Absorption in the Large Intestine

Functions of the Colon

The large intestine is lined by mucosa with Crypts of Lieberkühn, which contain glands and the 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.

By OpenStax CNX [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)], via Wikimedia Commons

Fig 1
Main functions of the large intestine

Water and Ion Absorption

The large intestine performs net absorption of sodium and chloride and net secretion of potassium and bicarbonate.

Sodium

Sodium is absorbed via several mechanisms:

• Sodium–hydrogen antiporters on the apical (luminal) membrane
• Epithelial sodium channels (ENaCs)
• Sodium–short-chain fatty acid symporters which facilitate sodium uptake alongside microbial fermentation products

Sodium absorption helps generate an electrochemical gradient that facilitates the passive absorption of chloride.

Chloride and Bicarbonate

Chloride ions are absorbed through chloride–bicarbonate exchangers, which simultaneously secrete bicarbonate into the lumen.

This leads to net bicarbonate secretion, which helps neutralise acidic bacterial metabolites.

Water

The absorption of sodium and chloride creates an osmotic gradient that drives the passive reabsorption of water into the colonic epithelial cells.

Potassium

In contrast to sodium and chloride, potassium is typically secreted into the lumen of the colon.

This is a passive process driven by the electrochemical gradient and is enhanced in states of diarrhoea, which can lead to hypokalaemia.

Vitamins and Short-Chain Fatty Acids

Commensal bacteria in the colon metabolise undigested carbohydrates and fibres to produce:

• Short-chain fatty acids (e.g., acetate, propionate, butyrate), which serve as an energy source for colonocytes and promote water and sodium absorption.
• Vitamin K – essential for the synthesis of clotting factors.
• B vitamins – including B6 and B12 (although B12 absorption requires intrinsic factor and primarily occurs in the ileum)

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.