Originally Posted by gregf83

The idea of these short intervals is to build anaerobic work capacity hence increased blood flow is not really necessary. Your HR will rise but it likely won't get close to your max if you're reasonably rested before you start.

For short intervals most of your energy output will be coming from anaerobic sources. One of the primary methods for measuring anaerobic capacity is to look at the maximal accumulated oxygen deficit (difference between predicted and actual oxygen demand during a high intensity interval). An early research paper on the subject,
Anaerobic capacity determined by maximal accumulated O2 deficit was co-authored by Tabata who many are familiar with.