Animal Organisation

• Organelle: A specialised unit within a cell which performs a specific function
• Cell: The basic building block of all living organisms
• Tissue: Groups of similar cells that work together to perform a particular function
• Organs: Tissues working together to form an organ, e.g. the heart is made up of cardiac muscle tissue.
• Organ system: A group of organs with related functions, working together to perform certain functions within the body
• Organisms must take in food,oxygen and water, and other essential substances, from the environment. Plants also need carbon dioxide for photosynthesis. Organisms also need to remove waste substances.
• Small organisms exchange these essential and waste substances between themselves and the environment. They do this over their body surface. Simple chemical substances can diffuse in and out of their bodies.
• Inside their bodies, in small organisms, substances don’t have to move far.

  The size of their surface, or surface area, defines how quickly they can absorb substances. The size of their volume defines how much of these substances they need.
• Modelling cells

  If we represent the cell of an organism with a cube:

  
• As the volume increases, surface area does not increase at the same rate.

  As multicellular organisms increase in size, they face two problems:

  • Their surface area does not increase as fast as the volume Problem: Insufficient surface area to meet their needs Solution: Body systems that add additional absorbing area to exchange surfaces
  • Their volume increases Problem: Diffusion is not quick enough to move substances to where they’re needed in the organism’s body Solution: A transport system
• he human gas exchange system

  The human lungs provide an exchange surface adapted for:

  • absorbing oxygen – needed for respiration – into the blood from the air
  • transferring carbon dioxide – produced by respiration – from the blood into the lungs then the air
• The lungs are organs enclosed within the chest or thorax. Air needs to be breathed in to be brought into contact with the exchange surfaces within the lungs. This process is called ventilation.
• The structure of the respiratory system:

  • The human respiratory system is adapted to allow air to pass in and out of the body, and for efficient gas exchange to happen.
  • The lungs are enclosed in the thorax, surrounded and protected by 12 pairs of ribs. The ribs are moved by two sets of intercostal muscles. There is a muscular diaphragm below the lungs. The lungs are sealed within two airtight pleural membranes. These wrap around the lungs and line the rib cage.
• The trachea, or windpipe, branches into two bronchi – one bronchus to each lung. Rings of cartilage in the walls of the trachea help to keep it open as air is drawn in.

  the bronchi split into smaller branches and then into smaller tubes called bronchioles. Each bronchiole ends in a cluster of microscopic air sacs called alveoli.
• Gaseous exchange

  The exchange of gases occurs between the alveoli and blood in the capillaries that supply the lungs. Capillaries cover 70% of the outside of alveoli, providing a large surface area for gases to diffuse across.

  
• The alveoli are adapted to provide a very large surface area for gaseous exchange:

  • small size - each alveolus is a small sphere about 300 μm in diameter, giving it a larger surface area to volume ratio than larger structures
  • number - there are around 700 million alveoli – ie 350 million per lung
• The total surface area of the alveoli is around 70 square metres.

  There is also a short diffusion path - the walls of blood capillaries and alveoli are just one cell thick. The alveoli are also lined with a thin film of moisture. Gases dissolve in this water, making the diffusion path even smaller.
• The ventilation of the lungs and the blood flow through the surrounding capillaries mean gases are being removed continually, and steep concentration gradients are set up for gases to diffuse.
• Breathing: Ventilation:

  Air is moved into and out of the lungs, as it is carried to and from the exchange surfaces of the alveoli.
• The diaphragm and rib cage move to create a lower air pressure in the lungs than that of the air outside the body. The air then rushes into the lungs
• The most important muscle when we inhale normally is the diaphragm. The external intercostal muscles are the second most important muscles. Breathing is a passive process resulting from pressure changes in the lungs.