Heart: Heart is the most important organ of circulatory system. It is a sort ofpump in the body of toad. This organ is situated in between the lungs and remains covered by the membrane named pericardium. The wall of the heart is quite thick and three layered Epicardium, Myocardium, and Endocardium.
The outer layer of the heart is epicardium. The middle layer is myocardium which is made of cardiac muscles and quite thick. The inner layer is known as endocardium.
The heart of a toad is divided into three main chambers: Two auricles and one ventricle. Besides there are two sub chambers named sinus venosus and conus arteriosus.
Auricle: The two upper chambers of the heart are named auricles. The twoauricles are divided into right and left parts by a septum. Right auricle is bigger than the left. By a common auriculo ventricular aperture the right and left auricles open into the ventricle.
Ventricle: In the heart of toad, there is one ventricle. The wall of the ventricleis quite thick and towards the inner side there are some longitudinal projections. These projections do not allow to mix the oxygenated blood and deoxygenated blood.
Sinus venosus: The triangular chamber situated on the dorsal side of auricle isnamed sinus venosus. Two anterior vena cavae and one posterior vena cava enter into it. Through the aperture named sinu-auricular aperture the sinus venosus opens into the right auricle. At the opening of this aperture a pair of one way valves is present.
Conus arteriosus: Placed obliquely form the right end of the ventricle of theheart up to the left auricle the chamber is named conus arteriosus. The two parts of conus arteriosus in front of the auricle named called truncus arteriosus or aorta. From here comes out the three main arteries. There. is a spiral shaped valve inside the conus called spiral valve. This valve divides the conus cavity into right and left parts.
Circulation of blood through the heart: Like all other land animals, the toadhas double circuit circulatory system. Blood is circulated through successive contraction and relaxation of heart. The two anterior vena cavae and the posterior vena cava carry the carbon-dioxide enriched blood into the sinuis venosus. This blood enters the right auricle through the sinu-auricular aperture Almost simultaneously the right auricle relaxes and from the lungs oxygen enriched blood enters into the left auricle through the pulmonary vein. Right and left auricles simultaneously contracts and from the right and left auricles carbon dioxide and oxygen enriched blood through the common auriculo-ventricular aperture enters the cavity of the ventricle. Later, the wall of the ventricle contracts and through the control of the semilunar valve, blood from the ventricle enters the cavity of the conus arteriosus. Afterwards oxygenated and carbon-dioxide enriched mixed blood through the right and left aorta circulates respectively through the carotid, systemic and pulmocutaneous arteries.