The lung's anatomical features include three boundaries, three surfaces, and an apex. Above the first rib is the apex. The anterior, posterior, and inferior borders are the three boundaries. A cardiac notch is formed in the left lung by the anterior edge of the lung, which is equivalent to the pleural reflection. The lung develops a concavity to provide room for the heart, known as the cardiac notch. The base of the lung and the costal surface are divided by the narrow inferior border. The thick posterior border runs from the apex of the lung to the inferior border, as well as from the C7 to the T10 vertebra. The primary purpose of the lungs, the foundational organs of the respiratory system, is to enable the exchange of gases from the surrounding environment into the circulation. After passing via the alveoli and into the capillary network, oxygen can enter the arterial system and eventually perfuse tissue. The nose, oropharynx, larynx, trachea, bronchi, bronchioles, and lungs make up the majority of the respiratory system. The lungs further split into separate lobes, each of which splits into more than 300 million alveoli. The main site of gas exchange is the alveoli. The nerve roots of C3, C4, and C5 innervate the diaphragm, the main breathing muscle, through the phrenic nerve. Exercise and respiratory distress are the main uses for the inspiratory muscles known as the external intercostals.