Sclerosis - A hardening of the walls of blood vessels.
 
Sinus Arrhythmia - An increase in heart rate during deep breathing. A normal response, more pronounced in children.
 
Stenosis - Narrowing of a canal, especially, a narrowing of one of the cardiac valves.
 
Sternum - The long flat bone that connects the breastbone with the cartilages of the first seven ribs.
 
Stroke - A hemorrhage into the brain.
 
Systolic Blood Pressure - The blood pressure reading that is on the top. The measurement of the contraction of the ventricles of the heart, which drives the blood through the aorta and pulmonary artery. The power of myocardic muscle is similar to that necessary to squeeze a tennis ball.