Controlling the Heart Rate

Heart rate must change to match the body's demands. During exercise, muscles respire faster, so they need more oxygen and glucose and produce more carbon dioxide and heat. The heart must pump more blood to supply them.The volume of blood pumped by one ventricle in one minute is the cardiac output:cardiac output = heart rate × stroke volumeHeart rate is measured in beats per minute, and stroke volume is the volume of blood pumped by the ventricle in one beat.Worked example: at rest, heart rate = 70 beats min⁻¹ and stroke volume = 70 cm³, so cardiac output = 70 × 70 = 4900 cm³ min⁻¹ (about 4.9 dm³ min⁻¹).Cardiac muscle is myogenic: it contracts without needing impulses from nerves. Each beat starts at the sinoatrial node (SAN), the pacemaker in the wall of the right atrium, which sends waves of excitation across the atria. The atrioventricular node (AVN) delays the impulse, then passes it down the bundle of His and the Purkyne fibres, so the ventricles contract from the apex upwards.Because the heart beats by itself, the nervous and hormonal systems do not start each beat. Instead they modify the rate at which the SAN fires. Heart rate is adjusted by:• nerves from the cardiovascular control centre in the medulla oblongata
• information from receptors that detect blood pressure and blood chemistry
• the hormone adrenalineChanging both heart rate and stroke volume allows cardiac output to rise to several times its resting value during intense exercise.