 You thought it was crazy? Well, just remember, we were in bronchialand with cartilages, and there can be up to 12 bronchi bifurcations. 12 layers of bronchi heading out there. How do we know they're bronchi? They have hyaline cartilage. The next layer, the next structure, does not have hyaline cartilage, and they are called bronchioles. Bronchioles. And bronchioles, no cartilage, but you know what they do have? They are surrounded by smooth muscle. Okay, structure determines function. What is going to be a function of a bronchiol if it is surrounded by smooth muscle? Bronchoconstriction. Bronchodilation. Done. Bronchioles can change the resistance. They can change the diameter of the tubing. If you have a bigger diameter, it's easier to get air in. If you have a smaller diameter, it's harder to get air in. You can actually have a smaller diameter and be fine as long as you get enough air to fuel your oxygen needs and to deal with your carbon dioxide waste products that you don't get rid of. So they're just more tubes, and we're not at the end because what hasn't even happened yet? What's the whole point of the respiratory system? Holy tubes going everywhere. We've had all these bronchies which are covered in cartilage to keep them open, and now we have bronchioles that can change diameter and affect the amount of air that gets in. And guess where we're going next? The hint. It starts with an A.