Heart rate is also affected by the hormone adrenaline. Adrenaline is secreted by the adrenal medulla (the inner part of the adrenal glands, which sit above the kidneys) when the sympathetic nervous system stimulates it during stress, fear, excitement or exercise.Adrenaline travels in the blood plasma to all parts of the body, but only affects target cells that have adrenaline receptors on their cell surface membrane. It acts as a first messenger and does not enter the cell. When it binds to its receptor, it activates the enzyme adenylyl cyclase, which converts ATP into cyclic AMP (cAMP). cAMP acts as a second messenger inside the cell and triggers the cell's response.Effects of adrenaline on the heart:• it binds to receptors on cells of the SAN, which then fires more frequently, so heart rate increases
• it makes cardiac muscle contract more forcefully, so stroke volume increasesBoth effects raise cardiac output, so more oxygen and glucose reach the respiring muscles.Comparison with nervous control: nervous control through the accelerator and vagus nerves is very fast and precise, acting within a second or so, and lasts only as long as the impulses continue. Hormonal control is slower to start, because adrenaline must be secreted and carried in the blood, but its effects are more widespread and last longer, until the adrenaline is broken down (mainly in the liver).Adrenaline therefore reinforces the sympathetic nervous system. It can prepare the body for 'fight or flight' in advance, for example raising heart rate before a race begins.
• it makes cardiac muscle contract more forcefully, so stroke volume increasesBoth effects raise cardiac output, so more oxygen and glucose reach the respiring muscles.Comparison with nervous control: nervous control through the accelerator and vagus nerves is very fast and precise, acting within a second or so, and lasts only as long as the impulses continue. Hormonal control is slower to start, because adrenaline must be secreted and carried in the blood, but its effects are more widespread and last longer, until the adrenaline is broken down (mainly in the liver).Adrenaline therefore reinforces the sympathetic nervous system. It can prepare the body for 'fight or flight' in advance, for example raising heart rate before a race begins.