EKG: Heart Anatomy and Physiology

Heart Anatomy and Physiology: Overview

Focus topic: Heart Anatomy and Physiology

The human heart is a hollow, cone-shaped, muscular organ roughly the size of its owner’s fist that weighs approximately 9 to 12 oz (250-350 g) (Size of the human heart). The heart measures about 5 in (13 cm) long from base to apex, 3.5 in (9 cm) wide (at the base or top of the heart), and 2.5 in (6 cm) thick. It comprises approximately 0.45% of a man’s and about 0.40% of a woman’s body weight. Variances that can be noted would be an athlete whose heart would most likely weigh more than someone who does not exercise regularly, a person with heart disease whose heart has increased in size and mass (hypertrophy), and an elderly individual whose heart would weigh less than someone in early adulthood. A person’s heart size and weight are influenced by body weight, physical build, exercise regimen, age, sex, and illnesses such as heart disease, scoliosis, and diabetes.

Heart Anatomy and Physiology: Size of the human heart

Focus topic:  Heart Anatomy and Physiology

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The heart is located in the chest slightly to the left of the breastbone. The heart resides in a space behind the sternum and in front of the spine between the lungs called the mediastinum. It sits above the diaphragm and tilts like an inverted triangle with two thirds of the heart lying to the left of the sternum. The remaining third of this organ extends into the right side of the sternum.

The top of the heart (the part closest to the head) is the base. The major portion of the base, which lies below the second rib, is established by the left atrium. The right atrium contributes a smaller portion of this base. The base is where the great vessels, including the aorta and the pulmonary artery, are attached to the heart. The heart lies in front of the esophagus and the trachea and the descending portion of the aorta lies behind this portion of the heart. The lower portion of the heart (the point of the triangle) is the apex and consists of the left ventricle which tilts down and forward, toward the left side of the body. It is positioned between the fifth and sixth ribs in the left midclavicular line and rests superior to or above the diaphragm (Position of the heart in the chest).

Heart Anatomy and Physiology: Position of the heart in the chest

Focus topic: Heart Anatomy and Physiology

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The position of the heart can vary considerably depending on body build, size and shape of the chest, and the level of the diaphragm. The heart of tall, slender people tends to hang vertically and be positioned centrally. Short, stocky people tend to have hearts that lie more left and horizontally. The infant heart lies in a more horizontal position until the age of approximately 7. The apex of the heart in the infant population is positioned at the fourth intercostal space.

Clinical Alert

The function of the heart is to pump blood through the network of arteries and veins called the cardiovascular system.

Heart Anatomy and Physiology: Heart Layers

Focus topic: Heart Anatomy and Physiology

The heart’s wall is made up of three tissue layers: endocardium, myocardium, and epicardium (Layers of the Heart Wall).

Heart Anatomy and Physiology: Layers of the Heart Wall

Focus topic: Heart Anatomy and Physiology

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  • Endocardium: Innermost layer of the heart wall made up of a thin layer of endothelial cells and connective tissues. This smooth endocardium lines the heart’s inner chambers and valves and is continuous with the interior layer of the arteries and veins. Capillaries, the connection between the arteries and veins in the body, are made up of one thin layer of endothelial cells. This allows for ease of movement of oxygen and other crucial elements to move across the membrane to nourish the cells and for waste products such as carbon dioxide to pass into the vascular space to be carried to their exit points.
  • Myocardium: Thick, muscular, middle layer of the heart wall consisting of cardiac muscle fibers which are found only in the heart. The myocardium is the largest portion of the heart’s wall and is responsible for the pumping action of the heart because this layer of muscle tissue contracts with each heartbeat. The muscle fibers in this layer are laid out in a spiral fashion which assists in the work of this muscle by creating a twisting or spiral motion that enhances each contraction. This is called the myocardial vortex. The myocardium is subdivided into two areas: the subendocardial area and the subepicardial area.
  • Subendocardial area: Innermost section of the myocardium
  • Subepicardial area: Outermost section of the myocardium

Clinical Alert

The myocardial layer of the ventricles is much thicker than that of the atria because the ventricles pump blood to the lungs and the rest of the body whereas the atria encounter little resistance when pumping blood through the valves to the ventricles. The left myocardium, which pumps blood against greater resistance to feed the arteries of the body as it exits the aorta, is also thicker than the right ventricle which propels blood into the nearby pulmonary system presenting a lesser degree of resistance.

