Absorption in the Large Intestine
The large intestine, or colon, has several roles including water absorption and immunity. The chyme that enters the colon is already very concentrated as most of the water has already been absorbed. Thus, the large intestine is specialised to work in the environment this produces.
In this article, we will review the functions of the intestine and how they are achieved.
The large intestine is lined by mucosa with crypts of Lieberkühn containing glands and mucus-producing goblet cells. These protect the intestinal wall from the plethora of anaerobic bacteria in the colon and from the pressure exerted on the walls by the concentrated chyme (soon to become faeces).
The walls also contain gastrointestinal lymphoid tissue (GALT) that contributes to the body’s immune defences.
The colon helps to absorb a small volume of water from the lumen (400ml/day). As the chyme is very concentrated by the time it reaches here, the colon must work against a larger osmotic pressure gradient than in the rest of the GIT. In other words, it must move water against the gradient for osmosis.
The colon also helps to transport ions.
Water and Ion Absorption
In the large intestine, there is a net absorption of sodium ions and chloride ions are actively absorbed.
Sodium – this ion may be absorbed by various methods:
- Sodium-hydrogen antiporter on the luminal membrane
- Epithelial sodium channels
- Enhanced by absorption of short-chain fatty acids in the colon via specialised symporters
Chloride and bicarbonate – the movement of sodium into the plasma produces an electrochemical gradient to allow absorption of chloride. Chloride ions are exchanged for bicarbonate ions (causing net bicarbonate secretion).
Water – the absorption of these electrolytes creates an osmotic gradient to allow further absorption of water.
Potassium – absorption of water along the length of the bowel concentrates potassium in the lumen. This provides an electrochemical gradient for the movement of potassium into the plasma. In the colon potassium may be absorbed or secreted depending on the remaining concentration in the lumen and the electrochemical gradient created by the active absorption of sodium. Secretion usually occurs when the luminal concentration of potassium ions is below 25mM.
Vitamins and fats – short-chain fatty acids, crucial B vitamins (such as B6 and B12) and vitamin K (required for blood clotting) are produced by the digestion of chyme by the commensal microbial flora of the colon.
Regulation of Absorption
Absorption in the gastrointestinal tract is regulated by neuroendocrine mechanisms.
In the colon endocrine mechanisms used include:
- Aldosterone – increases the net absorption of water and electrolytes by stimulating the basolateral sodium-potassium ATP-ase. This increases the electrochemical gradient and driving force for sodium absorption. It also increases transcription of epithelial sodium channels.
- Glucocorticoids and somatostatin – act to increase water and electrolyte absorption by increasing the action of the basolateral sodium-potassium ATP-ase.
The intestines are innervated by the enteric nervous system whose:
- Parasympathetic innervation promotes net secretion from the intestines
- Sympathetic innervation promotes net absorption from the intestines.