Nephron Structure and Ultrafiltration

A filtering unit inside the kidney

Each kidney contains many tiny nephrons. A nephron filters blood and then processes the filtered liquid to form urine. Its first part is a cup-shaped Bowman’s capsule in the kidney’s outer region, the cortex. Inside the cup is a knot of blood capillaries called the glomerulus. From the capsule, liquid passes along a tubule: first the proximal convoluted tubule, then the loop of Henle, the distal convoluted tubule and a collecting duct. The loop dips into the kidney’s inner region, the medulla, while the collecting duct carries the remaining liquid towards the renal pelvis.

How the glomerulus filters blood

Blood enters the glomerulus through an afferent arteriole and leaves through an efferent arteriole. The incoming arteriole is wider than the outgoing one, helping to maintain high pressure in the glomerular capillaries. This pressure forces some of the liquid and small dissolved substances out of the blood, through a filtering barrier and into Bowman’s capsule. This pressure-driven process is ultrafiltration.

The barrier lets small substances such as water, glucose, mineral ions and urea pass into the capsule. Blood cells and large molecules such as plasma proteins are too large to pass through, so they remain in the blood. The liquid collected in Bowman’s capsule is called filtrate. It is not yet urine: it contains useful glucose and water as well as waste urea.

From filtrate towards urine

Filtrate flows from Bowman’s capsule along the nephron tubule. As it travels, useful substances and some water are returned to the blood. The liquid left in the collecting duct becomes urine and passes towards the renal pelvis. This explains why the nephron needs both a pressure filter at its start and a long tubule afterwards: filtering removes small substances from the blood, while later processing prevents all of those substances from being lost in urine.

Structure and route

Glomerulus (capillary knot) inside Bowman’s capsule → proximal convoluted tubule → loop of Henle → distal convoluted tubule → collecting duct. Bowman’s capsule is in the cortex; the loop dips into the medulla.

Ultrafiltration

Blood enters by the wider afferent arteriole and leaves by the narrower efferent arteriole. High pressure forces water and small dissolved substances, including glucose, mineral ions and urea, into Bowman’s capsule. Blood cells and large proteins stay in the blood. The collected liquid is filtrate, not urine; it is processed further along the tubule.