  • Epicardium: Outermost layer of the heart wall formed by squamous epithelial cells and connective tissue containing nerve fibers, blood vessels, lymph capillaries, and fat. This is also the visceral layer of the serous  pericardium, establishing a continuum with the inner lining of the pericardial sac. The main coronary arteries are located on this layer. These arteries receive oxygenated blood first prior to entering the myocardium itself which then supplies the inner layers with oxygen rich blood.

Clinical Alert

The heart’s endocardial area is at the greatest risk for ischemia (decreased supply of oxygenated blood to an organ or body part). This area is fed by the distal branches of the coronary arteries that lie on the epicardial surface of the heart. It has excessive requirements for oxygen.

A layer of connective tissue called the pericardium, which is a tough, double-walled sac that protects the heart from injury and infection, encloses the heart. The pericardium consists of the outer fibrous pericardium and the inner serous pericardium.

  • Fibrous pericardium: Tough fibrous tissue that serves as the outer, protective layer. This layer fits around the heart in a loose fashion which allows the heart to expand with each heartbeat, but contains it within a specific area preventing expansion beyond acceptable limits. The fibrous pericardium anchors the heart to surrounding structures, such as the diaphragm and sternum with ligaments and prevents the heart from moving around excessively in the chest.
  • Serous pericardium: Thin, smooth, inner portion of the pericardium that has two layers. The inner layer is called the parietal layer. This layer lines the inner portion of the fibrous pericardium described above. The visceral layer lies on the outermost portion of the heart itself and is contiguous with the epicardium (Layers of the heart’s wall).

Heart Anatomy and Physiology: Layers of the heart’s wall

Focus topic: Heart Anatomy and Physiology

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Heart Anatomy and Physiology: Pericardial Space

Focus topic: Heart Anatomy and Physiology

Between the parietal and visceral layers of the serous pericardium is an area called the pericardial space. This space contains roughly 20 mL of thin, clear fluid secreted by the serous layers—called pericardial fluid—that acts as lubricant and cushion to prevent friction each time the heartbeats. Situations can occur in which too much pericardial fluid is present, blood is present within the space, or an exudate exists within the space leading to disruptions to the heart’s function.

  • Pericarditis: Inflammation of the pericardium that results in an excess of pericardial fluid production. This can occur due to rheumatoid arthritis, systemic lupus, a heart attack that destroys the heart muscle, viral or bacterial infection, or heart surgery.
  • Pericardial effusion: A condition where blood, a purulent exudate, and/or pericardial fluid builds up in the pericardial space compressing the heart and interfering with the heart’s ability to relax and fill with blood between heartbeats.
  • Cardiac tamponade: A life-threatening condition in which the extra fluid or blood within the pericardial space creates a situation in which the heart is restricted and cannot fill adequately with blood. This results in a decreased amount of blood which the ventricles are able to pump out to the body. If a decreased amount of blood is forced out through the arterial system (cardiac output), this will in turn lead to a decreased amount which can return to the heart through the venous system. This obstruction of blood flow results in an emergent condition which must be treated immediately. This can occur quickly from a traumatic event or can be produced over a longer period of time from disease processes such as malignancies or kidney disease. A pericardial tamponade associated with trauma can produce a life threat with as little as 100 mL of blood in the pericardial sac while a chronic situation can increase the fluid content to as much as 1000 mL with no significant consequences due to the ability of the pericardium to stretch over a period of time. The risk of death is dependent on the rate of the increase in fluid or blood within the space and the capacity of the sac to adjust to the increasing fluid buildup.

Pericardial effusion and cardiac tamponade are managed by a procedure called pericardiocentesis where a needle is inserted into the pericardial space and the excess fluid is removed (aspirated) through the needle. Pericarditis is treated with medications.

Clinical Alert

Pericardiocentesis is performed by a physician or mid-level provider who has specialized training, but, requires the entire team who is responsible for, among other things, watching the electrocardiogram during the process for both rhythm and EKG changes. In the emergent situation of trauma, removal of as little as 5 mL of blood can save the patient’s life.

