When water flows through pipes, it loses pressure. There are charts that will indicate how much pressure (usually expressed in PSI losses per every 100 feet of pipe) based on pipe type, pipe size, and water speed.
There are additional charts that will indicate for various pipes, what the equivilent pipe length is... and the last time I saw one of these charts for typical PVC fittings, it indicated a 90 elbow was equivilent in pressure loss to 1 foot of pipe. So if you are trying to compute the pressure loss in a 10ft long piece of pipe that includes 1 90 elbow, you have to pretent the pipe is 11ft long.