Urine then flows through the nephrons, through a system of converging tubules called collecting ducts. Birds have small glomeruli, but about twice as many nephrons as similarly sized mammals. Connecting tubules of some juxtamedullary nephrons may arch upward, forming an arcade. Blood is filtered by millions of nephrons, the functional units that comprise the kidneys. Horsfall mucoprotein secreted from the tubular epithelial cells of individual nephrons. Each kidney consists of functional units called nephrons. AVT triggers antidiuretic hormone (ADH) which targets the nephrons of the kidney. Focal segmental glomerulosclerosis may develop following acquired loss of nephrons from reflux nephropathy. This is followed by multiple changes in the filtration units of the kidneys, the nephrons. Cortical nephrons can be subdivided into "superficial cortical nephrons" and "midcortical nephrons". The blood is filtered by nephrons, the functional units of the kidney. The way the kidneys do this is with nephrons. The formation of urine begins within the functional unit of the kidney, the nephrons. Podocytes are found lining the Bowman's capsules in the nephrons of the kidney. A healthy adult has 1 to 1.5 million nephrons in each kidney.