Circulation, blood and lymph

The frog's blood vascular system is well developed and of the closed type. A lymphatic system is also present.

The blood vascular system has the heart, blood vessels and blood. The lymphatic system has lymph, lymph channels and lymph nodes.

The heart is muscular, lies in the upper part of the body cavity and is covered by a membrane, the pericardium. It has three chambers: two atria and one ventricle.

The sinus venosus, a triangular chamber, collects blood from the major veins (the vena cava) and joins the right atrium.

On the ventral side the ventricle leads into a sac-like conus arteriosus. Arteries carry blood from the heart to the body (arterial system); veins bring it back (venous system).

Frogs have special venous connections: the hepatic portal system between the liver and the intestine, and the renal portal system between the kidneys and the lower parts of the body.

Blood is plasma plus cells: RBCs (erythrocytes), WBCs (leucocytes) and platelets. The frog's RBCs are nucleated and contain the red pigment haemoglobin.

Lymph differs from blood: it lacks some proteins and has no RBCs.

Blood carries nutrients, gases and water to where they are needed, and the pumping of the muscular heart drives the circulation. The frog has a single circulation.