Structural overview — CMR phenotype documentation

The heart is divided by a central wall, or septum, into left and right halves. Each half functions as an independent pump that consists of an atrium and a ventricle. The atrium receives blood returning to the heart from blood vessels, and the ventricle pumps blood out into the blood vessels. The right side of the heart receives blood from the tissues and sends it to the lung for oxygenation through the pulmonary arteries. The left side of the heart receives newly oxygenated blood from the lungs and pumps it to tissues throughout the body through the large artery known as the aorta (Silverthorn et al. 2013).

<figure> <img src="/latex/images/anatomy/one%20way%20flow%20through%20the%20heart.png" id="fig:heart_structure" alt="One-way flow through the heart (Silverthorn et al. 2013)" /><figcaption aria-hidden="true">One-way flow through the heart <span>(Silverthorn et al. 2013)</span></figcaption> </figure>

Two sets of heart valves ensure the one-way flow: the first set is the atrioventricular valves between the atria and ventricles; while the second set is the semilunar valves that are located between the ventricles and arteries. Although the two sets of valves are very different in structure, they serve the same function: preventing the backward flow of blood (Silverthorn et al. 2013).

The two atrioventricular valves are not identical. The valve that separates the right atrium and the right ventricle has three flaps and is called the tricuspid valve. The valve between the left atrium and the left ventricle has only two flaps and is called the mitral valve because of its resemblance to the tall headdress, known as a miter (Silverthorn et al. 2013). Each of the atrioventricular complex consists of the orifice and its associated supporting annulus, the cusps, and a variety of chordae tendineae and papillary muscles. Harmonious interplay of these components, together with the myocardial mass, depends on the conduction tissues and mechanical cohesion provided by the cardiac skeleton. All parts change substantially in position, shape, angulation and dimensions during the cardiac cycle (Standring et al. 2005). Quantitative assessment of the mitral annulus (MA) and tricuspid annulus (TA) is essential for understanding the mechanisms underlying valvular regurgitation.

The semilunar valves separates the ventricles from the major arteries. The aortic valve is between the left ventricle and the aorta, and the pulmonary valve lies between the right ventricle and the pulmonary trunk. Each semilunar valve has three cup-like leaflets that snap closed when blood attempting to flow back into the ventricles fill them. (Silverthorn et al. 2013).

<figure> <img src="/latex/images/aorta/heart_valve1.png" id="fig:heart_valve2" alt="Summary of some of principle events for heart valves that occur in the cardiac cycle (Standring et al. 2005)" /><figcaption aria-hidden="true">Summary of some of principle events for heart valves that occur in the cardiac cycle <span>(Standring et al. 2005)</span></figcaption> </figure>

<figure> <img src="/latex/images/aorta/heart_valve2.png" id="fig:heart_valve2" alt="Summary of some of principle events for heart valves that occur in the cardiac cycle (Standring et al. 2005)" /><figcaption aria-hidden="true">Summary of some of principle events for heart valves that occur in the cardiac cycle <span>(Standring et al. 2005)</span></figcaption> </figure>

The heart is placed obliquely in the thorax (Standring et al. 2005). The pointed apex of the heart angles down to the left side of the body, while the broader base lies just behind the breastbone. Think of the heart as an inverted cone with apex down and base up, Within the thoracic cavity, the heart lies on the ventral side, with its apex resting on the diaphragm (Silverthorn et al. 2013).

<figure> <img src="/latex/images/anatomy/heart%20lies%20in%20the%20center%20of%20thorax.png" id="fig:apex_base" alt="Apex and base of heart (Silverthorn et al. 2013)" /><figcaption aria-hidden="true">Apex and base of heart <span>(Silverthorn et al. 2013)</span></figcaption> </figure>

Each cardiac cycle has two phases: diastole, the time during which cardiac muscle relaxes, and systole, the time during which the muscle contracts. The cardiac cycle can be divided into the five phases shown in the figure below:

