Bird This pericardial sac is filled with a serous fluid for lubrication. The heart itself is divided into a right and left half, each with an atrium and ventricle. The atrium and ventricles of each side are separated by atrioventricular valves which prevent back flow from one chamber to the next during contraction. Being myogenic, the heart's pace is maintained by pacemaker cells found in the sinoatrial node, located on the right atrium. The sinoatrial node uses calcium to cause a depolarising signal transduction pathway from the atrium through right and left atrioventricular bundle which communicates contraction to the ventricles. The
Sinoatrial node proteins called connexins. There are fewer gap junctions within the SA node and they are smaller in size. This is again important in insulating the SA node from the surrounding atrial cells. The sinoatrial node receives its blood supply from the sinoatrial nodal artery. This blood supply, however, can differ hugely between individuals. For example, in most humans this is a single artery, although in some cases there have been either 2 or 3 sinoatrial node arteries supplying the SA node. Also, the SA node artery mainly originates as a branch of the right coronary artery; however in some individuals