blood flow

• to cater to changes in requirements, blood flow to and from cells are changed by changing output of blood from heart or changing diameter of blood vessels supplying the tissues
• cardiac cycle is the cycle of events that occurs in one complete heartbeat
• systole is the pumping phase of the cycle where the heart muscle contracts
• diastole is the filling phase of the cycle where the heart muscles relaxes
• both atria and both ventricles contract simultaneously
• for a short period of time, both the atria and ventricles are in diastole. the atria fills with blood and so does the ventricles as the atrioventricular valves are open
• atrial systole forces remaining blood into the ventricles which fills with blood
• ventricular systole forces blood into arteries, while the atria relaxes and refills with blood
• How quick blood flows around the body depends on how fast the heart is beating and how much blood the heart pumps with each beat 
• Cardiac output(mL/min) = stroke volume(mL) x heart rate(beats/min)
• heart rate = number of times the heart beats per minute
• stroke volume = volume of blood forced from a ventricle of the heart with each contraction
• cardiac output = amount of blood leaving one of the ventricles every minute