Particularly, it is often stated that exercise is good for battling heart disease and controlling blood pressure. Does anyone have any insight on how/why exercise does this? Thanks.
Exercise does seem to have beneficial effects in treating and preventing many chronic diseases. As far as mechanism is concerned, we should probably narrow the scope:
What is the mechanism for the beneficial effects of exercise in people who have high blood pressure?
It seems to be multifactorial, and independent of any impact on weight loss. Regular exercises (with or without associated weight loss) causes a number of physiologic adaptations in metabolic, vascular, and cardiac health, all of which seem to contribute to the beneficial effects for people with high blood pressure (lower blood pressure, better long term disease outcomes). There are, of course, many types of exercise and many causes of high blood pressure, but these seem to apply generally.
Changes in sympathetic/parasympathetic tone: regular exercise causes decreased catecholamine production and (associated) decreased sympathetic mediated vasoconstriction. This leads directly to lower blood pressure because of the decrease in systemic vascular resistance
Increased perfusion and development of muscular capillary beds. Increased perfusion of muscular beds may be the primary mechanism for the acute decrease in blood pressure. With regular exercise, angiogenesis causes further development of muscular capillary beds as well. The opening of arterioles to perfuse these (now more developed) capillary beds also decreases systemic vascular resistance (and along with it, blood pressure).
Improved insulin sensitivity: insulin sensitivity is compromised in many people with high blood pressure, whether or not they have frank diabetes. The reduction in circulating insulin improves clearance of lipids and glucose from the blood, and theoretically reduces the downstream negative effects of high circulating lipids and glucose, including atherosclerosis. This has a direct effect on hypertension as well as the downstream negative effects of hypertension.
Decreased inflammation: regular exercise reduces circulating levels of inflammatory mediators, improving endothelial function (and decreasing atherosclerosis). This also has a direct effect on hypertension as well as its sequelae.
Improved cardiac function: regular exercise improves diastolic filling (directly, through beneficial adaptations in cardiac muscle, and, probably, indirectly, through the above effects on systemic vascular resistance, atherosclerosis, and endothelial function).
An excellent broad ranging review of the health benefits of physical activity discusses these benefits. You can find it for free here
A more recent review by the same first author is here, but it is behind a paywall.