Topography and Location
The foramen is located in the central tendon (centrum tendineum) of the diaphragm. This central tendon is a pale, triangular aponeurotic plate where all muscular fibers—sternal, costal, and lumbar parts—converge. Anteriorly, the central tendon lies adjacent to the pericardium.
- Skeletotopy: Projects at the level of the eighth thoracic vertebra (Th8).
- Relation to other hiatuses: The esophageal hiatus (hiatus oesophageus) is located at the level of Th10, and the aortic hiatus (hiatus aorticus) is at the level of Th12.
- Vascular syntopy prior to the foramen: Before entering the foramen venae cavae, the inferior vena cava courses along the posterior aspect of the diaphragmatic surface of the liver, resting in the groove for the inferior vena cava (sulcus venae cavae). The uppermost segment of the vein just below the diaphragm is slightly dilated and surrounded on three sides by the liver parenchyma.
Structure of the Foramen Margins
Unlike the esophageal hiatus, which is formed by muscular fibers of the crura of the diaphragm and exerts a sphincter-like function, the margins of the foramen venae cavae are fibrous.
- Fibrous tissue: The edges of the opening are composed of dense connective tissue.
- Attachment to the vessel: The adventitia of the inferior vena cava is firmly fused to the margins of the opening by connective tissue strands.
Contents of the Foramen
The following anatomical structures pass through this opening into the pericardial cavity:
- Inferior vena cava (v. cava inferior) — traverses the diaphragm into the pericardial cavity and immediately drains into the right atrium.
- Branches of the right phrenic nerve (n. phrenicus) — frequently accompany the vein through this opening.
Biomechanics and Function
Because the margins of the foramen are fibrous, they do not constrict during inspiration. The tethering of the inferior vena cava adventitia to the foramen edges via connective tissue strands keeps the lumen of the vessel open during diaphragmatic contraction, thereby facilitating venous return to the heart.
Clinical Distinction: Vena Cava Opening in the Heart
In anatomical nomenclature, there is a similarly named structure—the opening of the inferior vena cava in the right atrium (ostium venae cavae inferioris atrii dextri). It is crucial to differentiate these structures: the foramen venae cavae refers to the aperture in the central tendon of the diaphragm, whereas the ostium refers to the venous opening inside the right atrium of the heart.