NCLEX: Heart Anatomy and Physiology

Heart Anatomy and Physiology: Heart Nerves

Focus topic: Heart Anatomy and Physiology

The sympathetic and parasympathetic nervous systems that make up the autonomic nervous system are part of the heart’s physiology. This system does not cause the initiation of the electrical impulses within the cardiac tissue, but, can strongly impact the overall function. The sympathetic nervous system (also called the adrenergic nervous system) supplies norepinephrine and epinephrine which prepares the body to function under stress (“fight-or-flight” response).

When this system is activated, the following results can be expected related to heart function:

  • Increase in heart rate (chronotropic response)
  • Increase in contractility (inotropic response)
  • Increase in blood pressure through constriction of the blood vessels
  • Increase in cardiac output
  • Increase in speed of conduction
  • Increase in blood flow to the tissues
  • Increase in blood flow to organs necessary for survival (heart, lungs, brain)

Other systems throughout the body are also impacted when the sympathetic nervous system is stimulated. Some of these are:

  • Increase in size of the bronchi to enhance oxygenation
  • Increase in release of stored energy to supply the brain with glucose and muscles with fatty acids
  • Increase in sweating
  • Increase in size of pupils

The parasympathetic nervous system (also called the cholinergic nervous system) releases the chemical acetylcholine that slows heart rate, decreases automaticity of the AV node, decreases conduction of impulses through the AV node, decreases the force of contraction of the atria, and can mildly decrease the strength of ventricular contractions. The parasympathetic nerve that influences the heart is the vagus nerve. Baroreceptors, specialized nerve cells located in the aortic arch and the carotid arteries, also stimulate the vagus nerve, which then activates the parasympathetic response. These baroreceptors are sensitive to changes in vascular tone and blood pressure. As with the sympathetic nervous system, other organs in the body are also affected by the parasympathetic system including vascular dilation and pupillary constriction. The parasympathetic nervous system conserves and restores body resources (“feed-and-breed” or “rest and digest” response).

Clinical Alert

A maneuver called carotid sinus massage is sometimes performed in situations of rapid heartbeat in an intentional attempt to stimulate the baroreceptors, activate the vagus nerve, and slow the heart. This is known as a vagal maneuver and is performed by the physician or provider.

Heart Anatomy and Physiology: Blood Flow Through the Heart

Focus topic: Heart Anatomy and Physiology

The right atrium receives deoxygenated blood (low in oxygen and high in carbon dioxide) from the superior (from the head, neck and, upper extremities) and inferior (from the lower body) vena cavae and the coronary sinus, the largest vein that drains the heart. Blood flows through the tricuspid valve from the right atrium into the right ventricle. The right ventricle then contracts causing the tricuspid valve to close. The right ventricle propels blood through the pulmonic valve into the main pulmonary artery which then branches off to become the right and left pulmonary arteries. The deoxygenated blood reaches the lungs where oxygen and carbon dioxide are exchanged. The now oxygen-rich blood flows from the lungs through the four pulmonary veins into the left atrium. This completes the circuit called pulmonary circulation. Blood flows from the left atrium through the mitral valve (bicuspid) into the left ventricle. The left ventricle contracts and the mitral valve closes. Blood travels through the aortic valve from the left ventricle to the aorta. The aorta and its branches deliver blood throughout the body (systemic circulation) (Blood Flow Through the Heart). Blood also flows through the ostium, located near the aortic valve, into the coronary circulation. This occurs mainly during ventricular diastole when the aortic valve is closed and the ostium is more pronounced as it is partially hidden when the valve is in an open position (Blood flow through the heart).

Clinical Alert

In rapid heartbeat (tachycardia) situations, the resting period (diastole) for the ventricle is shortened. This can create a problem with a decrease in coronary blood flow as less oxygenated blood is allowed to enter these arteries. Also, the increase in contractions causes the vessels on the heart to be compressed adding to the decrease in coronary blood flow.

