Air passes into lungs through the nose and along the trachea, bronchi and bronchioles. Each part of this airway is adapted to its function of allowing the passage of air. Finally the air reaches tiny, air-filled sacs called alveoli. The walls of the alveoli is where the exchange of gases takes place.
The lungs are protected by the ribs. Movement of the ribs together with the diaphragm (a layer of muscle tissue beneath the lungs) help to produce breathing movements (ventilation).
How the lungs are adapted for exchange
The main adaptation is a large surface area. This provides more space for molecules to pass through. The individual alveoli are very small but there are so many of them that the total surface area of our lungs is much larger than that of our skin.
Another adaptation is the barrier permeable to oxygen and carbon dioxide. The plasma membranes that surround the thin cytoplasm of the cells form the barrier to exchange. These readily allow the diffusion of oxygen and carbon dioxide.
They also have a thin barrier to reduce diffusion distance.
- The alveolus wall is one cell thick.
- The capillary wall is one cell thick.
- Both walls consist of squamous cells - this means flattened or very thin cells.
- The capillaries are in close contact with the alveolus walls.
- The capillaries are so narrow that the red blood cells are squeezed against the Capillary wall, making them closer to the air in the alveoli and reducing the rate at which they flow past in the blood
- The total barrier to diffusion is two flattened cells thick - less than 1um thick.
In the lungs, oxygen diffuses from the air in the alveoli into the blood. CO2 diffuses in the opposite direction. This is gas exchange. To make this as efficient as possible, the air in the alveoli is continually refreshed so that the concentrations of O2 and CO2 are such that steep diffusion gradients are maintained between the air the the blood.
The blood brings CO2 from the tissues to the lungs. This ensures that the concentration of CO2 in the blood is higher than that in the air of the alveoli. It also carries oxygen away from the lungs. This ensures that the concentration of O2 in the blood is kept lower than the concentration in the air inside the alveoli.
Ventilation of the alveoli is achieved by moving the diaphragm and the rib cage when we breathe. Air moves through a system of airways to reach the alveoli. It brings a constant supply of oxygen to the lungs ensuring that the concentration of O2 remains higher than the concentration of blood. Ventilation also removes air containing CO2 from the alveoli. This ensures that the concentration of CO2 in the alveoli remains lower than that in the blood. (this is a way of keeping a steep diffusion gradient)
The movements of the diaphragm and the rib cage change the pressure in the lungs: when we breathe in the pressure is less than that of atmospheric air; when we breathe out the pressure is greater than that of atmospheric air. The difference in pressure is greater than that of atmospheric air. The difference in pressure between the air in the lungs 7 the atmosphere causes air to move in and out.
Diaphragm - Breathing in - Contracts and moves down
Breathing out - Relaxes and is pushed up by pressure in the abdomen.
Ribcage - Breathing in - External intercostal muscle contract to move ribs up and outwards
Breathing out - External intercostal muscles relax so that ribs fall with gravity
Volume of thorax - Breathing in - Increases Breathing out - Decreases
Pressure in lungs - Breathing in - Decreases Breathing out - Increases
Tissues in the lungs
Trachea, bronchi and bronchioles are airways that allow passage of air into the lungs and out again. To be effective, these airways must meet certain requirements;
- The larger airways must be large enough to allow sufficient air to flow without obstruction.
- They must also divide into smaller airways to deliver air to the alveoli.
- The airways must be strong enough to prevent them collapsing when the air pressure is low (this low pressure occurs during inhalation).
- They must be flexible, to allow movement.
- They must also be able to stretch and recoil.
The trachea and bronchi
Both have similar structures. They differ only in size - the bronchi are narrower than the trachea. They have relatively thick walls that have several layers of tissue.
- Much of the wall consists of cartilage.
- The cartilage is in the form of incomplete rings or C-rings in the trachea, but is less regular than in the bronchi.
- On the inside surface of the cartilage is a layer of glandular tissue, connective tissue, elastic fibres, smooth muscle and blood vessels. This is often called the 'loose tissue'.
- The inner lining is an epithelium layer that has two types of cell. Most of the cells have cilia. This is called ciliated epithelium. Among the ciliated cells are goblet cells.
The bronchioles are much narrower than the bronchi. The large bronchioles may have some cartilage, but smaller ones have no cartilage. The wall is made mostly of smooth muscle and elastic fibres. the smallest bronchioles have clusters of alveoli (air sacs) at their ends.
Functions of each tissue
- Cartilage - Provides strength to trachea and bronchus; holds open the airways so there is little resistance to air flow.
- Goblet cells - Secrete mucus. Mucus is sticky and collects particles of dust, spores and bacteria.
- Ciliated cells - Move mucus up the airways towards the mouth.
- Smooth cells - Contracts to narrow the airways.
- Elastic fibres - Stretch when breathing in a filling the lungs. Recoil when breathing out to help force air out of the lungs.
- Squamous epithelium - Gives a short diffusion pathway for oxygen and carbon dioxide in the alveoli.