The circulatory system of frog is well-developed and is of closed type. It also has a lymphatic system. The blood vascular system includes the heart, blood and blood vessels. The lymphatic system consists of lymph, lymph channels and lymph nodes.Heart:
It is a muscular structure situated in the upper part of body cavity. It has three chambers, i.e., two atria (sing, atrium) and one ventricle. The heart is covered by a membrane called pericardium.
There are two accessory chambers in the frog's heart, a tubular truncus arteriosus placed on the ventral side of the right auricle and a triangular sinus venosus present on the dorsal side of the heart.
Sinus venosus receives blood through the major veins called vena cava and opens into right auricle.
The left auricle receives oxygenated blood from the lungs by a common pulmonary vein. The auricles send their blood into ventricle where the blood gets mixed up. The ventricle opens into a sac-like conus arteriosus on the ventral side of the heart.
Special venous connection between liver and intestine as well as the kidney and lower parts of the body are present in frogs.
The former is called hepatic portal system and the latter is called renal portal system.
Blood Vessels:
The blood vessels found in frogs are arteries, arterioles, veins, venules and blood capillaries.
Arteries carry blood from the heart to different body parts. Veins bring blood from different body parts to the heart. Arteries further divide to form arterioles. The arterioles branch out to form capillaries, which further unite to form venules. The venules thus, join to form veins (which have valves to prevent backflow of the blood).
Blood:
The blood is composed of plasma and blood cells. The blood cells are RBCs (Red Blood Cells) or erythrocytes, WBCs (White Blood Cells) or leucocytes and platelets. RBCs are nucleated and contain red coloured pigment namely haemoglobin.