Topic 8 ~ Exchange and Transport in Animals

• The kidney efficiently removes waste such as urea and excess ions to prevent toxicity
• It is necessary to transport substances into organisms which are vital for life, and to transport waste products out of the organism to prevent them from accumulating
• Specialised exchange surfaces have a short distance for diffusion and a large surface area for efficient transport of substances
• Example of specialised exchange surfaces

  • The root hair cells of plants: these are specialised to take up water and nutrients from the soil
  • The walls of the nephrons in the kidney: required to efficiently reabsorb substances like water and glucose
  • In the lungs, oxygen is transferred to the blood and carbon dioxide is transferred to the lungs
  • In the small intestine, cells have projections called villi for absorbing digested food into the bloodstream
  • The gills in fish where gas exchange takes place
  • In the leaves of the plant, there are different tissues for gas exchange
• Excess carbon dioxide can build up and dissolve in the blood, causing it to become acidic, leading to a condition called acidosis
• Guard cells control the stomata in leaves, changing their size based on the plant's water intake
• Having a thin membrane provides a short diffusion pathway, allowing the process to occur faster
• The greater the surface area, the more particles can move through, resulting in a faster rate of diffusion
• Guard cells swelling with water and making the stomata larger


  The guard cells swell with lots of water and make the stomata larger
• An important measure of how well an organism or cell can transport substances is the Surface Area to Volume Ratio
• Larger organisms often have a small surface area to volume ratio, while smaller organisms have a larger surface area to volume ratio
• Single-celled organisms can use diffusion to transport molecules into their body from the air due to their relatively large surface area to volume ratio
• Alveoli in the lungs are adapted for gas exchange in a number of ways
• Having an efficient blood supply/being ventilated creates a steep concentration gradient, so diffusion occurs faster
• Lungs

  • Alveoli and capillary walls are extremely thin
  • Small intestine: villi have a single layer of surface cell
• Lungs

  • The lungs constantly supply oxygen to make the blood from alveoli capillaries oxygenated, by exchanging it for carbon dioxide that can be breathed out. This is a constant process meaning the concentration gradient is always steep
  • Fish: water flows in one direction and blood flows in the other - this means that a steep concentration gradient is maintained as the concentration of oxygen is always much higher in the water - so it will diffuse across
• Lungs

  • The small, spherical alveoli in the lungs create a very large surface area (approximately 75m² in humans)
  • Small intestine: the cells of the small intestine have millions of villi, which are projections that increase the surface area. This means digested food can be absorbed into the blood faster
  • Fish gills: these contain lamellae to increase the surface area
  • Leaves: the flattened shape increases the surface area. The air spaces inside the leaf increase the surface area, so more carbon dioxide can enter cells
• In multicellular organisms, adaptations are necessary as the surface area to volume ratio is small and diffusion alone is not sufficient
• If an organism increases its surface area, it can take in more nutrients and expel more waste products more efficiently
• The greater the surface area to volume ratio, the better adapted the organism is for diffusion
• Adaptation

  • Having a large surface area
  • Having a thin membrane
  • Having an efficient blood supply/being ventilated (in animals)
• Alveoli


  • They are very small and arranged in clusters, creating a large surface area for diffusion to take place over
  • The capillaries provide a large blood supply, maintaining the concentration gradient
  • The walls of the alveoli are very thin, meaning there is a short diffusion pathway
• White blood cells

  • Part of the immune system, body’s defence against pathogens
  • Have a nucleus
  • Types include those that produce antibodies against microorganisms, engulf and digest pathogens, and produce antitoxins to neutralize toxins produced by microorganisms
• Plasma


  • Liquid that carries components in the blood: red blood cells, white blood cells, platelets, glucose, amino acids, carbon dioxide, urea, hormones, proteins, antibodies, and antitoxins
• You should be able to calculate the rate of diffusion given surface area, difference in concentration between the two substances either side of a membrane, and the thickness of a membrane
• Gas exchange in the alveoli

  1. Take place over capillaries providing a large blood supply, maintaining the concentration gradient

  2. Walls of the alveoli are very thin, meaning there is a short diffusion pathway
• Types of blood vessels

  • Arteries
  • Veins
  • Capillaries
• Flow of blood in the heart

  1. Blood flows into the right atrium through the vena cava, and left atrium through the pulmonary vein

  2. The atria contract forcing the blood into the ventricles
• Veins

  • Carry blood towards the heart
  • Wide lumen to allow low-pressure blood flow
  • Have valves to ensure blood flows in the right direction
• The heart is an organ in the circulatory system that carries oxygen and nutrients to every cell in the body and removes waste products
• Red blood cells

  • Carry oxygen molecules from the lungs to all cells in the body
  • Biconcave disc shape provides a large surface area
  • No nucleus allowing more room to carry oxygen
  • Contain the red pigment haemoglobin, which binds to oxygen and forms oxyhaemoglobin
• Blood components

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


  • Help blood clot form at the site of a wound
  • Clot dries and hardens to form a scab, allowing new skin to grow underneath while preventing microorganisms from entering
  • Small fragments of cells
  • No nucleus
  • Without them, cuts would result in excessive bleeding and bruising
• Capillaries


  • Allow blood to flow very close to cells to enable substances to move between them
  • One cell thick walls create a short diffusion pathway
  • Permeable walls so substances can move across them
• Arteries


  • Carry blood away from the heart
  • Layers of muscle in walls make them strong
  • Elastic fibers allow them to stretch to withstand high pressure
• Structure of the heart

  • Muscular walls to provide a strong heartbeat
  • Left ventricle has a thicker muscular wall to pump blood all around the body
  • 4 chambers separate oxygenated and deoxygenated blood
  • Valves prevent backward blood flow
  • Coronary arteries cover the heart to provide its own oxygenated blood supply
• The heart pumps blood around the body in a double circulatory system with two circuits
• Deoxygenated blood

  Blood without oxygen
• Coronary arteries


  • Cover the heart to provide its own oxygenated blood supply
• Aerobic respiration


  Uses oxygen, yields the most energy, and most reactions occur in the mitochondria