If a large number of worms are present they can literally fill up the right atrium and caudal vena cava. (lbah.com)
This compresses the abdominal blood vessels such as the portal vein and caudal vena cava. (dog-health-guide.org)
The poorly oxygenated blood returns to the heart through the femoral vein, then the iliac veins, and eventually the caudal vena cava (labeled as Vein in this picture). (lbah.com)