Nephron Regions and Selective Reabsorption

Explained

How a nephron turns blood into urine

The kidney does not pick out the waste and leave everything else alone. It does the opposite. It filters almost everything small enough to fit through the capillary wall, then takes back what the body still needs. Filter first, recover second. Once you have that order in your head, most questions on this topic become straightforward.

The route out of the body

The kidneys form urine. The ureters carry it to the bladder, and the urethra carries it out of the body. Three structures, one direction. Questions often give you one of them and expect you to name the next, so it is worth learning them as a sequence rather than as separate labels.

Inside each kidney are around a million nephrons. A single nephron runs from Bowman's capsule, through the proximal tubule, round the loop of Henle, along the distal tubule and into the collecting duct.

Ultrafiltration: pressure does the sorting

Blood arrives at a tight knot of capillaries called the glomerulus, and it arrives under high pressure. That pressure forces water and small solutes out through the capillary wall and into Bowman's capsule. Glucose, mineral ions and urea all go through.

What stays behind is simply whatever is too big to pass: red and white blood cells, and the plasma proteins. Nothing here is being chosen. It is filtration by size, and the pressure is what drives it.

Selective reabsorption: taking the useful things back

The filtrate that has just been produced still contains a great deal the body cannot afford to lose, so the proximal tubule reclaims it. All of the glucose is reabsorbed into the blood, along with much of the water and many of the mineral ions.

That word "all" is worth pausing on. In a healthy person, no glucose reaches the urine at all. This is why a urine test can suggest diabetes: either there was more glucose in the blood than the tubule could handle, or the reabsorption itself is not working.

Why urea gets more concentrated as it travels

Students often expect urea to be added along the nephron, because its concentration rises. It is not. The urea was filtered out at the glomerulus and very little of it is reabsorbed, but a large amount of water is. Remove the water and the same quantity of urea sits in a smaller volume, so the concentration goes up.

Glucose behaves in the opposite way for the same reason. Its concentration falls to zero not because it is destroyed, but because it is taken back.

Reading a nephron data table

A common question gives you the concentration of protein, glucose and urea in three places: blood plasma, glomerular filtrate and urine. You can work out every row from the two processes above.

• Protein is present in plasma and absent from both filtrate and urine, because it is too large to be filtered.
• Glucose is present in plasma and filtrate but absent from urine, because it is filtered and then completely reabsorbed.
• Urea is present in all three and most concentrated in urine, because it is filtered, barely reabsorbed, then concentrated as water leaves.

If you can explain those three patterns, you can answer almost any table question on the nephron.