Control Of Heart Rate In Humans

In order to adapt and supply the body with more oxygen and glucose the heart -

• can increase the number of beats per minute
• can increase the strength of its contractions
• can increase the stroke volume (amount of blood pumped per beat)

The heart is supplied by neves from the medulla oblongata of the brain. These nerves connect to the brain. These can affect the frequencies of the contractions. Action potentials sent down the -

- accelerator nerve increases the heart rate

- vagus nerve decreases the heart rate

The SAN sends waves of excitation to regulate the heart rate. The frequency of these waves of excitation can controlled by the cardiovascular centre in the medulla oblongata.

• Movement of the limbs is detected by stretch receptors in the muscles. These send impulses to the cardiovascular centre, informing it that more oxygen may soon be needed. Heart rate then ususally increases. 
• When exercising we produce more carbon dioxide, some of which reacts with the water in the blood plasma and reduces its pH. The change of pH is detected by chemoreceptors in the carotid arteries, the aorta and the brain. The chemoreceptors send impluses to the cardiovascular centre, thus increasing the heart rate. 
• When we stop exercising - concentration of carbon dioxide in the blood falls so the activity of the accelerator pathway is reduced - heart rate declines.
• The hormone adrenaline is secreted in response to stress, shock, anticipation or excitement. Adrenaline increases heart rate. This helps prepare the body for activity. 
• Blood pressure is monitored by stretch receptors in the walls of the carotid sinus (a small sweliing in the carotid artery). If blood pressure raises too high, like during exercise, the strech receptos send signals to the cardiovascular centre and this results in a reduced heart rate.