50 - Motility of the colon and its regulation; secretion, digestion and absorption. Defecation.

sections

motility and regulation

parts of colon

mechanical digestion and motility of large intestine (colon)

absorption

secretion

stages of defecation

function

• absorption of water and electrolytes

• forming solid faeces

• leaving only about 100ml of fluid to be excreted in faeces.

• In all sections, there is no need for strong movements.

parts of colon

• absorbing colon- this is the proximal part for absorption

• storage colon- used for storing the faeces

mechanical digestion and motility of large intestine (colon)

• Motility of the large intestine includes:

  • Receptive relaxation (ascending colon and rectum)

  • Segmentation - Peristalsis

  • Mass peristalsis (lower propulsion)

  • Defection

• The muscles of the colon contract forming segmented motion

• Haustral contractions are Slow and persistent ,are mainly responsible for the propulsion/ mass movement in the caecum and ascending colon.

absorption

• Mainly water is absobed

• Chyme turns into faeces after remaining in the colon for 3-10 hrs, during this time it turns solid or semi solid

• Most of the water (5-8L/day) and electrolytes (Na+ ,Cl- ) in Chyme are absorbed in the colon

secretion

• Faeces is made of ¾ water and ¼ solid matter From the solid matter- 30% = dead bacteria the rest is like fat, protein, undigested food, dried digestive juices

  • The brown colour of faeces is caused by stercobilin and urobilin, derivatives of bilirubin.

• Mucosa of the large intestine secretes bicarbonate ions while it simultaneously absorbs an equal number of chloride ions. The bicarbonate helps neutralize the acidic end products of bacterial action in the large intestine.

• Defecation

  • Starts involuntarily- ends voluntarily or in the case of diarrhoea- involuntarily

    • When a mass movement forces faeces into the rectum, the desire for defecation occurs immediately

    • This is inhibited by constriction of the internal and external sphincters.

    • Subconsciously, the external sphincter is usually kept continuously constricted unless conscious signals inhibit the constriction.

stages of defecation

1. Filling of rectum

2. Stretching of the wall

3. Activating stretch receptors

4. Impulses to spinal cord

5. Parasympathetic impulses return to lower colon muscles, rectum and anus

6. Internal sphincter opens due to pressure by involuntary contraction of longitudinal rectal muscles

7. Voluntary relaxation of external sphincter