Heart Anatomy and Physiology: Heart Chambers

Focus topic: Heart Anatomy and Physiology

The heart consists of four major chambers—two atria and two ventricles—that are made of cardiac muscle. The right and left atria are receiving cavities that function as reservoirs prior to blood being pumped into the ventricles. The right atrium receives deoxygenated blood that has already traveled around the body and is returning to the heart via the inferior and superior vena cavae. This deoxygenated blood also returns from the heart itself through the coronary sinus. Four pulmonary veins direct newly oxygenated blood into the left atrium. These two atria are separated by the interatrial septum which assists in the force of contraction of the atria as they pump blood into the ventricles.

Cardiac cells that make up the heart muscle communicate with each other through the exchange of nutrients and other substances such as anions, cations, and metabolites. The cardiac muscle contracts as a result of calcium that travels from the interstitial fluid surrounding the heart’s cells into the heart’s muscle fibers, thus creating an electrical impulse that is quickly conducted throughout the wall of a heart chamber. Because of this, calcium plays an important role in the force of contraction. Without available calcium, the heart muscle remains in a relaxed state.

The “pumps” of the heart are the right and left ventricles and aid in blood flow through the heart. The right ventricle receives deoxygenated blood from the right atrium and pumps it into the pulmonary system by way of the pulmonary artery which branches into the right and left lungs, where it exchanges carbon dioxide for oxygen. This is the only time that an artery carries unoxygenated blood. The left atrium sends oxygenated blood to the left ventricle. The left ventricle pumps the now oxygenated blood out through the aorta to nourish the body. The ventricles are aided in their pumping action by the interventricular septum that separates the two distinct ventricles (The four chambers of the heart).

Heart Anatomy and Physiology: The four chambers of the heart

Focus topic: Heart Anatomy and Physiology

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Clinical Alert

The pumping action of the heart can be affected by many things including disease processes of the heart muscle, valves, or conduction system.

Heart Anatomy and Physiology: Heart Valves

Focus topic: Heart Anatomy and Physiology

The heart has four valves: two atrioventricular (AV) valves and two semilunar (SL) valves (Heart valves). The AV valves are the tricuspid valve on the right side of the heart and the mitral or bicuspid valve on the left side. The SL valves are the pulmonic and aortic valve located in each of these great vessels of the heart (the pulmonary artery and the aorta). Valves ensure that blood flows in a forward direction through the heart’s chambers, therefore, no backflow occurs. The valves open and close in response to the pressure in the corresponding heart’s chambers to which they connect. When the valves close, they prevent the blood from flowing backwards (called regurgitation) into the chamber. The heart sounds (“lub-dub” sounds) that are heard through a stethoscope are created by the closing of the heart valves.

Heart Anatomy and Physiology: Heart valves

Focus topic: Heart Anatomy and Physiology

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  • Atrioventricular valves: Controls flow of blood between the atria and their corresponding ventricles. These valves consist of tough, fibrous rings, cusps (also called leaflets or flaps) of endocardium, chordae tendineae, and papillary muscles. Pressure gradients control the opening and closing of these valves, propelling blood in a forward direction creating the opening and closing when then gradient pushes backward. The backward pressure gradient does not have to be great in order for the valves to close. A fibrous skeleton is present in the heart that includes thick connective tissue at the valves and also helps to shape and separate the atria from the ventricles. The AV valves have associated chordae tendineae, strands of connective tissue, which have the appearance of parachute lines. These strands are then attached to myocardial tissue called papillary muscles that extend into the ventricular floors. When the ventricle contracts the papillary muscles pull on the chordae tendineae which keep the valvular flaps from pulling back into the atrial chamber. Due to the appearance of the chordae tendineae, they are sometimes referred to as “heart strings” ((a) The tricuspid and (b) mitral (bicuspid) valves).

(a) The tricuspid and (b) mitral (bicuspid) valves

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  • Tricuspid valve: Lies between the right atrium and right ventricle. The tricuspid valve consists of three separate cusps and is thinner than the mitral valve.
  • Mitral valve: Lies between the left atrium and left ventricle. The mitral valve (also called bicuspid valve) has two cusps and takes its name from the fact that it bears the shape of a bishop’s hat called a mitre when it is open.
  • Semilunar valves: Located in the aorta and pulmonary artery to avoid back-flow of blood back into the ventricles. These valves are shaped like half-moons (therefore the name “semilunar”), have smaller openings than the AV valves, and have smaller, thicker cusps than the AV valves. The SL valves have three cusps. These valves do not have associated chordae tendinae or papillary  muscles. As ventricular contraction occurs, these valves open allowing blood to flow out of the ventricle into the connecting vessel. The SL valves close at the end of ventricular contraction when the pressure in the pulmonary artery and aorta exceeds that of the ventricles and pushes the cusps closed.
  • Pulmonic valve: Located where the pulmonary artery and right ventricle meet. The pulmonic valve allows deoxygenated blood to flow from the right ventricle into the pulmonary arteries without allowing blood to flow backwards into the right ventricle.
  • Aortic valve: Located where the left ventricle and aorta meet. The aortic valve allows freshly oxygenated blood to flow into the aorta and out to the remainder of the body without back-flow of blood into the left ventricle.

