Chapter 20–21: Heart Anatomy, Valves, Conduction, and Blood Vessels — Vocabulary

A vocabulary review of key terms from the heart anatomy, valves, conduction system, cardiac cycle, and blood vessel structure and function as presented in the lecture notes.

Describe the anatomical boundaries and key organs found within the mediastinum concerning the heart's position.

The mediastinum is the central thoracic region between the lungs, extending from the sternum anteriorly to the vertebral column posteriorly. It houses the heart, major blood vessels (aorta, vena cavae), trachea, esophagus, thymus, and lymph nodes.

Describe the anatomical landmarks of the heart's apex and base, indicating which chambers form them.

The apex is the pointed, inferior tip of the heart, formed by the left ventricle, directed anteriorly, inferiorly, and to the left, resting on the diaphragm. The base is the broad, superior and posterior aspect of the heart, formed mainly by the atria (primarily the left atrium).

Explain the structure and main physiological functions of both the fibrous and serous pericardium, including its layers.

The pericardium is a double-layered membrane. The fibrous pericardium is the tough, inelastic outer layer that anchors the heart in the mediastinum and prevents overstretching. The serous pericardium is a thinner, double-layered membrane with a parietal layer (fused to the fibrous pericardium) and a visceral layer (epicardium, adherent to the heart). Its primary function is to provide protection and a lubricated environment for the heart's movement.

What is the purpose of the fluid within the pericardial cavity in terms of cardiac function?

The pericardial cavity is a potential space between the parietal and visceral layers of the serous pericardium that contains a small amount of lubricating pericardial fluid. This fluid reduces friction between the pericardial layers as the heart beats, allowing it to move smoothly and effectively.

Contrast the three layers of the heart wall (epicardium, myocardium, endocardium) based on their histological composition and primary physiological roles.

1. Epicardium: The outermost layer, also the visceral layer of the serous pericardium; contains fat and coronary vessels. Its role is protective and houses vessels.
2. Myocardium: The thickest, muscular middle layer; composed of cardiac muscle. It is responsible for the heart's pumping action through contraction.
3. Endocardium: The smooth inner lining of the heart chambers and valves; continuous with the endothelial lining of blood vessels. It provides a smooth surface for blood flow and minimizes friction and clot formation.

What is the structural feature known as an auricle, and what functional advantage does it provide to the atria?

An auricle is a wrinkled, pouch-like extension of each atrium. Its primary functional advantage is to slightly increase the capacity of the atrium, allowing it to hold a greater volume of blood, especially when venous return is high.

Trace the pathway of deoxygenated blood as it enters the right atrium and then is pumped out by the right ventricle, naming the major vessels involved.

Deoxygenated blood from the body enters the right atrium via the superior vena cava, inferior vena cava, and coronary sinus. It then passes through the tricuspid valve into the right ventricle. From the right ventricle, blood is pumped through the pulmonary valve into the pulmonary trunk, which carries it to the lungs for oxygenation.

Trace the pathway of oxygenated blood as it enters the left atrium and then is pumped out by the left ventricle, naming the major vessels involved.

Oxygenated blood returns from the lungs to the left atrium via the four pulmonary veins. It then passes through the bicuspid (mitral) valve into the left ventricle. From the left ventricle, blood is pumped through the aortic valve into the aorta, which distributes it to the rest of the body.

Describe the location, number of cusps, and primary function of the tricuspid valve.

The tricuspid valve is the right atrioventricular (AV) valve, located between the right atrium and the right ventricle. It has three cusps. Its primary function is to prevent the backflow of blood from the right ventricle into the right atrium during ventricular systole (contraction).

Describe the location, number of cusps, and primary function of the bicuspid (mitral) valve.

The bicuspid (mitral) valve is the left atrioventricular (AV) valve, located between the left atrium and the left ventricle. It has two cusps. Its primary function is to prevent the backflow of blood from the left ventricle into the left atrium during ventricular systole (contraction).

Explain the mechanism by which the aortic valve opens and closes, and its crucial role in systemic circulation.

The aortic valve is a semilunar valve located between the left ventricle and the aorta. It opens when the pressure in the left ventricle exceeds the pressure in the aorta during ventricular systole, allowing blood ejection. It closes when ventricular pressure drops below aortic pressure during ventricular diastole, preventing backflow of blood into the left ventricle and maintaining pressure in the systemic circulation.

