How the Human Heart Beats
By Jeff Krawiec
The heart is an important organ in the human body and is responsible for pumping oxygenated blood
to its various tissues.
About 72 times every minute an average person’s heart pumps a new volume of blood throughout
the body. This blood can either be pumped to the lungs to become oxygenated if it is currently
deoxygenated or can be pumped to the rest of the body for use if it is oxygenated. Pumping blood
allows oxygen to be transported to different tissues allowing for energy to be produced within those
tissues. Without this constant supply of oxygen delivered by the blood, our tissues would not have
the necessary energy to perform their basic functions and die. When the heart stops beating, such as
during cardiac arrest, a person can die within a few minutes. There are various diseases that can
affect the heart which is one reason why it is an important organ to study in medicine. It is also a
great example of basic physiology since it can easily illustrate how an electrical stimulation can lead
to a mechanical response. This description should give anyone studying or interested in physiology a
general overview of how the human heart beats.
The Anatomy of the Heart
[Reference: www.rush.edu/rumc/page-1098987350664.html]
The human heart is divided into a left and right side each having two chambers, an input, and an
output. These two sides operate in synchrony but each pump blood to a different location in the
body. The right side of the heart is responsible for taking deoxygenated blood that has been
returned from the body and transporting it to the lungs where it can be refilled with oxygen. The
input for the right side of the heart is called the vena cava and the output is called the pulmonary
artery. The left side of the heart receives blood from the lungs and is responsible for transporting
this oxygenated blood to the body for use. The input for this side of the heart is called the pulmonary
vein and the output is called the aorta. The two chambers on either side of the heart are called the
atrium and the ventricle and are given a designation of left of right. The atrium is the first chamber to
be filled with blood and the ventricle is the second. There are also four valves within the heart and
each are responsible for opening and closing to seal off the different chambers of the heart. The
right side of the heart has the tricuspid valve and the pulmonary valve while the left side has the
mitral valve and the aortic valve.
[Reference: http://www.hrspatients.org/Patients/the_normal_heart/images/normalhrt4.jpg]
In combination with the structural components of the heart it also has electrical components. These
electrical components will create electrical signals, or action potentials, that will lead to muscle
contraction. The main components of the electrical system are:
SA node
AV node
Bundle of His
Bundle branches
Purkinje Fibers
The Electrical System of the Heart
[Reference: www.mayoclinic.com]
The heart runs on electrical activity and in most cases when the heart stops beating it is because
there is a problem in this electrical system. Patients get shocked with paddles when their heart stops
working or when they enter defibrillation to try and trigger the heart to start up its electrical system
or to get it back on track. The electrical activity of the heart is always what you see when you look at
an EKG reading. The electrical system functions like this:
1. The SA node creates an action potential which will start the electrical process.
2. The action potential travels through the each atrium to reach the AV node.
3. It then propagates through the ventricles traveling through the bundle of His, bundle branches,
and Purkinje fibers.
After this process has been completed one heartbeat will have occurred and the process is then
repeated. It is important to note that when the SA node fires an action potential it actually triggers
itself to prepare to fire another action potential. Most cells in the body can’t do this and is why
nerves are needed in order to apply electrical stimulation. The heart however is an exception due to
the SA node and can actually contract on its own even outside the body.
The Cardiac Cycle
When talking about physiology the term systole is given to the phase of the heartbeat when the
heart is contracting and diastole is given to the phase of the heartbeat when the heart is relaxing and
filling with blood. The cardiac cycles of the heart is as follows:
1. The heart begins in diastole where the atria fill with blood.
2. The action potential from the SA node fires and travels through the atria triggering them to
contract. This begins systole.
3. Blood is squeezed out of the atria due to this contraction, through the valves, and into the
ventricles.
4. The atria begin to relax.
5. The action potential continues to propagate through the heart’s electrical system and then
travels through the ventricles.
6. The ventricles contract causing pressure to build up in the ventricles.
7. The pressure becomes so strong that the blood is forced through the valves and out into the
body.
8. The ventricles begin to relax.
9. The blood circulates through the body where it is either oxygenated by the lungs or
deoxygenated by the rest of the body.
10. The blood returns to the heart where it fills the relaxed atria.
11. The process repeats over and over.
[Reference: http://academic.kellogg.cc.mi.us/herbrandsonc/bio201_McKinley/f22-11_cardiac_cycle_c.jpg]