Mechanical Heart Functioning

Mechanical heart function

Introduction to anatomy

The heart is a pump that beats 10,000 times per day. During a lifetime, the pumped volume equals that of three supertankers.

Heart location

The heart is located in the mediastinum, an area from the sternum to the vertebral column and between the lungs. The superior right point is at the superior border of the 3rd right costal cartilage; the superior left point is at the inferior border of the 2nd left costal cartilage, 3 cm to the left of the midline; the inferior left point is at the 5th intercostal space, 9 cm from the midline; the inferior right point is at the superior border of the 6th right costal cartilage, 3 cm from the midline.

Frontal view
Transversal view

Apex - directed anteriorly, inferiorly, and to the left. Base - directed posteriorly, superiorly, and to the right. Anterior surface - close to sternum and ribs. Inferior surface - rests on the diaphragm. Right border - faces right lung. Left border - faces left lung.

Surface anatomy

Sulci separate the grooves on the surface of the heart containing coronary blood vessels and fat:

• Coronary sulcus: Encircles the heart and marks the boundary between the atria and the ventricles.
• Anterior interventricular sulcus: Marks the boundary between the ventricles anteriorly.
• Posterior interventricular sulcus: Marks the boundary between the ventricles posteriorly.

Mechanical heart function

The pericardium allows the heart to move freely within its cavity. Each layer is composed of a single sheet of epithelial cells, known as mesothelium.

Outer layer: surrounds the heart. Continuous with the central tendon of the diaphragm, the fibrous pericardium is made of tough connective tissue and is relatively non-distensible. Its rigid structure prevents rapid overfilling of the heart but can contribute to serious clinical consequences.

Fibrous pericardium: Protects and anchors the heart, preventing overstretching (if pressure on the heart starts to increase, the heart tries to distend, and the fibrous pericardium stops it).

Serous pericardium: A thin, delicate membrane that contains:

• Parietal layer (outer layer): Lines the internal surface of the fibrous pericardium.
• Pericardial cavity: Contains pericardial fluid.
• Visceral layer (epicardium): Lubricates the heart to prevent friction.

The epicardium is the visceral layer of the serous pericardium. The myocardium is the cardiac muscle layer that forms the bulk of the heart. The endocardium lines the chambers and valves.

Cardiac tamponade (pathology)

Cardiac tamponade is caused by pericardial effusion (build-up of fluid inside the pericardium). This hampers cardiac filling (because the fibrous pericardium does not stretch), potentially leading to an emergency situation. Causes include pericarditis or myocardial rupture in the setting of acute myocardial infarction.

Chambers

The heart is divided into four chambers: two upper chambers called atria and two lower chambers called ventricles.

Right atrium

Receives blood from three sources: superior vena cava, inferior vena cava, and coronary sinus. The coronary sinus collects the majority of cardiac venous blood. It receives blood from the myocardium, a thick layer of muscle within the heart, and facilitates the movement of blood into the right atrium.

Interatrial septum: Partitions the atria from the ventricle. Here we find the fossa ovalis: a remnant of the fetal foramen ovale. If it remains open after birth, there's communication between the two atria, which could lead to various pathologies.

Tricuspid valve: Blood flows through into the right ventricle. It has three cusps composed of dense cartilage covered by endocardium.

Right ventricle

Forms most of the anterior surface of the heart. Papillary muscles are cone-shaped trabeculae carneae (raised bundles of cardiac muscle) that contract when the right ventricle is also contracting to help avoid backflow.

Chordae tendineae: Cords between valve cusps and papillary muscles. Interventricular septum: Partitions ventricles. Pulmonary semilunar valve: Blood flows into the trunk of the pulmonary artery.

Fossa ovalis: An oval depression on the septal wall of the atrium, corresponding to the situation of the foramen ovale in the fetus. It is situated at the lower part of the septum, above and to the left of the orifice of the inferior vena cava. This septum is less thick and is used by cardiologists to access the left atrium with catheters.

Left atrium

Receives blood from the lungs through four pulmonary veins (two right + two left). Around the valves of the pulmonary vein, there's often ectopic electrical activity causing atrial fibrillation.

Bicuspid valve: Blood passes through into the left ventricle. It has two cusps. To remember the names of this valve, use the mnemonic LAMB: Left atrioventricular, mitral, or bicuspid valve.

To prevent backflow through the mitral valve to the left atrium, the left ventricle contracts.

Left ventricle

Forms the apex of the heart. Chordae tendineae anchor the bicuspid valve to papillary muscles (it also has trabeculae carneae like the right ventricle). Aortic semilunar valve: Blood passes through the valve into the ascending aorta when the ventricle contracts; just above the valve are the openings to the coronary arteries.

To conclude the anatomy of the heart, we must mention:

• Ventricular myocardial band (Torrent-Guasp)
• Fibrous skeleton of the heart: Provides attachment for the leaflets and cusps of the valves. The myocardium is also attached. It acts as an electrical insulator between the atria and the ventricle and maintains patency of the AV and semilunar valves, preventing overdistension.

