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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Alveolar Adaptations
Your lungs contain millions of tiny air sacs called alveoli, which are where the actual gas exchange happens in your body. These microscopic structures have evolved several clever adaptations that make them incredibly efficient at their job - getting oxygen into your bloodstream and removing carbon dioxide. Let's explore how these tiny biological marvels work!
Key Definitions:
Alveoli are microscopic balloon-like structures at the end of the bronchioles (small airways). Each alveolus is about 0.2-0.5 mm in diameter - tiny, but their total surface area is enormous! An adult's lungs contain approximately 300-500 million alveoli, creating a total surface area of about 70-100 square metres (roughly the size of a tennis court).
Having a large surface area is crucial for efficient gas exchange. The more surface area available, the more gas molecules can diffuse across at the same time. This is why your lungs don't just have one big air sac - millions of tiny alveoli create much more surface area for gas exchange than one large chamber would.
Alveoli have several special features that make them perfectly suited for gas exchange. These adaptations ensure that oxygen and carbon dioxide can move quickly and efficiently between the air and your blood.
Alveolar walls are extremely thin - just one cell thick (about 0.5 micrometres). This creates a very short diffusion distance for gases to travel across, making gas exchange faster and more efficient.
The inner surface of alveoli is covered with a thin film of moisture. This is essential because oxygen and carbon dioxide must dissolve in water before they can diffuse across cell membranes.
Alveoli are surrounded by a dense network of capillaries (tiny blood vessels). This ensures a continuous supply of blood for gas exchange and maintains a steep concentration gradient.
If all the alveoli in your lungs were spread out flat, they would cover an area roughly the size of a tennis court! This enormous surface area is packed into the relatively small space of your chest by the folded, honeycomb-like structure of the alveoli.
Gas exchange in the alveoli works by diffusion - the movement of molecules from an area of high concentration to an area of low concentration. Several factors affect the rate of diffusion:
Alveoli are optimised for all these factors:
Your breathing constantly brings fresh air (high in oxygen, low in carbon dioxide) into the alveoli. Meanwhile, your blood flow continuously brings deoxygenated blood (low in oxygen, high in carbon dioxide) to the capillaries surrounding the alveoli. This maintains steep concentration gradients that drive diffusion in both directions.
The barrier between air in the alveoli and blood in the capillaries is extremely thin - just 0.5 micrometres thick. It consists of:
This thin barrier is known as the respiratory membrane and allows for rapid diffusion of gases.
Alveoli contain two special types of cells that help maintain their function:
These thin, flat cells make up about 95% of the alveolar surface. Their flattened shape minimises the diffusion distance for gases. They're specialised for gas exchange but are vulnerable to damage.
These rounded cells produce surfactant, a mixture of lipids and proteins that reduces surface tension in the alveoli. They can also divide to replace damaged type I cells, helping the lungs repair themselves.
Surfactant is a mixture of phospholipids and proteins produced by type II pneumocytes. It plays several vital roles in alveolar function:
Premature babies sometimes develop Infant Respiratory Distress Syndrome (IRDS) because their lungs haven't yet developed enough surfactant. Without sufficient surfactant, their alveoli collapse during exhalation, making breathing extremely difficult. Treatment involves giving artificial surfactant directly into the baby's lungs through a breathing tube. This highlights the crucial importance of surfactant in normal lung function.
Understanding alveolar adaptations helps us understand what happens when things go wrong in lung diseases:
In emphysema, alveolar walls break down, creating fewer, larger air spaces. This reduces the surface area available for gas exchange. It's commonly caused by smoking, which damages the elastic fibres in alveolar walls.
In pneumonia, infection causes inflammation and fluid buildup in alveoli. This increases the diffusion distance for gases and reduces the space available for air, making gas exchange less efficient.
In pulmonary fibrosis, lung tissue becomes scarred and thickened. This increases the diffusion distance between air and blood, making gas exchange more difficult.
To summarise, alveoli have several adaptations that make them incredibly efficient at gas exchange:
These adaptations work together to ensure that oxygen can quickly enter your bloodstream and carbon dioxide can be efficiently removed - all vital processes that keep your body functioning properly!
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