Topic 8

• Why do large multicellular organisms require specialised exchange surfaces?

  • Small SA/V ratio
  • Diffusion insufficient to provide all cells with the required oxygen and nutrients, and to remove all waste products
  • Exchange surfaces increase rate of diffusion and shorten diffusion distance
• Why do some multicellular organisms (e.g. trees) not require specialised exchange surfaces?

  Trees have a large number of leaves which provide a large SA/V ratio for diffusion
• The larger the organism


  The smaller the SA/V ratio
• Calculate the surface area to volume ratio of a cube that is 1 × 1 × 1 cm

  1. Surface area: (1 × 1) × 6 = 6 cm2

  2. Volume: 1 × 1 × 1 = 1 cm3

  3. Surface area : Volume = 6:1
• Calculate the surface area to volume ratio of a cuboid that is 2 × 3 × 4 cm

  1. Surface area: 2(2 × 3) + 2(2 × 4) + 2(4 × 3) = 52 cm2

  2. Volume: 2 × 3 × 4 = 24 cm3

  3. Surface area : Volume = 52:24 = 13:6
• Substances transported into and out of the human body

  • Oxygen
  • Carbon dioxide
  • Water
  • Dissolved food molecules
  • Urea
• How does oxygen enter and carbon dioxide leave cells?

  They diffuse into and out of cells
• How does water enter cells?

  It diffuses into cells by osmosis
• How do food molecules and mineral ions enter cells?

  They are dissolved in water which diffuses into cells
• Why must urea be excreted from the body?

  It is a waste product so must be excreted
• How is urea excreted from the body?

  1. Urea diffuses out of cells into the blood plasma

  2. The kidney filters urea out of the blood

  3. Urea is excreted in urine
• Why must the human body exchange oxygen and carbon dioxide with the environment?

  • Oxygen is required for respiration so diffuses into the body
  • Carbon dioxide is a toxic waste product of respiration so diffuses out of the body
• How does oxygen enter and carbon dioxide leave the bloodstream?

  1. Oxygen diffuses from air in the alveoli (high O2 conc) into blood in the capillaries (low O2 conc)

  2. Carbon dioxide diffuses from blood in the capillaries (high CO2 conc) into air in the alveoli (low CO2 conc)
• How are alveoli adapted for gaseous exchange?

  • Large surface area
  • Network of capillaries provide a good blood supply
  • Rapid blood flow maintains a steep concentration gradient
  • Thin walls give a short diffusion distance
  • Cell walls have partially permeable membranes enabling diffusion
  • Moist lining, enables gases to dissolve
• What factors affect the rate of diffusion?

  • Diffusion distance
  • Concentration gradient
  • Surface area
• How does diffusion distance affect the rate of diffusion?

  The greater the diffusion distance, the further the molecules must travel and the slower the rate of diffusion
• How does concentration gradient affect the rate of diffusion?

  The steeper the concentration gradient, the faster the rate of diffusion
• How does surface area affect the rate of diffusion?

  The larger the surface area, the greater the number of molecules that can diffuse across in a given time, so the faster the rate of diffusion
• How can the rate of diffusion be calculated?
• Circulatory system


  Network of organs and vessels that enables the flow of blood and transport of oxygen, carbon dioxide, nutrients and other molecules around the body
• Main components of blood

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


  Also known as erythrocytes
• Function of red blood cells

  • Transport O2 from lungs to tissues
  • Transport CO2 from tissues to lungs
• How red blood cells transport oxygen to body cells

  1. Lungs, haemoglobin in RBCs binds reversibly with oxygen to form oxyhaemoglobin

  2. Tissues, oxyhaemoglobin breaks down to form haemoglobin and oxygen which diffuses into cells
• Adaptations of red blood cells

  • Biconcave disk gives large SA/V ratio, increasing diffusion rate
  • Lack nucleus, allowing more space for haemoglobin molecules (increases oxygen carrying capacity of the cell)
  • Small and flexible so they can squeeze through capillaries
  • Thin giving a short diffusion distance
• White blood cells


  Provide immunological protection
• Types of white blood cells

  • Phagocytes
  • Lymphocytes
• Phagocytes


  Type of WBC that engulf pathogens and digest them in a process known as phagocytosis
• Lymphocytes


  Type of WBC that produce antibodies specific to a pathogen and antitoxins to neutralise toxins
• Function of platelets


  Role in blood clotting
• Plasma


  Pale-yellow liquid portion of the blood that contains proteins, nutrients, waste products, hormones, antibodies etc.
• Adaptation of plasma


  • Plasma consists mainly of water which acts as a solvent, enabling the transport of materials around the body
• Main types of blood vessel

  • Arteries
  • Capillaries
  • Veins
• Function of arteries


  Carry blood away from the heart under high pressure
• Adaptations of arteries


  • Narrow lumen maintains high pressure
  • Thick wall to withstand high pressure
  • Thick layer of smooth muscle provides strength
  • Thick layer of elastic fibres allow stretch and recoil
  • Smooth inner lining to reduce friction
  • No valves
• Function of veins


  Return blood to the heart under low pressure
• Adaptations of veins


  • Large lumen eases blood flow
  • Thin wall as blood at low pressure
  • Thin layer of smooth muscle and elastic fibres
  • Valves prevent backflow of blood
• Function of capillaries


  Allow the exchange of materials at tissues
• Adaptations of capillaries


  • Form large network ∴ greater surface area for diffusion
  • Walls one cell thick giving a short diffusion distance
  • Walls permeable allowing the exchange of substances
  • Narrow lumen decreases diffusion distance
• Double circulatory system in humans

  Blood flows through the heart twice in two circuits: Pulmonary circuit and Systemic circuit