Control by Chemoreceptors

The Control of Heart Rate

Heart rate is controlled by the autonomic nervous system.

Autonomic means self-governing, the autonomic nervous system controls the involuntary (or subconscious) workings of the internal muscles and glands.

It divides into two: 

1.       The sympathetic Nervous System

Used to help us cope with stressful situations by heightening our awareness and preparing us for activity

2.       The Parasympathetic Nervous System

In general, this inhibits effectors and slows down any activity. The parasympathetic nervous system also controls activities under normal resting conditions and its purpose is to conserve energy and to replenish the body’s reserves.

These systems are often antagonistic. If one system contracts a muscle, then the other relaxes it. Our internal glands and muscles are therefore regulated by a balance of the two systems.

One such example is heart rate:

·         The resting heart rate of an average human being is around 70 beats per minute

·         During exercise the resting heart rate may need to more than double to supply the active tissue with oxygen and food.

Changes to the heart rate are controlled by the Medulla Oblongata a region of the brain which has two centres.

1)      The first decreases heart rate- Parasympathetic Nervous System 

2)      The second that increases heart rate – Parasympathetic Nervous System

Both of these centres and linked to the Sino atrial node by their various nervous systems

The site that is stimulated depends upon the stimuli received by the receptor. In the case of heart rate it could be either

1)      Chemical changes in the blood, detected by chemoreceptors

2)      Pressure changes in the blood detected by Baroreceptors

Control by Chemoreceptors

·         Chemoreceptors detect chemical changes and are found in the walls of the carotid arteries which serve the brain.

·         These chemoreceptors are sensitive to changes in the pH of the blood that result from the changes in carbon dioxide concentration changes.

The process works as follows

1)      When pH falls the chemoreceptors in the walls of the carotid arteries and aorta detect this and increase the frequency at which they send nervous impulses to the centre in the medulla oblongata that is responsible for increasing heart rate.

2)      Via the sympathetic nervous system, the medulla oblongata in turn increases the rate at which it send nervous impulses to the Sino atrial node.

3)      The Sino atrial node as a result increases the rate at which it causes the muscles of the heart to contract.

4)      This means that the blood travels around the body faster and thus more carbon dioxide is removed from the blood by the lungs.

5)      With less carbon dioxide in the blood the pH returns to its normal value.

6)      This rise in pH is detected by the carotid arteries and they reduce the frequency at which they send impulses to the medulla oblongata causing the heart rate to return to normal.

Control by the pressure receptors

These receptors are also found in the carotid arteries and walls of the aorta.

They operate much the same as the chemoreceptors

1)      A higher than normal blood pressure is detected by the pressure receptors

2)      Nervous impulses are transmitted to the centre in the medulla oblongata

3)      The medulla oblongata in turn decreases the rate at which it sends nervous impulses to the Sino atrial node via the parasympathetic nervous system.

4)      The Sino atrial node stops the heart beating as fast or as powerful

5)      Blood pressure falls.