A double-walled epithelial capsule that is the bulbous closed proximal end of the kidney tubular system. It surrounds the cluster of convoluted capillaries of KIDNEY GLOMERULUS and is continuous with the convoluted PROXIMAL KIDNEY TUBULE.

The U-shaped portion of the renal tubule in the KIDNEY MEDULLA, consisting of a descending limb and an ascending limb. It is situated between the PROXIMAL KIDNEY TUBULE and the DISTAL KIDNEY TUBULE.