Erin Barnes

ENG 202 C
October 24, 2010
How the Heart Works

Introduction

A heartbeat is a basic bodily function familiar to every living person; the sounds of “lubb dubb”
and subtle thumps in our chests are essential to the maintenance of life. A cardiac cycle lasts from the
beginning of one heartbeat to the beginning of the next, consisting of the complete contraction (systole)
and relaxation (diastole) of the heart muscle. Electrical impulses generated by region of specialized
cardiac tissue control the heart contraction and rate, and thus regulates the actual heart “beat”. These
signals travel across the heart chambers via an electrical conductance pathway that is closely followed
by mechanical systole, in which the heart muscle physically contracts. Contraction is responsible for the
directional flow of blood throughout the organ itself and throughout the rest of the body’s circulatory
system. As with other physiological systems, the hearts beats because a signal causes a mechanical
response, and thus, a biological function. This document will provide introductory information
describing the anatomy of the human heart, the physiological mechanisms by which it works, and the
role it plays in the cardiovascular system.

Anatomy of the Heart

Understanding the structure of the heart is fundamental to the understanding of its function. A
human heart is divided into four chambers; the superior chambers are the left and right atria and the
inferior chambers are the left and right ventricles, as seen in Figure 1. The atria are the receiving
chambers, while the ventricles are the discharging chambers. The right and left sides of the heart are
divided by a muscular wall called the septum. The right side of the heart carries deoxygenated blood
received from body and pumps it to the lungs, where it becomes oxygenated. From there, the newly
oxygenated blood goes to the left side of the heart, from where it is pumped to the rest of the body.
The blood flow pathway is maintained due to a unidirectional valve system that regulates the passage of
blood from one chamber to the next. Shown in Figure 1 are the two sets of heart valves. The atria and
ventricles are separated by atrioventricular valves that only allow blood to travel from the atria into the
ventricles. Similarly, the semilunar valves in the ventricles only permit blood to pass from the ventricle
into major artery of the connected circulation system.

Figure 1. Basic Heart Anatomy. The major structures of the human heart are labeled,
with the right portion side colored in blue, and the left side in red.
The Heart and Circulation

The heart is the central organ of the cardiovascular system responsible for pumping blood
throughout the body by repeated, rhythmic contractions. The cardiovascular system is comprised of
two types of circulation through which blood flows, pulmonary and systemic. Pulmonary circulation
transports the deoxygenated blood from the right side of the heart to the lungs, where it becomes
oxygenated, and then is returned to the left heart. From the left ventricle, blood in pumped into
systemic circulation, which transports oxygenated blood to the body and returns deoxygenated blood to
the heart. This cycle is constantly in motion due to the heart’s ability to contract at regular intervals, a
process that is maintained by the electrical conductance system.

The Electrical Conductance System

Blood is pumped throughout the circulatory system by contraction of the heart muscle, and
similarly to other muscle contractions, this action is signaled by electrical stimulation. The cells
comprising heart tissue are modified muscle cells, and as such are called cardiomyocytes, or myocardial
cells. A specific type of cardiomyocyte is self-excitable, meaning no neural stimulation is required to fire
action potentials. These “pacemaker” cells set the rate and timing at which the heart muscle contracts,
and are the source of rhythmic electrical impulses that propagate through the other heart cells to signal
contraction. Cardiac cells are connected by intercalated disks, essentially acting as a syncytium that
allows electrical signals to spread rapidly to neighboring cells. The pacemaker cells are centralized at the
top of the right atrium at the “sinoatrial node”, where they spontaneously depolarize about 100 times
per minute. There is a basic electrical signaling pathway that action potentials take after conductance
at the SA node (# 1 in Figure 2). Depolarization spreads across and down the right and left atrium walls
until it reaches the atrioventricular node (#2) located at the junction of the atria and ventricles. This first
wave of conductance causes atrial systole, pushing blood from the atria to the ventricles. From the AV
node, an electrical impulse follows the Bundle of His (#3), which tapers down into the ventricular
septum. The signal then splits into the left and right bundle branches (#4) within the two ventricles,
and is rapidly carried throughout the rest of the chambers’ tissue via the Purkinje fibers (#5). The
resulting ventricular systole causes blood to be ejected into either the pulmonary or systemic
circulation. The complete electrical conductance pathway is illustrated in the figure below.

Figure 2. The Electrical Conductance System of the Heart. The pathway that electrical signals
follow after generation in the sinoatrial node is labeled in numerical order.

2
The Heartbeat

To bring all of these concepts together, we can discuss the course of one heartbeat. During
diastole, the heart is relaxed and the upper atria fill with blood. When an action potential depolarizes to
trigger the contraction of the atria, pressure builds and eventually opens the atrioventricular valves,
allowing blood to flow into the ventricles. As blood leaves the atria and the pressure decreases, the AV
valves snap shut, creating the first heart sound, or the “lubb” part of the “lubb dubb”. As discussed
before, the electrical signal travels next across the ventricles, causing ventricular systole. The pressure
created by the contraction of these chambers forces blood through the semilunar valves into either the
systemic or pulmonary circulatory systems. Again, the flow of blood out of the ventricles reduces the
pressure in the chambers, causing the valves to close. This is the second heart sound and the “dubb” of
“lubb dubb". This is the final stage of a single heartbeat, and the process is then repeated again about
sixty to eighty times per minute, and about two and a half billion times in an average human lifespan.
The anatomical structure and physiological mechanisms described in this document are what allow the
heart organ to function as a part of the cardiovascular system, and thus sustain human life.

References

Figure 1. Basic Human Heart Anatomy.

Figure 2. The Electrical Conductance Pathway

Bianco, Carl MD. “How Your Heart Works.” Discovery Health. 2010. 17 October 2010.
<http://health.howstuffworks.com/human-body/systems/circulatory/heart4.htm>

“Circulatory System.” Wikipedia.org. 20 October 2010.

“The Heartbeat.” Texas Heart Institute Heart Information Center. Texas Heart Institute. July 2010. 19
October 2010.
<http://www.texasheart.org/HIC/Anatomy/systole.cfm>

“The Human Heart.” The Franklin Institute. 2010. 17 October 2010.