The SA node (sinoatrial node) is a collection of tissue located in the hearts right atrium. This tissue, like all of the cells in the heart, is capable of producing electrical impulses. The unique function of the SA node however, is that it acts as the pacemaker for the heart.
For a normal heart, the SA node initiates an electrical impulse causing the heart to beat. This is why a normal heart rhythm is known as a Sinus rhythm. In the event that the sinoatrial node does not work, the heart is very resilient, and will use the AV node as the primary pacemaker. If the AV node also fails, then the Purkinje cells will function as the hearts pacemaker. When the SA node is not functioning correctly, though the hearts rate may be controlled by either the AV node or the Purkinje cells, the heart rate will typically be abnormal.
There are a number of arrhythmias that can be attributed to a problem with the SA node. Commonly you may see sinus bradycardia or sinus tachycardia. There are also a number of heart block issues that may be attributed to issues with the sinus node. These heart block issues include; SA node Wenckebach, SA node Mobitz II, and SA node exit block. Issues associated with the SA node can usually be diagnosed with an electrocardiogram, which is looking at the hearts electrical function.
When the SA node can no longer effectively act as the pacemaker for the heart, an artificial pacemaker may be used to control the hearts electrical system. Advanced Cardiac Life Support (ACLS) looks at the various conditions that can affect the SA node, and deals with diagnosis, treatment paths, and pharmacology for these various conditions. Most medical professionals that deal with the cardiovascular system, or function as emergency response have ACLS certification, to ensure they can properly deal with SA node and other heart problems.
The SA node is both controlled and influenced by both the parasympathetic nervous system and the sympathetic nervous system. This control/influence can adjust both the heart rate, and force of contractions. So while the heart beat occurs without conscious effort, it can also be influenced by your surroundings. For example, your heart rate will increase with exercise, slow down while you're sleeping, or increase in preparation for a fight or flight situation.
The SA node is supplied blood from the SA node artery, but can still be affected during myocardial infarction which can cause death to the SA node cells. This may cause a deficiency in the SA node's ability to act as a pacemaker, and can also lead to sick sinus syndrome.