Organisms respond to changes in their internal and external environments AQA A2 Biology PART 3 of 9 TOPICS: Control of heart rate

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Organisms respond to changes in their internal and external
environments (AQA A2 Biology) PART 3 of 9 TOPICS
Control of heart rate:
Action potentials originate in the sinoatrial node and travel across the wall of the atrium to the
atrioventricular node on the right side of the heart. This passes slowly giving time for the atria to
contract and empty all the blood into the ventricle. This is then passed along the atrioventricular
bundle into the interventricular septum along the bundle of His. This bundle separates into two at the
apex (bottom) of the ventricles and goes upwards along each of the ventricle walls. The action
potentials travel along this pathway and is carried deeper into the ventricular walls by the Purkinje
fibres. This provides unison of a strong contraction by both the ventricles and allowing the ventricles
to empty properly.
Chemoreceptors, located in the aorta and the carotid artery, monitor the CO2 levels, O2 levels and pH
levels in the blood. When the receptor picks up the stimulant it transfers it to the sensory neurone to
the medulla oblongata (also known as medulla) which also has chemoreceptors. The impulse then
travels along the sympathetic neurone or parasympathetic neurone depending on the stimuli to the
sinoatrial node. The effector, the cardiac muscle, can have two responses depending on the stimulus
and route it takes:
If there are low levels of O2 and pH with high levels of CO2 then the impulse will go through
the sensory neurone to the medulla and through the sympathetic neurone releasing
noradrenalin from the sinoatrial node (an excitatory neurotransmitter) which makes the heart
rate faster.
If there are high levels of O2 and pH with low levels of CO2 then the impulse will go through
the sensory neurone to the medulla and through the parasympathetic neurone releasing
acetylcholine from the sinoatrial node (an inhibitory neurotransmitter) which makes the heart
rate slower. This is so the levels of O2 and pH can lower and CO2 levels can be raised.
Baroreceptors, located in the carotid artery and aorta only, detect changes in the pressure in the
blood. The routes are exactly the same for which the nerve impulses take:
If there is high blood pressure the route it follows is through the parasympathetic neurone
which releases acetylcholine from the sinoatrial node slowing the heart rate down to lower
the blood pressure.
If there is low blood pressure the route it follows is through the sympathetic neurone which
releases noradrenalin from the sinoatrial node making the heart rate faster to raise the blood
pressure.

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