Circulatory system and blood

• Circulatory system


  System that pumps blood around a closed circuit made up of the heart and blood vessels
• Blood transports oxygen from the lungs to all other parts of the body, carbon dioxide from all parts of the body to the lungs, nutrients from the gut to all parts of the body, and urea from the liver to the kidneys
• Blood also transports hormones, antibodies and many other substances, and distributes heat around the body
• Single-celled organisms do not have circulatory systems
• Surface area to volume ratio

  Ratio that determines how much oxygen a single-celled organism can obtain and use
• Single-celled organisms have a high surface area to volume ratio, allowing them to obtain enough oxygen through diffusion
• Larger animals cannot get all the oxygen they need through their surface, so they have evolved special gas exchange organs and circulatory systems
• Single circulatory system


  Blood is pumped from the heart to the gas exchange organ and then directly to the rest of the body
• Double circulatory system


  Blood is pumped from the heart to the gas exchange organ, back to the heart and then to the rest of the body
• A double circulatory system is more efficient than a single circulatory system as the heart pumps the blood twice, maintaining higher pressures and allowing the blood to travel more quickly to organs
• Components of the human circulatory system

  • Heart (pump)
  • Blood vessels (carry blood around the body)
  • Blood (transport medium)
• Cardiac cycle


  Series of contractions and relaxations of the muscle in the walls of the four heart chambers
• Cardiac muscle can contract and relax continuously without becoming fatigued
• The coronary arteries can become blocked, cutting off the blood supply to an area of cardiac muscle, resulting in a heart attack
• Factors that increase risk of coronary heart disease


  • Heredity
  • High blood pressure
  • Diet high in saturated fat
  • Smoking
  • Stress
  • Lack of exercise
• Heart rate


  Number of times the heart beats per minute
• When we exercise


  Heart rate and stroke volume increase to deliver more oxygen to the muscles
• When we are angry or afraid


  Heart rate increases to supply extra blood to the muscles for the 'fight or flight' response
• When we sleep


  Heart rate decreases as organs are working more slowly
• Medulla


  Part of the brain that controls heart rate through nerve impulses
• Arteries carry blood from the heart to the organs at high pressure, while veins carry blood back to the heart at lower pressure
• Nerve impulses


  1. Sensory nerve to medulla

  2. Medulla responds by sending nerve impulses along accelerator nerve

  3. When carbon dioxide production returns to normal, medulla receives fewer impulses

  4. Medulla responds by sending nerve impulses along decelerator nerve
• The accelerator nerve increases the heart rate and causes the heart to beat with more force, increasing blood pressure
• The decelerator nerve decreases the heart rate and reduces the force of the contractions, returning blood pressure to normal
• These controls are both examples of reflex actions
• Arteries


  Carry blood from the heart to the organs of the body, pumped out by the ventricles at high pressure
• Arteries


  • Elastic tissue in the walls allows them to stretch and recoil, maintaining high blood pressure
  • Thick muscular wall helps control the flow of blood by dilating or constricting the vessels
• Veins


  Carry blood from organs back towards the heart, at much lower pressure than arteries
• Veins


  • Thinner walls than arteries, contain less elastic tissue and muscle
  • Contain semilunar valves to prevent backflow of blood
• Arterioles


  Small arteries that carry blood into organs, have a larger proportion of muscle in their walls and are supplied with nerve endings to allow them to dilate or constrict
• Arterioles constrict


  It is harder for blood to pass through them, increasing blood pressure
• Prolonged stress can cause arterioles to constrict, increasing blood pressure
• Arteries carry oxygenated blood, except the pulmonary artery and umbilical artery. Veins carry deoxygenated blood, except the pulmonary vein and umbilical vein
• Capillaries


  Carry blood through organs, allowing substances to be transferred between the blood and cells
• Capillaries


  • Small enough to fit between cells
  • Walls are one cell thick, allowing short diffusion distance for oxygen
• Red blood cells just fit through the tiny diameter of capillaries, so they are close to the capillary wall
• Blood


  A complex tissue containing different types of cells
• Types of blood cells


  • Red blood cells
  • White blood cells
  • Platelets
• Red blood cells


  Highly specialised cells made in the bone marrow, with the sole function of transporting oxygen
• Red blood cells


  • Contain haemoglobin to bind oxygen
  • Lack a nucleus to allow more haemoglobin
  • Biconcave shape for efficient oxygen exchange