Alveoli

The alveoli (singular: alveolus) are millions of microscopic air sacs located at the ends of the bronchioles within the human lungs. They constitute the primary gas exchange surface in mammals, crucial for transferring oxygen into the blood and removing carbon dioxide from the body. An adult human's lungs contain approximately 700 million alveoli, providing a vast total surface area estimated to be about 70-75 m², which is 30-40 times greater than the body's external skin surface. Some sources cite figures as high as 90 m² or 100 m².

Structure and Features Enhancing Gas Exchange

The structure of the alveoli is highly specialized to facilitate efficient gas exchange:

• Alveolar Wall (Alveolar Epithelium): Each alveolus is lined by a single layer of thin, flat squamous epithelial cells. This thinness creates a short diffusion pathway for gases, significantly speeding up the rate of diffusion.

• Capillary Network: Alveoli are densely surrounded by a rich network of blood capillaries. The walls of these capillaries are also only one cell thick (capillary endothelium). Gases must diffuse across both the alveolar epithelium and the capillary endothelium to enter or leave the blood. The capillaries are extremely narrow, often only wide enough for red blood cells to squeeze through, ensuring close contact between blood and the alveolar wall.

• Large Surface Area: The sheer number of alveoli (millions) provides an enormous surface area for gas exchange.

• Steep Concentration Gradients: A steep concentration gradient for both oxygen and carbon dioxide is continuously maintained between the alveolar air and the blood in the capillaries. This is achieved by the constant flow of blood through the capillaries, which continuously removes oxygenated blood and brings deoxygenated blood, and by ventilation (breathing), which replenishes the oxygen supply in the alveoli and removes carbon dioxide.

• Moist Surface: Oxygen must first dissolve in the thin film of moisture lining the alveoli before it can diffuse across the epithelium into the blood.

• Elastin Fibres: The walls of the alveoli contain elastic fibres (elastin). These fibres are stretched during inspiration as the alveoli expand, and then recoil during expiration, aiding in the expulsion of air and helping the alveoli return to their normal shape.

• Lung Surfactant: Specialized surfactant cells in the alveoli produce a detergent-like mixture of lipoproteins and phospholipids. This lung surfactant lowers surface tension, allowing the alveoli to flex easily during breathing and preventing them from collapsing on expiration.

Protective Mechanisms

The delicate alveolar structure is protected from inhaled foreign matter:

• Macrophages (Dust Cells): These phagocytic white blood cells are abundant on the surfaces of the airways and alveoli. They scavenge and engulf fine dust particles, bacteria, fungal spores, and other debris that may have bypassed the upper airway's cleaning mechanisms.

Impact of Lung Diseases

The efficiency of gas exchange in the alveoli can be severely compromised by lung diseases:

• Emphysema: This condition, often caused by smoking or long-term air pollution, involves the breakdown of elastin by enzymes from phagocytes. The loss of elastin impairs the alveoli's ability to recoil and expel air, leading to air trapping. Furthermore, the destruction of alveolar walls results in enlarged air spaces and a significant reduction in the surface area available for gas exchange. This reduces the rate of oxygen diffusion into the blood, leading to lower blood oxygen levels. Symptoms include shortness of breath and wheezing.

• Fibrosis: While not directly an alveolar disease, fibrosis involves the formation of thicker, less elastic scar tissue in the lungs. This can occur between the alveoli and capillaries, increasing the diffusion distance for gases and slowing down gas exchange.

• General Effects of Lung Diseases: Conditions affecting the alveoli, such as emphysema and fibrosis, reduce the rate of gas exchange, meaning less oxygen diffuses into the bloodstream. This leads to body cells receiving less oxygen, a reduction in aerobic respiration, and consequently, less energy being released, often causing tiredness and weakness in sufferers.

The alveoli are a critical component of the respiratory system, meticulously designed for the rapid and efficient exchange of gases, and their compromised function in disease highlights their importance for overall health.