Describe the ventral and dorsal side of the frog's heart.

HEART OF FROG:

 The heart of frog is a muscular and conical organ located in the body cavity between the oesophagus and sternum. It is enclosed in a membrane called pericardium. It contracts and expands continuously throughout the life. This contraction and expansion of heart is called heartbeat due to which blood keeps circulating in the body. The heart of frog consists of three chambers: 
(i) Right Auricle or Atrium 
(ii) Left Auricle or Atrium 
(iii) Ventricle.
 The two thin walled atria form the broader anterior part of the heart. Right atrium is larger than the left atrium. Ventricle forms the conical, thick, uneven walled posterior part of the heart. A broad vessel, truncus arteriosus originates from the ventral side of the ventricle and divides into two branches near atria. Each branch divides into three arches (arteries). A thin walled triangular sac called sinus venosus opens into the right atrium. Both the truncus arteriosus and sinus venosus are not true chambers of the heart but often called accessory chambers. However, some biologists consider them as chambers of heart which beat in a rhythmic manner. First of all sinus venosus contracts then the two atria followed by the contraction of the ventricle and in the end truncus arteriosus contracts. Deoxygenated blood from the whole body except lungs is carried to the sinus venous by two precavals and one postcaval. Sinus venosus opens through an aperture into the right atrium. Oxygenated blood from lungs is carried to the left atrium by two pulmonary veins. Both the atria open jointly by an aperture guarded by a valve into the ventricle. 

• The two atria, the right one with deoxygenated blood and the left one with oxygenated blood contract simultaneously to push their blood collectively into the ventricle. In the middle of the ventricle, there is some mixing of the two types of blood whereas on the two sides the blood remains unmixed as the blood passes very fast from the ventricle. Ventricle contracts to push the blood into truncus arteriosus through an aperture guarded by a valve. From truncus arteriosus the blood enters (a) the pulmonary arteries which carry the blood to lungs for oxygenation and (b) the systematic arteries which supply the pure blood to all parts of the body and (c) the carotid arteries which supply blood to the brain.