Why does the heart rate increases when you work hard?

A deep inspiration causes a decrease in intrathoracic pressure. This mediates a rush of blood into the lungs and right atrium as high pressure blood will move down the exaggerated pressure gradient. The lungs also expand and have a great capacity to hold blood, which will begin to pool there. Baroreceptors in the aortic and carotid sinus will sense a drop in pressure downstream and signal to controllers in the brain to lower inhibition of heart rate via the vagus nerve. This will cause a compensatory increase in heart rate, attempting to normalize blood pressure in the aorta and carotids.