The Heart
The heart is the pump station of the body and
is responsible for circulating blood throughout the
body. It is about the size of your clenched fist and
sits in the chest cavity between two lungs. Its walls
are made up of muscle that can squeeze or pump blood
out every time that the organ "beats" or
contracts. Fresh, oxygen-rich air is brought to the lungs through the trachea (pronounced tray-kee-ya) or windpipe every time that you take a breath. The lungs are responsible for delivering oxygen to the blood, and the heart circulates the blood to the lungs and different parts of the body.
The heart is divided into FOUR
chambers or "rooms". You can compare it
to a Duplex apartment that is made up of a right and
a left unit, separated from each other by a partition
wall known as a SEPTUM (pronounced sep-tum).
Each "duplex" is subdivided into an upper and a lower chamber. The upper chamber is known as an ATRIUM (pronounced ay-tree-yum) while the lower chamber is referred to as a VENTRICLE (pronounced ven-trickle).
The right atrium (RA) sits on top of the right ventricle (RV) on the right side of the heart while the left atrium (LA) sits atop the left ventricle (LV) on the left.
The right side
of the heart is responsible for sending blood to the
lungs, where the red blood cells pick up fresh oxygen.
This OXYGENATED blood is then returned to the left
side of the heart. From here the oxygenated blood
is transported to the whole body supplying the fuel
that the body cells need to function. The blood cells
of the body extract or removes oxygen from the blood.
The oxygen-poor blood is returned to the right atrium,
where the journey began. This round trip is known
as the CIRCULATION of blood.
The figure shown above is a section of the heart,
as viewed from the front. It demonstrates the four
chambers. You will also notice that there is an opening
between the right atrium (RA) and the right ventricle
(RV). This is actually a valve known as the TRICUSPID
(pronounced try-cus-pid) valve. It has three flexible
thin parts, known as leaflets, that open and shut.
The figure below shows the mitral and tricuspid valves,
as seen from above, in the open and shut position.
When shut, the edge of the
three leaflets touch each other to close the opening
and prevent blood from leaving the RV and going back
into the RA. Thus, the tricuspid valve serves as a
trapdoor valve that allows blood to move only in one
direction - from RA to RV. Similarly, the MITRAL valve
(pronounced my-trull) allows blood to flow only from
the left atrium to the left ventricle. Unlike the
tricuspid valve, the mitral valve has only two leaflets.
In the top diagram, you will also notice thin thread
like structures attached to the edges of the mitral
and tricuspid valves. These chords or strings are
known as chordae tendineae (do not even try to pronounce
it. However, if you really must, it is chord-ee tend-in-ee).
They connect the edges of the tricuspid and mitral
valves to muscle bands or papillary (pronounced pap-pill-lurry)
muscles. The papillary muscles shorten and lengthen
during different phases of the cardiac cycle and keep
the valve leaflets from flopping back into the atrium.
The chords are designed to control the movement
of the valve leaflets similar to ropes attached to
the sail of a boat. Like ropes, they allow the sail
to bulge outwards in the direction of a wind but prevents
them from helplessly flapping in the breeze. In other
words, they provide the capability of a door jamb
that allows a door to open and shut in a given direction
and NOT beyond a certain point.
When the three leaflets of the tricuspid bulge upwards
during contraction or emptying of the ventricles,
their edges touch each other and close off backward
flow to the right atrium. This important feature allows
blood to flow through the heart in only ONE direction,
and prevents it from leaking backwards when the valve
is shut. The two leaflets of the mitral valve functions
in a similar manner and allows flow of blood from
the left atrium to the left ventricle, but closes
and cuts off backward leakage into the left atrium
when the left ventricle contracts and starts to empty.
Confused? Continue to hang in there. We will clarify
this further in the next few pages. Please note that
the repetition is intentional! Rephrasing and repeating
an explanation often enhances the understanding and
retention of key concepts. Skip areas that are redundant
to you.
