The Circulatory System

• Water molecule


  Polar molecule - electrons are shared unequally within the bonds that hold a water molecule together
• Oxygen in water molecule


  • Gains electrons, pulls the electrons that make up the single covalent bond closer towards it (it is electronegative)
  • Gives oxygen a slight negative charge
• Hydrogen in water molecule


  • Has a slight positive charge
• Hydrogen bond


  Forms between the slightly negative oxygen atom of one water molecule and the slightly positive hydrogen atom on another molecule
• Ability of water molecules to hydrogen bond


  • Gives water some special properties
• Water as a solvent


  • Water is a good solvent because it can form hydrogen bonds with other polar molecules or charged ionic compounds
  • Water dissolves more substances than any other liquid, so it is referred to as the 'universal solvent'
  • Handy for humans, since our blood consists mostly of water which is able to dissolve hydrophilic molecules such as glucose, ions and some amino acids
• Latent heat of evaporation


  • A lot of energy is used to convert water from a liquid to a gas
  • This is why we feel cooler when we sweat, since the evaporation of water from our skin's surface takes a lot of heat energy with it
• Specific heat capacity of water


  • All those hydrogen bonds within water are great at absorbing energy, which means you have to add a lot of it to heat water up
  • This is really useful for marine life since it means that bodies of water are fairly resistant to changes in temperature, making it a stable habitat in which to live
• Cohesion of water molecules

  • Water molecules have a tendency to stick to other water molecules because the slightly positive hydrogen of one water molecule attracts the slightly negative oxygen on another water molecule
  • Cohesion helps water to flow, making it a good transport medium
• Bonds between water molecules:

  
• Mammals need mass transport systems:

  Organisms need oxygen and glucose to produce energy in aerobic respiration. Small organisms, like bacteria, can obtain these by diffusion because of the short diffusion distance. But in multicellular organisms, the diffusion distance is too large and diffusion would be too slow to reach the cells in the centre of their body, so they have mass transport systems.

  Mass transport systems, such as the circulatory system in mammals, deliver oxygen and glucose to all body cells. They also remove waste products (carbon dioxide and water). 
• Arteries


  Carry blood away from the heart to the various organs of the body
• Arteries


  • Need to cope with the high pressure generated from the heart forcing out blood with each heartbeat
  • Have a really thick muscular wall containing lots of elastic tissue
  • Inner lining (endothelium) is folded which allows the artery to expand (elastic recoil) and withstand high pressure
  • Small lumen ensures a high pressure is maintained
• Veins


  Carry blood from the organs of the body towards the heart
• Veins


  • Blood is flowing at a much lower pressure so veins have a large lumen and much thinner walls containing little elastic fibres or muscle tissue
  • Valves prevent the slow-moving blood from flowing backwards
  • Contraction of nearby body muscles helps blood to flow through veins
• Capillaries


  Connect arteries and veins
• Capillaries


  • Substances move out of the blood to the body tissues - things like oxygen, glucose and mineral ions
  • Waste products, such as carbon dioxide and water, will move out of the body tissues and into the capillaries
  • Small holes (pores) enable the exchange of substances
  • Walls are just one cell thick which reduces the diffusion distance for these substances
• Artery, Vein and Capillary:

  
• Heart


  Made up of four chambers divided into two sides
• Left side of the heart:


  • Has a thicker wall
  • Needs to pump more strongly to deliver blood all around the body
• Right side of the heart


  • Just needs to send the blood to the lungs
• Left side of the heart


  Carries oxygenated blood
• Right side of the heart


  Carries deoxygenated blood
• Atria

  Chambers at the top that receive the blood from the veins supplying the heart
• Blood flow


  1. Flows from the atria to the ventricles

  2. Separated from the atria by atrioventricular valves to prevent blood flowing in the opposite direction

  3. Another set of valves between the ventricles and the arteries called the semi-lunar valves as they look like little half-moons
• Aorta


  Main artery which takes oxygenated blood from the left side of the heart to the rest of the body
• Pulmonary artery


  Artery which delivers deoxygenated blood between the right side of the heart and the lungs
• Vena cava


  Major vein which returns blood from the body to the right side of the heart
• Pulmonary vein


  Vein which ferries blood from the lungs to the heart
• Coronary arteries


  Arteries that supply the heart muscle itself with blood, oxygen and glucose
• Blockage in coronary arteries


  Leads to a heart attack
• Cardiac cycle


  Coordinated sequence of contractions and relaxations by the heart muscle which causes blood to move from the atria, into the ventricles and then the arteries
• Systole


  Muscle contractions
• Diastole


  Muscle relaxation
• Cardiac cycle


  1. Atrial systole

  2. Ventricular systole

  3. Diastole
• Atrial systole


  1. Atria contract while ventricles relax

  2. Decreases volume inside atria which increases pressure

  3. Increased pressure forces atrioventricular valves open and pushes blood into ventricles
• Ventricular systole


  1. Ventricles contract while atria are relaxed

  2. Decreases volume inside ventricles which increases pressure

  3. Pressure higher in ventricles than atria forces atrioventricular valves closed and causes semi-lunar valves to open

  4. Closure of AV valve prevents backflow of blood into atria

  5. Blood forced out of ventricles and into arteries
• Diastole


  1. Atria and ventricles are relaxed so pressure is low in both chambers

  2. Pressure higher in arteries than heart chambers forces semi-lunar valves closed to prevent backflow

  3. Blood returned to heart and atria fill with blood
• Graph of cardiac cycle:

  
• Invertebrates


  Small animals without a backbone