The Large Bowel and Elimination of Faeces

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 colon3. List the 4 layers of the large bowel wall4. State the function of the mucous secreted in the large bowel5. 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 reflex8. Outline the function of gut flora 9. Outline the process of defecation10. Discuss the sources and functions of fibre in the diet11. Define the terms constipation and diarrhoea

Label the diagram (Objective 1)

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

Layers of the large bowel wall (Objective 3)

Mucosa

Submucosa

Muscularis

Serosa

Lumen

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).

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)

Layers of the large bowel wall (Objective 3)

Longitudinal muscles shorter than length of intestine sacculated appearance, forms haustra or small sacs

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

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

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

Absorption of water and electrolytes (Objective 5)

• H2O is absorbed in large amounts in the colon

• Approx. 1500 ml 2000ml of chyme enters caecum / day 90% of fluid reabsorbed

• 100 – 200 ml of fluid • eliminated in faeces daily

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+

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)

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

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

How the large bowel moves faeces through the tract (Gut motility: Objective 6)

Mass movements: 3 - 4 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).

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

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

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

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

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

Gut flora (Objective 8)

Normal gut flora reduce likelihood of invasion by pathogenic organisms

Faeces and stools

Faeces consists of:

• Unabsorbed food residue

• Sloughed off epithelial cells

• Mucous

• Digestive secretions

• Water

• Microorganisms (flatus is produced by bacterial action).

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.

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

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

Main Functions of Fibre (Objective 10)

1. 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

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

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

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