Basic Heart Anatomy
This guide will help to break down basic heart anatomy, or more medically correct “cardiac” anatomy, into simple understandable terms that will assist you in your pursuit of knowledge in the medical field.
The heart is broken up into four separate compartments or chambers. The two compartments that rest closest to the “superior” region (toward the human head) are referred to as Atria (pleural) or when referred to individually are the right and left Atrium (singular).
Resting generally “inferior,” (toward the foot of the patient), to both atrium are the right and left Ventricles.
The roles of the Atria and Ventricles are very different, and as such, their anatomy is different from one another. The primary role of the Atria are to receive, and to a degree collect blood that is returning from the body and lungs. The Ventricles on the other hand forcefully project blood into the lungs to become oxygen rich, and into the body to help “perfuse” (pump blood through) and supply oxygen to different cells of the body.
In between the Atria and Ventricles are valves, referred to as AV Valves or Atrioventricular Valves (makes sense). These valves shut closed when the ventricles contract to force blood out in a process called Ventricular Systole, and remain open to allow for the flow of blood from the Atria to the Ventricles during Ventricular Diastole or the time at which the ventricles are relaxed.
These AV valves, while similar in role, differ in the number of cusps, or leaflets that they have from right to left and are named accordingly. The right sided AV Valve, having three cusps, is referred to as the Tricuspid Valve and separates the right Atrium from the right Ventricle. The left sided AV Valve has two cusps, and as such is referred to as the Bicuspid Valve or Mitral Valve and separates the Left Atrium from the Left Ventricle.
One easy mnemonic device that I used when first learning the basics of anatomy was that when you make a purchase you “Try before you Buy.” Discussing that the Tricuspid Valve’s position “before” the Bicuspids Valve. (The cardiac cycle is a cycle and as such has no beginning or end; it is commonly taught however that the blood entering the Right Atrium marks the beginning of the cardiac cycle.)
There are two other valves that are located within the heart that control the flow of blood, and they are located between the Right Ventricle and the Pulmonary Artery which leads to the lungs (The blood pathway that begins at the heart and travels through the lungs back to the right Left Atrium is known as the Pulmonary Circuit), as well as between the Left Ventricle and the Aorta which leads to the body (The blood pathway that takes blood from the Aorta and travels throughout the body perfusing tissues and returning deoxygenated blood to the Right Atrium is known as the Systemic Circuit). While the AV Valves rest between the Atria and Ventricles, the valves that rest between the Ventricles and their respective Artery are known as Semilunar Valves, and both semilunar valves have a configuration similar to that of the tricuspid valve, having three cusps. The Semilunar Valve that rests between the Right Ventricle and the Pulmonary Artery is known as the Pulmonary Valve and the Semilunar Valve that rests between the Left Ventricle and the Aorta is known as the Aortic Valve.
As discussed earlier, the Atria and Ventricles play a different role within the heart, and as such their anatomy is different from one another. Such differences also exist between the right and left sides of the heart. The Right side of the heart collects deoxygenated blood which has been depleted of its nutrients from the body in the Right Atrium, and once collected in the Right Ventricle projects this deoxygenated blood through the Pulmonary Artery into the lungs where gas exchange will bind oxygen molecules to hemoglobin in the red blood cell, as well as take excess CO2 off of the hemoglobin. This oxygen rich blood will return to the Left Atrium, and get pushed out through the Aorta into the Systemic Circuit and to tissues and cells where it will drop off oxygen to be used by cellular structures of the body.
In this process it makes sense that the two sides would have minor variances within their anatomy as their roles are slightly different. One such instance is the force required by the ventricles to project blood being different between the two sides. For the Right Atrium to have a contraction forceful enough to pump blood to the lungs, its next door neighbor, it will require less force than the Left Ventricle, which has to pump blood to tissues much further away from the heart. Due to this difference the muscles or Myocardial layer of the Left Ventricle are much thicker than that of the Right Ventricle, to facilitate a stronger contraction.
While the Atria are separated from the Ventricles by the AV Valves, there are also walls that prevent the flow of blood between the Ventricles themselves. This wall that rests in between the Ventricles, keeping the compartments separate and preventing deoxygenated blood from mixing with oxygenated blood, is known as the Interventricular Septum. A similar wall prevents the flow of blood in between the two Atrium and is similarly named the Interatrial Septum.
Vessels of the Heart
When Discussing Basic Heart Anatomy it is important to have a fundamental understanding of the hearts vasculature and the vessels associated with the heart. While I go into greater detail about bloods pathway and direction in my Trace Billy Blood Drop article, understanding where these vessels rest and their role will greatly assist you in your knowledge of the heart and its function.
One point, often seen on the EMT National Registry and an important distinction when learning basic heart anatomy, is that Veins return blood in the direction of the heart, and Arteries move blood away from the heart. Often times tests will attempt to trick you into saying that veins carry deoxygenated blood or arteries carry oxygen rich blood. While this is most often the case, we will see in this article that there are exceptions.
Blood returns from the body or Systemic Circuit through two main portals of entry. The Superior Vena Cava is a vein which returns deoxygenated blood primarily from the upper extremities (arms) and head, or simply areas originating above the heart. The Inferior Vena Cava is a vein which returns deoxygenated blood from the abdomen, pelvis, and lower extremities, or simply from areas originating below the heart. A third, less often referenced vein which returns blood that perfuses the heart itself is the Coronary Sinus. This vessel is the main drainage point into the right atrium from the Coronary Arteries which quickly branch off of the Aorta and perfuse and oxygenate the heart itself.
Once the deoxygenated blood has entered the right side of the heart and descended into the right ventricle, it is then forcefully projected through the Pulmonary Semilunar Valve and into the Pulmonary Circuit through the Pulmonary Artery. When we discussed the distinction in classifying veins and arteries, the pulmonary circuit was the outlier we were referring to. This is due to the fact that the Pulmonary Artery, while an artery, contains deoxygenated blood. Once the blood enters the lungs and begins its route back it eventually enters the four Pulmonary Veins which then return blood to the Left Atrium. These Pulmonary veins, while veins, carry oxygen rich blood back to the heart.
After entering the Left Atrium and descending into the Left Ventricle, Ventricular Systole then projects blood through the Aortic Semilunar Valve and into the Aorta, marking the entrance to the systemic circuit. The blood from here perfuses tissues throughout the body, and returns via the Superior or Inferior Vena Cava, making the end, and beginning of the Cardiac Cycle.
Thank you for taking the time to read my Basic Heart Anatomy guide. For any questions or concerns regarding this content, please feel free as always to contact me using the contact us page. This guide is simply an introduction into the concepts of basic cardiac anatomy and more information regarding, EKG interpretation, coronary vessels, cardiac pathology, and all other elements of EMT related care are soon to come. If you enjoyed the contents of this article please sign up for our newsletter on the sidebar for very infrequent but interesting information regarding all aspects of Emergency care.