Final answer:
Alveoli have a circular shape, which is flexible due to their elastic walls. About 300 million alveoli in each human lung create an expansive surface for gas exchange with the bloodstream. Their design, together with the capillaries and red blood cells, ensures efficient diffusion of O₂ and CO₂.
Step-by-step explanation:
The alveoli have a unique shape best described as circular when inflated, although they may vary in shape as they can deform slightly due to the elasticity of their walls and the presence of alveolar pores that connect them. The alveoli are tiny sac-like structures and are a crucial part of the respiratory system, facilitating gas exchange.
There are approximately 300 million alveoli in a human lung, each alveolus approximately 200 μm in diameter. When inflated, their combined internal surface area is roughly the size of a tennis court, which dramatically increases surface area for gas exchange. The alveolar walls are only one-cell thick, and they are surrounded by a dense network of capillaries, also one-cell thick. This arrangement allows for rapid diffusion of oxygen (O₂) into the blood and carbon dioxide (CO₂) out of the blood.
An alveolar sac is a cluster of many individual alveoli that share a common duct. Given the vast number of alveoli, the lungs have a sponge-like consistency. The stretching of alveoli during air intake further maximizes the surface area available for gas exchange. In essence, the structure and function of alveolar sacs, alveoli, capillaries, and the red blood cells within them collectively enhance the efficiency of respiratory gas exchange.