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
Longitude of the ascending node it is not possible to tell which node is ascending and which is descending. In this case the orbital parameter which is recorded is the longitude of the node, Ω, which is the longitude of whichever node has a longitude between 0 and 180 degrees. In astrodynamics, the longitude of the ascending node can be calculated from the specific relative angular momentum vector h as follows: Here, n=<"n", "n", "n"> is a vector pointing towards the ascending node. The reference plane is assumed to be the "xy"-plane, and the origin of longitude is taken to be the positive "x"-axis. k