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How Your Heart Works
by Dr. Carl Bianco, M.D.

Everyone knows that the heart is a vital organ. We cannot live without our heart. However, when you get right down to it, the heart is just a pump. A complex and important one, yes, but still just a pump. As with all other pumps it can become clogged, break down and need repair. This is why it is critical that we know how the heart works, With a little knowledge about your heart and what is good or bad for it, you can significantly reduce your risk for heart disease.

Heart disease is the leading cause of death in the U.S. In 1997, 725,000 persons died of heart disease (31% of all deaths). Almost 2,000 Americans die of heart disease each day. That is 1 death every 44 seconds. The good news is that the death rate from heart disease has been steadily decreasing. The age adjusted death rate decreased by 3% from 1996 to 1997 and 34.6% from 1979 to 1997. Unfortunately, heart disease still causes sudden death and many people die before even reaching the hospital.

The heart holds a special place in our collective psyche as well. Of course the heart is synonymous with love. It has many other associations -- What other word has as many meanings as the word heart? Here are just a few examples:

  • have a heart - be merciful
  • change of heart - change your mind
  • to know something by heart - memorize something
  • broken heart - to lose love
  • heartfelt - deeply felt
  • have your heart in the right place - to be kind
  • cry your heart out - to grieve
  • heavy heart - sadness
  • have your heart set on - to want something badly
Certainly no other bodily organ elicits this kind of response. When was the last time you had a heavy pancreas?

In this edition of How Stuff Works we will look at this most important -- and most talked-about - organ so that you can understand exactly what makes your heart tick!

Anatomy of the Heart
The heart is a hollow, cone-shaped muscle located between the lungs and behind the sternum (breastbone). Two-thirds of the heart is located to the left of the midline of the body and 1/3 is to the right (see Figure 1).

Figure 1
[Please note - Medical illustrations assume that the patient is
facing you so that the right and left correspond to the patient's
right and left. That's why the left and right labels here seem backwards.]

The apex (pointed end) points down and to the left. It is 5 inches (12 cm) long, 3.5 inches (8-9 cm) wide and 2.5 inches (6 cm) from front to back, and is roughly the size of your fist. The average weight of the heart in a female is 9 ounces and in the male is 10.5 ounces. The heart comprises less than 0.5% of the total body weight.

The heart has 3 layers. The smooth inside lining of the heart is called the endocardium. The middle layer of heart muscle is called the myocardium. It is surrounded by a fluid filled sac call the pericardium.

Chambers and Valves
The heart is divided into 4 chambers: (see Figure 2)

  1. Right Atrium (RA)
  2. Right Ventricle (RV)
  3. Left Atrium (LA)
  4. Left Ventricle (LV)

Figure 2

Each chamber has a sort of one-way valve at its exit that prevents blood from flowing backwards. When each chamber contracts the valve at its exit opens. When it is finished contracting the valve closes so that blood does not flow backwards.

  1. Tricuspid valve- is at the exit of the Right Atrium.
  2. Pulmonary valve - is at the exit of the Right Ventricle.
  3. Mitral valve - is at the exit of the Left atrium.
  4. Aortic valve - is at the exit of the Left Ventricle.
When the heart muscle contracts or beats (called systole) it pumps blood out of the heart.  The heart contracts in two stages.  In the first stage the Right and Left Atria contract at the same time, pumping blood to the Right and Left Ventricles.  Then the Ventricles contract together to propel blood out of the heart.  Then the heart muscle relaxes (called diastole) before the next heartbeat.  This allows blood to fill up the heart again.

The right and left sides of the heart have separate functions. The right side of the heart collects oxygen-poor blood from the body and pumps it to the lungs where it picks up oxygen and releases carbon dioxide. The left side of the heart then collects oxygen rich blood from the lungs and pumps it to the body so that the cells throughout your body have the oxygen they need to function properly.

Flow of Blood
All blood enters the right side of the heart through two veins:

  1. The Superior Vena Cava (SVC) and
  2. Inferior Vena Cava (IVC) (see figure 3).
The SVC collects blood from the upper half of the body. The IVC collects blood from the lower half of the body. Blood leaves the SVC and the IVC and enters the Right Atrium (RA) (3).

