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Unlock This CourseYour lungs contain around 300 million tiny air sacs called alveoli. These microscopic structures are biological masterpieces - perfectly designed for one crucial job: swapping oxygen and carbon dioxide between your blood and the air you breathe. Every breath you take relies on these adaptations working flawlessly.
Gas exchange happens by diffusion - molecules naturally move from areas of high concentration to low concentration. The alveoli have evolved amazing adaptations to make this process as efficient as possible, ensuring your body gets the oxygen it needs and removes waste carbon dioxide.
Key Definitions:
Each alveolus is like a tiny balloon made of incredibly thin cells. The wall is only one cell thick - that's about 0.5 micrometres! This ultra-thin barrier allows gases to pass through easily whilst still being strong enough to handle the pressure changes during breathing.
Alveoli have four main adaptations that make them perfect for gas exchange. Think of them as the "fantastic four" of respiratory biology - each one plays a vital role in keeping you alive.
If you could unfold all your alveoli and lay them flat, they would cover about 70 square metres - roughly the size of a tennis court! This massive surface area is created by having millions of tiny alveoli rather than fewer large ones.
300 million alveoli create 70m² of surface area - about 40 times larger than your skin's surface area!
More surface area means more space for oxygen and carbon dioxide molecules to cross into and out of your blood.
It's like having millions of tiny windows instead of one huge window - much more efficient for air flow!
The alveolar wall consists of just one layer of flattened cells called squamous epithelium. These cells are so thin that gases can diffuse across them in milliseconds. The shorter the distance, the faster the diffusion.
The alveolar wall is 100 times thinner than a sheet of paper! This incredibly thin barrier is made of squamous epithelial cells that are perfectly flattened to minimise diffusion distance.
Each alveolus is surrounded by a dense network of capillaries - the tiniest blood vessels in your body. These capillaries are so narrow that red blood cells have to squeeze through in single file, bringing them as close as possible to the alveolar wall.
The pulmonary capillaries form a mesh around each alveolus. Blood flows slowly through these narrow vessels, giving maximum time for gas exchange to occur. The capillary walls are also just one cell thick, creating the thinnest possible barrier between air and blood.
For diffusion to work efficiently, there must always be a difference in concentration between the alveolar air and the blood. Your body maintains this through constant ventilation (breathing) and circulation.
Breathing brings in oxygen-rich air, maintaining high oxygen concentration in alveoli.
Breathing removes carbon dioxide-rich air, keeping CO₂ concentration low in alveoli.
Circulation brings deoxygenated blood to lungs and carries oxygenated blood away.
Understanding the process step-by-step helps you appreciate how perfectly adapted alveoli are for their job.
When you breathe in, oxygen-rich air fills your alveoli. The oxygen concentration in the alveolar air is higher than in the blood arriving from your body tissues. Oxygen molecules naturally diffuse across the thin alveolar wall into the blood, where they bind to haemoglobin in red blood cells.
Meanwhile, carbon dioxide works in reverse. Your blood arrives at the lungs carrying high concentrations of CO₂ from cellular respiration. Since the alveolar air has less carbon dioxide, CO₂ diffuses from the blood into the alveolar space, ready to be breathed out.
During exercise, your muscles need more oxygen and produce more carbon dioxide. Your breathing rate increases to bring in more fresh air and remove more waste. Your heart rate also increases to pump blood faster through the lungs. This shows how the body adapts to maintain efficient gas exchange even when demands change dramatically.
Scientists can measure how well your alveoli work using various tests. These measurements help doctors understand lung health and diagnose respiratory problems.
Doctors use spirometry to measure lung volumes and capacities. They can also measure how much oxygen and carbon dioxide are in your blood using pulse oximetry and blood gas analysis. These tests show whether your alveolar adaptations are working properly.
Healthy alveoli can exchange about 250ml of oxygen and 200ml of carbon dioxide per minute at rest. During exercise, this can increase to over 4000ml per minute! This shows just how efficient the alveolar adaptations are.
Understanding normal alveolar adaptations helps us understand what happens when things go wrong. Various diseases can affect the efficiency of gas exchange by damaging these crucial adaptations.
Emphysema destroys alveolar walls, reducing surface area. Pneumonia causes fluid buildup, increasing diffusion distance. Asthma narrows airways, reducing ventilation. Each problem shows how important the normal adaptations are for healthy gas exchange.
Your alveoli are so efficient that even when you're resting, they exchange enough oxygen in one day to fill about 500 party balloons! This incredible efficiency comes from the perfect combination of large surface area, thin walls, rich blood supply and maintained concentration gradients.