2.74B-2.75B - Nephron structure and ultrafiltration
Inside each kidney, blood is filtered by microscopic structures called nephrons. This lesson follows the named parts of a nephron, then zooms in on the first filtering step: ultrafiltration in the Bowman's capsule. The key exam skill is to separate what happens at the start of the nephron from later reabsorption, which belongs to the next kidney lesson.
The nephron map
A nephron is a microscopic filtering unit in the kidney. It has a blood side and a tubule side. Blood enters a knot of capillaries called the glomerulus, while the tubule side begins as the Bowman's capsule around that glomerulus.
Nephron
A nephron is a microscopic kidney structure that filters blood and begins the formation of filtrate.
The specification names these nephron structures:
- Bowman's capsule, a cup-shaped structure at the start of the nephron
- glomerulus, a knot of blood capillaries inside the Bowman's capsule
- convoluted tubules, the coiled tubule regions after the Bowman's capsule and after the loop of Henle
- loop of Henle, the U-shaped part of the tubule
- collecting duct, the duct that receives fluid from the tubule
[DIAGRAM: asset_name: Nephron structure map; asset_slug: b31_nephron_structure_and_ultrafiltration__diagram_01; recommended_method: deterministic_drawing; description: Monochrome labelled nephron diagram showing Bowman's capsule around the glomerulus, convoluted tubules, loop of Henle, collecting duct, blood entering and leaving the glomerulus, and arrows showing filtrate flow through the nephron.]
The fluid pathway is:
Bowman's capsule -> convoluted tubule -> loop of Henle -> convoluted tubule -> collecting duct
The blood pathway is different. Blood flows into and out of the glomerulus through small blood vessels. The filtrate forms in the Bowman's capsule and then moves along the tubule.
Bowman's capsule and glomerulus
The glomerulus and Bowman's capsule are closely linked, but they are not the same structure.
The glomerulus is a knot of tiny blood capillaries. Blood flows through these capillaries under pressure. The Bowman's capsule is the cup-shaped part of the nephron that surrounds the glomerulus and receives the filtered fluid.
The glomerulus is made of blood capillaries; the Bowman's capsule is the cup that receives filtrate.
This distinction matters in exam answers. If a question asks where ultrafiltration occurs, the safest answer is that substances are forced from the blood in the glomerulus into the Bowman's capsule. Naming only "the kidney" is too vague, and naming later parts such as the loop of Henle or collecting duct is wrong for ultrafiltration.
Ultrafiltration
Ultrafiltration is the filtering of blood under pressure at the start of the nephron. It happens as blood passes through the glomerulus inside the Bowman's capsule.
Ultrafiltration
Ultrafiltration is the forcing of water and small dissolved substances out of the blood in the glomerulus and into the Bowman's capsule.
Continue this lesson.
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