Assertion: Double circulation is incomplete in amphibians and reptiles.

Reason: Unlike in birds and mammals, in amphibians and reptiles, the left atrium receives oxygenated blood and the right atrium receives deoxygenated blood.

Reason: Valves are present in the heart which allows the movement of blood in one direction only.

Assertion: Sino-atrial node (SAN) is called the pacemaker.

Reason: SAN generates the maximum number of action potentials and is responsible for initiating and maintaining the rhythmic contractions of the heart.

Assertion: Atria act as primer pumps that increase the ventricular pumping.

Reason: About 80 percent of the blood flows directly through the atria into ventricles.

Assertion: There are $72-75$ heart beats per minute on an average when a person is performing normal work.

Reason: One heartbeat is completed in 0.8 second.

Assertion: The cardiac output of an ordinary man and of an athlete is the same.

Reason: It is impossible to alter the stroke volume as well as heart rate.

Assertion: The enlarged Q and R waves indicate myocardial infarction.

Reason: Any deviation in the normal recording of ECG indicates possible abnormality or disease.