Structure of the Kidney: Cortex, Medulla, Pelvis

The kidneys are two bean-shaped organs, each about 10-12 cm long, placed on either side of the backbone just below the ribs. Their job is to filter blood continuously, removing urea and extra salts while keeping useful substances. The internal structure of the kidney is arranged exactly for this filtering work.

Understanding kidney structure matters because every kidney disease — stones, infection, failure — affects a specific region. It also explains how the kidney can produce urine that is sometimes dilute and sometimes concentrated, depending on the body's needs.

Key Concepts

• Capsule is the tough outer covering protecting the kidney.
• Cortex is the outer, darker region containing the filtering heads of nephrons (glomeruli and Bowman's capsules).
• Medulla is the inner, lighter region arranged in cone-shaped pyramids; it contains the long loops of nephrons that concentrate urine.
• Renal pelvis is the funnel-shaped central cavity that collects urine from the pyramids and passes it to the ureter.
• Renal artery brings blood into the kidney; renal vein carries filtered blood away; the ureter carries urine to the bladder.
• The hilum is the notch on the inner side where vessels, nerves and the ureter enter or leave.
• Each kidney contains about one million tiny filtering units called nephrons, spread across cortex and medulla.

How It Works

1. Blood rich in urea enters through the renal artery and spreads into the cortex.
2. In the cortex, nephrons filter out water, urea, salts and glucose from the blood.
3. The filtrate travels down through the medulla, where water and useful salts are drawn back into the blood as needed.
4. The remaining liquid — urine — collects from the pyramid tips into the renal pelvis.
5. Urine flows down the ureter to the urinary bladder for storage, while cleaned blood leaves by the renal vein.

Worked Example

Each kidney receives roughly 20-25% of the heart's output shared between them; together they get about 1.2 litres of blood per minute. Over a day that is 1.2 x 60 x 24 = about 1,728 litres of blood flowing through the kidneys. From this, nephrons filter about 180 litres of fluid daily, yet urine output is only about 1.5 litres. So roughly 178.5 of 180 litres — over 99% — is reabsorbed. This arithmetic shows the kidney is not a simple drain; it is a careful recycling plant whose layered structure makes that recycling possible.

Common Misconceptions

• "Urine is made in the bladder." Wrong. The bladder only stores urine; the kidneys make it.
• "The cortex and medulla are just colour differences." Wrong. Cortex does filtration, medulla does concentration — different jobs, different structures.
• "Kidneys only remove urea." Wrong. They also balance water, salts, blood pH and even help control blood pressure and red blood cell production.

Real-World Applications

Ultrasound and CT scans read these regions to find stones, cysts and tumours. Kidney stones often lodge where the pelvis narrows into the ureter. In dialysis, an artificial filter copies the cortex's filtering job for patients whose kidneys fail. Surgeons use knowledge of the hilum and vessels during kidney transplant operations.

The kidney is layered for a reason: cortex filters, medulla concentrates, pelvis collects. This simple three-part plan lets a small organ clean the entire blood supply many times a day.