The airways (the tubes that carry air into and out of the lungs) are surrounded by a type of muscle called smooth muscle. In people with asthma, these muscles often tighten in reaction to certain things. When this happens, the airways become narrower, which blocks the flow of air and makes it harder to breathe. This narrowing of the airways is known as bronchoconstriction. Along with inflammation of the airways, it leads to symptoms such as coughing, wheezing, and shortness of breath.