ansa nephroni

a long, U-shaped part of the renal tubule, extending through the medulla from the end of the proximal convoluted tubule to the beginning of the distal convoluted tubule. It begins with a descending limb having a thick-walled segment called the proximal straight tubule, followed by a thin-walled segment called the thin or attenuated tubule; this is followed by the ascending limb, which sometimes includes the distal end of the attenuated tubule and always ends with a long thick-walled segment called the distal straight tubule. The loops vary in the lengths of their segments according to their locations in the kidney. Called also loop of Henle.