* Other substances are secreted or reabsorbed in the tubules passively by '''[[Transport Proteins - Physiology#Facilitated Diffusion |facilitated]]''', none facilitated [[Diffusion - Physiology| '''diffusion''']] or [[Transport Proteins - Physiology#Diffusion Through Water Filled Protein Channels |'''ion channels''']] down chemical or electrical gradients. | * Other substances are secreted or reabsorbed in the tubules passively by '''[[Transport Proteins - Physiology#Facilitated Diffusion |facilitated]]''', none facilitated [[Diffusion - Physiology| '''diffusion''']] or [[Transport Proteins - Physiology#Diffusion Through Water Filled Protein Channels |'''ion channels''']] down chemical or electrical gradients. |