Heart Anatomy and Physiology: Blood Flow Through the Heart

Focus topic: Heart Anatomy and Physiology

  • Blood low in oxygen and high in carbon dioxide travels from the body to the superior and inferior vena cavae and the coronary sinus into the right atrium.
  • Blood flows from the right atrium through the tricuspid valve into the right ventricle.
  • The right ventricle contracts and the tricuspid valve closes.
  • The right ventricle expels the deoxygenated blood through the pulmonic valve into the right and left pulmonary arteries.
  • Blood travels to the lungs via the pulmonary arteries.
  • Oxygen and carbon dioxide are exchanged in the lungs.
  • The oxygenated blood travels from the lungs to the left atrium via four pulmonary veins (two from the right lung and two from the left lung).
  • Blood flows from the left atrium through the mitral (bicuspid) valve into the left ventricle.
  • The left ventricle contracts and the mitral valve closes.
  • Blood leaves the left ventricle through the aortic valve and travels to the aorta.
  • Blood reaches the body through the aorta and the coronary arteries via the ostium.

Heart Anatomy and Physiology: Blood flow through the heart

Focus topic: Heart Anatomy and Physiology

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Heart Anatomy and Physiology: Coronary Circulation

Focus topic: Heart Anatomy and Physiology

The heart itself must also be nourished with oxygen rich blood and have an avenue available to remove waste products such as carbon dioxide. This is performed through the coronary circulation. As oxygenated blood passes through this system, approximately 60% to 75% of the oxygen within the blood stream is extracted as it passes through the heart. This is the highest amount of extracted oxygen for any organ and demonstrates the extreme importance of oxygen to cardiac tissue. When the demand for oxygen increases, there must be an increase in coronary artery blood flow in order to meet the needs of the myocardium. This coronary blood flow accounts for 4% to 5% of the total cardiac output or about 250 mL/min. The rate of blood flow through the entire cardiovascular system is about 5000 mL/min.

Clinical Alert

Coronary artery disease (CAD) occurs when there is a greater than 50% narrowing of the diameter in any of these major coronary arteries.

The coronary arteries lie on the surface (epicardium) of the heart and supply the heart muscle with blood and oxygen. The opening for the coronary arteries is known as the coronary ostium located near the aortic valve. The right and left coronary arteries branch off from this initial opening at the base of the aorta. From here these arteries then branch off into smaller arteries that extend into the heart’s muscle mass and supply it with blood. This network of smaller arteries is called collateral circulation. Even when the major coronary arteries become clogged with plaque, the collateral circulation continues to supply blood to the heart. The major coronary arteries are the right coronary artery (RCA) and the left coronary artery (LCA) which divides into the left anterior descending (LAD) artery and the circumflex (CX) artery (Major Coronary Arteries).

Heart Anatomy and Physiology: Major Coronary Arteries

Focus topic: Heart Anatomy and Physiology

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Heart Anatomy and Physiology: The coronary arteries and veins

Focus topic: Heart Anatomy and Physiology

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These arteries also supply various portions of the conduction system which will be discussed in Chapter 3.

The coronary veins travel alongside the arteries. Cardiac veins collect deoxygenated blood from the capillaries of the myocardium. This venous system includes the great cardiac vein, the middle cardiac vein, and the left marginal vein which empties into the coronary sinus. Eighty percent of returning deoxygenated blood returns through the coronary sinus. This large coronary sinus lies in the area between the atria and the ventricles and drains directly into the right atrium. The other 20% of blood returning to the circulation empties by way of multiple smaller veins that have direct pathways to the right atrium and ventricle (The coronary arteries and veins).

Heart Anatomy and Physiology: Cardiac Cycle

Focus topic: Heart Anatomy and Physiology

The cardiac cycle that is responsible for the blood flow through the heart consists of the systolic and diastolic phases (Cardiac Cycle). Systole occurs in both the atria and ventricles and is the period during which the chambers are contracting and projecting blood. Diastole also occurs in both the atria and the ventricles and is the phase of relaxation when the heart’s chambers fill with blood. Ventricular diastole is the phase when the myocardium receives its fresh supply of oxygenated blood from the coronary arteries.

