The ways stress affects the heart are both direct and indirect, so I think that needs to be part of your thesis. Many writers find it helpful to construct a thesis that leads naturally to the body paragraphs that will follow, so I might work with a thesis along these lines:
Because the body's systems are intricately interconnected, stress affects the heart both directly and indirectly.
You could then follow with a body paragraph that focus on the more direct links between stress and the heart. Stress raises blood pressures, making it necessary for the heart to exert more force to move the blood through the body. High blood pressure also has the potential to thicken the arteries in your heart or create bulges and weak spots, putting you at more risk for aneurysms and heart attacks. Stress can also change the way blood clots in your body, making a clot in the heart more likely.
But stress also indirectly affects heart health through the personal decisions people make when they are stressed. Consider some of the ways you see people coping with high levels of stress. Some people turn to comfort foods, consuming large quantities of high-fat, high-carb foods. Others turn to drinking, and some of that can be pretty heavy. Others turn to smoking or drug use. All of these coping methods for stress have proven to be bad for heart health. You could use the second body paragraph to examine the ways poor decisions in coping with stress indirectly create situations that are harmful to heart health. How does heavy drinking affect your heart? How does a high-carb diet affect your heart?
You could then conclude the paper with some ways to overcome these heart stressors and build a more heart-healthy lifestyle.