Transport in animals

• Circulatory systems


  Can be open (e.g. in insects) or closed (e.g. in fish and mammals where the blood is confined to blood vessels only)
• Closed circulatory systems


  • Can be single (heart with two chambers, blood passes through once per circuit) or double (heart with four chambers, blood passes through twice per circuit)
• Arteries


  • Adapted to carrying blood away from the heart to the rest of the body
  • Thick walled to withstand high blood pressure
  • Contain elastic tissue which allows them to stretch and recoil, smoothing blood flow
  • Contain smooth muscle which enables them to vary blood flow
  • Lined with smooth endothelium to reduce friction and ease flow of blood
• Arterioles


  • Branch off arteries
  • Have thinner and less muscular walls
  • Role is to feed blood into capillaries
• Capillaries


  • Smallest blood vessels
  • Site of metabolic exchange
  • Only one cell thick for fast exchange of substances
• Venules


  • Larger than capillaries but smaller than veins
• Veins


  • Carry blood from the body to the heart
  • Contain a wide lumen to maximise volume of blood carried to the heart
  • Thin walled as blood is under low pressure
  • Contain valves to prevent backflow of blood
  • No pulse of blood meaning there's little elastic tissue or smooth muscle as there is no need for stretching and recoiling
• Tissue fluid


  A liquid containing dissolved oxygen and nutrients which serves as a means of supplying the tissues with the essential solutes in exchange for waste products such as carbon dioxide
• Tissue fluid formation


  1. Hydrostatic pressure created when blood is pumped along arteries, into arterioles and then capillaries forces blood fluid out of the capillaries

  2. Only substances small enough to escape through the gaps in the capillary wall are components of the tissue fluid

  3. Osmotic pressure pushes some of the fluid back into the capillaries
• Lymphatic system


  • Carries the remaining tissue fluid not pushed back into the capillaries
  • Contains lymph fluid similar in content to tissue fluid but with less oxygen and nutrients as its main purpose is to carry waste products
  • Contains lymph nodes which filter out bacteria and foreign material from the fluid with the help of lymphocytes
• Myogenic


  The heart can initiate its own contraction
• Sinoatrial node


  • Region of specialised fibres in the wall of the right atrium
  • The pacemaker of the heart, initiating a wave of electrical stimulation which causes the atria to contract at roughly the same time
• Atrioventricular node


  • Located between the two atria
  • Passes on the electrical excitation to the ventricles
• Cardiac cycle


  1. Atrial systole - atria contract, forcing atrio-ventricular valves open and blood flows into ventricles

  2. Ventricular systole - ventricles contract, causing atrio-ventricular valves to close and semi-lunar valves to open, allowing blood to leave the ventricles

  3. Cardiac diastole - atria and ventricles relax, elastic recoil lowers pressure in heart chambers and blood is drawn from arteries and veins, causing semilunar valves to close
• Haemoglobin


  • A water soluble globular protein consisting of two alpha and two beta polypeptide chains each containing a haem group
  • Carries oxygen in the blood as oxygen can bind to the haem (Fe2+) group, and is then released when required
  • Each molecule can carry four oxygen molecules
• Partial pressure of oxygen

  Affects the affinity of oxygen for haemoglobin - higher partial pressure increases affinity, lower partial pressure decreases affinity
• Haemoglobin saturation


  Affected by affinity for oxygen - high partial pressure means high affinity and high saturation, low partial pressure means low affinity and low saturation
• Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, as it needs to absorb oxygen more effectively at the lower partial pressures in the placenta
• Presence of carbon dioxide

  Decreases the affinity of haemoglobin for oxygen (Bohr effect)