Short-term aerobic exercise such as running for 30 minutes will not increase the size of your heart muscle, nor will it thicken the walls of your heart. However, long-term aerobic exercise will increase the size and thickness of your heart, especially during moderately to vigorously intense exercise. These changes are reversible when you discontinue aerobic training.
When you take one aerobic class, the blood vessels going to your working skeletal muscles increase in size, or dilate so more blood can flow to these muscles. Long-term aerobic exercise improves the elasticity of your blood vessels, or the ability of your vessels to expand and contract. The improved elasticity delivers more oxygen and glucose to your muscles at a faster rate. The number of capillaries in your working muscles also increases as an adaptation to long-term aerobic exercise.
The amount of blood circulating in your body increases within 24 hours after your first exercise session. According to William McArdle and colleagues in the book, "Exercise Physiology Energy, Nutrition & Human Performance," the long-term effect of aerobic exercise is an approximately 20 percent increase in blood volume. An increase in blood volume means your body can deliver more oxygen to your working muscles. Your body will also be able to better regulate your body temperature during exercise.
Cardiac output is the amount of blood your heart pumps out in one minute. It is regulated by the amount of blood your heart pumps out in one contraction, or stroke volume, and the number of times your heart beats in one minute, or your heart rate. Cardiac output increases as a short-term and a long-term effect of regular aerobic exercise. Initially, cardiac output increases due to an increase in your heart rate. Then, as your aerobic fitness improves, cardiac output increases due to an increase in stroke volume and a decrease in heart rate.
Your systolic pressure is the pressure against your artery walls when your heart contracts. Diastolic pressure is the pressure against your artery walls when your heart relaxes. When you first start to aerobic exercise, your blood pressure increases as your cardiovascular system works to deliver more oxygen and glucose to your working muscles. A long-term adaptation to aerobic exercise is a decrease in both your systolic and diastolic blood pressures during rest and during sub-maximal exercise. The American College of Sports Medicine encourages regular aerobic exercise as a method of controlling and reducing high blood pressure due to its effects on lowering your blood pressure.