Download How the Heart Pumps Blood

How the Heart Pumps Blood
Michael Brinton
The heart is a muscular organ in the circulatory system of all mammals. In humans, it is about
the size of a fist and is located in the center of the thoracic cavity, directly behind the sternum
(breastbone). The muscles of the heart form four chambers. Coordinated contractions of the
heart muscles pump blood through these four chambers and out to the pulmonary and systemic
circulation. As with all muscles, electrical impulses control the contractions of the heart. This
document will first examine the anatomy of the human heart and the path of blood flow through
the heart. It will then move on to the electrical pathways controlling the contractions of the heart
muscles. Finally, these electrical pathways will be related to an electrocardiogram, a diagnostic
tool used in medicine to observe the electrical events of the heart.
Anatomy and Blood Flow
The heart is separated down the center by the septum, forming the left and right sides. Each side
has an upper chamber, the atrium, and a lower chamber, the ventricle. Each chamber has a oneway valve at its exit. These valves control the blood flow by inhibiting any backward flow. The
atria function to collect blood that is returning to the heart from the body (right atrium) or from
the lungs (left atrium) and to push that blood into the ventricles. The ventricles function to pump
blood out to other parts of the body. The walls of the left ventricle are thicker than the walls of
the right ventricle, because the left ventricle must pump blood to the whole body, whereas the
right ventricle only pumps blood to the pulmonary circulation (in the lungs). Figure 1 illustrates
the anatomy of the heart, with arrows showing blood flow.
Figure 1. Anatomy and Blood Flow of the Heart
(Source: texasheartinstitute.org)
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Deoxygenized blood returns to the heart through a series of veins, dumping into the right atrium
via the vena cava. The right atrium pushes blood to the right ventricle. The right ventricle then
pumps blood to the lungs through the pulmonary artery. In the lungs, red blood cells refill with
oxygen, while at the same time carbon dioxide and other waste products leave the blood. As
blood leaves the lungs, it travels back to the heart through the pulmonary vein, dumping into the
left atrium. The left atrium pushes blood into the left ventricle, which then pumps the blood out
to the rest of the body. Blood delivers oxygen and picks up waste products in the tissues of the
body. Then it travels back to the heart, and the cycle starts again.
The Electrical System
Although the heart is made mostly of muscle tissue, it also contains tissues that direct electrical
impulses to coordinate the contractions of the heart. These impulses originate in the sinoatrial
(SA) node, a specialized group of cells located at the back of the right atrium. From the SA
node, impulses travel around both atria, causing them to contract. Impulses are then paused at
the atrioventricular (AV) node. As can be guessed by the name, this node is located between the
atria and the ventricles. The AV node pauses the electrical impulse for one to two tenths of a
second to allow the blood time to pass from the atria to the ventricles. The impulse then travels
down the septum (between the ventricles) and then back up through the outer walls of the
ventricle via the Purkinje fibers. This way, the ventricle contracts from the bottom up, allowing
as much blood as possible to exit into the arteries. Figure 2 below illustrates the electrical
pathways in the heart.
Figure 2. Electrical Pathways of the Heart
(Source: heartrhythmclinic.com)
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The Clinical Picture
The electrical pathways above can be viewed in a clinical setting using a device called and
electrocardiogram (EKG or ECG). An EKG consists of several wires which are connected to the
patient’s chest, generally with a sticky, electricity-conducting patches. The number and
placement of these wires varies greatly, depending on what the clinician wishes to see.
However, all placements view the heart in the same way, by measuring electric potential between
different wires. A “normal” four-lead EKG reading is shown in Figure 3, with the waveforms
labeled. This readout can be very helpful to diagnose problems with the heart, but first one must
know what a normal reading signifies.
Figure 3. Normal EKG (Source: a-fib.com)
The first upward wave of the EKG, the P wave, represents the contraction of the atria (the upper
chambers). The flat segment before the next wave, the PR segment, shows the time that the
electrical impulse is paused at the AV node. The next set of waves, the QRS complex, represents
the contraction of the ventricles (the lower chambers). There is another pause, and then the T
wave is formed when the ventricles electrically reset to be ready for the next impulse. Note that
the atria also reset, but this happens at the same time that the ventricles contract, and thus it is
covered by the much more intense electrical activity of the ventricles.
While a printout of an EKG can be useful to a trained medical provider in an ambulance, a
hospital, or a doctor’s office, the EKG is also used in some other devices without needing to be
interpreted by a medical provider. For example, an AED (Automated External Defibrillator) is a
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tool that can be used by medical providers and laymen alike to deliver shocks during CPR for a
person in cardiac arrest. However, those who will benefit from a shock in cardiac arrest still
have electrical activity in their heart. It just is not normal or coordinated. The AED actually
contains a computer that analyzes the electric potential between the pads (which were placed on
the patient’s chest by the rescuer) in a similar way as a regular EKG machine. If the computer
identifies an abnormal, but “shockable” rhythm, the rescuer can administer that shock without
ever having to see or interpret the rhythm. This way, life-saving treatment can be delivered by
those without the knowledge to properly interpret an EKG.
Conclusion
This document has provided the basics of how the heart pumps blood and how the events of the
cardiac cycle can be recorded using EKG technology. One could get lost in the intricacies of
EKG interpretation, cardiac dysrhythmias, and hemodynamics. As such, those topics require a
much deeper study than what this document can provide. However, the anatomy and basic
function of the heart, contained above, can serve as foundational knowledge, should one wish to
continue their study of the heart beyond the information contained in this document.
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