The Crucial Idea The respiratory and circulatory systems transport oxygen and nutrients to and from the cells. The bronchi, bronchioles, and alveoli are all found in the lungs. These organs play a crucial role in removing harmful substances from the body by way of exhalation (breathing out) and inhalation (breathing in). The circulatory system is responsible for delivering blood to every part of the body via the arteries, capillaries, veins, and lymphatic vessels. The heart is the main driver of this system; it sends electrical signals to various muscles in the body that control the opening and closing of valves allowing blood to flow in one direction only. These signals are also responsible for causing hearts to contract and relax at regular intervals so that they can keep up with daily activities and sleep cycles. There are two main types of respiratory diseases: chronic and acute. Chronic diseases include asthma, emphysema, and cystic fibrosis. With these conditions, the airways become swollen or damaged over time, making it difficult for them to clear mucus or expel pollutants. As a result, symptoms such as coughing, shortness of breath, wheezing, chest tightness, and pain may occur frequently. Acute diseases include pneumonia and influenza.