A hilus (also spelled hilum) is the indented, often slit-shaped area on the surface of an organ where blood vessels, nerves, lymphatic vessels, and ducts enter or exit. Think of it as a loading dock: it is the single organized gateway through which an organ receives its blood supply and connects to the rest of the body. Nearly every solid organ has one, from the kidneys and lungs to the liver, spleen, and ovaries, and the term even extends to a specific region inside the brain. Because so much critical traffic passes through a small space, the hilus plays an outsized role in surgery, diagnostic imaging, and disease.
Why Organs Need a Gateway
Solid organs are wrapped in a tough outer capsule or membrane that holds their shape and protects the tissue inside. That capsule cannot be entirely sealed off, though, because the organ needs blood flowing in and out, lymphatic fluid draining away, and sometimes a duct carrying the organ’s product (bile from the liver, urine from the kidney) to its destination. Rather than having vessels and ducts pierce the capsule at scattered, unpredictable points, the body funnels them through one organized opening. That opening is the hilus.
The arrangement keeps things tidy in a surgical sense, too. A surgeon who needs to temporarily shut off blood flow to an organ, or who needs to remove an organ altogether, can focus on one compact bundle of structures rather than tracking vessels all over the organ’s surface. This bundling is not just convenient; it is often the difference between a safe operation and a dangerously complicated one.
The Renal Hilum
The kidney’s hilum sits on the organ’s concave, medial side, roughly facing the spine. Through this notch pass the renal artery (bringing oxygenated blood in), the renal vein (carrying filtered blood out), the ureter (draining urine toward the bladder), lymphatic vessels, and a network of nerves. Surrounding these structures is a pad of fat sometimes called hilar fat, which is anatomically distinct from the fat encasing the kidney’s outer surface. Dissection studies have identified a fibrous connective tissue gap between the perirenal fat and the fat sitting inside the hilum, suggesting the two fat compartments are more separate than they appear on standard imaging.1PubMed Central. Zoning inside the renal fascia: The anatomical relationship between the urinary system and perirenal fat
The renal hilum gets particular attention during partial nephrectomy, a kidney-sparing surgery used for small renal tumors. To control bleeding and get a clear view, the surgeon typically clamps the hilum, temporarily cutting off blood flow. Whether to clamp just the renal artery or both the artery and vein together has been debated for years. Research comparing the two approaches found that clamping the artery alone led to slightly less kidney function loss during the first week after surgery, but by about two weeks out, total kidney function was statistically the same regardless of technique.2PubMed Central. Comparison of Hilar Clamping Techniques in Partial Nephrectomy: Is Artery Only Clamping Beneficial? The artery-only approach also came with more blood loss, muddying any early advantage.
What matters more than which structures are clamped is how long they stay clamped. A large retrospective study found that each additional minute of warm ischemia time (the period with no blood flowing to the kidney) raised the odds of acute kidney failure and long-term chronic kidney disease. A threshold of roughly 25 minutes best separated patients who had complications from those who did not.3PubMed. Every minute counts when the renal hilum is clamped during partial nephrectomy The phrase “every minute counts” has since become something of a mantra in kidney surgery, and it underscores how concentrated and consequential the hilum’s vascular bundle really is.
The Pulmonary Hilum
Each lung has its own hilum on the medial surface, facing the central chest cavity. The pulmonary arteries (carrying deoxygenated blood from the heart to be oxygenated), pulmonary veins (returning oxygen-rich blood to the heart), the main bronchus (the airway tube), bronchial arteries, lymphatic vessels, and nerves all pass through this region. The left and right hila are not mirror images; because the heart sits slightly to the left, the left hilum is positioned a bit higher and has a somewhat different arrangement of vessels and bronchi.
Anatomical variations at the pulmonary hilum are surprisingly common. A cadaveric study documented variations in about 16 percent of right lungs and roughly 48 percent of left lungs, including cases of doubled upper pulmonary veins, two pulmonary arteries, or extra bronchi passing through the hilum.4PubMed. Anatomical variations of the arrangement of structures at the pulmonary hilum: a cadaveric study These variations rarely cause symptoms on their own, but they matter enormously to a thoracic surgeon who needs to isolate and divide individual vessels or airways during a lobectomy or pneumonectomy. An unexpected extra vessel that the surgeon does not anticipate can lead to serious bleeding.
