Selective reabsorption and urineSpec 2.76B, 2.77B, 2.79B
The glomerular filtrate contains useful substances as well as waste. The body cannot afford to lose them, so the useful substances are taken back into the blood. This is selective reabsorption.
Glucose: proximal convoluted tubule
All the glucose in the filtrate is reabsorbed into the blood at the proximal convoluted tubule. It is reabsorbed by active transport, which uses energy from respiration. Glucose is needed by cells as a fuel for respiration, so none must be lost in the urine. Normal urine contains no glucose.
Water: collecting duct
Water is reabsorbed back into the blood from the collecting duct. It moves out of the collecting duct by osmosis, into the more concentrated tissue around it, and then into the blood capillaries. The amount that is reabsorbed depends on the body's needs, and is controlled by ADH (next section). The more water that is reabsorbed, the less water is left in the urine.
What is in urine
What remains after reabsorption is urine. It contains water, urea and ions. It flows from the collecting duct to the ureter, then the bladder.
| Substance | Filtered at the glomerulus? | In urine? |
|---|---|---|
| Water | Yes | Yes, the amount not reabsorbed |
| Glucose | Yes | No, all reabsorbed |
| Urea | Yes | Yes |
| Ions | Yes | Yes, the excess |
| Proteins and blood cells | No | No |
For a mark on glucose, give the place (proximal convoluted tubule), the process (active transport) and the reason (needed for respiration).