When the RA contracts, the blood goes through the Tricuspid Valve (4) and into the Right Ventricle (RV) (5). When the RV contracts blood is pumped through the Pulmonary Valve (6), into the Pulmonary Artery (PA) (7) and into the lungs where it picks up oxygen.

Figure 3

Why does it happen this way? Because blood returning from the body is relatively poor in oxygen. It needs to be full of oxygen before being returned to the body. So the right side of the heart pumps blood to the lungs first to pick up oxygen before going to the left side of the heart where it is returned to the body full of oxygen.

Blood now returns to the heart from the lungs by way of the Pulmonary Veins (8) and goes into the Left Atrium (LA) (9). When the LA contracts, blood travels through the Mitral Valve (10) and into the Left Ventricle (LV) (11). The LV is a very important chamber that pumps blood through the Aortic Valve (12) and into the Aorta (13) to the rest of the body. The Aorta is the main artery of the body. It receives all the blood that the heart has pumped out and distributes it to the rest of the body. The LV has a thicker muscle than any other heart chamber because it must pump blood to the rest of the body against much higher pressure in the general circulation (blood pressure).

Here is a recap of what we just discussed. Blood from the body flows:

  • to the Superior and Inferior Vena Cava,
  • then to the Right Atrium
  • through the Tricuspid Valve
  • to the Right Ventricle
  • through the Pulmonic Valve
  • to the Pulmonary Artery
  • to the Lungs
The blood picks up oxygen in the lungs, and then flows from the lungs:
  • to the Pulmonary Veins
  • to the Left Atrium
  • through Mitral valve
  • to the Left Ventricle
  • through the Aortic Valve
  • to the Aorta
  • to the body

Electrical System
Have you ever wondered what makes your heart beat? How does it do it automatically, hour after hour, day after day?

The answer lies in a special group of cells that have the ability to generate electrical activity on their own. These cells separate charged particles. Then they spontaneously leak certain charged particles into the cells. This produces electrical impuses in the pacemaker cells which spread over the heart, causing it to contract. These cells do this more than once per second to produce a normal heart beat of 72 beats per minute.

The natural pacemaker of the heart is called the Sinoatrial node (SA node). It is located in the Right Atrium. The heart also contains specialized fibers that conduct the electrical impulse from the pacemaker (SA node) to the rest of the heart (see Figure 4).

Figure 4

The electrical impulse leaves the SA node (1) and travels to the right and left Atria, causing them to contract together. This takes .04 seconds. There is now a natural delay to allow the Atria to contract and the Ventricles to fill up with blood. The electrical impulse has now traveled to the Atrioventricular Node (AV node) (2). The electrical impulse now goes to the Bundle of His (3), then it divides into the Right and Left Bundle Branches (4) where it rapidly spreads using Purkinje Fibers (5) to the muscles of the Right and Left Ventricle, causing them to contract at the same time.

Any of the electrical tissue in the heart has the ability to be a pacemaker. However, the SA node generates an electric impulse faster than the other tissue so it is normally in control. If the SA node should fail, the other parts of the electrical system can take over, although usually at a slower rate.

Although the pacemaker cells create the electrical impulse that causes the heart to beat, other nerves can change the rate at which the pacemaker cells fire and the how strongly the heart contracts. These nerves are part of the Autonomic Nervous System. The Autonomic Nervous System has 2 parts - The Sympathetic Nervous System and the Parasympathetic Nervous System. The Sympathetic Nerves increase the heart rate and increase the force of contraction. The Parasympathetic Nerves do the opposite.

All this activity produces electrical waves we can measure. The measurement is typically represented as a graph called an Electrocardiogram (EKG). Here is an example of three heartbeats from an EKG (Figure 5):

Figure 5

Each part of the tracing has a lettered name:

  1. P wave - coincides with the spread of electrical activity over the Atria and the beginning of it's contraction.
  2. QRS complex - coincides with the spread of electrical activity over the ventricles and the beginning of it's contraction.
  3. T wave - coincides with the recovery phase of the ventricles.
Electrical system abnormalities can range from minor premature beats (skipped beats) that do not require treatment, to slow or irregular beats that require an artificial pacemaker.

