The human gas exchange system - Structure and alveoli
The human lungs provide an An area of the lungs that is adapted for efficient transfer of gases. adapted for:
- absorbing Gaseous element making up about 20% of the air, which is needed by living organisms for respiration. - needed for respiration - into the blood from the air in the lungs
- 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 (which then goes into the air)
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.
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 m2 - capillaries cover 70 per cent of the surface of the alveoli
There is also a short The movement of molecules from an area of higher concentration to an area of lower concentration. 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.