A malfunctioning heart valve can disrupt the flow of blood through the heart. When this happens a murmur can be heard. Types of valvular heart disease as a result of a malfunctioning of heart valve include:

  • Valvular regurgitation: Blood flows back into the heart chambers when a heart valve does not close properly. For instance, blood would flow back into the right atrium from the right ventricle if the tricuspid valve did not have complete closure. Other terms for this problem are valvular incompetence or valvular insufficiency.
  • Valvular stenosis: A stenosed valve occurs when a valve thickens, constricts, or for some reason becomes rigid. This can cause the heart to work harder to pump blood through the narrower or less flexible valve.
  • Valvular prolapse: A valvular prolapse occurs when a valve cusp inverts. This typically occurs when one valve cusp is larger than the other(s). Elongated or ruptured chordae tendineae can also cause valvular prolapse.

Clinical Alert

Rupture of papillary muscles, which can be caused by an acute myocardial infarction, can cause incomplete closure of the valves resulting in regurgitation or prolapse of the valve. When rupture of the papillary muscles occurs, cardiac output can be decreased and place the patient into potentially fatal complications. Emergency surgery may be necessary.

Heart Anatomy and Physiology: Heart Sounds

Focus topic: Heart Anatomy and Physiology

As the heart contracts and relaxes and blood flow increases and decreases within the chambers and the valves open and close, vibrations are created. These vibrations that occur in the heart tissue as the valves close create heart sounds.

  • During one heartbeat cycle (“lub-dub”), the heart’s ventricles contract (called ventricular systole) and relax (called ventricular diastole). More specifically, during ventricular systole, the atria relax (atrial diastole) and fill with blood while the mitral and tricuspid valves close. This closure of the mitral and tricuspid valves causes the first heart sound (“lub”). This first heart sound is referred to as S1. The aortic and pulmonic valves are then forced open by a rise in ventricular pressure. This rise in ventricular pressure causes the ventricles to contract, and blood flows through the circulatory and pulmonic systems. During diastole, the ventricles relax while the atria contract, which forces blood through the open tricuspid and mitral valves. The aortic and pulmonic valves close during this time causing the second heart sound—S2—“dub” (Opening and closing of the heart valves creating heart sounds). It is thought that the actual creation of the sounds occurs due to turbulence of the blood flow and heart wall motion.
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Heart Anatomy and Physiology: Extra Heart Sounds

Focus topic: Heart Anatomy and Physiology

A third heart sound (S3), occurs at the beginning of diastole after S2 (ventricular filling). S3 is not caused by valve closure, but instead is caused by blood moving quickly back and forth between the walls of the ventricles due to blood rushing in from the atria. S3 is lower in pitch than S1 or S2. A third heart sound occurring in youth is benign, but is considered abnormal in persons older than 30 to 40 years of age and is frequently associated with heart failure or a mitral valve regurgitation. An S1-S2- S3 sequence sounds like “lub-dub-ta”, “slosh-ing-in” or “Ken-tuck-y” and is referred to as a ventricular gallop or gallop rhythm.

A fourth heart sound (S4) is rare but when heard in an adult is referred to as a presystolic gallop or atrial gallop. S4 sounds like “ta-lub-dub”, “Ten-es-see”, or “a-stiff-wall” and is a sign of a failing left ventricle possibly due to a stiffening or enlargement of the wall. The sound occurs after atrial contraction at the end of diastole and immediately before S1. The sequence with this gallop rhythm is S4-S1-S2.

Clinical Alert

The combination of an S3 and S4 is called a “Hello-Goodbye” gallop.

Heart Anatomy and Physiology: Opening and closing of the heart valves creating heart sounds

Focus topic: Heart Anatomy and Physiology

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