The lungs
The human lungs provide an A surface where substances, eg gases, food substances, wastes, are moved across membranes. adapted for:
- absorbing Gaseous element making up about 20% of the air, which is needed by living organisms for respiration. – needed for The chemical change that takes place inside living cells, which uses glucose and oxygen to release the energy that organisms need to live. Carbon dioxide is a by-product of respiration. – into the blood from the air
- transferring A gaseous compound of carbon and oxygen, which is a by-product of respiration, and which is needed by plants for photosynthesis. – produced by respiration – from the blood into the lungs then the air
The transfer of these two gases between organisms and their environment is called gas exchange.
The lungs are organs enclosed within the chest or The ribs and upper backbone, and the organs found in the chest.. Air needs to be breathed in to be brought into contact with the exchange surfaces within the lungs. This process is called Breathing in and out..
The structure of the respiratory system
The human The organ system where air is taken into and out of the body, and gas exchange happens. 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 Sets of muscles between the ribs which raise and lower the rib cage.. There is a muscular A large sheet of muscle that separates the lungs from the abdominal cavity. below the lungs. The lungs are sealed within two airtight Thin, moist membranes surrounding the lungs that make an airtight seal.. These wrap around the lungs and line the rib cage.
The The windpipe, the tube that leads from the mouth towards the lungs., or windpipe, branches into two The plural of 'bronchus'. The bronchi are the two major air tubes in the lungs. – one bronchus to each lung. Rings of A strong, flexible type of connective tissue. It forms C-shaped rings in the trachea that keep the airway open. 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 The many small, branching tubules into which the bronchi subdivide.. Each bronchiole ends in a cluster of microscopic air sacs called Tiny air sacs in the lungs, where gas is exchanged during breathing..
Gaseous exchange
The exchange of gases occurs between the alveoli and blood in the Tiny blood vessels with walls one-cell thick where exchange of materials occurs. that supply the lungs. Capillaries cover 70% of the outside of alveoli, providing a large surface area for gases to The movement of molecules from an area of higher concentration to an area of lower concentration. across.
In cross section:
The alveoli are adapted to provide a very large surface area for gaseous exchange:
- small size - each of the alveoli 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 The difference in the concentration of a chemical across a membrane. are set up for gases to diffuse.
