ER Nurse’s Guide to the Anatomy of the Heart

The heart is one of the most brilliant organs in the body. It normally works very efficiently and requires very little from the person who houses it. However, there are times when the heart does not work as expected. It is the nurse’s job to know why, and also how to fix it. You can’t do that without a firm grasp of the anatomy of the heart.

The heart is the main organ of the cardiovascular system. This system is responsible for pumping blood throughout the body. It pumps about 2,000 gallons of blood through the body every single day. It also works with other systems to help control your heart rate and blood pressure. So yeah, the heart is kind of a big deal.

The normal heart weighs about 10 ounces in humans. It lives in the front of your chest, right behind and slightly to the left of the sternum (the breastbone). The left lung is slightly smaller than the right to accomodate the heart. It is protected by the ribcage.

Anatomy of the Heart

The major anatomical peices of the heart include

• Walls
• Chambers
• Valves
• Blood Vessels
• Electrical Conduction System

Walls

The walls of the heart are the muscles that contract (squeeze) to pump blood through the body. There is also a layer of muscular tissue within the heart that divides the heart into the right and left sides.

The heart walls have three layers, each with a specific function.

Epicardium-outermost layer, offers protection to the heart, helps maintain cardiac cell development during development, and helps with cardiac cell injury response and regenration.

Myocardium-middle layer, the “pump” of the heart, responsible for contraction and relaxation which keeps blood moving throughout the body.

Endocardium-innermost layer, lines and protects the heart chambers and valves.

Chambers

The heart has 4 chambers, 2 on the top and two on the bottom. The chambers are responsible for storing blood momentarily between contractions before it is pumped to it’s next destination. We will talk about the flow of blood through the body a little later in this post.

The upper chambers, the atria, are responsible for recieving blood. The right atrium recieves deoxygenated blood from the body via the vena cavas. The superior vena cava brings blood from the upper body, while the inferior vena cava brings blood up from the lower body The left atrium recieves oxygen-rich blood from the lungs via the pulmonary veins. Both atria hold blood until the next cardiac contraction, when it is pumped to the ventricles.

The ventricles are the bottom 2 chambers of the heart. They are responsbile for recieving blood from the atria and holding it momuntarily until it is pushed out of the heart. The right ventricle holds deoxygenated blood until it can be pumped to the lungs. The left atrium holds oxygenated blood until it can be pumped out to the systemic circulation.

Valves

The heart also contains 4 heart valves, which are responsible for allowing blood to flow to it’s next destination with each contraction. The tricuspid valve is between the right atrium and the right ventricle. The pulmonic valve is between the right ventricle and the pulmonary artery. (The pulmonic valve is also referred to as the pulmonary valve. TomAYto, tomAHto.) The mitral valve is bewteen the pulmonary vein and the left atrium. Finally, the aortic valve is between the left ventricle and the aorta.

Not that you will ever need to know this (if you actually do please let me know), but the tricuspid, pulmonic and aortic valves all have three “leaflets”, while the mitral valve only has two. The mitral valve is often the site of regurgitation of blood (blood leaks backwards instead of forward). I’m no doctor, but I bet the lack of that extra leaflet may be part of the reason.

The mitral and tricuspid valves are atrioventricular valves, meaning they seperate the atria and ventricles. The aortic and pulmonary valves are semilunar valves, seperating the ventricles from the greater arteries.

Blood flow through the heart

Superior and inferior vena cava > right atrium > through tricuspid valve > right ventricle > through mitral valve > pulmonary arteries > lungs to pickup some O2 and deliver carbon dioxide > pulmonary veins > left atrium > through the mitral valve > left ventricle > through the aortic valve > aorta > throughout the body to deliver the O2 and pick up carbon dioxide > superior and inferior vena cava > rinse and repeat

A helpful way to remember which valve is where in this adventure is the phrase “Toilet Paper My Ass”. T=Tricuspid P=Pulmonic M=Mitral A=Aortic

Blood Vessels

The heart has it’s own circulation as well (thankfully because otherwise you’d die).

There are two main coronary arteries that supply blood to the heart itself.

The left main coronary artery (LMCA) supplies blood to the left side of the heart (the left atrium and ventricle). The LMCA divides into the left anterior descending artery and the circumflex artery.

The left descending artery is the site of a heart attack called “The Widowmaker”. The LAD carries about 50% of the coronary arteries blood supply, so a 100% blockage of the LAD can cause the heart to stop working very quickly. The survival rate of a 100% blockage of the LAD is approximately only 12%, meaning it kills almost all of the people who experience it (thus the name).

The right coronary artery (RCA) supplies blood to the right side of the heart (right atrium and ventricle) as well as the SA and AV nodes (we will talk about those in a bit) and the septum (the wall that divides the heart into right and left sides. The RCA divides into the smaller branches as well, including the right posterior descending artery and the acute marginal artery.

Electrical Conduction System of the Heart

The heart’s conduction system is a series of nodes (groups of cells), special cells and electrical signals that cause the heart to beat.

To begin a heartbeat, the sinoatrial (SA) node generates some electricity. The SA node sits in the right atrium and is the natural “pacemaker” of the heart. There is a lot to do with ions and channels and electrolytes at this step, but I’ll save you from that in this post. We’ll just say “the SA node generates some electricity”.

That electricity travels through the atria, causing them to contract and pushing blood forward to it’s next destination. The electricty then hits the atriventricular (AV) node, which sits in bwtween the atria and ventricles. The AV node pumps the breaks just a smidge and allows the atria to empty, then sends the electricity through the ventricles by way of the Bundle of His and the Perkinje fibers. The ventricles contract when the electrical impulse hits the Perkinje fibers, allowing the ventricles to push blood on in it’s journey.

Interesting little aside: The SA node is responsible for allowing the heart to beat at roguhly 60-100 beats per minute. If the SA fails, the AV node can generate electrical impulses at 40-60 beats per minute. Other heart cells outside of the SA and AV nodes can also generate electrical impulses, which is why there are 610,000 different irregular heartbeats(OK, not really that many…… but a lot.)

Some Other (Non-Anatomy) Facts About The Heart

• Every cell in the body gets blood from the heart except the corneas. The corneas do not contain blood vessels.
• The average woman’s heartrate is 8 beats per minjte faster than the average man’s.
• The left ventricle has the thickest heart wall. That’s because this chamber has to push blood out the the entire body via the circulatory system.
• More heart attacks happen on Monday than any other day of the week.
• Heart muscle cells don’t divide. This makes heart cancer incredibly rare.

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