Let us now follow the circulation
of blood through the heart. As noted earlier, oxygenated
blood is pumped by the left ventricle to all parts
of the body, other than the lungs. The body tissue
removes much of the oxygen for its own need. The blood,
which is now carrying less oxygen, returns to the
heart. Blood from the head, neck and arms return to
the right atrium (RA) via the SVC or SUPERIOR VENA
CAVA. On the other hand, blood from the lower portion
of the body returns to the RA via the IVC or INFERIOR
VENA CAVA (pronounced vee-nah cave-ah).
The RA contracts when filling
is completed. This builds up pressure within that
chamber and pushes the tricuspid valve open. Blood
now rushes from the RA to the right ventricle (RV).
When the RV is filled, the walls begin to contract
and raises pressure within the RV. The increased pressure
shuts the tricuspid valve and pumps blood into the
pulmonary (pronounced pull-mun-narey) artery through
the pulmonic valve (PV, pronounced pull-mon-nick)
which is pushed open by the increased pressure. The
diagram below once again shows the four heart valves
as viewed from the top, standing in front of the heart,
i.e., we are looking down at the two ventricles with
the right atrium and left atrium removed.
When the RV contracts to empty, the pressure within the chamber rises above that of the pulmonary artery. This forces open the three cusps of PV and blood rushes through the pulmonary arteries and is sent to the lungs. Here the red blood cells pick up oxygen
The oxygenated blood from the lungs now returns to the left atrium (LA) via four tubes that are known as pulmonary veins. They empty into the back portion of the LA. When the LA contracts after it is completely filled. This opens the mitral valve and forces blood into the left ventricle (LV).
When the LV is completely filled, it starts to empty its contents by contacting the walls. This increases pressure within the chamber, shuts the mitral valve and opens the aortic valve (AV, pronounced a-ortic). The sequence is similar to that described for the RA, RV and pulmonic valve.
Blood now rushes through the aorta (pronounced a-or-tah).
The aorta is the main "highway" blood vessel
that supplies blood to the head, neck, arms, legs,
kidneys, etc. Thus, blood is brought to each of these
organs and limbs via branches that originate from
the aorta. The cells within each part of the body
pick up oxygen and nutrients from the blood. The oxygen-poor
blood then returns to the RA, via the superior and
inferior vena cava, and the beat goes on!!
The animation above demonstrates
the flow of blood through the heart and lungs, as
explained above. Notice that the mitral and the right
side of the heart works in synchrony with the left,
but that each atria contracts while the ventricle
fills
Less confused? Good! Continue to hang in there as
we further clarify these concepts
The various parts of the heart, including
its chambers, valves and arteries are shown in the
figures displayed below.
The diagram on the right shows a longitudinal
(cut from top to bottom) section of the heart. The
flow of blood is represented by arrows. The narration
will describe different phases of the cardiac circulation.
The animation will serve as a revision or reinforcement of the various stages or steps of the circulation, with arrows and labels serving as a reminder of what takes place. You may click on the stop, rewind and play buttons to control the animation.
The animation will serve as a revision or reinforcement of the various stages or steps of the circulation, with arrows and labels serving as a reminder of what takes place. You may click on the stop, rewind and play buttons to control the animation.
The pictures
below represent a heart that is cut along the horizontal
axis. The picture on the left shows the plane along
which the heart is cut. That is, the top of the heart,
including the right and left atria (atria is plural
for atrium), the pulmonary artery and aorta are removed
on the picture on the right (below). It shows the
heart as you would look down at it from the front.
The tricuspid and mitral valves are represented right
and left, respectively (you can see the right and
left ventricles through the two valves). The aortic
and pulmonic valves are shown up and down, respectively,
in the bottom half of the picture. The heart size
increases and decreases during the filling (DIASTOLE,
pronounced die-as-tull-ee) and contraction or emptying
(SYSTOLE, pronounced sis-tull-ee) of the heart chambers.
The animation
on the bottom left shows a longitudinal section of the beating heart,
together with valve structures that open and shut to let blood pass
through the atria, ventricles and the great vessels. The animation on
the right shows a cross-sectional view of the heart.
