control of the heart rate

• There's a small mass of tissue in the wall of the right atrium of the heart called the sino-atrial node (SAN). The SAN generates electrical impulses that cause the cardiac muscles to contract. The rate at which the SAN fires (i.e. heart rate) is unconsciously controlled by the cardiovascular centre in the medulla oblongata (a structure in the brain)
• Animals need to alter their heart rate to respond to internal 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.
• control of the heart rate

  A) cardiovascular centre

  B) sensory neurones

  C) SAN

  D) carotid artery

  E) aorta

  F) sympathetic neurone

  G) parasympathetic neurone

  H) chemoreceptors

  I) baroreceptors

  
• Internal stimuli are detected by pressure receptors and chemical receptors:

  • There are pressure receptors called baroreceptors in the aorta and the carotid arteries. They're stimulated by high and low blood pressure.
  • There are chemical receptors called chemoreceptors in the aorta, the carotid arteries and in the medulla oblongata. They monitor the oxygen level in the blood and also carbon dioxide and pH (which are indicators of O, level).
• Nerve impulses from receptors are sent to the cardiovascular centre along sensory neurones. The cardiovascular centre processes the information and sends impulses to the SAN along motor neurones.
• Baroreceptors detect high blood pressure and send impulses along 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.
• Baroreceptors detect low blood pressure 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 speed up in order to increase blood pressure back to normal.
• Chemoreceptors detect chemical changes in the blood and send impulses along 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 decrease in order to return oxygen, carbon dioxide and pH levels back to normal.
• Chemoreceptors detect chemical 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.
• When an organism is threatened 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.