Gross Structure of the Human Excretory System

The human excretory system is centred on a pair of kidneys — the principal organs responsible for forming urine — together with the ducts and single organ that carry that urine out of the body: the ureters, the urinary bladder, and the urethra.

Each kidney is a reddish-brown, bean-shaped organ, roughly 10-12 cm long, 5-7 cm wide and 2-3 cm thick in an adult, lying against the posterior wall of the abdominal cavity, one on either side of the vertebral column, at the level of roughly the last thoracic to third lumbar vertebrae. The kidneys lie behind the peritoneum (retroperitoneal) rather than within the abdominal cavity proper, and the right kidney sits slightly lower than the left because the bulk of the liver above it takes up space that would otherwise be available. The outer, convex border of each kidney is smoothly curved, while the inner border is concave and bears a deep notch called the hilum (or hilus), the single point through which the renal artery enters the kidney to supply it with blood, and through which the renal vein and the ureter both leave — the renal vein carrying filtered blood back toward the heart, and the ureter carrying newly formed urine onward toward the bladder. Each kidney is also loosely wrapped in a tough fibrous capsule and cushioned within a protective layer of perirenal fat.

A longitudinal section through the kidney reveals two clearly distinct internal regions. The outer region, the cortex, has a granular appearance under low magnification because it is packed with the coiled, convoluted portions of many thousands of nephrons and their associated blood vessels. The inner region, the medulla, is darker and is organised into 8-12 cone-shaped structures called renal pyramids, whose broad bases face outward toward the cortex and whose narrow, blunt apices — called renal papillae — point inward toward the centre of the kidney; the pyramids appear striped because they are made up of long, straight tubules (the loops of Henle and collecting ducts) running in parallel. Extensions of cortical tissue, called renal columns, dip down between adjacent pyramids. Urine draining from the papilla of each pyramid is collected by a small, cup-shaped chamber called a minor calyx; several minor calyces converge to form a larger major calyx, and the major calyces in turn open into a single funnel-shaped chamber, the renal pelvis, which lies roughly at the level of the hilum and continues onward as the ureter. …