Heart Anatomy and Location Overview

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Questions and Answers

What percentage of ventricular filling occurs passively before atrial contraction?

80% (correct)

60%

50%

40%

What event follows atrial depolarization during the cardiac cycle?

Atrial relaxation

Ventricular contraction (correct)

Isovolumetric contraction

Ventricular repolarization

During the isovolumetric contraction phase, what happens to the blood volume in the ventricles?

It is expelled into the arteries

It decreases rapidly

It increases

It remains constant (correct)

What is the end diastolic volume (EDV) referred to?

The maximum blood volume the ventricles can contain Signup and view all the answers

Which event occurs as ventricular pressure exceeds the pressure in large arteries?

SL valves are forced open Signup and view all the answers

What happens to the semilunar valves when the ventricles contract?

They close due to increased intraventricular pressure. Signup and view all the answers

Which statement accurately describes the function of the aortic semilunar valve?

It prevents backflow of blood into the aorta. Signup and view all the answers

What unique characteristic do cardiac muscle fibers possess?

They are connected by intercalated discs. Signup and view all the answers

During the contraction of cardiac muscle, what role do gap junctions play?

They allow ions to pass between adjacent cardiac cells. Signup and view all the answers

What initiates contraction in cardiac muscle cells?

Autorhythmic cardiac muscle cells. Signup and view all the answers

What does the term 'automaticity' refer to in cardiac muscle?

The spontaneous generation of an action potential. Signup and view all the answers

What event occurs immediately after depolarization of cardiac muscle cells?

The influx of sodium ions into the cells. Signup and view all the answers

How does the heart prevent tetanic contractions during the contraction cycle?

Through a long absolute refractory period. Signup and view all the answers

What is the primary function of the fibrous pericardium?

Anchors the heart to surrounding structures Signup and view all the answers

Which layer of the heart wall is responsible for the contractile action?

Myocardium Signup and view all the answers

What separates the two atria in the heart?

Inter-atrial septum Signup and view all the answers

What vessels carry blood to the right atrium?

Superior vena cava and inferior vena cava Signup and view all the answers

What is the role of the papillary muscles in the heart?

Anchor the AV valves in place Signup and view all the answers

Which valve prevents backflow into the left atrium when the left ventricle contracts?

Bicuspid valve Signup and view all the answers

During which phase does blood flow from the left atrium to the left ventricle?

Ventricular filling Signup and view all the answers

What distinguishes the pulmonary circuit from the systemic circuit?

Shorter and lower pressure Signup and view all the answers

Which of the following arteries branches from the left coronary artery?

Circumflex artery Signup and view all the answers

Which heart valve has three cusps?

Tricuspid valve Signup and view all the answers

What is the primary function of the atrioventricular (AV) node in the cardiac conduction system?

Delay impulses to allow atrial contraction before ventricular contraction Signup and view all the answers

How many times per minute does the AV bundle depolarize in the absence of AV node input?

Which cardiac structure conducts impulses into the ventricular walls and is more elaborate in the left ventricle?

Purkinje fibers Signup and view all the answers

What effect does the cardioacceleratory center have on heart function?

Increases heart rate and pumping force Signup and view all the answers

What does the QRS complex in an electrocardiogram represent?

Ventricular depolarization Signup and view all the answers

What are the heart sounds associated with in the cardiac cycle?

Closing of heart valves Signup and view all the answers

What does the term 'systole' refer to in the cardiac cycle?

Period of contraction Signup and view all the answers

Which interval measures the duration from the beginning of atrial excitation to the beginning of ventricular excitation?

P-Q (P-R) interval Signup and view all the answers

During which phase of the cardiac cycle do the ventricles fill with blood?

Diastole Signup and view all the answers

Which component obscures the atrial repolarization wave in an ECG?

QRS complex Signup and view all the answers

What occurs during isovolumetric relaxation in the cardiac cycle?

Ventricular pressure decreases, causing backflow of blood Signup and view all the answers

What is the effect of increased venous return on stroke volume?

It increases stroke volume due to increased preload Signup and view all the answers

Which factor is NOT a major determinant of stroke volume?

Heart rate Signup and view all the answers

What is the primary effect of increased afterload on stroke volume?

Decreased stroke volume and increased ESV Signup and view all the answers

Which hormonal factor primarily increases heart rate during physical stress?

