Gaseous exchange
Gaseous exchange occurs by diffusion between air in the alveoli and blood in the capillaries surrounding their walls. The concept of partial pressure applies the diffusion of gases from a gas mixture to a gas in solution and vice versa. Gases in contact with a liquid dissolve into solution by diffusion until equilibrium is achieved. At equilibrium, the partial pressure of the gases is the same in both gaseous and liquid states, and the gases are diffusing in and out of each state at the same rate. blood entering the capillaries from the pulmonary arteries has a lower oxygen content and a higher carbon dioxide content than the air in the alveoli. oxygen diffuses into the blood via the surface of the alveoli, through the thin walls of the capillaries, through the red blood cell membrane and finally latches on to haemoglobin. carbon dioxide diffuses in the opposite direction, the blood into the alveoli.
Mechanisms of breathing
pulmonary ventilation, or breathing, is the process by which air is transported into and out the lungs, and it can be considered to have two phases. breathing is regulated by the respiratory centres in the brain and stretch receptors within the air passages and lungs. it requires the thorax to increase in size to allow air to be taken in, followed by a decrease to allow air to be forced out.
Inspiration
with inspiration the intercostal muscles contract to lift the ribs upwards and outwards, while the diaphragm is forced downwards and the sternum forwards. this expansion of the thorax in all directions causes a drop in pressure below that of atmospheric pressure, which encourages air to flood into the lung. at this point, oxygen is exchanged for carbon dioxide through the capillary walls.
Expiration
Expiration follows inspiration as the intercostal muscles relax, the diaphragm extends upwards and the ribs and sternum collapse. at that point, pressure within the lungs is increased the air is expelled. when the body is in action, greater amounts of oxygen are required, requiring the intercostal muscles and diaphragm to work harder.
Lung volumes
Your respiratory rate is the amount of air you breathe in one minute. for a typical 18-year old, this represents about 12 breaths per minute at rest, during which time about 6 litres of air passes through the lungs. it can increase significantly during exercise, by as much as 30 to 40 breaths per minute.
Tidal volume
tidal volume is the term used to describe the amount of air breathed in and out with each breath. under normal conditions this represents about 500cm3 of air breathed, both inhaled and exhaled. of this, approximately two-thirds (350cm3) reaches the alveoli in the lungs where gaseous exchange takes place. the remaining 150cm3 fills the pharynx, larynx, trachea, bronchi and bronchioles and is known as dead stationary air.
during exercise, tidal volume increases to allow more air to pass through the lungs. the volume of air passing through the lungs each minute is known as the minute volume and is the product of breathing rate and the amount of air taken in with each breath.
the lungs normally contain about 350cm3 of fresh air, 150cm3 of dead air and 2,500 cm3 of air that has already undergone gaseous exchange with the blood.
Inspiratory reserve volume
by breathing in deeply, it is possible to take in more than the usual 350cm3 of fresh air that reaches the alveoli. this is especially important during exercise. in addition to the tidal volume, you can also breathe in up to an additional 3,000cm3 of fresh air. this is known as the inspiratory reserve volume.
Expiratory reserve volume
this can be up to 1,500cm3 and is the amount of additional air that can be breathed out after normal expiration. at the end of a normal breath, the lungs contain the residual volume plus the expiratory reserve volume. if you then exhale as much as possible, only the residual volume remains.
vital capacity
vital capacity is the amount of air that can be forced out of the lungs after maximal inspiration. the volume is around 4,800cm3
Residual volume
the lungs are never fully emptied of air otherwise they would collapse. the air that remains in the lungs after maximal exspiration, when you breathe out as hard as you can, is referred to as residual volume. the volume is around 1,200cm3 for an average male
Total lung capacity
this is your total lung capacity after you have inhaled as deeply and as maximally as you can, after maximal inspiration. it is normally around 6,000 cm3 for an average-sized male.
Control of breathing
neutral control
although breathing seems simple, its control is complex, it involves neurones, cells that conduct nerve impulses, in the reticular formation and pons, both parts of the brain stem. neurones in two areas of the medulla are critical in respiration. these are the dorsal respiratory group (DRG) and the ventral respiratory group (VRG). the VRG is thought to be responsible for the rhythm generation.
Chemical control
other factors that control breathing are the continually changing levels of oxygen and carbon dioxide. sensors responding to such chemical fluctuations are called chemoreceptors. these are found in the medulla and in the aortic arch and carotid arteries.
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