HEART:
- Heart muscle is myogenic (impulse to contract the heart muscles originates from within
the heart) resulting in depolarisation
- Does not require any external stimulus
The control of cardiac cycle:
- SAN receives impulses from the cardiovascular center in the medulla oblongata
- SAN acts as a pacemaker
- SAN initiates depolarisation which stimulates the contraction of the atria
- This forces oxygenated blood into ventricles
- The impulse then spreads to AV node to delay impulse*
- Impulse then travels through the bundle of His through the Purkinje fibres
- This depolarises the ventricles and causes ventricular contraction (forces oxygenated
blood into aorta)
*delays impulse due to:
- Allows time for atria to contract fully
- Ensures all the blood can flow from atria into ventricle
- Allows sufficient time for AV valves to close (prevents backflow of blood)
- Ensures ventricles do not contract too early
Why is there a change in the cardiac cycle during exercise?
- chemoreceptors (eg. in aortic/carotid body) detect decrease in blood pH, (due to
high CO2
- SAN is stimulated to generated more impulses per second
- Therefore, cardiac cycle is shorter
Heart is supplied by two nerves:
- Decelerator nerve (parasympathetic nerve): DECREASES the rate of polarization at
SAN. The synapse releases acetylcholine
- This SLOWS DOWN the rate of heartbeat
- Accelerator nerve (sympathetic nerve): INCREASES the rate of depolarization at SAN.
The synapse releases nor-adrenaline
- This INCREASES the rate of heartbeat
When controlling the heart rate, chemical and pressure receptors in carotid artery and aorta
send electrical impulses to the cardiovascular centre in the medulla oblongata
This is the area of control for increasing and decreasing heart rate
The events leading to a change in heart rate:
, - If chemoreceptors (eg. in aortic/carotid body) detect decrease in blood pH, this
means there is a high concentration of CO2
- They send nerve impulses to the cardiovascular center (in medulla oblongata) for
increasing heart rate
- Impulses travel via sympathetic nerve to SAN
- Noradrenaline released onto SAN
- Rate of depolarisation at SAN increases
- Increases force of contraction of cardiac muscles
- Blood flows through lungs more frequently and hence more CO2 is removed
- As pH in blood returns to normal, nerve impulse are sent from chemoreceptors to
cardiovascular centre for decreasing heart rate, which sends nerve impulses through a
parasympathetic nerve to the SAN to decrease heart rate to normal
- This is an example of a negative feedback mechanism
Measuring heart rate
Electrocardiography (ECG) is an interpretation of electrical activity of the heart over a period of
time. It helps diagnose cardiovascular diseases
P wave: depolarisation of the atria, electrical impulses sent from SAN to AVN (atrial systole)
QRS complex: depolarisation of the ventricles, higher amplitude due to ventricle having more
muscle (ventricular systole)
T wave: repolarisations of the atria and the ventricles (diastole)
How does an ECG help calculate a person’s heart rate:
- ECG shows the electrical activity of the heart
- Time taken for one heartbeat (is from one P wave to the next)
- Count the number of peaks in a set time
- Heart rate is number of beats divided by time
BREATHING RATE:
Inhalation:
- Inspiratory centre in the medulla oblongata sends impulses to intercostal
muscles/diaphragm for contraction
- Increases volume of lungs, decreases pressure in lungs
Exhalation:
- Stretch receptors in lungs are stimulated
- Stretch receptors send nerve impulses back to the medulla oblongata. These impulses
inhibit the action of the inspiratory centre
- Heart muscle is myogenic (impulse to contract the heart muscles originates from within
the heart) resulting in depolarisation
- Does not require any external stimulus
The control of cardiac cycle:
- SAN receives impulses from the cardiovascular center in the medulla oblongata
- SAN acts as a pacemaker
- SAN initiates depolarisation which stimulates the contraction of the atria
- This forces oxygenated blood into ventricles
- The impulse then spreads to AV node to delay impulse*
- Impulse then travels through the bundle of His through the Purkinje fibres
- This depolarises the ventricles and causes ventricular contraction (forces oxygenated
blood into aorta)
*delays impulse due to:
- Allows time for atria to contract fully
- Ensures all the blood can flow from atria into ventricle
- Allows sufficient time for AV valves to close (prevents backflow of blood)
- Ensures ventricles do not contract too early
Why is there a change in the cardiac cycle during exercise?
- chemoreceptors (eg. in aortic/carotid body) detect decrease in blood pH, (due to
high CO2
- SAN is stimulated to generated more impulses per second
- Therefore, cardiac cycle is shorter
Heart is supplied by two nerves:
- Decelerator nerve (parasympathetic nerve): DECREASES the rate of polarization at
SAN. The synapse releases acetylcholine
- This SLOWS DOWN the rate of heartbeat
- Accelerator nerve (sympathetic nerve): INCREASES the rate of depolarization at SAN.
The synapse releases nor-adrenaline
- This INCREASES the rate of heartbeat
When controlling the heart rate, chemical and pressure receptors in carotid artery and aorta
send electrical impulses to the cardiovascular centre in the medulla oblongata
This is the area of control for increasing and decreasing heart rate
The events leading to a change in heart rate:
, - If chemoreceptors (eg. in aortic/carotid body) detect decrease in blood pH, this
means there is a high concentration of CO2
- They send nerve impulses to the cardiovascular center (in medulla oblongata) for
increasing heart rate
- Impulses travel via sympathetic nerve to SAN
- Noradrenaline released onto SAN
- Rate of depolarisation at SAN increases
- Increases force of contraction of cardiac muscles
- Blood flows through lungs more frequently and hence more CO2 is removed
- As pH in blood returns to normal, nerve impulse are sent from chemoreceptors to
cardiovascular centre for decreasing heart rate, which sends nerve impulses through a
parasympathetic nerve to the SAN to decrease heart rate to normal
- This is an example of a negative feedback mechanism
Measuring heart rate
Electrocardiography (ECG) is an interpretation of electrical activity of the heart over a period of
time. It helps diagnose cardiovascular diseases
P wave: depolarisation of the atria, electrical impulses sent from SAN to AVN (atrial systole)
QRS complex: depolarisation of the ventricles, higher amplitude due to ventricle having more
muscle (ventricular systole)
T wave: repolarisations of the atria and the ventricles (diastole)
How does an ECG help calculate a person’s heart rate:
- ECG shows the electrical activity of the heart
- Time taken for one heartbeat (is from one P wave to the next)
- Count the number of peaks in a set time
- Heart rate is number of beats divided by time
BREATHING RATE:
Inhalation:
- Inspiratory centre in the medulla oblongata sends impulses to intercostal
muscles/diaphragm for contraction
- Increases volume of lungs, decreases pressure in lungs
Exhalation:
- Stretch receptors in lungs are stimulated
- Stretch receptors send nerve impulses back to the medulla oblongata. These impulses
inhibit the action of the inspiratory centre