Explain the mechanism by which the pulmonary valve opens and closes, and its crucial role in pulmonary circulation.

The pulmonary valve is a semilunar valve located between the right ventricle and the pulmonary trunk. It opens when the pressure in the right ventricle exceeds the pressure in the pulmonary trunk during ventricular systole, allowing blood ejection to the lungs. It closes when ventricular pressure drops below pulmonary trunk pressure during ventricular diastole, preventing backflow into the right ventricle and maintaining pressure in the pulmonary circulation.

During ventricular contraction (systole), what prevents blood from flowing backward into the atria, involving what specific structures?

During ventricular systole, the increase in ventricular pressure forces the atrioventricular (AV) valves (tricuspid and bicuspid/mitral) closed, preventing backflow into the atria. The papillary muscles contract, tightening the chordae tendineae attached to the valve cusps, which prevents the cusps from everting (prolapsing) into the atria under the high pressure.

During ventricular relaxation (diastole), what mechanism prevents blood from flowing backward into the ventricles from the great arteries?

During ventricular diastole, as the ventricles relax and pressure drops, blood in the aorta and pulmonary trunk attempts to flow backward into the ventricles due to the higher pressure in the arteries. This backward pressure fills the crescent-shaped cusps of the semilunar valves (aortic and pulmonary valves), forcing them to close tightly and prevent regurgitation into the ventricles.

What is the primary anatomical and physiological significance of the interventricular septum?

The interventricular septum is a muscular partition that completely separates the right and left ventricles. Anatomically, it ensures no mixing of deoxygenated and oxygenated blood. Physiologically, its muscular composition allows for a coordinated and efficient contraction of both ventricles simultaneously, maximizing pumping efficiency for both pulmonary and systemic circuits.

Describe the interatrial septum and its most notable feature, the fossa ovalis, explaining the embryological origin of the latter.

The interatrial septum is the partition separating the right and left atria. Its most notable feature is the fossa ovalis, a shallow depression. The fossa ovalis is a remnant of the foramen ovale, an opening in the fetal heart that allowed blood to bypass the non-functional fetal lungs by shunting directly from the right atrium to the left atrium. It normally closes shortly after birth.

Identify the main external grooves of the heart (coronary, anterior, posterior interventricular sulci) and describe their anatomical significance regarding blood vessels.

1. Coronary sulcus: Encircles the heart, marking the boundary between the atria and ventricles; houses the coronary sinus and major coronary arteries.
2. Anterior interventricular sulcus: Located on the anterior surface, marking the division between the right and left ventricles; houses the anterior interventricular artery (LAD) and great cardiac vein.
3. Posterior interventricular sulcus: Located on the posterior surface, marking the division between the ventricles; houses the posterior interventricular artery and middle cardiac vein.

These sulci are anatomically significant as they contain the major coronary blood vessels that supply and drain the myocardium.

Trace the path of the left coronary artery, listing its main branches (LAD and Circumflex) and the significant heart regions each supplies with blood.

The left coronary artery branches off the aorta and typically divides into two major branches:

1. Anterior Interventricular Artery (LAD): Runs in the anterior interventricular sulcus, supplying the anterior walls of both ventricles and the interventricular septum.
2. Circumflex Artery: Travels in the coronary sulcus, supplying the left atrium and the posterior and lateral walls of the left ventricle.

Trace the path of the right coronary artery, listing its main branches and the significant heart regions each supplies with blood.

The right coronary artery branches off the aorta and typically runs in the coronary sulcus. Its main branches include:

1. Marginal Artery: Supplies the right ventricle.
2. Posterior Interventricular Artery: Runs in the posterior interventricular sulcus, supplying the posterior walls of both ventricles and the interventricular septum (in most individuals).

It primarily supplies the right atrium, most of the right ventricle, and parts of the left ventricle and conduction system.

How does deoxygenated blood from the heart muscle tissue return to the general circulation, mentioning the coronary sinus and great cardiac vein?

Deoxygenated blood from the myocardium is collected by cardiac veins. The great cardiac vein drains areas supplied by the left coronary artery, primarily running in the anterior interventricular sulcus. Most cardiac veins drain into the coronary sinus, a large venous sinus located in the coronary sulcus on the posterior surface of the heart. The coronary sinus then empties this deoxygenated blood directly into the right atrium, completing the coronary circulation.

Explain how desmosomes and gap junctions within intercalated discs contribute to both the structural integrity and synchronized contraction of cardiac muscle cells.

