Figure 51.9 The Human Excretory System (A) The kidneys lie against the back wall of the abdominal cavity, in the region of the middle back. The adrenal glands (see Figure 40.16) are positioned on top of the kidneys; they are the source of the hormone aldosterone, which plays a role in the regulation of kidney function. (B) A highly organized internal structure is the basis for kidney function. Certain parts of the nephrons are in the organ’s outer region, the cortex; other parts are in the internal region, the medulla. (C) The glomeruli and the proximal and distal convoluted tubules are located in the cortex of the kidney. The loops of Henle run in parallel as straight sections down into the renal medulla and back up to the cortex. Collecting ducts run from the cortex to the inner surface of the medulla, where they open into the ureter. The vasa recta are peritubular capillaries that parallel the loops of Henle and are permeable to salts and water.