The hilar lymph nodes clustered around the pulmonary hilum are a frequent focus in both lung cancer staging and inflammatory diseases. In sarcoidosis, enlarged lymph nodes in the bilateral hilar stations and the mediastinum are the most common imaging finding, showing up on CT scans in about 90 percent of cases at presentation.5Seminars in Ultrasound, CT and MRI. Imaging of Diseases Involving Pulmonary Lymphatics: Focus on Pulmonary Sarcoidosis Distinguishing enlarged hilar lymph nodes caused by sarcoidosis from those caused by lymphoma or metastatic lung cancer is a common diagnostic challenge, and the specific pattern of node involvement helps radiologists tell them apart.6PubMed Central. Differentiation between sarcoidosis and Hodgkin’s lymphoma based on mediastinal lymph node involvement pattern: Evaluation using spiral CT scan
The Hepatic Hilum, or Porta Hepatis
The liver’s hilum has its own traditional name: the porta hepatis, Latin for “gate of the liver.” It sits on the liver’s underside and serves as the entry and exit point for three structures that are central to everything the liver does. The portal vein delivers nutrient-rich blood absorbed from the intestines. The hepatic artery proper supplies oxygenated blood. And the common hepatic duct carries bile, produced by liver cells, out toward the gallbladder and small intestine.7PubMed Central. Porta hepatis in relation to portal vein among Indians
This triple bundle makes the porta hepatis one of the most surgically significant spots in the abdomen. During liver surgery, a technique called the Pringle maneuver involves compressing everything at the porta hepatis to temporarily stop blood flow into the liver, controlling bleeding while the surgeon works. Bile duct cancers that arise right at the hilum (called hilar cholangiocarcinoma or Klatskin tumors) are among the most challenging tumors to treat, in part because the cancer sits at the exact junction where bile drainage, arterial supply, and portal blood flow all converge. Getting a clear surgical margin without destroying one of those essential conduits is extremely difficult.
The Splenic Hilum
The spleen’s hilum is on its medial, visceral surface. The splenic artery and vein, along with lymphatic vessels and nerves, pass through here. One clinically relevant quirk of the splenic hilum is that it is the most common location for accessory spleens, small nodules of splenic tissue that developed separately during embryonic life. Cadaveric dissections have confirmed that accessory spleens frequently sit right at the hilum.8PubMed Central. Accessory Spleen in the Splenic Hilum: a Cadaveric Study with Clinical Significance
Accessory spleens are usually harmless and most people never know they have one. But they become a real problem if a splenectomy (complete removal of the spleen) is being done to treat a blood disorder like immune thrombocytopenic purpura. In those cases, any surviving splenic tissue can continue the destructive immune process the surgery was meant to stop. A surgeon who misses a small accessory spleen tucked into the hilar fat may find that the patient’s platelet counts do not improve as expected. This is why careful exploration of the hilum and its surrounding fat is standard during splenectomy for hematological conditions.
Rare developmental anomalies can also affect the splenic hilum. A bifid spleen, in which the organ is partially split into two lobes, results from incomplete fusion of the separate tissue buds (splenic primordia) that form the organ during embryonic development. These cases sometimes come with unusual vascular branching patterns at the hilum.9Adolescência E Saúde. Bifid Spleen With A Novel Hilar Vascular Pattern: A Rare Cadaveric Observation With Embryological, Biochemical, Radiological, And Surgical Implications
Other Organs With a Hilum
The kidneys, lungs, liver, and spleen get the most attention, but a hilum is a general anatomical feature of most solid organs. Each ovary has a hilum along its mesovarium (the fold of tissue connecting the ovary to the broad ligament), and this is where the ovarian artery, vein, nerves, and lymphatic vessels enter. Lymph nodes throughout the body also have hila where blood vessels pass in and out. Even the adrenal glands have a hilum through which the adrenal vein exits.
Less intuitively, the term hilus is also used for a region inside the brain. The hilus of the dentate gyrus is a zone within the hippocampus, the brain structure most associated with memory. This hilus is not a surface indentation like the hila of solid organs; instead, it is a deep layer of tissue sandwiched inside the dentate gyrus, densely packed with a distinctive type of neuron called the mossy cell. Mossy cells send strong excitatory inputs to the granule cells of the dentate gyrus, and they also communicate with local inhibitory neurons, giving them a complex role in regulating hippocampal circuits.10Hippocampus. Towards an Understanding of the Dentate Gyrus Hilus Partial loss of these mossy cells is a hallmark of temporal lobe epilepsy, and research in mice has shown that selectively killing mossy cells triggers spontaneous seizures and spatial memory deficits.11PubMed Central. Dentate gyrus mossy cells control spontaneous convulsive seizures and spatial memory The hippocampal hilus is a good reminder that “hilus” can describe internal anatomical landmarks, not just organ-surface gateways.
Imaging the Hilus
The hilum of an organ is often one of the first things a radiologist evaluates on a chest X-ray or CT scan, particularly in the lungs. Enlarged hilar shadows on a plain chest film can signal anything from lung cancer to sarcoidosis to heart failure (which engorges the pulmonary veins passing through the hilum). CT with intravenous contrast is the standard tool for detailed evaluation, because the contrast agent lights up blood vessels and makes it easier to distinguish a normal vessel from an abnormal lymph node or mass.
