Sharks take a long time to mature. Some range in age from 8 to 18 before they can reproduce. Sharks have no mammary gland, because they are fish, so the pups must be ready to fend for themselves from birth without the benefit of any parental care. Not all sharks swim continuously. Some sharks have the ability to pump water through their mouth over their gills to get oxygen for breathing.
Sharks have low blood pressure.
Swimming allows their muscles to contract which in turn circulates their blood.