The role of chemoreceptors and pressure receptors, the autonomic nervous system and effectors in controlling heart rate.

The autonomic nervous system carries messages around without concious input. One example being that heart rate is controlled in this way, without having to think about it.

The autonomic nervous system has two different roles:

• To stimulate, fight or flight,(the sympathetic nervous system): increase blood pressure, increase cardiac functions, dilate pupils, increase ventilation, relax the bladder
• To inhibit and slow down, rest and digest (the parasympathetic nervous system): the opposite of above.



In the control of heart rate, chemical and pressure receptors in the carotid artery and aorta send signals to the medulla oblongata in the brain. This is the area in control of heart rate and has a center for increasing and a center for decreasing heart rate.

The events leading to a change in heart rate go as follows:

• If the chemo receptors sense a low PH level in the blood, it means there is a high concentration of CO2.
• They send signals to the center for increasing heart rate through sensory neurones.
• A sympathetic nerve then takes a signal from this center to the SAN which increases the heart rate.
•  Blood flows through the lung more frequently and therefore more CO2 is removed.
• As the PH in the blood returns to normal.
• signals are sent from the chemo receptors that the PH is normal, to the center for decreasing heart rate, which sends a signal through a parasympathetic nerve to the SAN to decrease heart rate to normal.

In a similar way if blood pressure is too high a signal will be sent with the response of a decreased heart rate by the parasympathetic nerves and if it is too low the sympathetic nerves will be used to increase it.