Tuesday, October 23, 2012

The Heart

 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.

    The pulmonic valve is made up of three cusps or flexible cup like structures. When the pressure in the right ventricle is low (as is the case during the filling phase of the chamber) the three cusps are full of blood and their sides touch each other to close the opening. This prevents blood from leaking into the pulmonary artery while the RV is filling.
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 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.
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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.
    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.

    Once again, no labels are provided. If we have done our job, you should not have any problems following the flow of blood. If we have failed to clarify it for you, please accept our apologies and go through the previous sections again - if you so desire.
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.
Electrical activation 1
Electrical activation 2

   The sequence of electrical activity within the heart is displayed in the diagrams above and occurs as follows:
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.

    In summary, the heart constantly generates a sequence of electrical activity with every single heart beat. This can be recorded on paper or displayed on a monitor by attaching special electrodes to a machine that can amplify and record an EKG or ECG (electrocardiogram). The animation (above) shows the sequence of electrical activity throughout the heart. Note how the chambers of the heart contract when they are electrically stimulated. This in turn makes the heart valves open and shut.
Click on the NEXT button below to move to the EKG section . .

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