(Atrial and ventricular diastole) At the moment during which both the atria and ventricles are relaxing, the atria are filling with blood from the veins, and the ventricles have just completed a contraction. As the ventricles relax, the atrioventricular (AV) valves between the atria and ventricles open. Blood flows by gravity from the atria into the ventricles. The relaxing ventricles expand to accommodate the entering blood (Silverthorn et al. 2013).
(Atrial systole) Most blood enters the ventricles while the atria are relaxed, but the last 20% of filling is accomplished when the atria contract and push blood into the ventricles. Atrial systole, or contraction, begins following the wave of depolarization that sweeps across the atria. The pressure increase that accompanies contraction pushes blood into the ventricles. During atrial systole, a small amount of blood is forced backward into the veins, because there are no one-way valves to block backward flow (Silverthorn et al. 2013).
At the end of atrial systole, the ventricles contain the largest volume they will hold during the cycle. This maximal volume is called the end-diastolic volume (EDV) (Silverthorn et al. 2013).
(Isovolumetric ventricle contraction) While the atria are contracting, the depolarization wave is moving slowly through the conducting cells of the atrioventricular node, then down the Purkinje fibers to the apex of the heart. Ventricular systole begins there as spiral bands of muscle squeeze the blood upward toward the base. Blood pushing against the underside of the AV valves forces the closed so that blood cannot flow back into the atria. Vibrations following the closure of the AV valves create the first heart sound. With both sets of AV and semilunar valves closed, blood in the ventricles has nowhere to go. Nevertheless, the ventricles continue to contract, which is similar to an isometric contraction, in which muscle fibers create force without movement. Such naming of phase is to underscore the fact that the volume of blood in the ventricle is not changing (Silverthorn et al. 2013).
While the ventricles begin to contract, the atrial muscle fibers are repolarizing and relaxing. When atrial pressure falls below that in the veins, blood flows from the veins into the atria again. Closure of the AV valves isolates the upper and lower cardiac chambers, meaning that atrial filling is independent of events taking place in the ventricles (Silverthorn et al. 2013).
(Ventricular ejection) As the ventricles contract, they generate enough pressure to open the semilunar valves and push blood into the arteries. The pressure created by ventricular contraction becomes the driving force for blood flow. High-pressure blood is forced into the arteries, displacing the low-pressure blood that fills them and pushing it farther into the vasculature. During this phase, the AV valves remain closed and the atria continue to fill. The heart does not empty itself completely of blood each time the ventricle contracts. The volume of blood left in the ventricle at the end of contraction is known as the end-systolic volume (ESV) (Silverthorn et al. 2013).
(Isovolumetric ventricular relaxation) At the end of the ventricular ejection, the ventricles begin to repolarize and relax. As they do so, ventricular pressure decreases. Once ventricular pressure falls below the pressure in the arteries, blood starts to flow backward into the heart. This backward of blood fills the cup-like cusps of the semilunar valves, forcing them together into the closed position. The vibration created by semilunar valve closure are the second heart sound. In this period the volume of blood in the ventricles is not changing.

<figure> <img src="/latex/images/anatomy/heart_cycle.png" id="fig:heart_cycle" alt="The heart cycles between contraction and relaxation (Silverthorn et al. 2013)." /><figcaption aria-hidden="true">The heart cycles between contraction and relaxation <span>(Silverthorn et al. 2013)</span>.</figcaption> </figure>

As the cardiac cycle begins with both atria and ventricles at rest, the electrocardiogram (ECG) begins with atrial depolarization. Atrial contraction starts during the latter part of the P wave and continues during the P-R segment. During the P-R segment, the electrical signal is slowing down, as it passes through the AV node and AV bundle. Ventricular contraction begins just after the W wave and continues through the T wave. The ventricles are repolarizing during the T wave, which is followed by ventricular relaxation. During the T-P segment, the heart is electrically quiet (Silverthorn et al. 2013).

<figure> <img src="/latex/images/anatomy/heart_cycle_ecg.png" id="fig:heart_cycle_ecg" alt="Correlation between an ECG and electrical events in the heart (Silverthorn et al. 2013)." /><figcaption aria-hidden="true">Correlation between an ECG and electrical events in the heart <span>(Silverthorn et al. 2013)</span>.</figcaption> </figure>

Silverthorn, Dee Unglaub, William C Ober, Claire W Garrison, Andrew C Silverthorn, and Bruce R Johnson. 2013. Human Physiology: An Integrated Approach. Vol. 3. Pearson Education Indianapolis, IN.

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Standring, Susan, Harold Ellis, J Healy, D Johnson, A Williams, P Collins, and C Wigley. 2005. “Gray’s Anatomy: The Anatomical Basis of Clinical Practice.” American Journal of Neuroradiology 26 (10): 2703.

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