Which organ is primarily responsible for water reabsorption in the GI tract?

• A. Colon
• B. Stomach
• C. Duodenum
• D. Jejunum

Answer: (A)

Water balance along the GI tract is achieved as liquid contents move from the small intestine toward the colon, where the final adjustment of water content occurs. The colon’s primary job is to reclaim water and electrolytes from indigestible material to form solid stools and help maintain hydration. It accomplishes this by absorbing sodium through epithelial transporters and pumps, with water following passively through osmosis and via channels like aquaporins. The stomach does little water reabsorption, and while the small intestine (duodenum and jejunum) does take up a lot of water early on as nutrients are absorbed, the colon is the key organ that completes most of the water reabsorption needed to produce formed stool.