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

Motility and regulation + digestion

The colon's main functions are:

• Absorption of water and electrolytes

• Formation and storage of faeces (only ~100 mL fluid remains in final stool)

Colon parts:

• Absorbing colon (proximal): water/electrolyte absorption

• Storage colon (distal): faeces storage

Mechanical digestion

Motility of the large intestine includes:

Types of movements:

• Receptive relaxation – ascending colon and rectum

• Segmentation – mixes contents, aids absorption

• Haustral contractions – slow, persistent propulsion, mainly in caecum & ascending colon

• Peristalsis – coordinated movement

• Mass peristalsis – infrequent, forceful, moves contents to rectum

Regulation:

• Controlled by enteric nervous system, parasympathetic stimulation (pelvic nerves) increases motility

• Hormones (e.g., gastrin) can also enhance mass movements after meals (gastrocolic reflex)

digestion

• Minimal enzymatic digestion occurs in the colon.

• Bacterial fermentation breaks down undigested carbohydrates, fiber, and proteins → producing gases (H₂, CO₂, CH₄) and short-chain fatty acids (e.g. butyrate, acetate) which can be absorbed and used for energy.

absoerption

Mainly water is absobed

Absorption

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

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

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.

1. Water Absorption

• Passive

• Follows osmotic gradient created by active transport of electrolytes (especially Na⁺)

• Up to 5–8 liters/day of water is absorbed

2. Electrolyte Absorption

• Sodium (Na⁺): Active transport via Na⁺ channels and Na⁺/K⁺ ATPase

secretion

Secretion

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

Stages Defecation

1. Filling of rectum

2. Stretching of the wall

3. Activating stretch receptors

4. Impulses to spinal cord

5. Parasympathetic impulses return to contract colon muscles, rectum and anus + relax sphincter

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

7. Voluntary relaxation of external sphincter