Epinephrine Signup and view all the answers

What is the heart rate when cardiac output is calculated at rest, given a stroke volume of 70 ml/beat?

75 beats/min Signup and view all the answers

What physiological process causes the dicrotic notch in aortic pressure?

Backflowing blood bouncing off closed valve cusps Signup and view all the answers

How does sympathetic stimulation directly affect cardiac output?

It increases heart rate and enhances contractility Signup and view all the answers

Which condition primarily leads to increased ESV?

Hypertension Signup and view all the answers

What effect does acetylcholine have on heart rate?

It hyperpolarizes pacemaker cells, decreasing heart rate Signup and view all the answers

Study Notes

Heart Location

Located in the mediastinum
Between the 2nd rib and the 5th intercostal space
On the superior surface of the diaphragm
Anterior to the vertebral column, posterior to the sternum
Two-thirds to the left of the midsternal line

Heart Anatomy

Coverings of the Heart:
- Pericardium:
  - Double-walled sac surrounding the heart
  - Fibrous Pericardium:
    - Tough and dense CT
    - Protects the heart
    - Anchors it to surrounding structures
    - Prevents excess filling with blood
  - Serous Pericardium:
    - Thin and slippery serous membrane
    - Consists of two layers:
      - Parietal layer: Covers the internal surface of the fibrous pericardium
      - Visceral layer: Covers the external heart surface, also known as the epicardium
Layers of The Heart Wall:
- Epicardium: Outermost layer, also known as the visceral layer of the serous pericardium
- Myocardium: Middle layer composed of cardiac muscle
  - Cardiac muscle cells arranged in spiral or circular bundles
  - CT fibers:
    - Collagen and elastic fibers that provide support
    - Reinforces the myocardium, anchors muscles fibers
    - Supports the great vessels and valves
    - Directs the spread of action potentials to specific pathways
- Endocardium: Innermost layer
  - Endothelial layer that lines the heart chambers and covers the fibrous skeleton of the valves
  - Continuous with blood vessels

Chambers of the Heart

Four chambers:
- Atria: Two superior chambers
  - Separated by the inter-atrial septum
  - Coronary sulcus: Encircles the junction of atria and ventricles
  - Auricles: Increase atrial volume
- Ventricles: Two inferior chambers
  - Separated by the inter-ventricular septum
  - Anterior and posterior inter-ventricular sulci: Mark the position of the septum externally

Atria: The Receiving Chambers

Thin walls ridged by bundles of muscle tissue called pectinate muscles
Vessels entering right atrium:
- Superior vena cava
- Inferior vena cava
- Coronary sinus
Vessels entering left atrium:
- Two right and two left pulmonary veins

Ventricles: The Discharging Chambers

Thick walls ridged by irregular ridges called trabeculae carneae
Papillary muscles: Project into the ventricular cavities and function in valve regulation
Vessels leaving the right ventricle:
- Pulmonary trunk: Carries blood to the lungs for gas exchange
Vessels leaving the left ventricle:
- Aorta: Carries oxygenated blood to the systemic circulation

Pathway of Blood Through The Heart

The heart functions as two side-by-side pumps, serving two separate blood circuits:
- Right side: Pulmonary circuit (carries blood to and from the lungs)
- Left side: Systemic circuit (carries blood to and from all body tissues)
Complete Pathway:
- Superior & inferior vena cava → right atrium → tricuspid valve → right ventricle → pulmonary semilunar valve → pulmonary trunk → pulmonary arteries → lungs
- Lungs → pulmonary veins → left atrium → bicuspid valve → left ventricle → aortic semilunar valve → aorta → systemic circulation
Pulmonary circuit is short and low-pressure
Systemic circuit is longer and encounters five times more resistance

Coronary Circulation

The blood supply to the heart muscle itself
Arteries:
- Right coronary: Supplies blood to the right ventricle, right atrium, and posterior part of the left ventricle. Branching arteries include the marginal and posterior interventricular arteries.
- Left coronary: Supplies blood to the left ventricle, left atrium, and the interventricular septum. Branching arteries include the circumflex and anterior interventricular arteries.
- Numerous anastomoses provide alternative pathways for blood when one route is blocked.
Veins:
- Small cardiac, middle cardiac, and great cardiac veins: These drain into the coronary sinus, which empties into the right atrium.
- Anterior cardiac veins: Drain directly into the right atrium.

