The rib cage is the bony structure that forms the chest cavity. This protective framework is composed of 24 ribs, the sternum, and the thoracic vertebrae of the spine. Its purpose is to shield organs housed within the chest from external forces. Beyond protection, this structure is engineered for mobility, allowing the expansion and contraction necessary for breathing. It also provides structural support to the upper body.
The Bony Components of the Thorax
The rib cage is an assembly of bones that includes 12 pairs of ribs, the sternum, and 12 thoracic vertebrae. Each pair articulates posteriorly with a specific thoracic vertebra. Rib classification is determined by the forward attachment to the sternum.
The first seven pairs are “true ribs” because they connect directly to the sternum via their own strips of costal cartilage. This flexible tissue bridges the gap between the rib and the sternum, providing elasticity and stability to the chest wall.
The “false ribs” (pairs eight, nine, and ten) do not connect directly to the sternum. Instead, their costal cartilage joins the cartilage of the rib immediately above them. This indirect connection provides slightly more flexibility.
The final two pairs (11th and 12th) are “floating ribs.” They only attach to the thoracic vertebrae in the back. These ribs lack any anterior connection, ending freely within the musculature of the abdominal wall.
How the Rib Cage Facilitates Breathing
The rib cage facilitates respiration by changing the volume of the thoracic cavity. The expansion and contraction of the chest wall work in concert with the diaphragm, the large muscle situated beneath the rib cage. The diaphragm flattens and moves downward during inhalation, increasing the vertical dimension of the chest.
Simultaneously, the ribs execute two distinct mechanical movements to further increase the chest capacity. The upper ribs (pairs one through six) move in a “pump handle” movement. Their elevation during inhalation causes the sternum to move up and forward, increasing the chest’s front-to-back dimension.
The lower ribs (pairs seven through ten) move in a “bucket handle” motion, which increases the side-to-side, or transverse, dimension of the chest. This movement ensures that lung expansion occurs across all three dimensions: vertical, anterior-posterior, and transverse.
The intercostal muscles, located between the ribs, also play a role. The external intercostal muscles contract during inhalation to help pull the ribs up and out, while the internal intercostals are more active during forced exhalation. Exhalation during quiet breathing is usually a passive process, relying on the elastic recoil of the lungs and the chest wall.
Key Organs and Their Placement
The rib cage protects the organs of the cardiorespiratory system and the upper organs of the abdomen. The heart and lungs are centrally located and receive comprehensive protection. The lungs fill the majority of the thoracic cavity, wrapped by the rib cage.
The heart is situated slightly to the left of the center, nestled between the lungs, shielded by the sternum and the true ribs. The dome of the diaphragm sits immediately below, separating the thoracic and abdominal cavities.
The lower portion of the rib cage covers several organs in the upper abdomen. The liver, the body’s largest internal organ, occupies a large area under the lower right ribs (7th through 11th).
On the left side, the spleen is tucked high up beneath the 9th, 10th, and 11th ribs. The stomach is also located primarily in the upper left quadrant, sheltered by the lower left ribs.
Moving to the back, the kidneys are positioned against the posterior abdominal wall, below the diaphragm. The upper poles of the kidneys receive direct protection from the 11th and 12th floating ribs.