Need help about a bridge driver electronic diagram

Hello,
I am a french student and I need some help in electronics. For my class I had to study this system :
http://img.villagephotos.com/p/2008-2/1300357/FP1_Tx_push_pull.gif

I know which it is a bridge driver. When Q1 and Q3 are "ON" Q2 and Q4 are "OFF". I don't know how to explain the operating : Icsat= ? ; Ibsat= ? ; etc.

Between B3 and B4 there are an ultrasonic transducer.
Vcc_B=5V
Tx=0/5V

Please help me ! Thank you
Yours faithfully

__________ __________ __________ __________

Romain from Saint-Etienne (France)