How does the gallbladder concentrate bile?

The gallbladder concentrates bile primarily through the active reabsorption of electrolytes, especially sodium, via Na/K ATPase pumps located in the epithelial cells of the gallbladder wall. This process creates an osmotic gradient that facilitates the passive movement of water out of the gallbladder lumen, leading to a reduction in the volume of bile while increasing the concentration of bile salts and other solutes.

As sodium is actively transported out of the cells into the interstitial fluid, water follows passively due to osmosis, resulting in the concentration of bile. This mechanism is essential because the gallbladder serves as a storage reservoir for bile produced by the liver, allowing the body to control bile release during digestion.

The other processes listed, such as passive diffusion of electrolytes and filtration through biliary ductules, do not adequately describe the primary mechanism of concentration within the gallbladder. While bile salts may be reabsorbed in the intestines, the gallbladder's primary method of concentrating bile is through this active transport and subsequent osmotic effect on water.