The Heart

• Atria

  Have thin muscular walls and receive blood under low pressure returning to the heart in veins
• Ventricles

  Have thick muscular walls and pump blood at high pressure into arteries
• Right atrium

  Receives deoxygenated blood from the body (except the lungs) via the vena cavae
• Right ventricle

  Pumps deoxygenated blood into the pulmonary artery leading to the lungs
• Left atrium

  Receives oxygenated blood from the lungs via the pulmonary vein
• Left ventricle

  Pumps oxygenated blood into the aorta leading to the rest of the body
• The left ventricle has a much thicker wall than the right ventricle, because it pumps blood a greater distance than the right ventricle which only supplies blood to the lungs
• Atrio-ventricular valves


  Open to allow blood into the ventricles, close as the ventricles contract, preventing back-flow of blood into the atria
• Tendinous cords


  Prevent the Atrio-ventricular valves 'turning inside out' as the ventricular pressure increases
• Semilunar valves

  Open to allow blood into the pulmonary artery and aorta as the ventricles contract, close as the ventricles relax, preventing the backflow of blood into ventricles
• Left ventricle contracts (systole)


  1. Pressure increases above that of the atrium

  2. Bicuspid valve closes
• Semilunar valve opens


  1. Pressure in the ventricle is greater than that in the aorta

  2. Blood flows into the aorta
• Ventricle relaxes (diastole)


  1. Semilunar valve closes

  2. Pressure in the aorta is greater than that in the ventricle
• Pressure inside the ventricle falls below that in the atrium

  1. Atrio-ventricular valve opens

  2. Blood flows into the ventricle
• SAN


  Patch of modified muscle cells in the wall of the right atrium
• SAN function

  1. Produces regular bursts of electrical impulses

  2. Waves of depolarisation across the atria

  3. Causing them to contract together
• Impulses from SAN


  • Do not pass directly to the ventricles
  • Reach the atrioventricular node (AVN) between the atria and ventricles
• Impulses reaching AVN

  1. 0.15 second delay before AVN reacts

  2. Ensures ventricles contract after atria

  3. Allows time for atria to fill with blood
• Impulses from AVN

  1. Travel rapidly through Purkyne fibres

  2. Organised into the Bundle of His

  3. To all parts of the ventricles
• Ventricles contracting

  1. Stimulated to contract together

  2. Starting at the bottom

  3. To push the blood up and out into the arteries
• Heart rate

  Controlled by the cardiac centre in the medulla of the brain
• Cardiac centre

  • Cardioacceleratory centre
  • Cardioinhibitory centre
• Autonomic (involuntary) Nervous system


  Divided into the Sympathetic Nervous System and Parasympathetic Nervous System
• Increase in heart rate (e.g. during exercise)

  1. Cardioacceleratory centre sends more impulses along sympathetic neurones to the SAN

  2. Noradrenaline is released by sympathetic neurones stimulating the SAN
• Decrease in heart rate (e.g. during rest)

  1. Cardioinhibitory centre sends more impulses along parasympathetic neurones to the SAN

  2. Acetylcholine is released by parasympathetic neurones inhibiting the SAN
• CO2 dissolves in the blood

  1. Produces carbonic acid

  2. Lowers the pH
• Chemoreceptors

  Receptors in the medulla and in the aortic and carotid bodies that are stimulated by the lowered pH
• Chemoreceptors in the aortic and carotid bodies


  1. Transmit impulses to

  2. Respiratory centre in the medulla increasing the rate of ventilation

  3. Cardiac centre in the medulla, which increases the heart rate by transmitting more impulses from the cardioacceleratory centre via the sympathetic neurones to the SAN
• During exercise muscles contract strongly

  1. Pressing on veins

  2. Increasing the rate at which blood returns to the heart in veins (venous return)
• Increased venous return

  Causes the cardiac muscle to contract more strongly, pumping out an increased volume of blood (i.e. an increase in stroke volume)
• Pressure receptors (stretch receptors)


  Receptors in the wall of the aorta and the carotid artery (carotid sinus) that detect an increase in blood pressure
• If the blood pressure increases too far above normal

  1. Pressure receptors send more impulses to the medulla

  2. Stimulating the cardioinhibitory centre

  3. Inhibiting the cardioacceleratory centre

  4. More impulses are sent from the cardiac centre along parasympathetic neurones to the SAN

  5. Causing a decrease in heart rate thereby reducing blood pressure
• Blood circulation

  1. Pumped from heart (right ventricle) to lungs

  2. Returns to heart (pulmonary circulatory system)

  3. Pumped (left ventricle) to rest of body (systemic circulatory system)
• Double circulatory system


  • Ensures blood is pumped at high pressure to body after pressure reduced passing through lungs
• Arteries

  Carry blood away from the heart at high blood pressure
• Aorta (main artery)

  • Large amount of elastic tissue
  • Distends (enlarges) due to its elasticity when the left ventricle contracts (systole)
  • Retains some of the blood forced out of the ventricle
• Blood flow in arteries

  1. Most of the blood is forced along the aorta to the body tissues (systolic pressure)

  2. When the left ventricle relaxes (diastole) the artery wall recoils due to its elasticity and forces blood to the body tissues (diastolic pressure)

  3. This helps to provide a smooth flow of blood and maintain a relatively high pressure when the ventricle relaxes
• Arteries

  • Thicker wall and a smaller lumen (inner cavity) than veins
  • Contain more elastic fibres and smooth muscle tissue
  • Do not possess valves except for the aorta and pulmonary artery
  • Transport blood at a higher pressure than veins
  • Carry oxygenated blood except for the pulmonary artery
• Aorta and larger arteries

  • Higher ratio of elastic fibres to smooth muscle fibres
  • To withstand the high blood pressure
  • To help smooth out blood flow
• Arterioles

  • Do not have to stand the very high pressure found in main arteries
  • Possess a higher proportion of smooth muscle
  • Can regulate the flow of blood to different tissues or organs by contraction or relaxation of the smooth muscle in their walls