The heart is a biological pump designed to move blood through the brain and body. It has four chambers: two "upper" chambers called the atria, and two "lower" chambers called the ventricles. Anatomically, the atria are more posterior to the ventricles, but for ease of understanding, are often drawn "above" them. The atria are separated from the ventricles beneath by the atrioventricular valves, which open to allow blood into the ventricles and close when ventricular pressure exceeds atrial pressure. Blood is transferred into the ventricles in two steps: in the first step, as the ventricle relaxes from the previous systolic phase, the pressure in the ventricle becomes less than the pressure in the atria. This causes the leaflets of the atrioventricular valves (tricuspid on the right, mitral on the left) to open like trap doors, and blood falls into the ventricles. On the left side, the velocity at which the blood moves during this initial action is called the "E" ( for early) filling velocity. Early filling is responsible for roughly 80% of total ventricular filling. Some blood always remains in the atria, so toward the end of ventricular relaxation (diastole), the atria actively contract to empty the rest, causing the ventricle walls to "stretch", which allows for a stronger contraction during the upcoming systolic phase. This is called the "atrial kick". The velocity of the blood filling the ventricle in this step is the "A" (for atrial) filling. Phases of diastole: # Isovolumic relaxation time (abbreviated as IVR) # early filling # diastasis # atrial contraction
E/A ratio
QRS complex
Ka/Ks ratio
NC ratio
Extraction ratio
J wave
U wave
P wave 