Gut flora (Objective 8)
Normal gut flora reduce likelihood of invasion by pathogenic organisms

24. Faeces and stools Faeces consists of: Unabsorbed food residue
Sloughed off epithelial cells
Mucous
Digestive secretions
Water
Microorganisms (flatus is produced by bacterial action).

25. Stools may be
Loose, watery and runny  Mushy, flattened surface, definite flow  Mushy, heaped surface  Collapsed, remnants of the original shape visible  Snake-like, coiled or cylindrical shape with a smooth surface  Cylindrical with superficial cracks  Cylindrical with deep cracks  Fragmented, segments, pellet-like, sheep droppings, button-like discs.

26. The process of defecation (Objective 9)
This is the expulsion of faeces from the rectum and has both involuntary and voluntary elements
Mechanism
Faeces enters the rectum 
Distension and pressure 
Sensory impulses 
Spinal reflex 
Relaxation of internal sphincter
and contraction of rectal muscle 
Faeces moves into anal canal
External anal sphincter is under voluntary control
and must relax for evacuation of the rectum

27. The process of defecation (Objective 9)
Pressure within the abdomen can be raised by:
a) holding breath / contracting diaphragm
b) contracting muscles of abdominal wall

28. Main Functions of Fibre (Objective 10)
Increases bulk and softness of stools
2. Increased bulk  increased peristalsis
3. Increased peristalsis  decreased transit time
4. Increased satiety
5. Decreased absorption of carbohydrates

29. Main Functions of Fibre (Objective 10)
Fibre helps to prevent: constipation, haemorrhoids, diverticular disease, cancer of the large bowel, diabetes, obesity
Good sources of fibre: fruit and vegetables, brown bread, brown rice, lentils, dried fruit etc

30. Altered patterns of elimination (Objective 11): DIARRHOEA
Excessive, frequent passage of stools.
Stools may be loose  liquid.
Rapid movements of the intestine result in decreased H2O absorption.
May be acute or chronic.
? Manifestations (identified via assessment)
? Complications (also identified via assessment)
? Causes
? Nursing interventions

31. Altered patterns of elimination (Objective 11): CONSTIPATION
Infrequent passage of stools.
Stools are hard and difficult to pass.
Increased reabsorption of water occurs due to the stool being in the intestine too long.
May be acute or chronic.
? Manifestations (identified via assessment)
? Complications (also identified via assessment)
? Causes
? Nursing interventions