The axillary artery officially becomes the brachial artery at the lower border of the teres major muscle, a thick band of muscle running from the shoulder blade to the upper arm bone. This naming convention is one of anatomy’s cleaner boundaries: a single vessel carrying blood from the torso into the arm simply changes its name as it passes a defined muscular landmark. But the transition is not always as tidy as textbooks suggest, because anatomical variations in branching patterns can shift the effective starting point of the brachial artery, and the surrounding nerves and muscles make this zone clinically significant for surgeons, radiologists, and emergency physicians alike.
The Textbook Transition Point
The subclavian artery, which runs beneath the collarbone, is renamed the axillary artery once it crosses the outer border of the first rib. From there, the axillary artery travels through the armpit region and is classically divided into three parts based on its relationship to the pectoralis minor muscle: the first part sits above (medial to) the muscle, the second part lies behind it, and the third part sits below (lateral to) it.1Scholars International Journal of Anatomy and Physiology. A Rare Bifurcation of the Axillary Artery into Deep and Superficial Branches: A Clinically Relevant Anatomical Variation: A Case Report Each part gives off different branches: the first part typically sends off one branch (the superior thoracic artery), the second part gives off two (the thoracoacromial trunk and lateral thoracic artery), and the third part gives off three (the subscapular artery and the anterior and posterior circumflex humeral arteries).
Once the vessel passes the lower edge of the teres major, it enters the arm proper and is called the brachial artery. The brachial artery then runs along the medial side of the upper arm, nestled in a groove between the biceps and triceps, until it reaches the cubital fossa (the pit of the elbow), where it typically splits into the radial and ulnar arteries. In practical terms, this is a continuous tube of blood vessel. No valve, no sudden change in diameter, and no structural feature within the vessel wall marks the spot where one name ends and the other begins. The distinction exists because surgeons and anatomists need a shared vocabulary for describing where injuries, diseases, and procedures occur.
Why the Teres Major and Not Some Other Landmark
Anatomical naming conventions rely on landmarks that can be reliably identified in dissection, imaging, and surgery. The lower border of the teres major was chosen because it is a consistent, palpable structure that corresponds to the point where the vessel leaves the axilla (armpit) and enters the arm compartment. Other candidate landmarks, like the humeral head or the edge of the latissimus dorsi tendon, either sit too high, overlap too much with other structures, or vary more between individuals. The teres major is a broad, flat muscle whose lower edge forms a clean boundary between the axillary and brachial regions in most people, making it a practical dividing line.
This convention also aligns with the vessel’s relationship to surrounding nerves. In the axilla, the artery is surrounded by the three cords of the brachial plexus (lateral, medial, and posterior), which are named for their typical positions around the second part of the axillary artery.2PubMed Central. Cords of the Brachial Plexus and Their Branches Positioned Laterally to the Axillary Artery Below the teres major, these cords have already given off their terminal branches, and the median nerve, ulnar nerve, and other individual nerves travel alongside the now-renamed brachial artery in a more predictable arrangement. The naming transition, then, roughly tracks a shift in the neurovascular neighborhood from the complex plexus of the axilla to the more orderly anatomy of the arm.
How the Vessel Forms During Development
In the embryo, the arteries of the upper limb do not spring into existence fully formed. Instead, they develop from a mesh of tiny capillaries that extends outward from the aorta as the limb bud grows. Research on staged human embryos has mapped this process in detail. An initial capillary network appears from the dorsal aorta during stage 12 (roughly the fourth week of development) and grows at the same pace as the limb itself. By stage 13, parts of this mesh begin to enlarge and differentiate, and this remodeling sweeps outward in a proximal-to-distal sequence: the subclavian and axillary segments are recognizable by stage 15, the brachial artery has formed as far as the elbow by stage 17, and by stage 21 the entire arterial pattern of the arm, forearm, and hand has taken its adult shape.3PubMed Central. Development of the arterial pattern in the upper limb of staged human embryos: normal development and anatomic variations
This stepwise remodeling explains why variations in the brachial and axillary arteries are so common. If some capillary channels persist or enlarge at the wrong time, the result can be an artery that branches too early, too late, or in an unusual pattern. The process also makes it clear that the axillary and brachial arteries were never truly separate structures during development; they are segments of a single trunk that was sculpted from a capillary network. Their different names reflect adult anatomy, not a developmental distinction.
