Vena Cavae consist of the two largest veins within the human body and are divided into two separate blood vessels, the superior vena cava and the inferior vena cava.
The superior vena cava (SVC), is a large valveless venous channel formed by the union of the brachiocephalic veins. It receives blood from the upper half of the body (except the heart) and returns it to the right atrium. 
The inferior vena cava (IVC), also empties into the right atrium of the heart and . The inferior vena cava runs posterior, or behind, the abdominal cavity. This vein also runs alongside the right vertebral column of the spine and unlike the SVC contains a valve. 
These blood vessels enable the return of depleted deoxygenated blood from all parts of the body to the heart to be pumped to the lungs for reoxygenation via the pulmonary arteries.
 - Bashir, O. (2017). Superior vena cava | Radiology Reference Article | Radiopaedia.org. [online] Radiopaedia.org. Available at: https://radiopaedia.org/articles/superior-vena-cava [Accessed 19 Nov. 2017].
 - Healthline Medical Team, (2015). Inferior Vena Cava Function, Anatomy & Definition | Body Maps. [online] Healthline.com. Available at: https://www.healthline.com/human-body-maps/inferior-vena-cava [Accessed 19 Nov. 2017].