Excretion

A vertical cross-section through a kidney shows three regions:

• Cortex is the outer layer. Contains the Bowman's capsules, glomeruli, proximal convoluted tubules and distal convoluted tubules (covered in section 5).
• Medulla is the inner darker layer. Contains the loops of Henle and most of the collecting ducts.
• Renal pelvis is the funnel-shaped cavity at the very centre. Urine drains into the renal pelvis from all the collecting ducts and flows from there into the ureter.

Each kidney contains roughly one million tiny filtering units called nephrons. Each nephron starts in the cortex, dips down into the medulla, and ends at the renal pelvis. Together they filter the entire blood plasma volume around 60 times every day.