Same intensity - heart rate increase over time

Quoted message said:

I talked to friends about this and they seem to experience the same.
How come the body's oxygen-demand increases over time?

Because you're a punk, and your friends are punks.

Try the opposite:

Intervals, increase intensity in segments, then
resume back to baseline.

I am guessing that the body accommodates to steady state,
and BP drops;

the body's homeostatic response to decreased BP is to increase
heart rate to maintain vessel tone for upper body, eg, brain.

Cardiovascular drift.

http://home.hia.no/~stephens/hrchngs.htm

Quoted message said:

If I cycle with a constant intensity, my heart rate gradually
increases. To keep the rate constant, I have to decrease the intensity
step-by-step.

I talked to friends about this and they seem to experience the same.
How come the body's oxygen-demand increases over time?

Thanks in advance.

One possibility is that heart rate increases directly proportional to
hydration levels. The more dehydrated you are, the higher the heart
rate.