Smooth Muscle which is found in the trachea, bronchus and the terminal bronchioles, is a Circumferential layer: This becomes increasingly prominent as the airway diameter and cartilage decreases. Circular smooth muscle is most prominent in the terminal bronchioles. Smooth muscle tone: Controls the diameter of the conducting passages and controls resistance to the passage of air within the respiratory system. Creating pressure differences for breathing. (contraction and relaxation) There is no smooth muscle present in the alveoli, due to the fact that this would take up space, therefore minimising the rate of gaseous exchange. (the elastic fibres within the alveoli tend to do the contracting and relaxing.)
Goblet cells is a form of defence, attacking the germs/bacteria, before they enter the alveoli. They are only found in the bronchus and the trachea lining, secreting mucus, to prevent germs/bacteria which would lead to infection. The mucus, traps the “pollutants.”
This breathing in (inspiration) and breathing out (expiration) is controlled via nervous impulses from the respiratory centre in the medulla of the brain. Both the intercostal muscles (in between the ribs) and the diaphragm receive impulses from the respiratory centre. Stretch receptors in the lungs send impulses to the respiratory centre in the brain giving information about the state of the lungs. As the lungs fill with air the stretch receptors send impulses to the expiratory part of the respiration centre to end breathing in. (The ribs however, prevent the lungs from becoming damaged.)
There are also chemoreceptors in the medulla and certain blood vessels that are sensitive to changes in carbon dioxide levels in the blood. If the level is too high (the pH would drop, enzyme action would be affected with serious results), impulses are sent from these cells to the inspiratory part of the centre so that breathing rate increases. This means that carbon dioxide is got out of the body as quickly as possible and more oxygen comes in.
Other Components of Lung Structure, is the actual environment in which the lungs are situated. The blood supply is extensive, which means that oxygen is carried away to the cells as soon as it has diffused into the blood. The lung has a dual blood supply: pulmonary (venous blood) and systemic (arterial blood). The pulmonary circulation delivers deoxygenated blood from the right side of the heart and returns oxygenated blood to the left side of the heart. The pulmonary circulation consists of larger elastic and smaller muscular arteries, as it is handing less pressure. This environment is perfect for breathing.