Intercalated discs are specialized cell-cell junctions in cardiac muscle fibers. Desmosomes (adhering junctions) provide strong adhesion between adjacent cells, preventing them from pulling apart during vigorous contraction, thus ensuring structural integrity. Gap junctions (electrical connections) allow ions to pass directly from one cardiac muscle cell to the next, facilitating the rapid spread of action potentials throughout the myocardium. This electrical coupling ensures that all cardiac cells contract in a synchronized, coordinated manner, enabling the heart to act as a functional syncytium.

Describe the unique physiological characteristic of autorhythmic fibers and their essential role in the heart's ability to beat independently.

Autorhythmic fibers are specialized cardiac muscle fibers that possess the unique physiological characteristic of spontaneous depolarization (pacemaker activity). Unlike contractile muscle cells, they do not require external nervous stimulation to generate action potentials. This property is essential because it allows the heart to initiate its own rhythmic contractions, forming the intrinsic cardiac conduction system and ensuring the heart can beat independently.

As the heart's natural pacemaker, what is the role of the SA node in initiating the cardiac electrical impulse, and what is its intrinsic firing rate?

The Sinoatrial (SA) node, located in the right atrial wall, is the heart's natural pacemaker. Its role is to spontaneously generate the electrical impulses (action potentials) that initiate each heartbeat. It has the fastest intrinsic depolarization rate, typically firing at 60-100 times per minute in a healthy adult. This rate sets the overall pace for the entire heart, driving the cardiac conduction system.

Imagine the electrical impulse from the SA node reached the ventricles simultaneously with the atria. What physiological impairment would occur, and why is the AV node delay crucial?

If the electrical impulse reached the ventricles simultaneously with the atria, both chambers would attempt to contract at the same time. This would physiologically impair ventricular filling, as the atria would not have propagated all their blood to the ventricles before ventricular contraction began. The AV node delay is crucial because it slows the conduction of impulses from the atria to the ventricles, providing a brief but essential pause. This pause allows the atria to complete their contraction (atrial systole) and fully eject blood into the ventricles, optimizing ventricular filling and thus maximizing the heart's pumping efficiency (stroke volume).

What is the 'Bundle of His,' and why is it considered the only electrical connection between the atria and ventricles?

The Bundle of His (Atrioventricular or AV bundle) is a specialized electrical conduction pathway originating from the AV node. It is considered the only electrical connection between the atria and ventricles because the fibrous skeleton of the heart acts as an electrical insulator, preventing direct electrical transmission between these chambers. All impulses from the atria must pass through the AV node and then the Bundle of His to reach the ventricles, ensuring a controlled and unidirectional conduction pathway.

Explain how the rapid conduction through the Purkinje fibers ensures an efficient and coordinated ventricular contraction.

The Purkinje fibers form an extensive network that rapidly distributes the electrical impulse from the Bundle branches throughout the ventricular myocardium. Their large diameter and numerous gap junctions facilitate extremely fast conduction. This rapid and widespread propagation of the action potential ensures that virtually all ventricular contractile muscle cells depolarize almost simultaneously, leading to a strong, synchronized contraction of the ventricles from the apex upward. This coordinated contraction is essential for efficiently ejecting blood into the great arteries.

Describe the primary electrical event and corresponding mechanical activity of the heart represented by the P wave, QRS complex, and T wave on an ECG.

1. P wave: Represents atrial depolarization (electrical excitation of the atria), leading to atrial systole (atrial contraction).
2. QRS complex: Represents rapid ventricular depolarization (electrical excitation of the ventricles), leading to ventricular systole (ventricular contraction). Atrial repolarization also occurs but is masked.
3. T wave: Represents ventricular repolarization (electrical recovery of the ventricles), leading to ventricular diastole (ventricular relaxation).

Explain the physiological significance of the P-Q interval, S-T segment, and Q-T interval on an ECG regarding cardiac conduction and muscle activity.

1. P–Q interval: Represents the time from the start of atrial excitation to the start of ventricular excitation. It includes the time taken for the impulse to travel through the atria, the AV node delay, and the Bundle of His.
2. S–T segment: Represents the period when the ventricular contractile fibers are fully depolarized. This is the plateau phase of the ventricular action potential, during which ventricular contraction is sustained and blood is ejected.
3. Q–T interval: Represents the total time from the start of ventricular depolarization to the end of ventricular repolarization, reflecting the duration of ventricular systole and subsequent recovery.

