When following a training programme your cardiovascular system is going to adapt to the required stimulus. Aerobic training has been shown to increase the size of the ventricles, which allows the heart to pump more blood to the body. Resistance training thickens the myocardial wall; thus when the muscles of the heart contract, more blood is pushed through the arteries compressed by the contractions.
The thickening (or hypertrophy) is reversible and non-pathological. If the stress on the body through resistance training is volume overload, the sarcomeres in the ventricle must lengthen rather than thicken, dilating the ventricle (expanding the heart) and thus allowing a greater volume of blood to the heart. This process of thickening of the ventricular wall is called eccentric hypertrophy.
The increase in thickness is proportional to the increase in the chambers radius. The chamber radius might not change with pressure overload (e.g., resistance training compresses arteries and thus increases resistance to the blood flow). The ventricular wall does, however, get much thicker as new sarcomeres are added in parallel to existing sarcomeres this is called concentric hypertrophy. Ventricles with concentric hypertrophy can generate greater forces and higher pressures, whilst maintaining normal stress to the muscle wall.
Long-term effects of aerobic exercise on the heart include increased stroke volume and consequently lowered resting heart rate. Training increases the number of capillaries in the heart, thereby increasing blood supply to the hearts own muscles. The net results of cardiovascular training are reduced risk of heart disease, reduction of high blood pressure and improved blood cholesterol.
Klabunde, R.E. 2011. Cardiovascular physiology concepts. Baltimore: Lippincott Williams & Wilkins.
Smith, D., and B. Fernhall. 2010. Advanced Cardiovascular Exercise Physiology. Champaign, IL: Human Kinetics.