The axillary region, commonly known as the armpit or underarm, is a key intersection point in human anatomy. It serves as a protected passageway for major structures traveling between the torso and the upper limb. Understanding the location and contents of the axilla provides insight into the body’s vascular, nervous, and lymphatic systems.
Precise Anatomical Location
The axilla is a complex, four-sided, pyramidal space situated beneath the shoulder joint, defined by boundaries formed from bone, muscle, and fascia. The apex (top) of this pyramid is positioned superiorly, opening toward the base of the neck. It is bordered by the first rib, the clavicle, and the scapula.
The base (floor) of the axilla is visible as the armpit, consisting of skin and a layer of tough connective tissue called axillary fascia. The walls of this space are formed by large muscles. The anterior wall, which forms the front fold, is composed of the pectoralis major and pectoralis minor muscles.
The posterior wall, forming the back fold, includes the latissimus dorsi, the teres major, and the subscapularis muscles. The medial wall is formed by the upper four ribs and the serratus anterior muscle, which lies against the chest wall. The lateral wall is narrow, formed by the upper part of the humerus in the intertubercular sulcus, along with the coracobrachialis and biceps muscles.
Critical Internal Structures
The axilla’s protected position houses important structures that supply the entire arm. These structures are embedded within fat and connective tissue, which offers cushioning and allows for the wide range of motion of the shoulder joint. The major blood supply to the upper limb, the axillary artery, passes through this region.
The axillary artery begins at the lateral border of the first rib and continues to the lower border of the teres major muscle, where it becomes the brachial artery. Running parallel is the axillary vein, which carries deoxygenated blood from the arm toward the heart. Both vessels, along with the major nerve bundle, are encased in a protective sheath of fascia.
The main nerve supply to the shoulder, arm, and hand travels through the axilla in the brachial plexus. This complex network originates from spinal nerves C5 through T1 and is responsible for nearly all motor and sensory function in the upper limb. Damage to the brachial plexus can result in loss of muscle control or sensation in the arm.
The axillary lymph nodes are a significant component, acting as a filter for lymphatic fluid. These nodes are arranged in five main groups and collect lymph from the arm, the chest wall, and the breast tissue. They contain immune cells that remove foreign material and damaged cells before the fluid is returned to the circulation.
Common Medical Concerns
The presence of the axillary lymph nodes makes the region highly relevant for monitoring disease spread, particularly breast cancer, which frequently metastasizes here first. Swelling of these nodes, known as axillary lymphadenopathy, is a common clinical finding. This swelling can be caused by a variety of conditions, ranging from minor infections to systemic diseases.
Temporary swelling may indicate a localized infection in the arm or breast, a reaction to a recent vaccination, or a systemic viral illness like mononucleosis. Persistent or painless lymphadenopathy, however, may signal a serious underlying condition, such as lymphoma or metastatic cancer. Healthcare providers evaluate the size, consistency, and mobility of the nodes to determine the cause.
Another frequent medical concern is hyperhidrosis, or excessive sweating. The axilla contains a high concentration of both eccrine and apocrine sweat glands. Eccrine glands produce a watery, odorless fluid primarily for cooling the body.
Apocrine glands, concentrated in the armpit, produce a thicker fluid containing proteins and fats. Body odor occurs when skin bacteria break down this apocrine secretion. In primary focal hyperhidrosis, the sweat glands are overactive due to a hyperactive sympathetic nervous system, leading to excessive sweating in the underarm area.