Heart disease is the leading cause of death in the United States. Almost 2,000 Americans die of heart disease each day. That is one death every 44 seconds. The good news is that the death rate from heart disease has been steadily decreasing. 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, too. 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
In this article, we will look at this important organ so that you can understand exactly what makes your heart tick.
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 a female human heart is 9 ounces and a male's heart is 10.5 ounces. The heart comprises less than 0.5% of the total body weight.
The heart has three 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 four chambers: (see Figure 2)
- right atrium (RA)
- right ventricle (RV)
- left atrium (LA)
- 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.
- The tricuspid valve is at the exit of the right atrium.
- The pulmonary valve is at the exit of the right ventricle.
- The mitral valve is at the exit of the left atrium.
- The 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.
Blood Flow
All blood enters the right side
of the heart through two veins: The superior vena
cava (SVC) and the 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). 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
- to the pulmonary veins
- to the left atrium
- through the mitral valve
- to the left ventricle
- through the aortic valve
- to the aorta
- to the body
Click the play button to see how blood flows through the body.
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