the circulatory system

• Arteries


  • Transport blood away from the heart, usually at high pressure, to the tissues
• Arterioles


  • Arteries branch into narrower blood vessels that transport blood into capillaries
• Veins


  • Transport blood to the heart, usually at low pressure
• Venules


  • Narrower blood vessels that transport blood from the capillaries to the veins
• Capillaries


  • Microscopic blood vessels that carry blood to the cells
• Lumen


  The size of the opening or channel within a blood vessel
• Artery walls


  • Consist of three layers: endothelium, middle layer, outer layer
• Endothelium


  One cell thick layer that lines the lumen of all blood vessels, is very smooth to reduce friction
• In arteries, the endothelium is highly folded, enabling it to expand
• Artery walls


  • Contain elastic fibres and smooth muscle to allow expansion and contraction
• Vein walls


  • Thinner than artery walls, contain less elastic tissue and smooth muscle
• Veins have valves to prevent backflow of blood
• Capillaries have a very thin wall, only one cell thick, to allow rapid exchange of substances
• Venules


  Narrower blood vessels that transport blood from the capillaries to the veins
• Capillaries


  Microscopic blood vessels that carry blood to the cells
• Blood flow through the lumen of a blood vessel

  The size of the lumen varies depending on the type of blood vessel
• Arteries


  Have a narrow lumen
• Veins


  Have a wider lumen
• Layers of artery walls


  • Endothelium (tunica intima)
  • Smooth muscle and elastic tissue (tunica media)
  • Outer wall (tunica adventitia, tunica externa)
• Endothelium


  • One cell thick and lines the lumen of all blood vessels
  • Very smooth and reduces friction for free blood flow
  • In arteries it is highly folded, enabling it to expand under high pressure
• Smooth muscle and elastic tissue


  • Thick in arteries
  • Strengthens arteries to withstand high pressure
  • Enables constriction and narrowing of the lumen for reduced blood flow
  • Helps maintain blood pressure by stretching and recoiling to even out fluctuations
• Outer wall


  Contains the structural protein collagen which protects blood vessels from damage by over-stretching
• Arteries


  • Have a narrow lumen which helps maintain high blood pressure
  • Have a pulse as they stretch to accommodate increased blood volume with each heartbeat
• Veins


  • Receive blood that has passed through capillary networks, so the blood pressure is very low
  • Have the same layers as arteries but in different proportions
  • Have a much thinner smooth muscle and elastic layer as they don't need to withstand high pressure
  • Have a wider lumen to ensure blood returns to the heart at an adequate speed and reduce friction
  • Have valves to prevent backflow of blood, helping return blood to the heart
  • Have no pulse due to the increased distance from the heart
• Capillaries


  • Have thin, permeable walls allowing substances to leave the blood and reach body tissues
  • Can form capillary beds which are important exchange surfaces
  • Have a very narrow lumen forcing red blood cells to pass through in single file, slowing blood flow to allow more time for diffusion
  • Have a wall that is a single layer of endothelial cells, reducing the diffusion distance for oxygen and carbon dioxide
  • Have gaps called pores in the wall allowing blood plasma to leak out and form tissue fluid, and allowing white blood cells to combat infection
• Capillary walls are one cell thick and do not have cell walls
• Cardiac cycle


  The series of events that take place in one heartbeat, including muscle contraction and relaxation
• Systole


  The contraction of the heart
• Diastole


  The relaxation of the heart
• There is no gap between cardiac cycles where blood stops flowing
• Volume and pressure changes

  • Contraction of the heart muscle causes a decrease in volume in the corresponding chamber, which then increases again when the muscle relaxes
  • Volume changes lead to corresponding pressure changes - when volume decreases, pressure increases, and when volume increases, pressure decreases
• Heart valves


  • Open when the pressure of blood behind them is greater than the pressure in front of them
  • Close when the pressure of blood in front of them is greater than the pressure behind them
  • An important mechanism to stop blood flowing backwards
• Atrial systole


  1. The walls of the atria contract

  2. Atrial volume decreases

  3. Atrial pressure increases

  4. The pressure in the atria rises above that in the ventricles, forcing the atrioventricular (AV) valves open

  5. Blood is forced into the ventricles

  6. There is a slight increase in ventricular pressure and chamber volume as the ventricles receive the blood from the atria

  7. The ventricles are relaxed at this point; ventricular diastole coincides with atrial systole
• Ventricular systole


  1. The walls of the ventricles contract

  2. Ventricular volume decreases

  3. Ventricular pressure increases

  4. The pressure in the ventricles rises above that in the atria, forcing the AV valves to close

  5. The pressure in the ventricles rises above that in the aorta and pulmonary artery, forcing the semilunar (SL) valves open so blood is forced into the arteries and out of the heart

  6. During this period, the atria are relaxing; atrial diastole coincides with ventricular systole

  7. The blood flow to the heart continues, so the relaxed atria begin to fill with blood again
• Diastole


  1. The ventricles and atria are both relaxed

  2. The pressure in the ventricles drops below that in the aorta and pulmonary artery, forcing the SL valves to close

  3. The atria continue to fill with blood

  4. Blood returns to the heart via the vena cava and pulmonary vein

  5. Pressure in the atria rises above that in the ventricles, forcing the AV valves open

  6. Blood flows passively into the ventricles without need of atrial systole

  7. The cycle then begins again with atrial systole
• The events of the cardiac cycle allow the heart to pump blood to the lungs and body
• Pressure changes in the cardiac cycle


  • Left atrium
  • Aorta
  • Left ventricle
• The points where the lines cross indicate when valves open and close
• Calculating heart rate

  1. Work out the length of one heart beat

  2. Calculate how many heart beats occur per second

  3. Calculate how many heart beats occur per minute
• Factors that influence heart rate


  • Drugs
  • Caffeine
  • Alcohol
  • Sex (male or female)
  • Weight
  • Height
  • Temperature
  • Diet
  • Dehydration