Structures and Functions in Living Organisms · 6 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 12% of your exam marks.
The kidney and urea production appear regularly; dialysis is a common application question.
Three main processes turn blood into urine as the filtrate flows along the nephron:
The Bowman's capsule surrounds a small knot of capillaries called the glomerulus. Two clever design features set up high-pressure filtration here:
The high blood pressure forces the small components of plasma out of the capillaries and across the filter into the Bowman's capsule:
Larger molecules and cells are too big to cross the filter and stay in the blood:
The fluid that collects inside the Bowman's capsule is called the glomerular filtrate. It is blood plasma without the proteins and cells.
High blood pressure in the glomerulus can damage the filter, allowing proteins to get through. Finding protein in urine is therefore an early sign of high blood pressure or kidney disease.
The glomerular filtrate is full of valuable molecules that the body cannot afford to wash away. As the filtrate flows along the proximal convoluted tubule (PCT), these useful substances are pulled back into the bloodstream through the surrounding capillaries:
It is selective because only some of the molecules are pulled back: useful ones move into the blood while waste substances (such as urea) are left behind in the filtrate.
Active transport of glucose against its gradient needs ATP, so the cells lining the PCT are packed with mitochondria. The wall of the PCT also has microvilli on its inner surface, dramatically increasing the surface area for reabsorption.
In a healthy person, all of the glucose in the filtrate is reabsorbed in the PCT, so there should be no glucose in the urine at all. If glucose is found in the urine, it usually means blood glucose has risen so high that the PCT cannot keep up. This is a classic sign of diabetes.
After the PCT, the filtrate flows down through the loop of Henle into the medulla and back up. The loop of Henle uses a clever counter-current arrangement (beyond IGCSE in detail) to make the surrounding tissue of the medulla very salty. This means that when the filtrate later flows through the collecting duct back down through the same salty tissue, water moves out of the filtrate by osmosis into the blood.
The amount of water that is reabsorbed here depends on how much the body needs to keep, and that is controlled by the hormone ADH (section 7).
| Process | Where | What happens | Key substances |
|---|---|---|---|
| Ultrafiltration | Glomerulus → Bowman's capsule | High blood pressure forces small molecules through a partially-permeable filter | Water, glucose, urea, ions filtered out; proteins and cells stay in blood |
| Selective reabsorption | Proximal convoluted tubule | Useful molecules are reabsorbed by active transport into the blood | All glucose, all amino acids, most ions, much water |
| Water reabsorption | Collecting duct | Water moves into the blood by osmosis; amount controlled by ADH | Water |
What is left in the collecting duct at the end of all of this becomes urine: water + urea + excess ions + a few other waste products. The urine flows into the renal pelvis, down the ureter, into the bladder, and eventually out through the urethra.