The cardiac cycle is propelled by a pressure relationship where blood flows from one heart chamber to another from higher to lower pressure. In diastole, the pressure in the heart chambers decreases while it increases in systole. The heart’s valves keep the blood moving forward. Precise timing of the contractions is an important aspect regarding the pressure relationships. The timing of atrial and ventricular systole is dependent upon the heart’s conduction system that controls the nerve pathways and electrical impulses.

Heart Anatomy and Physiology: Cardiac Cycle

Focus topic: Heart Anatomy and Physiology

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The atria work in concert with each other and the ventricles also are working synchronously during the cardiac cycle. Each cardiac cycle takes about 1 second to complete. The four stages (Phases of the cardiac cycle) of the cardiac cycle are:

1. Ventricular Filling: During ventricular filling, two activities occur. First, the pressure in the ventricles becomes less than the atria and the AV valves (tricuspid and mitral) open to allow blood to enter the ventricles. Initially there is a rapid filling followed by a slower period of movement of blood volume from the atria into the ventricles. This second slower filling period is known as diastasis. At the end of diastasis, the atria contract (atrial systole) providing “atrial kick” which propels an extra 10% to 30% of blood into the ventricles. This amount of blood that is now in the ventricles is known as the end-diastolic volume (EDV). This is different in individuals but is usually approximately 130 mL.

2. Isovolumetric ventricular contraction: Pressure in the ventricles increases due to ventricular depolarization and causes the mitral and tricuspid valves to close. During this phase the pulmonic and aortic valves remain closed due to a higher amount of pressure in the aorta and pulmonary vessels, therefore, no blood is being ejected into the systems at this time. The term isovolumetric is used to help describe this phase because no change in ventricular volume occurs. During this phase, the atria are in a relaxed state (atrial diastole).

3. Ventricular ejection: The aortic and pulmonic valves open in response to ventricular pressure that exceeds that of the aortic and pulmonary arterial pressure. The SV valves (aortic and pulmonic) open and the ventricles eject blood at this stage (ventricular systole). The entire end-diastolic volume (approximately 130 mL) is not expelled. About 70 mL is ejected during this phase and is known as the stroke volume. The percentage of the amount of ejected blood is known as the ejection fraction which should be at least 55%. The blood that remains in the ventricle at the end of this phase is known as the end-systolic volume (ESV).

Clinical Alert

An echocardiogram is one way to assess cardiac contractile functionality. This testing modality provides information regarding performance of the cardiac valves, levels of different chamber volumes, and contractile malfunctions. A clinical measurement obtained from this piece of equipment is called the ejection fraction as listed above. Ejection fractions (EF) can range from 55% to 80%. This is calculated from the stroke volume (SV) and end-diastolic volume (EDV)(EF = SV divided by EDV). Exercise can increase ejection fraction up to 90% in some individuals. Cardiac disease will produce a decreased ejection fraction. EF measuring less than 55% indicate a decline in the contractile strength of the myocardium.

4. Isovolumetric relaxation: The aortic and pulmonic valves close in response to a decrease in ventricular pressure below that of the aorta and pulmonary artery (ventricular diastole). All valves are closed at this stage, atrial diastole takes place, and blood fills the atria. This phase is also known as isovolumetric since no blood is passing through any valves again.

Clinical Alert

Loss of atrial kick can seriously diminish the amount of blood pumped out into the vascular system with each cardiac cycle (cardiac output). If less blood is pumped into the ventricles, less blood will be available to be ejected into the systemic circulation. This can occur with certain heart rhythms such as atrial fibrillation.

