Systemic veins carry CO2-rich blood from different parts of the body to the ductus cuveae. Blood then enters the sinus venosus from the ductus cavae. The different types of systemic veins are as follows.
(i) Anterior cardinal vein: It brings blood from different parts of the brain via ductus cuveae.
(ii) Inferior jugular vein: It supplies blood from different parts of the brain through the ductus cuveae.
(iii) Posterior Cardinal Vein: The right and left posterior cardinal veins are joined by the anastomosis vein. It carries blood from the back of the body through the ductus cuveae.