Short-term effects of exercise on the heart are as follows:
Increased cardiac rate (pulse)
Increased stroke volume (output of blood with each heart contraction)
Dilatation of coronary arteries
Increased energy and O2 consumption by heart
Re-direction of blood flows (via vasodilatation and vasoconstriction)
This answer is limited to discussion of effects of short-term exercise only. The long-term benefit to the heart of physical training is not included.
The heart under conditions of exercise requires increased oxygen (provided by increased respiratory rate and depth) that is brought to the heart muscle in increased quantity due to vasodilatation of the coronary arteries.
The exercising heart needs more energy as well. This is supplied through the ATP system.
The major overall effect on the heart of short-term exercise is increased blood flow (cardiac output). This is accomplished by increased heart rate and stroke volume:
BF = HR x SV
Where BF is blood flow, HR is heart rate, and SV is stroke volume
The magnitude of the effect of exercise on the heart depends on the following:
Exercise intensity (aerobic versus non-aerobic)
In this regard, a hot environment merely magnifies the effect of exercise on the heart. Whatever effect is found at a given temperature, it is increased with increasing environmental temperature. A secondary way in which a hot environment affects the heart is humidity, which has a similar, but less dramatic effect compared to heat.
Negative effects on the heart of short-term exercise may occur with heart disease. Examples would be chest pain (angina) in a patient with coronary artery disease, shortness of breath in patients with congestive heart failure and cardiac arrhythmia or even death in patients with coronary artery disease or under the influence of amphetamines or cocaine.
The reference lists cardiac benefits of short-term exercise, and, for those who may be interested, has a link to long-term benefits of cardiac training.