Body Fluids and Circulation

To study blood corpuscles and plasma, the blood must be kept from clotting.
Calcium ions ($Ca^{2+}$) are essential for blood coagulation (they are needed for the conversion of prothrombin into thrombin and in other steps of the clotting cascade).
A test tube containing calcium bicarbonate would supply extra $Ca^{2+}$ ions and promote clotting, so it must not be used.
Sodium oxalate (and citrate) removes $Ca^{2+}$ by forming insoluble calcium oxalate, so it prevents clotting.
Heparin is a natural anticoagulant that prevents the conversion of prothrombin into thrombin.
Chilling slows down the enzymatic reactions of clotting, so a chilled tube also delays coagulation.
So the test tube containing calcium bicarbonate is the one you will not use.

The human heart is myogenic: the heart beat starts in the heart muscle itself.
The sinoatrial (SA) node, a patch of nodal tissue in the right upper corner of the right atrium, generates the maximum number of action potentials (70–75 per minute). It initiates and maintains the rhythm of the heart, so it is called the pacemaker.
From the SA node the impulse spreads through the atria to the AV node, then along the bundle of His (AV bundle) and the Purkinje fibres to the ventricles. These parts conduct the impulse but do not normally set the rhythm.
When the natural pacemaker fails, the artificial pacemaker takes over its function. So it is placed to act at the site of the sinoatrial node.
So it will be grafted at the site of the sinuatrial node.