Click the
button to apply and remove label heart labels. You may also proceed
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The mitral and tricuspid valves
open and the aortic and pulmonic valves are shut while
the ventricles fill during diastole. In contrast,
the mitral and tricuspid valves shut while the aortic
and pulmonic valves open during ventricular systole.
This sequence ensures that the ventricles are filled
to capacity before the aortic and pulmonic valves
are opened. At this time, the mitral and tricuspid
valves are shut so that blood does not leak back into
the the two atria. Yes, the heart is an ingenious
device that could have inspired design of the modern
day mechanical pump and integrated valves.
Did you stop and wonder why each side of the heart has two pumping chambers (atrium and ventricle)? Why not just have a ventricle to receive blood and then pump it straight out? The reason is that the atrium serves as a "booster pump" that increases the filling of the ventricle. Filling a normal ventricle to capacity translates to more vigorous contraction or emptying. You can compare this to a strong spring, and imagine that the heart muscle is made up of tiny little "springs" known as ACTIN and MYOSIN. Within reasonable limits, the more you stretch a spring, the more vigorously will be its contraction or recoil. In medical terms, this is known as "Frank-Starling's" law.
Next, we will show you the heart sections in the
frontal and top views again. However, the labels and
arrows will be removed so that you can use your imagination
to follow the flow of blood through the heart. As
you visualize the flow, name the various chambers,
valves, arteries and veins. Remember that ARTERIES
(pronounces art-trees) carry blood away from the heart.
The pulmonary arteries carry oxygen-poor blood to
the lungs, while the AORTA (pronounced ay-or-ta) carries
oxygen rich blood to the rest of the body. The tubes
that return blood to the heart are known as VEINS
(pronounced vaynes). The pulmonic (pronounced pull-monic)
veins return oxygenated blood from the lungs to the
left atrium. They connect into the back of the left
atrium.Did you stop and wonder why each side of the heart has two pumping chambers (atrium and ventricle)? Why not just have a ventricle to receive blood and then pump it straight out? The reason is that the atrium serves as a "booster pump" that increases the filling of the ventricle. Filling a normal ventricle to capacity translates to more vigorous contraction or emptying. You can compare this to a strong spring, and imagine that the heart muscle is made up of tiny little "springs" known as ACTIN and MYOSIN. Within reasonable limits, the more you stretch a spring, the more vigorously will be its contraction or recoil. In medical terms, this is known as "Frank-Starling's" law.
The superior vena cava brings
oxygen-poor blood from the head, neck and arms to
the right atrium, while the inferior vena cava returns
oxygen-poor blood to the same chamber from lower portions
of the body. Remember that both the superior and inferior
vena cavae (cavae is plural for cava and is pronounced
cay-veeh). Yup! us docs have our own secret language
that originate from the Latin roots of medical terminology.
That is why the plural for cava is "cavae"
and not "cavas." Go figure!!
The superior vena cava connects to the top of the right atrium (and hence the term superior) while the inferior vena cava connects to the bottom of the chamber.
Shown below is the same
figure that was presented to you a few pages ago.
It shows the circulation of blood through the heart
and lungs. The superior vena cava connects to the top of the right atrium (and hence the term superior) while the inferior vena cava connects to the bottom of the chamber.
The aorta is the major blood vessel that arises from the left ventricle and is separated from it by the aortic valve. The left main coronary artery arises from above the left portion of the aortic valve and then usually divides into two branches, known as the left anterior descending (LAD) and the circumflex (Circ) coronary arteries. In some patients, a third branch arises in between the LAD and the Circ. This is known as the ramus (pronounced ray-muss), intermediate , or optional diagonal coronary artery.
The LAD travels in the groove (known as the inter-ventricular groove) that runs in the anterior or front portion the heart. It sits between the right and the left ventricles or the two lower chambers of the heart.
The LAD gives rise to the following two sets of branches:
- The diagonals are branches of the LAD that runs diagonally away from the LAD and towards the left edge in front of the heart.
