Hormonal Control of Heart Rate

Heart rate is also controlled by hormones, especially during the fight or flight response. When a threat is detected, the hypothalamus activates the sympathetic nervous system, which stimulates the adrenal medulla to release adrenaline into the blood.Adrenaline travels in the blood to the heart and binds to receptors on the cells of the sinoatrial node (SAN). This increases the frequency of excitation of the SAN, so heart rate increases. Adrenaline also increases the force of contraction of the ventricles, so stroke volume increases.Noradrenaline released by the accelerator nerve endings has a similar effect directly at the SAN.The result is a greater cardiac output:cardiac output = heart rate × stroke volumeWorked example: at rest, heart rate is 70 beats min⁻¹ and stroke volume is 70 cm³, giving a cardiac output of 4900 cm³ min⁻¹. After adrenaline release, heart rate is 120 beats min⁻¹ and stroke volume is 90 cm³, so cardiac output is 120 × 90 = 10 800 cm³ min⁻¹.More oxygen and glucose are delivered to the muscles and more carbon dioxide is removed, preparing the body for intense activity. Adrenaline has other fight or flight effects, such as dilating the bronchioles and stimulating glycogenolysis in the liver, but its effect on the heart is to raise both rate and force.