Blood supply
Coronary arteries are the ones that we try to keep clear by eating a healthy diet. If Coronary arteries are blocked a heart attack results.

The heart, just like any other organ, requires blood to supply it with oxygen and other nutrients so that it can do its work. The heart does not extract oxygen and other nutrients from the blood flowing inside it. The heart gets its blood from coronary arteries that eventually carry blood within the heart muscle. Approximately 4-5% of the blood output of the heart goes to the coronary arteries (7 ounces/minute or 225 ml/min).

There are two main coronary arteries (figure 6) - The Left Main Coronary artery (1) and the Right Coronary Artery (2) which arise from the Aorta. The Left Main Coronary Artery divides into the Left Anterior Descending (3) and the Left Circumflex Arteries (4). Each Artery supplies blood to different parts of the heart muscle and the electrical system.

Figure 6

The heart also has veins that collect oxygen poor blood from the heart muscle. Most of the major veins of the heart (great cardiac vein, small cardiac vein, middle cardiac vein, posterior vein of the Left Ventricle, and oblique vein of the Left Atrium) drain into the coronary sinus which opens into the Right Atrium.

Coronary artery disease is caused by a blockage in one of the coronary arteries. When a coronary artery is partially blocked that artery cannot supply enough blood to the heart muscle to meet its needs during exertion. When someone with coronary artery disease exerts himself or herself, it causes chest pain. This is due to lack of blood and oxygen to that part of the heart muscle. This is called angina. If the angina worsens (more frequent angina episodes, with less exertion) a condition called unstable angina exists. A worsening of the obstruction causes this. A heart attack occurs when a coronary artery is completely blocked and no blood or oxygen is getting to the heart muscle served by that artery. This also causes chest pain and causes death to the heart muscle served by that artery.

Interesting Heart Facts
What causes the sound your heart makes?
When someone listens to your heart with a stethoscope the sound is often described as - lub-dub lub-dub. The 1st heart sound (lub) is caused by the acceleration and deceleration of blood and vibration of the heart at the time of the closure of the Tricuspid and Mitral Valves. The 2nd heart sound (dub) is caused by the same acceleration and deceleration of blood and vibrations at the time of closure of the Pulmonic and Aortic Valves.

How may times does you heart beat?
The average heartbeat is 72 times per minute. In the course of one day it beats over 100,000 times. In one year the heart beats almost 38 million times, and by the time you are 70 years old, on average, it beats 2.5 billion times!

Does your heart rate change as we age?
Everyone's pulse (average heart rate per minute) changes as we age. Here is a chart of average pulse at different ages:

Age Pulse
Newborn 130
3 months 140
6 months 130
1 year 120
2 years 115
3 years 100
4 years 100
6 years 100
8 years 90
12 years 85
adult 60 - 100

How much blood does your heart pump?
An average heart pumps 2.4 ounces (70 milliliters) per heartbeat. An average heartbeat is 72 beats per minute. Therefore an average heart pumps 1.3 gallons (5 Liters) per minute. In other words it pumps 1,900 gallons (7,200 Liters) per day, almost 700,000 gallons (2,628,000 Liters) per year, or 48 million gallons (184,086,000 liters) by the time someone is 70 years old. That's not bad for a 10 ounce pump!

I hope this information has helped you to learn about the heart in a fun way.  This will serve as a good foundation for future articles about heart disease. 

About the Author
Dr. Carl Bianco, M.D. is an Emergency Physician practicing at Dorchester General Hospital in Cambridge, MD, located on the Eastern Shore of Maryland. Dr. Bianco attended Medical school at Georgetown University School of Medicine and he received his undergraduate degree from Georgetown University majoring in nursing and pre-med. He Completed an internship and residency in Emergency Medicine at Akron City Hospital in Akron, Ohio.

Dr. Bianco lives near Baltimore with his wonderful wife and two wonderful children.


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