Biological Systems MCAT Practice Exam

What is the main function of the colon in the excretory system?

1. Filtration of blood

2. Decomposition of large waste products

3. Reabsorption of water and ions

4. Production of sweat

The correct answer is: Reabsorption of water and ions

The primary function of the colon within the excretory system is the reabsorption of water and ions. As digested food passes through the colon, it undergoes further processing where a significant amount of water is absorbed back into the body, which is essential for maintaining hydration and electrolyte balance. This reabsorption helps to convert liquid waste into solid stool, facilitating its eventual elimination from the body. The colon also plays a role in the fermentation of undigested materials by bacteria, somewhat reflecting aspects of decomposition, but its main function ties directly to water and ion reabsorption. Consequently, while some aspects of decomposition occur, they are secondary to the crucial role of water and ion absorption. The functions of blood filtration and sweat production are handled by other organs in the excretory system, such as the kidneys and sweat glands, respectively.