Structure of the Nephron: Kidney's Functional Unit

The nephron is the microscopic functional unit of the kidney — the tiny machine that actually makes urine. Each kidney contains about a million nephrons, and each one works independently, filtering blood and adjusting what the body keeps and what it throws out.

This topic matters because nearly every kidney test and disease is described in nephron language. Protein in urine points to one part, salt imbalance to another. Unless you know the nephron's parts in order, urine formation makes no sense.

Key Concepts

• Glomerulus is a ball of capillaries where blood pressure forces fluid out of the blood.
• Bowman's capsule is the cup-shaped structure surrounding the glomerulus; together they form the Malpighian body in the cortex.
• Proximal convoluted tubule (PCT) is the first coiled section; it reabsorbs most glucose, amino acids, salts and water.
• Loop of Henle is the U-shaped part dipping into the medulla, with descending and ascending limbs; it concentrates urine.
• Distal convoluted tubule (DCT) fine-tunes salts and pH under hormone control.
• Collecting duct gathers urine from many nephrons and carries it to the renal pelvis; its water absorption is controlled by ADH hormone.
• The peritubular capillaries and vasa recta are blood vessels hugging the tubule, taking back reabsorbed materials.

How It Works

1. Blood enters the glomerulus under high pressure, and water with small molecules filters into Bowman's capsule.
2. Cells, proteins and platelets stay in blood because they are too large to pass.
3. As filtrate flows through the PCT, useful substances are pumped back into surrounding capillaries.
4. The loop of Henle removes more water and salt in a pattern that builds concentration in the medulla.
5. In the DCT and collecting duct, hormones decide the final amounts of water and sodium to keep, and the leftover fluid leaves as urine.

Worked Example

In one day, all nephrons together filter about 180 litres. The PCT alone reabsorbs roughly 65% of this: 0.65 x 180 = 117 litres returned to blood in the first segment itself. Glucose is normally 100% reabsorbed in the PCT, so urine has zero glucose. If blood glucose rises too high in uncontrolled diabetes, the PCT transporters saturate — say they can handle the load from normal blood but the filtrate now carries extra glucose beyond their capacity — and the surplus appears in urine, the classic test result doctors use.

Common Misconceptions

• "The nephron is a single straight tube." Wrong. It is a sequence of specialised regions, each with different cells and jobs.
• "Filtration happens along the whole tubule." Wrong. Filtration happens only at the glomerulus; the tubule does reabsorption and secretion.
• "One nephron failing means kidney failure." Wrong. Kidneys have huge reserve; symptoms appear only after a large fraction of nephrons is damaged.

Real-World Applications

Urine tests are read by nephron logic: glucose in urine points to PCT overload, protein points to glomerular damage. Many blood pressure medicines (diuretics) act on specific tubule segments to remove extra salt and water. Kidney biopsy examines glomeruli directly, and dialysis timing is judged by how much total nephron function remains.

A nephron is an assembly line: glomerulus filters, tubule reclaims, duct decides the final urine. A million of these units working together give the kidney both its power and its precision.