Heart rate

• The heart rate is the number of times the heart beats in one minute. In adults, the average heart rate is 60 to 100 beats per minute. Heart rates are usually higher in children and women. Differences in gender, size, age and fitness level can affect the heart rate, as can some medications and conditions. Very fit people have lower resting heart rates, as the heart muscle is stronger and can pump more blood with each beat. If you are upset or excited, your heart rate will increase.
  
  

Stroke Volume

• Stroke volume is the amount of blood pumped with each heartbeat. An average resting stroke volume is about 2 fluid ounces per beat, or about 60-80ml. Usually only 65% of the blood in the ventricles is pumped out during each beat. Starling's Law of the Heart and the inotropic effect are two things that can alter the force of the contraction, causing more of the blood to be expelled with each beat.
  
  

Starling's Law

• Starling's Law depends on the amount of stretch that is in the cardiac muscle fibers. If there is an increase in the volume of the blood pumped into the heart, the ventricle will stretch, which in turn causes a greater force of contraction and increases the cardiac output. If less blood volume enters the heart, the ventricle does not stretch as much, and the contraction is less forceful. This lowers the cardiac output. The process defined by Starling's Law ensures that the heart pumps out only what it receives at each point in time.
  
  

Inotropic Effect

• If the strength of the contraction is increased without increasing the stretch of the cardiac fibers, cardiac output will be increased. Certain hormones and medications can cause this to happen. Sympathetic nerve stimulation of the heart -- for example, when a person is scared or startled or excited -- is another mechanism of the inotropic effect. Some drugs may also cause a negative inotropic effect and will decrease cardiac output, which can lead to heart failure. It is extremely important to closely monitor the use of any medication that has a negative inotropic effect on the heart.
  
  

Volume and Output

• The amount of blood pumping though the body of the average adult is about 5 liters. That's equivalent to the average cardiac output. That means that the heart pumps the total amount of blood in the body every minute.