Sino-Auricular node (S.A node) present in the walls of right auricle has a myogenic initiation of heartbeat in a regular fashion and controls the pace of heartbeat called pacemaker.

Higher vertebrates exhibit double circulation of blood namely systemic and pulmonary circulation. Systemic circulation involves the distribution of oxygenated blood via the aorta from the heart to all parts of the body, and the collection of deoxygenated blood from all parts of the body through the great veins into the heart. Pulmonary circulation denotes the route impure blood takes from the heart to the lungs for purification and then back to the heart, the left auricle receives oxygenated blood and passes it to the left ventricle. The left ventricle pumps the pure blood to all parts of the body through aorta.Hence these two chambers involved in systemic circulation are referred to as systemic heart.

Periplaneta (P.americana,the Indian Cockroach) is an insect & insects do not have closed blood circulation.Their blood (called haemolymph) even does not serve for respiration. They do not possess blood vessels in this circulatory system and hence it is an open system.

In human, blood passes from the post caval to the diastolic right atrium of heart due to suction pull.

The ABO blood group system of human very is the important in blood transfusion. It is named ABO and not ABC, because blood group O (or blood group zero in some countries) individuals do not have either A or B antigens on the surface of their RBCs, but their blood serum contains IgM anti-A antibodies and anti-B antibodies against the A and B blood group antigens.Therefore, a group O individual can receive blood only from a group O individual, but can donate blood to individuals of any ABO blood group (i.e.A, B, O or AB).

Lymph is a mobile connective tissue comprising lymphplasma and lymph corpuscles. Its composition is just like blood plasma except that it lacks RBCs and large plasma proteins.