The medulla model

A teaching model says that the cortex is at 300 mOsmol/L and that each level below it adds a fixed step. You will write the model, find how concentrated the urine can become, and find the volume of fluid that leaves a collecting duct.

The idea. The osmolarity at level $n$ is $300 + \text{step} \times n$. In a duct, only water leaves, so $\text{volume} \times \text{osmolarity}$ stays the same.

Your tasks.

1. Finish osmolarity_at. It gives the osmolarity at a level.
2. Finish concentration_ratio. It gives how many times the cortex the tip of a loop is.
3. Finish volume_out. It gives the litres that leave a duct when volume_in litres at conc_in leave at conc_out.