Colon physiology

Bacterial digestion

Fibre \(\rightarrow H_2 + CH_4 + \text{short chain fatty acids}\)

Urea + amino acids \(\rightarrow NH_4\)

Bilirubin → urobilirogen

Cystine + methionine \(\rightarrow H_2S\)

Bile acid fermentation \(\rightarrow N_2 + H_2 + CO_2\)

Na+/water absorption

• Stimulated by aldosterone
• Cl moves paracellularly (secreted in small intestine)
• Na+ 2nd active transport
• Creates osmotic gradient for water 

 

• K+ can undergo secretion + absorption in the colon - usually net secretion
• Balance determined by plasma [K+], aldosterone + cAMP
• People with diarrhoea can become hypokalemic

 

Motility

Slow contractions squeeze contents backwards and forwards as a segment - haustration

A few times a day becomes organised into peristatic wave - mass movement 

Haustra disappear during mass movement then reform