Briefly describe the major events (systole and diastole for atria and ventricles) that constitute one complete cardiac cycle.

A cardiac cycle includes all events of one heartbeat. It begins with atrial systole (atrial contraction and emptying into ventricles), followed by ventricular systole (ventricular contraction and blood ejection into the arteries). Both atria and ventricles then undergo diastole (relaxation and filling), with ventricular diastole being particularly important for ventricular filling. This cycle ensures the continuous and efficient pumping of blood throughout the body.

What is the physiological contribution of atrial systole to overall ventricular filling, and when does it occur in the cardiac cycle?

Atrial systole (atrial contraction) occurs at the end of ventricular diastole. Physiologically, it contributes the final 20-30\% of ventricular filling, often referred to as the 'atrial kick'. While not essential at rest, it becomes more crucial during exercise to maximize ventricular preload and cardiac output. This contraction occurs just before ventricular systole.

During ventricular systole, describe the pressure changes that occur and how they lead to the ejection of blood into the great arteries.

During ventricular systole, the ventricles contract, rapidly increasing pressure within their chambers. Initially, ventricular pressure rises above atrial pressure, closing the AV valves (isovolumetric contraction). As ventricular pressure continues to rise, it eventually surpasses the pressure in the great arteries (aorta and pulmonary trunk). This pressure gradient forces the semilunar valves (aortic and pulmonary) open, leading to the ejection of blood into the systemic and pulmonary circulations, respectively.

How is stroke volume (SV) physiologically calculated, and what does it represent?

Stroke volume (SV) is the volume of blood ejected by a ventricle during one heartbeat. Physiologically, it is calculated as the difference between the End-Diastolic Volume (EDV) and the End-Systolic Volume (ESV): SV = EDV - ESV. It represents the effectiveness of the heart's pump per beat.

Differentiate between End-Diastolic Volume (EDV) and End-Systolic Volume (ESV), considering their physiological significance in cardiac output.

1. End-Diastolic Volume (EDV): The volume of blood in a ventricle at the end of its filling phase (diastole), typically about 130\ mL at rest. It represents the maximum amount of blood the ventricle holds and is a key determinant of preload.
2. End-Systolic Volume (ESV): The volume of blood remaining in a ventricle after it has contracted and ejected blood (systole), typically about 60\ mL at rest. It represents the blood not ejected, indicating the efficiency of ventricular emptying.

Both are critical for calculating stroke volume (SV), which directly influences cardiac output.

A patient's venous return increases significantly. How would this physiologically affect their stroke volume, according to the Frank–Starling law of the heart, and what is the underlying mechanism?

According to the Frank–Starling law of the heart, an increase in venous return leads to a higher End-Diastolic Volume (EDV), which in turn increases the preload (stretch) of the cardiac muscle fibers. Physiologically, this stretch brings the sarcomeres closer to their optimal length for contraction. The underlying mechanism is that slightly stretched cardiac muscle fibers contract more forcefully. Consequently, the ventricle ejects a greater volume of blood, leading to an increased stroke volume. This mechanism helps the heart match its output to the venous return.

Define preload and contractility, and explain how an increase in each factor would physiologically affect the force of ventricular contraction.

1. Preload: The degree of stretch on the cardiac muscle fibers just before they contract, primarily determined by the End-Diastolic Volume (EDV). An increased preload (e.g., from higher venous return) stretches the sarcomeres more, leading to a stronger force of contraction (Frank–Starling law).
2. Contractility: The intrinsic strength of contraction of the heart muscle at any given preload. An increase in contractility (e.g., due to sympathetic stimulation or certain medications acting on Ca^{2+} availability) leads to a more forceful contraction and a greater ejection fraction, independent of muscle stretch.

If a patient has significantly elevated systemic blood pressure (hypertension), how would this affect the afterload on the left ventricle, and what would be the physiological consequence for the heart over time?

Significantly elevated systemic blood pressure (hypertension) increases the afterload on the left ventricle. Afterload is the pressure that must be exceeded by the ventricle before blood can be ejected into the aorta. With higher systemic pressure, the left ventricle has to generate greater force (work harder) to open the aortic valve and eject blood. Over time, this chronic increase in afterload leads to left ventricular hypertrophy (thickening of the heart muscle) as it struggles to overcome the resistance. Persistent, unmanaged hypertrophy can eventually lead to heart failure due to reduced ventricular compliance and impaired relaxation and filling.