- The septal perforators (SP) runs into the septum (partition that separates the two ventricles) and provides its blood supply.
Circumflex Coronary Artery |
The Circumflex
(Circ) coronary artery is a branch of the left
main coronary artery. It travels in the left atrio-ventricular
groove that separates the left atrium from the left
ventricle. The Circ moves away from the LAD and wraps
around to the back of the heart. The major branches
that it gives off in the proximal or initial portion
are known as obtuse (pronounced Ob-tews) marginal
or OM coronary arteries. As it makes
its way to the back or posterior portion of the heart,
it gives off one or more left postero-lateral (PL) branches.
In 85% of cases, the Circ terminates
at this point and is known as a non-dominant
left coronary artery system. In the other 15% of cases,
a dominant Circ supplies the PDA or
posterior descending artery, which run in the bottom
of the heart within a groove that separates the left
from the right ventricle.
Right Coronary Artery
The right
coronary artery or RCA travels
originates above the right portion of the aortic valve
and runs in the groove that separates the right atrium
from the right ventricle, as it moves towards the
bottom or inferior portion of the heart.
The acute marginal coronary
artery is given off in the proximal or early course
of the artery. While the terminal or distal portion
of the RCA gives off the posterior descending artery
or PDA. The PDA runs in the bottom of the heart in
a groove that separates the left and right ventricles,
as it supplies branches to the lower portion of the
septum (partition between the two ventricles. In 15%
of cases, RCA is "non-dominant" and the
Circ supplies the PDA branch.
The RCA also supplies the
postero-lateral artery or PLA to the lower back portion
of the left ventricle and the right ventricular branch
to the right ventricle.
Heart Electrical Activity
The heart has a natural pacemaker that regulates
the pace or rate of the heart. It sits in the upper
portion of the right atrium (RA) and is a collection
of specializes electrical cells known as the SINUS
or SINO-ATRIAL (SA) node. Like the spark-plug of an automobile it generates a number of "sparks" per minute. Each "spark" travels across a specialized electrical pathway and stimulates the muscle wall of the four chambers of the heart to contract (and thus empty) in a certain sequence or pattern. The upper chambers or atria are first stimulated. This is followed by a slight delay to allow the two atria (atria is plural for atrium and pronounced ay-tree-ya) to empty. Finally, the two ventricles are electrically stimulated.
In an automobile, the number of sparks per minute generated by a spark plug is increased when you press the gas pedal or accelerator. This revs up the motor. In case of the heart, adrenaline acts as a gas pedal and causes the sinus node to increase the number of sparks per minute, which in turn increases the heart rate. The release of adrenaline is controlled by the nervous system. The heart normally beats at around 72 times per minute and the sinus node speeds up during exertion, emotional stress, fever, etc., or whenever our body needs an extra boost of blood supply. In contrast, it and slows down during rest or under the influence of certain medications. Well trained athletes also tend to have a slower heart beat.
As the SA node fires, each electrical impulse travels through the right and left atrium. This electrical activity causes the two upper chambers of the heart to contract. This electrical activity and can be recorded from the surface of the body as a "P" wave" on the patient's EKG or ECG (electrocardiogram).
The electrical impulse then moves to an area known as the AV (atrio-ventricular) node. This node sits just above the ventricles. Here, the electrical impulse is held up for a brief period. This delay allows the right and left atrium to continue emptying it's blood contents into the two ventricles. This delay is recorded as a "PR interval." The AV node thus acts as a "relay station" delaying stimulation of the ventricles long enough to allow the two atria to finish emptying.
Following the delay, the electrical impulse travels through both ventricles (via special electrical pathways known as the right and left bundle branches). The electrically stimulated ventricles contract and blood is pumped into the pulmonary artery and aorta. This electrical activity is recorded from the surface of the body as a "QRS complex". The ventricles then recover from this electrical stimulation and generates an "ST segment" and T wave on the EKG.
Click on the NEXT button below to move to the EKG section . .
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