MRI offers an interesting advantage at the hilum. Because flowing blood produces a natural signal void on certain MRI sequences, blood vessels appear dark while soft-tissue masses appear bright, even without injecting a contrast agent. Early studies comparing MRI with CT for hilar and mediastinal masses found that MRI could distinguish masses from blood vessels without contrast and, in some cases, showed a greater extent of disease than CT did.12PubMed. NMR evaluation of hilar and mediastinal lymphadenopathy A study focused specifically on the pulmonary hilum found that lymph nodes about a centimeter in diameter were more easily seen on MRI than on contrast-enhanced CT in several patients.13PubMed. Magnetic resonance imaging of the normal and abnormal pulmonary hila This contrast-free capability is especially useful for patients who are allergic to iodinated contrast dye or who have impaired kidney function, where injecting contrast could cause further damage.14PubMed. Magnetic resonance imaging of mediastinal and hilar masses: comparison with CT
Today, CT remains the workhorse for routine hilar evaluation because it is faster and more widely available. But MRI is a go-to problem solver when CT findings are ambiguous or when contrast injection is risky. PET-CT, which maps metabolic activity, adds another layer by revealing whether hilar lymph nodes are merely enlarged or are metabolically active in a way that suggests cancer or active inflammation.
Why Hilar Anatomy Varies So Much
One of the underappreciated aspects of hilar anatomy is how much it varies from person to person. As noted earlier, the pulmonary hilum shows anatomical variations in close to half of all left lungs.4PubMed. Anatomical variations of the arrangement of structures at the pulmonary hilum: a cadaveric study The renal hilum is similarly variable: the number, branching pattern, and position of renal arteries and veins differ enough from one person to the next that surgeons routinely order preoperative imaging to map the vasculature before kidney surgery. Some people have a single renal artery entering the hilum, while others have two or even three, and the veins can be just as unpredictable.
The porta hepatis is another hot spot for variation. The “textbook” arrangement of portal vein, hepatic artery, and bile duct is actually found in only a portion of people; the hepatic artery, in particular, has a wide range of origin and branching variants that can change the surgical approach to liver resection or transplantation. This variability is a major reason why cross-sectional imaging (CT angiography or MR angiography) before hepatobiliary surgery has become essentially mandatory. Surgeons cannot assume they will find the standard anatomy when they open up the hilum.
Accessory structures at the splenic hilum add to the variability picture. Beyond accessory spleens, extra branches of the splenic artery or unusually short splenic vessels can complicate splenectomy or distal pancreatectomy. None of these variants are “abnormalities” in a disease sense; they are simply part of the normal range of human anatomy. But they make the hilum of any organ a place where preoperative planning and careful dissection are not optional.
Common Misconceptions About the Hilus
One frequent point of confusion is the plural form. “Hilum” is the standard singular in modern anatomical terminology, and its Latin plural is “hila.” You will also see “hilus” as an alternative singular (the Latinized Greek form), with “hili” as its plural. Both singular forms are used in the medical literature, sometimes interchangeably within the same textbook. The choice between hilum and hilus is largely a matter of convention, and neither is wrong.
Another misconception is that the hilum is just a hole in the organ’s surface. It is better described as a structured gateway with its own connective tissue scaffolding, fat pad, and sometimes its own cluster of lymph nodes. In the kidney, the hilum opens into a deeper internal space called the renal sinus, where the renal pelvis (the funnel that collects urine) and the major branches of the renal vessels sit. The hilum is the visible entrance, but the sinus is the room inside.
People sometimes assume that because the hilum is a single concentrated point, it is inherently a weak spot in an organ’s defenses. There is a grain of truth here: infections, cancers, and inflammatory conditions often show up at the hilum first, because the lymph nodes clustered there are the first to encounter pathogens or tumor cells draining from the organ. But the hilum is also richly supplied with immune tissue for exactly that reason. Hilar lymph nodes in the lung, for example, are part of the body’s frontline surveillance system, filtering material that comes up from the airways and flagging threats for an immune response. The hilum is not a weakness so much as a checkpoint.
The Hilum in Transplant Surgery
Organ transplantation puts the hilum front and center. When a kidney is transplanted, the surgeon connects the donor renal artery and renal vein from the hilum to the recipient’s iliac vessels in the pelvis, and the ureter is implanted into the bladder. Everything flows through that one hilar bundle. If the donor kidney has multiple renal arteries, each one needs its own anastomosis (connection), which increases operative complexity and time. Preoperative imaging of the donor’s hilar anatomy is essential, especially in living-donor transplants, to determine which kidney to take and how to handle the vascular connections.
Liver transplantation is even more complex at the hilum. The porta hepatis must be carefully dissected so that the donor’s portal vein, hepatic artery, and bile duct can be connected to the recipient’s corresponding structures. Variations in hepatic artery anatomy can force the surgical team to improvise vascular reconstructions during the transplant. A replaced right hepatic artery (one that arises from the superior mesenteric artery instead of the common hepatic artery) is among the more common variants, and missing it during organ procurement can compromise blood supply to part of the graft.
Lung transplantation similarly revolves around the hilum: the donor bronchus, pulmonary artery, and pulmonary veins are connected to the recipient’s corresponding structures at or near the hilar region. Bronchial complications at the anastomosis site are among the most common surgical issues after lung transplant, in part because the bronchial blood supply, which runs through the hilum, is often disrupted during the procedure and takes time to regenerate.