Heart Anatomy and Physiology: Phases of the cardiac cycle

Focus topic: Heart Anatomy and Physiology

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Heart Anatomy and Physiology: Cardiac Output

Focus topic: Heart Anatomy and Physiology

The term cardiac output is used to describe the amount of blood pumped into the systemic circulation through the aorta each minute. This is measured by multiplying the stroke volume (SV) (amount of blood ejected from a ventricle with each contraction) times the heart rate. Cardiac output in most adults is between 4 and 8 L/min. Decreased cardiac output can place individuals at risk for life threatening problems and can occur for a variety of reasons. Symptoms that can be noted are listed in Manifestations of Decreased Cardiac Output.

Heart Anatomy and Physiology: Manifestations of Decreased Cardiac Output

Focus topic: Heart Anatomy and Physiology

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Heart Anatomy and Physiology: Stroke Volume

Focus topic: Heart Anatomy and Physiology

Preload, afterload, and myocardial contractility affect stroke volume and must be in balance for optimal cardiac output to occur.

  • Preload: The amount of force placed on the walls of the ventricles causing the cardiac muscle to stretch. This is established by the pressure exerted and the volume of blood present within the left ventricle at the end of its resting phase (diastole). It is affected by the amount of blood returning to the right atrium; therefore, an increase or decrease in preload can be appreciated with increases and decreases in blood volume. Larger volumes will stretch the cardiac fibers prior to contraction allowing the ventricles to eject the increased volume through an increase in the force of contraction. In a normal heart, the greater the preload, the greater the force of ventricular contraction and the greater the stroke volume, resulting in increased cardiac output. The heart adjusts its pumping capacity in response to venous return. This is important when individuals are exercising because it allows the fibers to stretch before contracting. It can also be stretched beyond normal limits at which time it could produce a decrease in cardiac output. This could occur with an overload of volume causing a failing of the work of the heart.
  • Afterload: The pressure against which the ventricles must work to pump blood. Three factors affect afterload—arterial blood pressure, arterial resistance, and the ability of the arteries to stretch. Lower resistance means that blood flow is more easily ejected. When resistance is increased, the heart must work harder to eject blood. This increase in resistance can occur with increased viscosity (thickness) of the blood, increased blood pressure, or a rigid (stenosed) aortic valve. Pulmonary disorders such as chronic obstructive pulmonary disease can cause scarring within the lung fields that can increase the resistance against which the right ventricle must work. This causes an increased afterload for the right side of the heart as well.
  • Contractility: The ability of cardiac muscle cells to respond to stimulation causing contraction. This is not the tension itself, but, rather the responsiveness that occurs within the cardiocytes (heart cells). This occurs after depolarization (response of a myocardial cell to an electrical impulse that triggers myocardial contraction). The amount the muscle fibers are stretched at the end of diastole affects contractility and the volume of blood pumped out of the ventricles. Too much or too little stretch affects the actual SV that is delivered. A term used when discussing contractility is “inotropic”. Positive inotropes will increase contractile strength and negative inotropes will decrease contractility. Calcium is a positive inotropic agent. It increases the strength of contraction. Patients with a low calcium level will not have good contractile strength. However, in extremely high levels of calcium, cardiac arrest can occur. Negative inotropic effects can occur with many things including high levels of potassium.

Heart Anatomy and Physiology: Heart Rate

Focus topic: Heart Anatomy and Physiology

The second piece of the formula for cardiac output is heart rate. This is the chronotropic response. Things that increase heart rate have positive chronotropic responses and those that decrease heart rate have negative chronotropic reactions. Both high and low pulse rates will have a strong bearing on cardiac output. Sympathetic stimulation, low levels of calcium, and increased levels of thyroid hormone or caffeine will produce positive chronotropic effects (fast heart rate). Parasympathetic stimulation, high levels of calcium and potassium as well as decreased levels of potassium will cause negative chronotropic effects (slow heart rate). Infants respond to a decrease in cardiac output with fast heart rates (tachycardia) because they have fixed stroke volumes. Therefore, the only way for them to attempt to increase their cardiac output is to increase their heart rates.

