After about three minutes of exercise the heart rates response would gradually begin to steady out, it’s what’s called a Plateaux. Initially when you begin exercise the heart rate increases rapidly to almost in a way get ahead of it’s self by creating more blood circulating around the body, because it has to change it’s amount of heart beats rapidly after being at rest, so as the exercise continues the intensity doesn’t decrease it’s just that the heart is able to cope with the demand of blood needed to be supplied around the body much more efficiently because it adapts it’s heart beats.
So it is very similar when the heart is at rest, the heart beat is at a steady pace because it recognises that there is a consistency within the rest, and it works the same when exercising at the same intensity for a long period at a time, the heart balances enough heart beats to supply the demand of oxygen and sources needed to be carried around the body, this is why interval training feels so intense because your heart does not manage to plateaux, and therefore the cycle may not be balanced as there may be more deoxygenated blood within the body as there is oxygenated.
The venous return during stead state exercise is increased because the heart recognises that there is an increased demand for oxygen for a long period of time, so to cope with this it pumps more blood out than possibly needed so that when it is returned there is a high amount of blood to load off oxygen to therefore it results in a higher amount of oxygenated blood. This is also why this increased volume of blood has the effect upon the hearts muscular walls.
It is commonly known that when there is more blood within the muscles they become more flexible, this is similar to the heart, because the Venous return has been increased the heart walls have to in fact stretch more to enable the increased amount of blood to fit within the walls of the heart, this is why there is also a larger volume of blood within the heart, because as more is required to become oxygenated more is also sent back which is deoxygenated.
On average the cardiac output supplies approximately 5 litres of blood per minute however during exercise there are certain organ and working muscles which receive more blood than others to supply them with the high demands of energy, so therefore during steady-state exercise the body is having to redirect blood flow by increasing the amount of constriction the blood vessels carry out which are leading to organs which do not need such a large blood flow.
This would also explain why dilation of blood vessels would increase because they need to increase within size to supply the muscles with the increased volumes of blood supply. So that blood pressure is not increased Dilation of the blood vessel help to assist within feeding the working muscles that help reduce blood pressure however this is all in vein because the cardiac output has been increased to cope with the demand of blood therefore blood pressure increases anyway.