When the Boundary Gets Blurry
Textbooks present the teres major rule as though it is always clear-cut, but anatomical variations can confuse the picture. The most clinically discussed variant is the superficial brachial artery, where the axillary artery bifurcates high, sometimes in its third (infrapectoral) segment, into two trunks rather than continuing as a single vessel. One study of cadaveric dissections found the superficial brachial artery in about 5% of cases. In these individuals, the third part of the axillary artery split into a superficial brachial artery and a deep brachial (profunda brachii) artery, with the two branches named by their relationship to the median nerve.4PubMed Central. Vascular patterns of upper limb: an anatomical study with accent on superficial brachial artery In some of those cases, the superficial brachial artery descended to the elbow pit and resumed the usual course and branching pattern of the brachial artery. In others, it split into radial and ulnar arteries that ran superficially over the forearm flexors instead of beneath them.
A systematic review of brachial artery variations organized the main categories as the superficial brachial artery, the brachioradial artery (where the radial artery takes off much higher than normal), and the accessory brachial artery (an extra trunk running alongside the standard one).5PubMed Central. Morphological variations of the brachial artery and their clinical significance: a systematic review Each of these patterns effectively shifts the functional transition from axillary to brachial anatomy. If the axillary artery forks well above the teres major, asking “where does the brachial artery begin?” becomes a naming problem with no neat answer. In clinical practice, surgeons and interventionalists must work with whatever anatomy they find, and pre-procedural imaging has become the standard way to map these variants before cutting or threading a catheter.
The Deep Brachial Artery as a Key Landmark
Once the brachial artery begins, its first and largest branch is the deep brachial artery (also called the profunda brachii). This vessel dives posteriorly, running alongside the radial nerve through the spiral groove of the humerus to supply the triceps and the bone itself. A meta-analysis of multiple studies found that in roughly 93% of cases, the deep brachial artery originates directly from the brachial or axillary artery, while in about 7% it arises indirectly as a common trunk shared with other arteries.6PubMed. The deep brachial artery-A meta-analysis of its origin and diameter with a review of the literature Its average diameter is around 2 mm.
The deep brachial artery’s origin point matters because it is the most reliable anatomical confirmation that you have crossed from the axillary segment into the brachial segment. A systematic review of cadaveric data found that in the vast majority of cases, this artery arises from the brachial artery itself, with only about 1.4% of cadavers showing an origin from the axillary artery.7Asian Journal of Pharmaceutical and Clinical Research. Variations in the Origin of Profunda Brachii Artery from Brachial Artery – Systematic Review That same review noted duplication of the deep brachial artery in about 11% and early branching in about 17% of dissected specimens. In rare cases, it has been found originating from the subscapular artery via a common trunk with the posterior circumflex humeral artery, a configuration discovered bilaterally in at least one reported cadaver dissection.8Jornal Vascular Brasileiro. Anomalous origin of the deep brachial artery (profunda brachii) observed in bilateral arms: case report
These variations are not merely academic curiosities. Surgeons repairing humeral shaft fractures need to know where the deep brachial artery runs, because it is vulnerable in the spiral groove. If it takes off from an unusual point, the artery may lie in an unexpected position relative to the fracture site.
Choosing Between Axillary and Brachial Access for Procedures
The distinction between the axillary and brachial arteries has direct consequences for vascular procedures. When the groin (femoral) approach is not an option, doctors thread catheters through the upper limb instead. Here the choice between puncturing the axillary artery and puncturing the brachial artery involves real trade-offs in safety and reach.
A retrospective study comparing the two access sites found that complication rates for percutaneous puncture via the brachial artery ran around 29%, while surgical cutdown via the axillary artery showed complications in about 11% of cases. Major complications (those requiring intervention) occurred in roughly 14% of percutaneous brachial cases versus about 4% of axillary cutdown cases.9PubMed. Brachial and Axillary Artery Vascular Access for Endovascular Interventions The authors acknowledged that patient selection may have influenced these numbers, since the axillary group was not randomly assigned. A separate comparison of trans-brachial and trans-axillary access in a smaller cohort found no statistically significant difference in major access-site complications between the two groups, with rates of about 17% and 15% respectively.10PubMed Central. Axillary compared to brachial access for endovascular procedures
Why does access site matter if the two vessels are really the same tube? The key difference is the surrounding anatomy. The axillary artery sits deeper within the armpit, surrounded by soft tissue that can help tamponade (naturally compress) a bleeding puncture site. The brachial artery, by contrast, lies more superficially in the arm, where it is easier to access but harder to compress effectively if something goes wrong, and where a hematoma can more readily press on the median nerve. The brachial artery is also smaller in diameter than the axillary artery at the point where it is typically punctured, which raises the risk of vessel spasm or dissection when large-bore sheaths are used.
