14 Control of the heart rate 0.0 / 5 ? BiologyOrganisms respond to changes in their environmentA2/A-levelAQA Created by: lee8444Created on: 01-03-20 11:30 Autonomic nervous system Sympathetic nervous system stimulates effectors speeds up activities emergency controller strenuous exercise powerful emotions cope with stressful situations heightens awareness prepares us for activity (flight or fight) Parasympathetic nervous system inhibits effectors slows down activity controls activities under resting conditions conserves energy replenishes body's reserves They are antagonistic to each other, one contracts a muscle, the other relaxes it 1 of 5 Control of heart rate Cardiac muscle is myogenic - contraction is initiated from the muscle itself - not from nervous impulses from the outside (neurogenic) SAN (sinoatrial node) is in the wall of the right atrium This initiates the stimulus for contraction Has a basic rhythm that determines the heart rate - referred to as the pacemaker A wave of electrical excitation spreads from the SAN across both atria Atria contract Non-conductive tissue (atrioventricular septum) prevents the wave from passing to the ventricles The wave enters the AVN (atrioventricular node) in between the atria After a short delay, the AVN conveys a wave between the ventricles through Purkyne tissue which collectively makes the bundle of His The wave has now been conducted to the bottom of the ventricles where the bundle branches into smaller fibres of Purkyne tissue The wave is passed up from the Purkyne tissue causing the ventricles to contract quickly from the bottom of the heart upwards 2 of 5 Modifying the resting heart rate Typically around 70bpm Changes are controlled by the medulla oblongata which has 2 centres a centre that increases the heart rate which links to the SAN via the sympathetic nervous system a centre that decreases the heart rate which links to the SAN via the parasympathetic nervous system 3 of 5 Control by chemoreceptors Chemoreceptors are found in the wall of the carotid arteries (ones that serve the brain) Sensitive to changes in blood pH due to carbon dioxide concentrations Increase in carbon dioxide causes a decrease in pH Chemoreceptors in carotid arteries and aorta detect this and increase the frequency of nervous impulses to medulla oblongata's sympathetic centre This increases the number of impulses to the SAN This increases the number of waves sent out by the SAN Increased blood flow More carbon dioxide is removed by the lungs Carbon dioxide concentrations decrease The pH of the blood rises to normal levels Carotid arteries and aorta send fewer impulses to the medulla oblongata Fewer impulses are sent by the brain to the SAN decreasing the heart rate 4 of 5 Control by pressure receptors When blood pressure is too high pressure receptors transmit nervous impulses to the parasympathetic centre in the medulla oblongata this centre sends impulses to the SAN this decreases the number of waves sent out by the SAN this decreases the heart rate this decreases blood pressure When blood pressure is too low pressure receptors transmit more nervous impulses to the centre in the medulla that increases the heart rate this centre sends more nervous impulses to the SAN this increases the number of waves sent out by the SAN this increases the heart rate this increases blood pressure 5 of 5
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