Question

chemoreceptors in blood vessels

Accepted Answer

sensitive to changes in blood pH (e.g more CO2 in blood, more interacts with water to from carbonic acid so pH decreases), chemoreceptors detect this and response increases heart rate so blood travels quicker to lungs and more CO2 exhaled
if low CO2, pH increases and chemoreceptors detect change, results in less impulses sent to medulla and so less impulses sent to SAN via sympathetic NS, and so heart rate returns to normal

Chemoreceptors Blood Ph Co2 Detect Increases Heart Rate