The superior and inferior venae cavae carry relatively deoxygenated blood from the upper and lower systemic circulations, respectively.
This arch of the azygos vein ("arcus venae azygos") is an important anatomic landmark.
Increasing the heart rate serves to decrease the pressure in the superior and inferior venae cavae by drawing more blood out of the right atrium.
It is accompanied by venae comitantes (accompanying veins).
In the right atrium, the stretch receptors occur at the junction of the venae cavae.
In mammals, it exists distinctly only in the embryonic heart (where it is found between the two venae cavae); in nonmammalian chordates, however, the sinus venosus persists in the adult.
