The Common Grass Frog
Circulatory System
The heart of the frog has three chambers, consisting of one ventricle and two atria. Blood leaves the heart from the ventricle through a single truncus arteriosus which is short and soon branches into two aortic arches which loop left and right and dorsal to the heart to rejoin as a single aorta in the mid dorsal region of the body cavity. Each aortic arch has a branch leading to the lungs and skin where oxygenation occurs. Carotid arteries also branch off the aortic arches and supply the head region. Veins bring blood to the left and right atria. Both atria then empty into the single ventricle. Blood from the ventricle thus enters either the pulmonary or body circulation.
Since there is only a single ventricle there is some mixing of oxygenated and deoxygenated blood. This mixing is not complete since there is a special valve system which partially keeps the two separate. This is clearly a less efficient system than in the double circulatory system of the rat, in which the heart has four chambers and the two types of blood do not mix; the lungs receiving only deoxygenated blood, the body tissues only oxygenated blood. The circulatory system of animals such as the frog in which a three chambered heart is present and in which oxygenated and deoxygenated blood mix before leaving the heart is called an incomplete double circulatory system.
The heart is surrounded by a thin connective tissue sheath, the pericardium. If not done previously, this sheath should be removed to fully expose the heart and associated structures. The sinus venosus is a thin-walled structure on the dorsal surface of the heart. The sinus venosus drains into the right atrium, while the left atrium receives blood directly from the lungs via the pulmonary veins. The sinus venosus receives its blood from the two anterior vena cavae and the posterior vena cava. The truncus arteriosus lies between the two atria. Posterior to the atria, note the single thick-walled ventricle. Follow the truncus arteriosus till it branches into the two aortic arches. Trace the two aortic arches till they reunite to form the dorsal aorta. See if you can locate any of the branches off the aortic arches such as the carotid or pulmocutaneous artery.
Blood
(Campbell 6th Ed. 882-884, Fig. 42.14; 7th Ed. 879-883, Fig. 42.15)
Blood is a tissue in which the extracellular matrix, the plasma, is a liquid. The cellular components are the erythrocytes and the leucocytes, the former outnumbering the latter 600 to 1. Erythrocytes, or red blood corpuscles, contain the respiratory pigment hemoglobin and have an orange tint when seen under the microscope; leucocytes, or white cells are colorless. Remember, however, that you are studying stained slides.
Erythrocytes:
Leucocytes:
Examine the following stained blood smears:
