What fluid is responsible for carrying dissolved substances between capillaries and cells?

The correct answer is interstitial fluid. This fluid acts as the medium through which nutrients, oxygen, and other dissolved substances are transported from the capillaries to the cells and vice versa. It exists in the spaces between the cells in tissues, effectively bridging the gap between the circulatory system and the individual cells.

Interstitial fluid is dynamic; it continuously bathes and supports the cells, providing them with the necessary components for metabolic functions and helping to remove waste products.

Although plasma, tissue fluid, and serum may seem relevant to this question, they serve different roles in the body. Plasma is the liquid component of blood that carries cells and macromolecules throughout the bloodstream. Tissue fluid refers to the fluid in the tissue spaces, but it’s more of a general term that encompasses interstitial fluid. Serum is the liquid that remains after blood coagulates, containing many proteins and nutrients but lacking the cellular components of blood. Therefore, while these other fluids have specific functions in circulation and homeostasis, interstitial fluid is specifically responsible for the transfer of dissolved substances between capillaries and cells.