The kidneys are an organ into which infection can come both "from below" and "from above". This feature is manifested by pyelonephritis. It is a nonspecific infectious and inflammatory process characterized by simultaneous or sequential involvement of the calyx-lobar system and the kidney parenchyma (main tissue). Pyelonephritis can be "ascending" - this is when the infection comes to the kidney "from below", from the lower parts of the urinary system (bladder, ureter, etc.), where the pathogen originally entered. And "descending" - in this case the infectious agent enters the kidney through the blood, "from above". Most often pyelonephritis develops as a result of infection through the blood, as in the animation☝🏻