Imaging the Transition Zone
Identifying the axillary-to-brachial transition and mapping its branches is straightforward with modern imaging. Conventional catheter angiography remains the gold standard for arterial injuries, but noninvasive alternatives have become the workhorses of everyday practice. Color-flow Doppler ultrasound is widely used for evaluating peripheral arteries and has been reported to achieve sensitivity between 94% and 100%, and specificity between about 83% and 100%, for detecting arterial damage when compared with CT angiography.11PubMed Central. Sonography Doppler ultrasound diagnosis of brachial artery injury due to blunt trauma: A Case Report CT angiography excels at showing vessel reconstruction and collateral pathways but cannot determine the direction of blood flow, a limitation ultrasound handles well.
For planning endovascular procedures, CT angiography is typically the go-to because it creates a detailed 3D map of the entire upper-limb arterial tree. This makes it possible to spot high bifurcations, superficial brachial arteries, or accessory trunks before any catheter is introduced. Ultrasound, on the other hand, is portable, radiation-free, and can be done at the bedside, making it the first-line tool in trauma settings when you need a quick answer about whether blood flow in the brachial artery is intact.
Shoulder Instability and Axillary Artery Compression
Because the axillary artery runs directly behind the shoulder joint, it is vulnerable to compression during extreme arm positions or in people with unstable shoulders. A study of subjects with varying levels of shoulder laxity found that applying an anterior glide to the shoulder joint caused a significant reduction in axillary artery diameter and a corresponding increase in blood-flow velocity, the classic signs of vessel compression. Subjects who showed clinically significant compression (more than a 50% reduction in vessel diameter with the arm overhead) had significantly greater ranges of anterior translation at the glenohumeral joint than those with minimal compression.12PubMed. Anterior translation at the glenohumeral joint: a cause of axillary artery compression?
This finding matters for overhead athletes, particularly baseball pitchers and volleyball players, as well as for patients with recurrent shoulder dislocations. The artery’s proximity to the humeral head means that a dislocated shoulder can stretch, kink, or even tear the vessel, especially in older patients whose arteries have lost some elasticity. Although axillary artery injury from shoulder dislocation is uncommon, it is a recognized emergency that requires prompt vascular repair. Checking the radial pulse and capillary refill after any shoulder reduction is standard practice precisely because the artery sits in harm’s way at this transition point.
The Same Pattern Across Primates
The axillary-to-brachial transition is not unique to humans. Comparative dissection studies have shown that the same basic pattern, where the axillary artery gives off circumflex and subscapular branches and then continues as the brachial artery, is conserved across a wide range of primates, from small New World monkeys to great apes. In the Japanese macaque, for example, the axillary artery gives off the subscapular, circumflex scapular, circumflex humeral, and thoracodorsal arteries before continuing as the brachial artery, a pattern shared by rhesus macaques, olive baboons, capuchins, and all apes studied so far.13PubMed Central. Comparative Gross Anatomy of the Forelimb Arteries of the Japanese Monkey (Macaca fuscata) and a Comparative Pattern of Forelimb Arterial Distribution in Primates Where species differ is in details further down the limb: in the macaque, the brachial artery splits into radial and ulnar arteries at a point about one-third of the way above the elbow, rather than at the elbow pit as in humans.
This deep evolutionary conservation suggests that the branching pattern is tightly constrained by the functional demands of the forelimb. The shoulder and upper arm need reliable, high-volume blood supply regardless of the specific arrangement of muscles, and the teres major boundary (or its anatomical equivalent in species with different shoulder geometry) marks the natural handoff between the deep axillary space and the exposed arm. The high rate of minor variations in humans, like the superficial brachial artery or unusual deep brachial artery origins, likely reflects the inherent flexibility of the capillary-remodeling process during embryonic development working against this otherwise conserved blueprint.
Why the Brachial Plexus Arrangement Varies Too
Although the naming convention treats the cords of the brachial plexus as reliably positioned around the axillary artery (lateral cord laterally, medial cord medially, posterior cord behind), real anatomy is not always so orderly. A cadaveric study described a case in which all three cords and their terminal branches were positioned lateral to the third part of the axillary artery, rather than surrounding it on three sides as expected.2PubMed Central. Cords of the Brachial Plexus and Their Branches Positioned Laterally to the Axillary Artery Variations like this change the risk profile of procedures in the axilla: a nerve block aimed at the tissue surrounding the artery might miss all three cords if they are bunched together on one side instead of encircling the vessel.
For anesthesiologists performing ultrasound-guided nerve blocks, these variations mean that relying on the artery as a sole landmark can be misleading. The standard approach is to visualize both the artery and the individual nerves in real time, adjusting needle placement accordingly. Understanding that the axillary artery is a hub around which nerves, veins, and lymph nodes are all packed in a small space reinforces why this transition zone commands so much clinical attention despite being, in vessel-wall terms, just another centimeter of the same arterial pipe.