Trace Billy Blood Drop Through The Body
In determining fundamental EMT-Basic level Cardiology knowledge many employers, during their interview process, will request you to trace the path a “drop” of blood will take in completing a full circuit through the circulatory system. This question comes in many forms, but one common somewhat lighthearted way to phrase this question is requesting you trace the path of Billy Blood drop through the body. This guide will assume you know the fundamental anatomical structures that compose the heart, but if you need a refresher, or are brand new to cardiac anatomy please view my Basic Heart Anatomy guide.
As the circulatory system is just that, a circuit, there is no true starting point, but the most common starting point is the entrance of deoxygenated blood into the Right Atrium from the Superior and Inferior Vena Cava where blood is at its lowest pressure. While these two large venous entry points are the most frequently cited, there is a third, less mentioned entry point for deoxygenated blood to enter the Right Atrium, the Coronary Sinus, which returns blood from the Cardiac Veins, and respectively the Coronary Arteries.
With the exception of the Azygos Vein, nearly all major vessels draining into the Superior Vena Cava (SVC) stem from above armpit level, while the major vessels draining into the Inferior Vena Cava (IVC) consist of the vessels of the abdomen, pelvis, and lower extremities. Due to this fact it is commonly stated during recitation of blood’s pathway that blood from above the heart returns via the SVC and the blood below the heart returns via the IVC.
Right Atrial Systole
As the deoxygenated blood from the Superior Vena Cava, Inferior Vena Cava, and Coronary Sinus enter the Right Atrium it collects and passively drains down to the Right Ventricle. This passive filling occurs until a forceful contraction of both atria (atrial systole) projects the blood pooled in both atria downward, actively increasing the volume of both Ventricles as well as increasing ventricular pressure. As the blood passes from the Right Atria to the Right Ventricle during both active and passive filling it passes the Tricuspid Valve, which remains open during ventricular diastole (the time at which the ventricles are at rest).
The Tricuspid Valve, situated between the Right Atrium and Right Ventricle, gets its name for its three cusps or leaflets and is one of the two AV Valves which get their name Atrioventricular as the sit between the Atria and the Ventricles. The other AV Valve, which sits between the Left Atrium and Left Ventricle, is known as the Bicuspid, or Mitral Valve as it has two cusps or leaflets. In memorizing the order that these valves are introduced to blood in the heart, a simple memory device commonly used is the mnemonic “Try it before you Buy it.” This easy to remember statement will assist you in remembering that the blood entering the heart will encounter the Tricuspid, before the Bicuspid Valve.
Right Ventricular Systole: Isovolumetric Ventricular Contraction
Once the Right Ventricle is sufficiently filled with blood and has adequate pressure, the Ventricle will then begin to contract, and the Tricuspid Valve slams shut, preventing blood from projecting back into the Artium from the Ventricle during Ventricular contraction. The slamming shut of the AV Valve is a pivotal component of Ventricular Systole, as it ensures the proper projection of blood out of the heart and into the pulmonary or systemic circuit. This closure of the AV Valve is ensured due to the ventricular contraction stemming from the apex of the heart (the inferior, left lateral region toward the “tip” of the heart) which pushes the high pressure high volume blood upward closing the AV Valve. This stage of Ventricular systole in which the AV Valve and Semilunar Valve are closed is called Isovolumetric Contraction, as volume within the ventricle is remaining the same. During this stage of contraction, pressure is increasing greatly within the ventricle but volume remains the same as the blood remains within the right ventricle.
Right Ventricular Systole: Ejection down the Pressure Gradient
The continuing contraction of the Right Ventricle eventually pushes with enough force to push its way through the Pulmonary Semilunar Valve, and into the Pulmonary Artery. Due to the proximity of the lungs to the heart, the pressure required by the right ventricle to perfuse and enter the pulmonary circuit is significantly less than that of the left ventricle, which must project blood throughout the systemic circuit or entire body.
Pulmonary Circuit Fundamentals
The blood then travels down the Pulmonary Artery which then branches off into the Right and Left Pulmonary Artery, which head toward their respective lungs. Arteries branch off becoming arterioles, which are similar (though not identical) in structure and function to arteries. These arterioles similarly branch off and eventually become capillaries. At the capillary level gas is exchanged from the alveoli in the lungs to the capillaries themselves, and the CO2 rich, Oxygen depleted blood that is passing through the capillaries within the lungs releases its excess CO2 from its hemoglobin while hemoglobin binds onto O2 (oxygen) molecules. Up until this transaction the blood has been Deoxygenated but once the oxygen binds to the hemoglobin, the happy red blood cells become Oxygenated and begin their trip back to the heart.
This Oxygen rich blood returns to the heart through a venous process similar to that of the arteriole process described above, and travels from the capillary level into venules, veins and finally into the primary vessels that return oxygenated blood back to the Left Atrium of the heart, the four Pulmonary Veins.
Left Atrial and Ventricular Diastole
As the blood is now entering the left side of the heart into the Left Atrium the process becomes nearly a mirror image of the process which occurred on the right side. This process begins with blood entering the Left Atrium from the four Pulmonary Veins and passing inferoriorly passively filling the Left Ventricle, which enters the Ventricle by passing through the Atrioventricular valve that separates the Left Atrium and Left Ventricle, the Mitral or Bicuspid Valve. This passive filling accounts for around 80% of the total volume that will collect in the Left Ventricle prior to its contraction.
Left Atrial Systole
Similar to the sequence of events that occurred on the right side of the heart, a contraction of the Atrium projects blood downward into the Ventricle, increasing the volume within the ventricle (the remaining 20%) as well as the pressure within the Ventricle.
Left Ventricular Systole: Isovolumetric Ventricular Contraction and Ejection
The next step involved is once again the beginning of ventricular systole, or isovolumetric contraction, and the Mitral Valve that was allowing the passage of blood from the Atrium to the Ventricle slams shut. The Ventricle then continues contraction until there is pressure within the Ventricle sufficient to project the blood within the Ventricle through the Aortic Semilunar Valve and into the Aorta, marking bloods entrance into the Systemic Circuit, and the point at which blood is at its highest pressure.
Systemic Circuit Fundamentals
The blood carried within the Aorta is then projected into a series of different arteries, where it follows its pathway until branching off into Arterioles and finally into Capillaries where red blood cells will exchange O2, nutrients, gasses, water, waste products, hormones, and proteins to tissues and cells within the body.
The now depleted Deoxygenated red blood cells will now collect into Venules from the Capillaries, which will in turn collect into veins, and finally the Superior and Inferior Vena Cava as well as the Coronary Sinus. Marking the traditional end, and beginning of this cycle.
Right Atrial and Ventricular Diastole
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