The human lungs are the primary organs of respiration, responsible for the continuous exchange of oxygen and carbon dioxide between the body and the outside air. The uppermost regions, known as the lung apices, possess distinct characteristics that set them apart. The apices are the rounded, superior tips of the lungs, and understanding their unique anatomy and function is important for comprehending both normal respiratory physiology and certain disease patterns. This specific area acts differently from the rest of the organ due to physical forces like gravity and surrounding anatomical restraints.
Anatomy and Exact Location
The apices of the lungs are the dome-shaped, pointed caps that represent the highest point of each lung. This part of the lung extends upward through the superior thoracic aperture, the narrow opening at the top of the rib cage. The apices project approximately 2.5 to 5 centimeters above the first rib and the medial third of the clavicle, or collarbone.
This superior extension means the apices are positioned close to the structures of the neck and the base of the throat. The lung tissue in this region is covered by the cervical pleura and is situated near the first rib and the lower cervical and upper thoracic vertebrae (C7 and T1 segments). The proximity of the apices to these bony and nervous structures in the cervicothoracic junction gives this area its unique clinical relevance. The superior sulcus, a groove formed by the subclavian artery, sits immediately below the apex.
Unique Structural Characteristics
The apices function under physical conditions that are significantly different from the lower parts, or bases, of the lungs, largely due to the effects of gravity. In an upright person, the apices experience lower blood flow, or perfusion, compared to the lung bases. This occurs because gravity makes it more difficult for blood to travel upward against resistance to reach the pulmonary capillaries at the lung tips.
This reduced blood flow results in a high ventilation-to-perfusion (V/Q) ratio, which is a measure of how well air delivery matches blood flow. While the overall lung V/Q ratio is generally around 0.8, the ratio in the apices can be much higher, sometimes reaching 3.3. This means that air is relatively abundant compared to the blood available for gas exchange. Even though the V/Q ratio is high, the absolute amount of blood flow and air movement is lower in the apices compared to the bases, which handle the majority of gas exchange.
The intrapleural pressure also contributes to this difference. In the upright position, the pressure is more negative at the apex, which causes the alveoli, the air sacs, to be more expanded and less compliant at rest. Because these apical alveoli are already partially stretched, they do not expand as much during a normal breath, which contributes to the physiological differences in air distribution across the lung fields.
Clinical Significance
The unique environment of the lung apices, characterized by high oxygen levels due to the high V/Q ratio and reduced blood flow, makes them a favored site for certain diseases. Primary pulmonary tuberculosis (TB), caused by the bacterium Mycobacterium tuberculosis, preferentially affects the apices. The bacterium thrives in high-oxygen environments, making the highly ventilated but poorly perfused apical tissue an ideal location for the infection to reactivate and flourish.
The anatomical location of the apices at the top of the chest cavity also makes them susceptible to specific types of tumors. A Pancoast tumor is a rare form of lung cancer that develops in the superior sulcus, near the apex. These tumors are particularly challenging because they often invade or compress nearby structures like the first rib, upper thoracic vertebrae, and the brachial plexus, a network of nerves that supplies the arm and hand.
Compression of the surrounding nerves by a Pancoast tumor can lead to a specific set of symptoms, including severe shoulder and arm pain, which may radiate down the arm. The tumor can also affect the sympathetic nervous system, leading to a condition called Horner syndrome, which involves a drooping eyelid and constricted pupil on the same side. Because the symptoms can mimic common musculoskeletal pain, a Pancoast tumor can sometimes be misdiagnosed initially, leading to a delay in treatment.