Heart Valves

Four valves enforce the one-way flow of blood through the heart
Type of Valves:
- Atrioventricular (AV) Valves:
  - Tricuspid valve (right): Three cusps prevent backflow into the right atrium during ventricular contraction.
  - Mitral valve (left): Bicuspid (two cusps) valve prevents backflow into the left atrium during ventricular contraction.
  - Chordae tendineae: Anchor the AV valve cusps to papillary muscles which protrude from the ventricular walls. This helps keep the valves closed during ventricular contraction.
- Semilunar (SL) Valves:
  - Aortic semilunar valve: Located at the base of the aorta, prevents backflow into the left ventricle during ventricular relaxation.
  - Pulmonary semilunar valve: Located at the base of the pulmonary trunk, prevents backflow into the right ventricle during relaxation.

Cardiac Muscle Fibers

Characteristics:
- Striated, short, fat, branched, and interconnected
Intercalated Discs:
- Junctions between cardiac cells
- Anchor cardiac cells together
- Desmosomes: Prevent cells from separating during contraction
- Gap junctions: Allow ions to pass, electrically coupling adjacent cells, allowing the myocardium to behave as a single, coordinated unit
Intercellular space: Contains loose connective tissue matrix called endomysium
- Connects to the fibrous skeleton and contains capillaries
Mitochondria: Numerous and large (25-35% of cell volume), enabling muscle cells to resist fatigue
T tubules: Wider but less numerous than skeletal muscle
SR: Simpler than skeletal muscle

Contraction of Cardiac Muscle

Automaticity/autorhythmicity:
- Contraction is initiated by autorhythmic cardiac muscle cells
  - These cells make up about 1% of all cardiac cells
  - Have the ability to depolarize independently, pacing the heart
Depolarization:
- Rhythmic and spontaneous
- Gap junctions ensure the heart contracts as a unit
Long absolute refractory period (250 ms):
- Prevents tetanic contractions (sustained muscle contractions)
Steps in Depolarization:
1. Depolarization opens fast Na+ channels in the sarcolemma.
2. Depolarization opens Ca2+ channels in the sarcolemma (slow Ca2+ channels).
3. Depolarization wave and Ca2+ influx induce the release of Ca2+ from the SR.
4. The surge in Ca2+ prolongs the depolarization phase (plateau).
Sequence of Excitation:
- SA node: Initiates the heart beat
- Atrial depolarization: Depolarization wave travels through the atria
- AV node:
  - Serves as the only electrical connection between the atria and ventricles.
  - Delays impulses for approximately 0.1 seconds, allowing the atria to contract before the ventricles.
  - Conducts impulses slower than other parts of the conduction system.
- AV bundle (bundle of His):
  - The only electrical connection between the atria and ventricles, no gap junctions.
- Right & left bundle branches:
  - Two pathways along the interventricular septum, carrying impulses towards the apex of the heart.
- Purkinje fibers:
  - Complete the pathway through the interventricular septum, into the apex, and then into the ventricular walls. More elaborate in the left ventricle.
  - Depolarization of ventricular muscle cells with the help of gap junctions.

Extrinsic Innervation of the Heart

The autonomic nervous system (ANS) modifies heartbeat
Cardiac Centers: Located in the medulla oblongata
- Cardioacceleratory center: Sympathetic neurons
  - Innervates SA and AV nodes, heart muscle, and coronary arteries
  - Increases heart rate and pumping force
- Cardioinhibitory center: Parasympathetic fibers in the vagus nerves
  - Innervates SA and AV nodes
  - Slows down heart rate

Electrocardiography

Electrocardiogram (ECG or EKG): Graphic record of heart activity
- Composite of all action potentials generated by nodal and contractile cells at a given time
Waves:
- P wave: Atrial depolarization, followed by atrial contraction
- QRS complex: Ventricular depolarization, followed by ventricular contraction
- T wave: Ventricular repolarization

Normal and Abnormal ECG Tracings

P-Q (P-R) interval: Beginning of atrial excitation to the beginning of ventricular excitation
S-T segment: Ventricular myocardium is totally depolarized, elevation or depression indicates cardiac ischemia
Q-T interval: Beginning of ventricular depolarization through complete ventricular repolarization, prolonged Q-T can indicate repolarization abnormality.