Heart Anatomy and Physiology: Blood Pressure

Focus topic: Heart Anatomy and Physiology

Blood pressure is the pressure of circulating blood that is exerted on the walls of the arteries and is a principal vital sign. Blood pressure refers to the arterial pressure of the systemic circulation. At each heartbeat, blood pressure changes due to varying systolic and diastolic pressures that is maintained by the pumping action of the heart. It is equal to cardiac output multiplied by peripheral vascular resistance. Peripheral vascular resistance is the resistance to the flow of blood. Two factors influence this resistance—blood vessel diameter and the tone (balanced tension) of the vascular musculature. Therefore, any condition that changes either cardiac output or peripheral vascular resistance affects blood pressure. An increase in blood pressure is due to an increase in cardiac output or peripheral vascular resistance, whereas a decrease in blood pressure is due to a decrease in cardiac output or peripheral vascular resistance.

Clinical Alert

When blood pressure drops below 80 mm Hg (systolic), decreased cardiac output can cause a deficiency in the coronary artery blood flow. This can produce a lack of oxygenated blood to cardiac tissue and dysrhythmias may occur.

Blood pressure is expressed in terms of the systolic pressure over diastolic pressure and is measured in millimeters of mercury (mm Hg), for example, 118/68 mm Hg. Ideal blood pressure for adults is 90 to 119 mm Hg systolic and 60 to 79 mm Hg diastolic. Blood pressure may be affected by a person’s age, weight, medications, life-style choices, medical conditions, nutrition, exercise habits, and environmental factors, such as responses to stress. Hypertension refers to arterial pressure that is abnormally high, as opposed to hypotension, which is abnormally low blood pressure.

Clinical Alert

Risk factors for developing high blood pressure are family history, advanced age, gender-related risk patterns, lack of physical activity, poor diet (especially one that includes too much salt), overweight and obesity, and consumption of too much alcohol. Possible contributing factors include stress, smoking and second-hand smoke, and sleep apnea.

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Heart Anatomy and Physiology: Conclusion

Focus topic: Heart Anatomy and Physiology

The heart acts as a pump that moves blood through the circulatory system. Note these key points about the heart:

  • The human heart is a hollow, cone-shaped, muscular organ, roughly the size of its owner’s fist that weighs between 250 and 350 g (roughly 9-11 oz).
  • The heart is located in the chest slightly left of the breastbone in a space called the mediastinum.
  • The heart’s wall is made up of three tissue layers: endocardium, myocardium, and epicardium.
  • 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 serous fluid—called pericardial fluid—that acts like a lubricant and cushion to prevent friction as the heartbeats.
  • The heart contains four chambers—two atria and two ventricles—that are made of cardiac muscle and act as two separate pumps that work in concert with each other.
  • The heart has four valves: two atrioventricular valves (tricuspid valve and mitral valve) and two semilunar valves (aortic valve and pulmonic valve). These valves ensure that blood flows in a forward direction through the heart’s chambers without creating a backflow of blood.
  • The heart has two normal heart sounds (S1 and S2) that can be heard as the heart valves close, and they sound like “lub-dub.” Extra heart sounds are S3 and S4.
  • The sympathetic and parasympathetic nervous systems that make up the autonomic nervous system contribute to the heart’s function.
  • Blood flow through the heart consists of pulmonary and systemic circulations.
  • The cardiac cycle that is responsible for the blood flow through the heart consists of the systolic and diastolic phases that occur in four stages.
  • Cardiac output is equal to stroke volume times the heart rate.
  • Three factors affect stroke volume—preload, afterload, and contractility.
  • The heart has its own vascular system which provides oxygenated blood to the coronary arteries.
  • An echocardiogram provides information related to contraction of the heart muscle.
  • Blood pressure is equal to cardiac output multiplied by peripheral vascular resistance and is affected by any condition that increases cardiac output or peripheral vascular resistance.
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