Describe how and why systolic and diastolic blood pressure should
change from the beginning to the end fo the Astrand test?

As with any excercise, the sympathetic system will be in dominance,
thus causing positive ionotropic, chronotropic, and dromotropic
effects on the heart, all of which of course help increases blood
pressure (esp. systolic). Furthermore, sympathetic activation of
arterioles causes more resistance in the blood vessels, and thus an
even higher blood pressure (esp. diastolic). How does this all happen
in the first place? Your muscles are releasing more CO2 and consuming
more oxygen, and from here you can see my response to another
question:
http://answers.google.com/answers/threadview?id=370493. I think that
briefly  sums up what you are asking.