While the basal heart rate is determined within the heart by the pacemaker, it can be regulated by external signals
Nerve signals from the medulla of the hindbrain can trigger rapid changes, while endocrine signals can trigger more sustained changes
NERVE SIGNALLING
The pacemaker is under autonomic (involuntary) control from the brain, specifically the medulla oblongata (brain stem)
Two nerves connected to the medulla regulate heart rate by either speeding it up or slowing it down:
The sympathetic nerve releases the neurotransmitter norepinephrine to increase heart rate
The parasympathetic nerve (vagus nerve) releases the neurotransmitter acetylcholine to decrease heart rate
Since the sympathetic nerve and the vagus nerve have opposite effects, we say they are antagonistic.
C3.1.14: Feedback control of heart rate following sensory input from baroreceptors and chemoreceptors.
- Outline the structures and functions of nervous tissue that can regulate heart rate.
- Describe factors that will increase heart rate.
- Describe factors that will decrease heart rate.
- Chemoreceptors in carotid arteries, the aorta and the brain monitor the level of carbon dioxide in the blood and pH level.
- Increased carbon dioxide or decreased pH levels causes the chemoreceptors to signal the heart to beat faster. By increasing blood flow, carbon dioxide can be moved more quickly to the lungs where it is removed by diffusion into the alveolar spaces. This lowers carbon dioxide levels and returns blood pH to safe limits.
- When exercising, more CO2 is present in the blood. This is detected by chemoreceptors in the brain’s medulla oblongata, resulting in a nerve signal being sent to the SA node to speed the heart rate
When CO2 levels fall, another nerve (Vegus) reduces heart rate - Nerves supplying the cardiovascular centre bring impulses from baroreceptors (stretch receptors) located in the walls of the aorta, in the carotid arteries, and in the wall of the right atrium, when change in blood pressure at these positions is detected.
- When blood pressure is high in the arteries, the heart rate is lowered by impulses from the cardiovascular centre, via the vagus nerve.
When blood pressure is low, the heart rate is increased.
NOTE:
- Electrical impulses are sent from the chemoreceptors and baroreceptors to the medulla and, depending on the message, the medulla responds by stimulating either the sympathetic or parasympathetic nerves.
- the cardiovascular control centre of the medulla INCREASES/DECREASES the rate of sinoatrial (SA) node firing through activation of the sympathetic nervous system.
C3.1.12: Epinephrine secretion by the adrenal glands to prepare the body for vigorous activity.
- Outline conditions that will lead to epinephrine secretion.
- Explain the effect of epinephrine on heart rate.
- strong emotions like stress or excitement will trigger the adrenal gland to release epinephrine/exercise to prepare the body for physical activity
- epinephrine/adrenaline increases heart rate HORMONALLY (causes the sympathetic nervous system to be activated)
B3.2.16: Stages in the cardiac cycle.
State the cause of systolic and diastolic pressure.
Describe how sound is used to measure blood pressure.
- The systolic pressure is a higher value that represents the pressure in the vessel when the heart is contracting (i.e. pulse flow)
- The diastolic pressure is a lower value that represents the pressure in the vessel when the heart is relaxing
-Blood pressure is measured in arteries using a sphygmomanometer
- The sphygmomanometer is a blood pressure cuff that cuts off circulation to a region (typically the brachial artery in the arm)
- The pressure of the cuff is slowly released until a pulse can be audibly detected with a stethoscope (systolic pressure)
- The pressure continues to be released from the cuff until a pulse can no longer be audibly detected (diastolic pressure)
NOTE:
- receptors to medulla to SA node to parasympathetic/sympathetic nerve to decrease/increase heart rate
- CO2 AND HEART RATE - DIRECTLY PROPORTIONAL
- BP AND HEART RATE - INDIRECTLY PROPORTIONAL
