The lungs perform the continuous work of respiration, allowing the body to exchange carbon dioxide for life-sustaining oxygen. This exchange requires a constant, controlled connection between the lungs and the main structures of the chest cavity, known as the mediastinum. The hilum serves as the central point of entry and exit for all the necessary vessels and airways that sustain the lung’s function. This arrangement of structures is often referred to as the lung root, securing the organ in place while providing the critical channels for air and blood flow.
Defining the Hilum and Its Position
The hilum is a wedge-shaped depression located on the medial surface of each lung, facing the center of the chest. It acts as the specific gateway through which essential structures pass, connecting the lung to the central mediastinum. The hilum is typically situated roughly between the fifth and seventh thoracic vertebrae, positioned slightly towards the back of the chest cavity.
The lung is encased by the pleura, a protective, two-layered membrane consisting of the inner visceral layer and the outer parietal layer. At the hilum, these two layers meet and become continuous, creating a sleeve that encloses the structures of the lung root. This reflection of the pleura extends downward as the pulmonary ligament, which helps anchor the lower lobe of the lung and allows for movement during breathing.
The Structures That Form the Lung Root
The main bronchus is the most prominent structure passing through the hilum, conducting inhaled air into the lung. This air is distributed throughout the lung tissue to reach the air sacs where gas exchange occurs.
Blood flow is managed by the pulmonary vasculature, which includes one pulmonary artery and two pulmonary veins for each lung. The single pulmonary artery carries deoxygenated blood to the lungs from the right side of the heart for oxygenation. Conversely, the two pulmonary veins carry the newly oxygenated blood from the lungs back to the left side of the heart for distribution to the rest of the body.
There is a structural difference in the arrangement of these major components between the two lungs. On the right side, the pulmonary artery is positioned anterior to the main bronchus. On the left side, the pulmonary artery arches over the left main bronchus. The lung root also contains the smaller bronchial arteries, which supply oxygenated blood directly to the lung tissue itself, and the pulmonary plexuses of nerves, which control the muscle tone of the airways and vessels.
Clinical Significance in Health and Disease
The hilum holds considerable importance in medical diagnosis due to the concentration of lymph nodes in this area. These small, bean-shaped structures, known as hilar lymph nodes, are part of the body’s immune system, filtering lymphatic fluid and housing immune cells. When these nodes become enlarged, a condition known as hilar lymphadenopathy, it serves as a common indicator of various diseases.
Enlargement can be caused by infections, such as tuberculosis or fungal diseases, as the immune system responds to foreign organisms. Systemic inflammatory conditions, like sarcoidosis, often cause a characteristic bilateral enlargement of the hilar lymph nodes. The hilum is also a site for the spread of cancer, as malignant cells can metastasize from lung tumors or other parts of the body to these nodes.
The appearance of the hilum is a standard focus in diagnostic imaging, particularly on chest X-rays and Computed Tomography (CT) scans. On a chest X-ray, the hila appear as soft tissue shadows, and abnormal changes in their size, shape, or contour suggest underlying pathology. CT scans provide a more detailed view, allowing clinicians to differentiate between an enlarged blood vessel, a mass, or swollen lymph nodes. This differentiation is essential for accurate diagnosis and staging of diseases like lung cancer.