Making the Heart Pump Harder
Your heart, also a muscle, gets a workout during exercise, too, and its job is to get more blood out to the body's hard-working muscles. The heart's blood flow increases by about four or five times from that of its resting state. Your body does this by increasing the rate of your heartbeat and the amount of blood that comes through the heart and goes out to the rest of the body. The rate of blood pumped by the heart (cardiac output) is a product of the rate at which the heart beats (heart rate) and the volume of blood that the heart ejects with each beat (stroke volume). In a resting heart, the cardiac output is about 5 liters a minute (0.07 L x 70 beats/min = 4.9 L/min). As you begin to exercise, sympathetic nerves stimulate the heart to beat with more force and faster; the heart rate can increase about threefold. Also, the sympathetic nerve stimulation to the veins causes them to constrict. This, along with more blood being returned from the working muscles, increases the amount of blood returned to the heart (venous return). The increased venous return helps to increase the stroke volume by about 30 to 40 percent. When the heart is pumping at full force, the cardiac output is about 20-25 liters per minute.
Breathing Faster and Deeper
So far, we have talked about getting more blood to working muscle. Your lungs and the rest of your respiratory system need to provide more oxygen for the blood, too. The rate and depth of your breathing will increase because of these events:
- Sympathetic nerves stimulate the respiratory muscles to increase the rate of breathing.
- Metabolic byproducts from muscles (lactic acid, hydrogen ions, carbon dioxide) in the blood stimulate the respiratory centers in the brainstem, which, in turn, further stimulates the respiratory muscles.
- Slightly higher blood pressure, caused by the increased force of each heartbeat and by the elevated cardiac output, opens blood flow to more air sacs (alveoli) in the lungs. This increases the ventilation and allows more oxygen to enter the blood.
As the lungs absorb more oxygen and the blood flow to the muscles increases, your muscles have more oxygen.