Define cardiac output and mathematically express its relationship to stroke volume and heart rate.

Cardiac output (CO) is the volume of blood ejected by each ventricle per minute. It represents the total amount of blood the heart pumps to the body in a minute. Mathematically, its relationship to stroke volume (SV) and heart rate (HR) is expressed as: CO = SV \times HR.

Explain the physiological concept of Mean Arterial Pressure (MAP) and why it is a better indicator of tissue perfusion than systolic or diastolic pressure alone.

Mean Arterial Pressure (MAP) is the average pressure exerted by the blood on the arterial walls during a single cardiac cycle. It is approximated as: MAP = Diastolic\ BP + 1/3(Systolic\ BP - Diastolic\ BP). MAP is a better indicator of tissue perfusion (blood flow to tissues) than isolated systolic or diastolic pressures because it accounts for the duration of both systole and diastole. Diastole typically lasts longer than systole, so MAP more accurately reflects the average pressure driving blood into the capillaries and perfusing organs.

Compare and contrast the three types of capillaries (continuous, fenestrated, sinusoids) based on their structure, location, and the physiological types of exchange they facilitate.

1. Continuous Capillaries:
   • Structure: Tight junctions between endothelial cells with an intact basement membrane; a few intercellular clefts.
   • Location: Found in muscle, skin, lungs, central nervous system (forms blood-brain barrier).
   • Exchange: Limited exchange; allow small molecules (water, glucose, amino acids) to pass, but restrict larger proteins and cells via intercellular clefts or pinocytosis.
2. Fenestrated Capillaries:
   • Structure: Endothelial cells have pores (fenestrations) covered by a diaphragm; intact basement membrane.
   • Location: Found in kidneys (glomeruli), small intestine (villi), endocrine glands (hormone absorption/secretion).
   • Exchange: Facilitate rapid filtration and absorption of fluids and solutes due to fenestrations.
3. Sinusoids (Discontinuous Capillaries):
   • Structure: Wide, leaky capillaries with large fenestrations, incomplete or absent basement membrane, and large intercellular clefts/gaps.
   • Location: Found in liver, spleen, bone marrow, anterior pituitary gland.
   • Exchange: Allow the passage of large molecules, proteins, and even blood cells, facilitating extensive material exchange and cell migration.

A patient presents with symptoms of a myocardial infarction. Diagnostic imaging reveals a blockage in the Left Anterior Descending (LAD) artery. Explain the physiological consequence of this blockage, and why this specific artery's occlusion is often termed the 'widowmaker'.

A blockage in the Left Anterior Descending (LAD) artery would lead to myocardial ischemia (lack of oxygen) and potentially necrosis (infarction) of a significant portion of the ventricular myocardium. The LAD artery supplies the anterior two-thirds of the interventricular septum and a large part of the anterior and lateral walls of the left ventricle, which is the heart's main pumping chamber. The physiological consequence is a severe reduction in the heart's ability to pump blood effectively, leading to potentially fatal arrhythmias or pump failure. It's termed the 'widowmaker' because occlusion of this artery causes extensive damage to the left ventricle, resulting in a very high mortality rate.

If the Purkinje fibers were significantly damaged due to a heart attack, describe the physiological impact on ventricular contraction and systemic blood flow.

Significant damage to the Purkinje fibers would severely impair the rapid and coordinated depolarization of the ventricular muscle cells. Physiologically, this would result in a disorganized and inefficient ventricular contraction (e.g., ventricular arrhythmia or fibrillation) rather than a synchronized beat. The ventricles would not contract effectively to eject blood, leading to a drastic reduction or cessation of cardiac output. This would critically compromise systemic blood flow, leading to immediate and severe ischemia in body tissues, organ failure, and if uncorrected, death.

If a patient's papillary muscles or chordae tendineae were damaged, describe the physiological consequence on valve function during ventricular systole.

If a patient's papillary muscles or chordae tendineae were damaged, their primary physiological consequence would be the inability of the atrioventricular (AV) valves (tricuspid or mitral) to close properly during ventricular systole. Without the tension provided by these structures, the valve cusps would prolapse or evert into the atria due to the high ventricular pressure. This would lead to valvular regurgitation, where blood flows backward from the ventricles into the atria during contraction. This reduces the effective stroke volume, increases atrial pressure, and ultimately diminishes cardiac output, potentially leading to heart failure.