controlling heart rate#

• the human heart pumps blood around the circulatory system, it has several important roles:

  • transport of oxygen and nutrients, like glucose, fatty acids, and amino acids to the tissues
  • removal of waste products like carbon dioxide from the tissues to prevent the build up of toxic materials
  • transport of urea from the liver to the kidneys
  • distribution of heat around the body or to deliver it to the skin to be radiated away.
• It is essential that the circulatory system can adapt to meet the needs of the body's tissues- part of this adaptation is controlling the heart's activity:

  • raising or lowering heart rate, number of beats per minute will change
  • altering force of contractions of the ventricles
  • altering stroke volume, the volume of blood pumped per beat.
• cardiac muscle in the heart is myogenic, it can initiate its own heartbeat at regular intervals. however, the atrial, muscle has a higher myogenic rate than the ventricular muscle. The two pairs of chambers must contract in a coordinated fashion or the heart's action will be ineffective. thus a coordination system is needed.
• the heart contains it's own pacemaker called the sinoatrial node (SAN). the SAN initiates waves of excitation that usually override the myogenic action of the cardiac muscle. the SAN is a region of tissue that can initiate an action potential that travels as a wave of excitation over the atrial walls through the atrioventricular node (AVN) and down the purkyne fibers to the walls of the ventricles to cause them to contract.
• the heart muscle also responds directly to the hormone adrenaline in the blood, it increases the heart rate.
• At rest, the heart rate is controlled by the sino atrial node (SAN). this has a set frequency that varies from person to person which is typically 60 - 80 beats per minute. The frequency of these frequency waves is altered by the cardiac centre in the medulla oblongata
• Nerves from the cardiovascular center of the medulla oblongata of the brain supply the sino atrial node (SAN) These nerves are part of the autonomic system. The nerves do not initiate contractions, but can affect the frequency of the contractions:

  • action potentials are sent down a sympathetic nerve, the accelerans nerve) causing the release of the neurotransmitter noradrenaline at the SAN to increase the heart rate.
  • action potentials sent down the vagus nerve release the neurotransmitter acetylcholine to reduce the heart rate.
• a range of environmental factors can affect heart rate. input from sensory receptors is fed to the cardiovascular center in the medulla oblongata. some inputs increase heart rate, others decrease it. the interaction of these inputs is coordinated by the cardiovascular center to ensure the output to the SAN is appropriate to the overall conditions.
• how do stretch receptors in the muscles of the limbs imp[act heart rate?


  These send impulses to the cardiovascular center in the medulla oblongata to inform the brain that extra oxygen may be soon needed- leading to an increase in heart rate.
• how to chemoreceptors in the carotid arteries, aorta and brain impact heart rate?

  The chemoreceptors in the carotid arteries, aorta and brain monitor the pH of the blood. when under stress or exercise, the muscles will produce more carbon dioxide. Some react with water in the blood plasma to produce carbonic acid. This reduces the pH of the blood, impacting the transportation of oxygen. action potentials are sent to the cardiovascular centre of the medulla oblongata causing an increased heart rate.
• how do stretch receptors of the carotid sinus impact heart rate?

  the stretch receptors of the carotid sinus monitor blood pressure. the carotid sinus is a small swelling in the carotid artery. an increase in blood pressure is detected by its stretch receptors. if pressure is too high, the receptors send action potentials to the cardiovascular center so the heart rate is reduced.