The blood vessels which carry the blood towards the heart constitute the venous system.
The venous system in frog constitutes the following main veins:
Pulmonary Veins:
The oxygenated blood from two lungs is returned by two pulmonary veins, which open dorsally into the left auricle after uniting with each other.
Caval Veins:
The deoxygenated blood from the rest of the body comes to the sinus venosus through two precavals (anterior venae cavae) and one postcaval vein (posterior vena cavae).
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(i) Precavals or Anterior Vena Cavae:
Each precaval or anterior vena cava is formed by the union of three veins:
(a) External jugular receiving branches from the tongue (lingual) and floor of the mouth (mandibular),
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(b) Innominate receiving branches from the brain and orbit (internal jugular), and from shoulder and back of arm (subscapular),
(c) Subclavian vein receiving branches from arm (brachial) and from skin and muscles of the abdomen and also mucous membrane of mouth and head muscles (musculo-cutaneous).
(ii) Postcaval or Posterior Vena Cava:
The postcaval vein receives blood from the kidneys by 5 to 6 pairs of renal veins and a pair of gonadial veins (spermatic in male and ovarian in female) from gonads. It arises between the kidneys and runs forward ventral to the dorsal aorta in the mid-ventral line. Before it is communicated with the sinus venosus it also receives a pair of hepatic veins from the liver.
Renal Portal Vein:
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The veins which collect the blood from the posterior side of the body constitutes renal portal system. Two large veins, the femoral and sciatic, return the blood from each leg. On entering the body cavity each femoral vein divides into a ventral pelvic and a dorsal renal portal vein. The pelvic meets its fellow from the opposite side forming the anterior abdominal vein.
It receives blood from the bladder and central abdominal wall and runs forward to enter into liver. Before entering the liver, it is connected with the hepatic portal vein. The renal portal unites with the sciatic and runs along the outer margin of the kidneys sending branches to it. The renal portal vein receives the blood from the body wall of lumbar region through dorso-lumbar vein.
Hepatic Portal System:
Hepatic portal system collects the blood from the alimentary canal through many branches and carries it to the liver where the veins break up into capillaries and the blood is collected by hepatic veins to pour into the postcaval.