The body’s adaptation to the demands of aerobic activity involves a complex sequence of heart-healthy events. Cardiorespiratory endurance improves and the body delivers oxygen more efficiently. With cardio-respiratory endurance, the total blood volume and the number of red blood cells increase, so the blood can carry more oxygen. The heart muscle becomes stronger and larger, and its cardiac output increases. Each beat empties the heart’s chambers more completely, so the heart pumps more blood per beat, in other words it’s stroke volume increases. This makes fewer beats necessary, so the pulse rate falls. The muscles that inflate and deflate the lungs gain strength and endurance so breathing becomes more efficient. Blood moves easily through the blood vessels because the muscles of the heart contract powerfully, and contraction of the skeletal muscles pushes the blood through the veins. Such improvements keep resting blood pressure normal. The improvements that come with this type of training also raise blood HDL, the lipoprotein associated with lower heart disease risk.
Which activities produce these beneficial changes? Effective activities elevate the heart rate, are sustained for longer than 20 minutes, and use most of the large-muscle groups of the body (legs, buttocks, and abdomen). Examples are swimming, cross-country skiing, rowing, fast walking, jogging, fast bicycling, soccer, hockey, basketball, inline-skating, lacrosse, beach volleyball, and rugby.
An informal pulse check can give you some indication of how conditioned your heart is. The average resting pulse rate for adults is around 70 beats per minute. Active people can have resting pulse rates of 50 or even lower.

To recap, cardio-respiratory endurance training enhances the ability of the heart and lungs to deliver oxygen to the muscles. With this type of training the heart becomes stronger, breathing becomes more efficient, and the health of the entire body improves.