The Kidney and Nephron

Explained

The kidney and the nephron: structures and filtration

The kidneys do two jobs at once. They remove urea, which is the waste produced when the liver breaks down excess amino acids, and they help regulate the water and ion content of the blood. Both happen in the same structures, which is why the anatomy is worth learning before the processes.

The structures, in the order the filtrate meets them

• Kidney: filters the blood, removes urea and helps control water and ion content.
• Glomerulus: a knot of capillaries where high pressure causes ultrafiltration.
• Bowman's capsule: the cup shaped structure around the glomerulus that receives the filtrate.
• Convoluted tubules: the coiled sections the filtrate passes through.
• Loop of Henle: the U shaped section between the two convoluted tubules.
• Collecting duct: receives fluid from the nephrons, and some water is reabsorbed from it.

Each kidney contains many thousands of nephrons. A nephron filters blood at the glomerulus, then alters the filtrate as it travels along the tubule.

Ultrafiltration, and what the filter actually separates

Blood arrives at the glomerulus under high pressure, and that pressure forces small molecules out of the capillary and into Bowman's capsule. Water, glucose, ions and urea all pass through.

Blood cells and plasma proteins do not. They are too large to cross the capillary wall, so they stay in the blood. This is the single most examined idea in the topic, and the reason must be size. An answer that says protein "is not filtered" without saying why it cannot pass is describing the result rather than explaining it.

Why the word ultra is there

Ordinary filtration separates a solid from a liquid using a mesh. Ultrafiltration separates by molecular size, under pressure, and the boundary sits between small solutes and large proteins. Nothing is being selected on the basis of whether the body needs it. That choosing happens later, further along the tubule.

What happens next

After filtration, the filtrate still contains a great deal the body needs, and that is recovered further along the nephron. Selective reabsorption, where glucose is returned to the blood, and the role of ADH in controlling water, are covered in the specification points that follow this one and on the nephron regions and selective reabsorption page.