The term “hilum” refers to a distinct, often concave, depression or fissure on the surface of an organ, similar to the indentation on a bean. The adjective “hilar” describes this specific region. This anatomical feature serves a functional purpose, acting as the point where structures functionally connected to the organ pass through.
The Hilum as the Organ’s Gateway
The hilum acts as a gateway for the essential components that sustain an organ’s function. This arrangement is observed across multiple organs, including the lungs, kidneys, liver, spleen, and lymph nodes. The collective bundle of structures passing through the hilum is often referred to as the organ’s root or pedicle.
These vital structures typically include arteries and veins, nerves, and lymphatic vessels. For organs that filter or produce substances, the hilum also accommodates ducts or tubes, such as the ureter in the kidney. This centralized connection point ensures the organ is efficiently integrated into the body’s circulatory and regulatory networks.
Anatomy of the Lung Hilum
The pulmonary hilum is a large, wedge-shaped depression located on the medial surface of each lung. It serves as the attachment point for the “root of the lung,” which connects the lung to the trachea and heart. This root contains the primary components necessary for respiration and blood circulation.
The structures that enter and exit include the main bronchus, the pulmonary artery, and the two pulmonary veins. The main bronchus carries air, the pulmonary artery transports deoxygenated blood for gas exchange, and the pulmonary veins return oxygenated blood to the heart.
The arrangement differs slightly between the left and right lung. On the left side, the single pulmonary artery typically arches over the main bronchus, positioning the left hilum slightly higher than the right. The pulmonary veins are usually situated in the most inferior and anterior positions on both sides. Bronchial arteries and veins, nerves, and lymph nodes also pass through this region.
Anatomy of the Kidney Hilum
The renal hilum is the concave fissure found on the medial border of the kidney. This indentation provides a protected entry and exit point for the structures that form the renal pedicle, connecting the kidney to the circulatory and urinary systems. The arrangement of these structures follows a consistent pattern.
From anterior to posterior, the structures are typically ordered as the renal vein, the renal artery, and the renal pelvis, which narrows to become the ureter. The renal vein drains filtered blood, while the renal artery supplies blood to be filtered and is positioned centrally. The ureter, the most posterior structure, carries urine from the renal pelvis down to the bladder.
When the Hilar Region is Affected
The concentration of vessels, airways, and lymphatic tissue makes the hilar region a frequent focus of medical imaging and diagnosis. Issues in this area are often referred to as “hilar enlargement.” The most common clinical presentation is hilar lymphadenopathy, which is the swelling of the lymph nodes located within the hilum, particularly in the lungs.
This lymph node enlargement is a non-specific sign indicating infection (like tuberculosis or fungal illness) or inflammatory disease (like sarcoidosis). Hilar lymphadenopathy can also signal malignancy, such as lung cancer or lymphoma, since cancer cells often spread through the lymphatic system. Imaging tests like chest X-rays and computed tomography (CT) scans are routinely used to assess the size and density of hilar structures and determine the underlying cause.