Valves

A-V valves open and allow blood to flow from atria into ventricles when ventricular pressure is lower than atrial pressure (a form of passive opening). This occurs when ventricles are relaxed, chordae tendineae are slack, and papillary muscles are relaxed. A-V valves close preventing backflow of blood into atria - occurs when ventricles contract, pushing valve cusps closed, chordae tendineae are pulled taut, and papillary muscles contract to pull cords and prevent cusps from reverting (prevents backflow from the ventricle to the atria).

Semilunar valves open with ventricular contraction - allow blood to flow into the pulmonary trunk and aorta. SL valves close with ventricular relaxation, preventing blood from returning to ventricles; blood fills valve cusps, tightly closing the SL valves.

Valves during systole and diastole

Phases of the heartbeat

• SL valves close "dub": AV valves open and filling of atria and ventricles begins.
• AV valves open; passive filling of ventricles, ventricular filling stage, and contraction of atria (atrial kick) contraction of ventricles.
• AV valves close "lub": SL valves open, and blood goes to lungs and body.

Heart murmur: The atria also contract, and this has hemodynamic significance because it contributes to the cardiac output, to the pump function of the heart. The atria contract and push some blood into the right ventricle and to the left ventricle before the right and left ventricles contract themselves.

It is called atrial kick (occurs as the atria contract prior to ventricular contraction, it contributes to the cardiac output. Atrial kick contributes 15-35% to the volume of blood in the ventricle. This extra volume in turn increases cardiac output by a similar 15-35%. Note: as we age, atrial kick tends to be a more significant contributor to cardiac output (closer to 35%).

Heart sounds

Auscultation is performed through a stethoscope. This technique listens for sounds of heartbeat coming from turbulence in blood flow caused by valve closure. The first heart sound (LUB) is created with the closing of the atrioventricular valves. We hear the tricuspid valve, and the ventricle starts to contract. The second heart sound (DUB) is created with the closing of semilunar valves. When the contraction is over, the pulmonary and aortic valves are closed in order to prevent backflow from the aorta and pulmonary artery.

Heart sound in a normal subject: LUB-DUB
At the end of the ejection phase, the semilunar valves (the aortic valve – here not visible – and the pulmonary valve) close. Heart sound BUB. When the ventricular contraction begins, the AV valves (mitral and tricuspid valves) close. Heart sound LUB.

Coronary arteries and veins

Coronary arteries

• Branches of aorta above aortic semilunar valve
• Left coronary artery is split into:
  • Circumflex branch (LCx): In coronary sulcus, supplies left atrium and left ventricle.
  • Anterior interventricular artery (LAD): Supplies both ventricles.
• Right coronary artery (RCA) is split into:
  • Marginal branch: In coronary sulcus, supplies right ventricle.
  • Posterior interventricular artery: Supplies both ventricles.

If there are some problems connected to the coronary artery, part of the heart has a lack of blood, and this phenomenon is called ischemia.

Coronary veins

Collects wastes from cardiac muscle. Drains into a large sinus on the posterior surface of the heart called the coronary sinus. The coronary sinus is important for the introduction of electrodes during electrophysiological studies. The coronary sinus empties into the right atrium.

Innervation of the heart

Information about blood pressure, generated by baroreceptors in the carotid sinuses and in the aortic arch, travels via afferent fibers in cranial nerves IX and X to the cardiovascular center of the autonomic nervous system in the brain stem. This center responds dynamically by adjusting efferent sympathetic outflow (travelling to the heart via the spinal cord) and adjusting efferent parasympathetic outflow (travelling to the heart via cranial nerve X).

Thus, heart rate and cardiac contractility are adjusted to counteract blood pressure changes. From the book: Autonomic innervation of the heart plays an important role in regulating cardiac function. The heart is innervated by parasympathetic (vagal) and sympathetic efferent fibers. The right vagus nerve preferentially innervates the sinoatrial (SA) node, whereas the left vagus nerve innervates the AV node; however, significant overlap can occur in the anatomical distribution. Atrial muscle is also innervated by vagal efferent; the ventricular myocardium is only sparsely innervated by vagal efferent. Sympathetic efferent nerves are present throughout the atria (especially in the SA node) and ventricles, and in the conduction system of the heart.

Vagal activation of the heart decreases heart rate (negative chronotropy), decreases conduction velocity (negative dromotropy), and decreases contractility (negative inotropy) of the heart. Vagal-mediated inotropic influences are moderate in the atria and relatively weak in the ventricles. Activation of the sympathetic nerves to the heart increases heart rate, conduction velocity, and inotropy. Sympathetic influences are pronounced in both the atria and ventricles.

Contraction mechanism

The contraction mechanism is one of the determinants of the mechanical heart functions. Cardiac functions are determined by different factors:

• Contraction mechanism
• Excitation-contraction: Calcium influences on the contraction.
• Electrical activity
• Neurohormonal systems: They exert their influence on contractility.
• Cardiovascular interactions & loading conditions: These are pre-load and after-load, so how will the heart fill, and which pressure will the heart see when it pumps the blood into the aorta and pulmonary arteries. These are all together called the loading conditions, and they have an important impact.