Control of Heart Rate

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Structure of the Nervous System

Structure of the Nervous System

The nervous system is split into two different systems- the central nervous system (CNS) and the peripheral nervous system. The CNS is made up of the brain and spinal, whereas the peripheral nervous system is made up of the neurones that connect the CNS to the rest of the body.

The peripheral nervous system also has two different systems- the somatic and autonomic nervous system. The somatic nervous system controls conscious activities, e.g. running and playing video games. The autonomic nervous system controls unconscious activities, e.g. heart rate.

The autonomic nervous system is split into the sympathetic and parasympathetic nervous systems, which have opposite effects on the body. The sympathetic nervous system is the 'fight or flight' system that gets the body for action. Sympathetic neurones release a neurotransmitter called noradrenaline. The parasympathetic system is the 'rest and digest' system that calms the body down. Parasympathetic neurones release a neurotransmitter called acetlcholine.

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Control of heart rate- the nervous system

Control of heart rate- the nervous system

There is a small mass of tissue in the wall of the right atrium of the heart called the sinoatrial node (SAN). The SAN generates electrical impulses that cause the cardiac muscles to contract. The rate at which the SAN fires is unconsciously controlled by the cardiovascular centre in a part if the brain called the medulla oblongata.

Animals need to alter their heart rate to respond to intenral stimuli, e.g. to prevent fainting due to low blood pressure or to make sure the heart rate is high enough to supply the body with enough oxygen. Internal stimuli are detected by pressure receptors and chemical receptors:

  • There are pressure receptors called baroreceptors in the aorta, the vena cava and caroid arteries. they are stimulated by high and low blood pressure
  • There are chemical receptors called chemoreceptors in the aorta, the carotid arteris and in the medulla oblongata. They monitor the oxygen level in the blood and also carbon dioxide and pH.

Electrical impulses from receptors are sent to the cardiovascular centre along sensort neurones. The cardiovascular centre processes the information and sends impulses to the SAN along motor neurones.

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Control of heart rate to different stimuli

Control of heart rate in response to different stimuli

1) High blood pressure
      Baroreceptors detect high blood pressure and send impulses aling sensory neurones to the cardiovascular centre, which sends impulses along parasympathetic neurones. These secrete acetylcholine, which binds to receptors on the SAN. This causes the heart rate to slow down in order to reduce blood pressure back to normal.

2) Low blood pressure
      Baroreceptors detect low blood pressure and send impulses along sensort neurones to the cardiovascular centre, which sends impulses along sympathetic neurones. These secrete noradrenaline, which binds to receptors on the SAN. This causes the heart rate to speed up in order to increase blood pressure back to normal. 

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Control of heart rate to different stimuli

Control of heart rate in response to different stimuli

3) High blood oxygen, low carbon dioxide or high blood pH levels
      Chemoreceptors detect changes in the blood and send impulses along sensory neurones to the cardiovascular centre, which sends impulses along parasympathetic neurones. These secrete acethylcholine, which binds to receptors on the SAN. This causes the heart rate to decrease in order to return oxygen, carbon dioxide and pH levels back to normal.

4) Low blood oxygen, high carbon dioxide and low blood pH levels
       Chemoreceptors detect changes in the blood and send impulses along sensory neurones to the cardiovascular centre, which sends impulses along sympathetic neurones. These secrete noradrenaline, which binds to receptors on the SAN. This causes the heart rate to increase in order to return oxygen, carbon dioxide and pH levels back to normal. 

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Control of heart rate- the hormonal system

Control of heart rate- the hormonal system

When an organism is threatended (e.g. by a predator) the adrenal glands release adrenaline. Adrenaline binds to specific receptors in the heart. This causes the cardiac muscle to contract more frequently and with more force, so heart rate increases and the heart pumps more blood.

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