[Solved] What is clear air turbulence (CAT) and why can it represent a hazard to aviation? Kidney: A pair of organs in the urinary system that filter waste products from the blood and excrete them in urine. Microscopic Units: Small units of measurement typically used to describe dimensions of cells and microorganisms, not visible to the naked eye. Calyx: The cuplike division of the kidney pelvis. Medulla: A central region of an organ, notably in the brain (medulla oblongata) affecting vital functions like breathing and heart rate, and in the kidney involved in regulating electrolyte balance.