Heart Sounds

Two sounds (lub-dup) are heard:
- First sound (lub): Closure of AV valves
- Second sound (dup): Closure of SL valves

Mechanical Events of the Cardiac Cycle

Includes all events occurring during the blood flow through the heart during a complete heartbeat.
Alternating Cycles:
- Systole: Contraction phase
- Diastole: Relaxation phase
Sequence of Events:
- Atrial systole → atrial diastole → ventricular systole → ventricular diastole
Electrical Events: Electrical events always precede mechanical events.

Cardiac Cycle

1. Ventricular Filling (Diastole):
- Passive filling (80%): Blood flows passively through the atria and AV valves, SL valves are closed.
- Atrial contraction (20%): Atrial depolarization (P wave) causes atria to contract, increasing atrial pressure, pushing blood into ventricles.
- EDV (End Diastolic Volume): Maximum blood volume ventricles can contain.
2. Ventricular Systole (Contraction):
- Ventricular pressure rises: This causes the AV valves to close (lub)
  - Isovolumetric contraction phase: Brief moment when ventricles are completely closed and blood volume remains constant.
- Ventricular pressure rises further: When ventricular pressure exceeds aortic pressure, the SL valves open, blood is ejected into the aorta and pulmonary trunk (ventricular ejection phase).
3. Isovolumetric Relaxation (Early Diastole):
- Ventricular repolarization (T wave): Ventricles relax, ventricular pressure drops.
- SL valves close (dup): When pressure in the aorta and pulmonary trunk is higher than ventricular pressure, blood backflows toward the heart closing the SL valves.
- ESV (End Systolic Volume): Blood remaining in ventricles after contraction.

Cardiac Output (CO)

Amount of blood pumped out by each ventricle in one minute.
Determinants of CO:
- Heart rate (HR): Number of beats per minute.
- Stroke volume (SV): Volume of blood pumped out by one ventricle with each beat.
Formula: CO = HR x SV

Cardiac Output (CO): Regulation of Stroke Volume

Stroke volume (SV): SV = EDV - ESV
Factors influencing SV:
- Preload: Degree to which cardiac muscle cells are stretched just before contraction.
  - According to the Frank-Starling law of the heart, the greater the preload, the greater the SV.
  - Venous return plays a significant role in preload
    - Increased venous return ➔ Increased EDV ➔ Increase SV
- Contractility: Contractile strength at a given muscle length, independent of muscle stretch and EDV.
  - Enhanced contractility ➔ increased SV and decreased ESV.
  - Positive ionotropic agents (increase strength): Sympathetic stimulation, hormones (thyroxine, glucagon, epinephrine).
  - Negative ionotropic agents (decrease strength): Acidosis, increased extracellular K+, calcium channel blockers.
- Afterload: Back pressure exerted by arterial blood that the ventricles must overcome to eject blood
  - Not a major determinant in healthy individuals.
  - Increased afterload ➔ increased ESV and decreased SV
Cardiac reserve: Difference between resting and maximal CO.

Cardiac Output (CO): Regulation of Heart Rate

Positive chronotropic factors: Increase heart rate
Negative chronotropic factors: Decrease heart rate
Autonomic Nervous System Regulation:
- Sympathetic Nervous System: Activated by emotional or physical stressors
  - Norepinephrine release at the cardiac synapses ➔ increased pacemaker firing ➔ increased heart rate and contractility.
- Parasympathetic Nervous System: Opposes sympathetic effects
  - Acetylcholine hyperpolarizes pacemaker cells ➔ slows heart rate.
- Atrial (Bainbridge) reflex: Sympathetic reflex triggered by increased venous return and atrial filling.
  - Stretching of the atrial walls stimulates the SA node and atrial stretch receptors, triggering sympathetic reflexes that increase heart rate.
Chemical Factors:
- Hormones: Epinephrine enhances heart rate and contractility, thyroxine increases heart rate and enhances epinephrine and norepinephrine effects.
- Ions: Ca2+ and K+ levels must be maintained for normal heart function.
Other factors: Age, gender, exercise, body temperature.

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This quiz covers the important aspects of heart anatomy, including its location within the mediastinum and the various layers of the heart wall. Key structures such as the pericardium and its components will also be discussed. Test your knowledge of how the heart is situated and protected in the thoracic cavity.