THE HEART

INTRODUCTION TO THE CARDIOVASCULAR SYSTEM: THE HEART

Chapter Objectives

Identify and describe the interior and exterior parts of the human heart.
Describe the path of blood through the cardiac circuits.
Describe the size, shape, and location of the heart.
Compare cardiac muscle to skeletal and smooth muscle.
Explain the cardiac conduction system.
Describe the process and purpose of an electrocardiogram (ECG).
Explain the cardiac cycle.
Calculate cardiac output.
Describe the effects of exercise on cardiac output and heart rate.
Name the centers of the brain that control heart rate and describe their functions.
Identify other factors affecting heart rate.
Describe fetal heart development.

The Heart as a Pump

The heart functions primarily as a pump that develops pressure to eject blood into major vessels (aorta and pulmonary trunk), distributing blood throughout the body.
Although "pump" suggests a mechanical device, the heart is actually a sophisticated muscle.
The term "heart" is derived from the Latin word "kardia".
Cardiology is the medical study of the heart, with specialists known as cardiologists.

Figure 19.1

The human heart depicted as a powerful engine, emphasizing its muscular pump function.

19.1 Heart Anatomy

Learning Objectives

Describe the location and position of the heart within the body cavity.
Describe the internal and external anatomy of the heart.
Identify the tissue layers of the heart.
Relate the structure of the heart to its function as a pump.
Compare systemic circulation to pulmonary circulation.
Identify the veins and arteries of the coronary circulation system.
Trace the pathway of oxygenated and deoxygenated blood through the chambers of the heart.

Importance of the Heart

Average heart contractions: 75 times/minute - Total: ~108,000 contractions/day, ~39 million/year over a lifespan of 75 years.
Each major pumping chamber ejects ~70 mL of blood/contraction: - 5.25 liters/min - Approximately 14,000 liters/day or 10 million liters/year (2.6 million gallons).

Location of the Heart

Situated within the thoracic cavity, medially between the lungs, in the mediastinum.
Separated from other structures by a tough membrane called the pericardium.
Pericardial cavity houses the heart, lined by serous fluid for lubrication.
Base of the heart: level with 3rd costal cartilage.
Apex of the heart: located just left of the sternum, between the 4th and 5th ribs.
Cardiac notch: depression on the medial surface of the left lung, accommodating the heart.

CPR Connection

CPR Technique: Pressure applied on sternum between T4 and T9 to maintain blood flow if heart stops.
Current standards: compress at least 5 cm deep, at 100 compressions/min.

Size and Shape

Shape resembles a pinecone: broad at the top, tapering to the apex.
Typical heart measurements: - Length: 12 cm (5 in), Width: 8 cm (3.5 in), Thickness: 6 cm (2.5 in).
Female heart: 250-300 grams (9-11 oz). Male heart: 300-350 grams (11-12 oz).
Athletic hearts: may be larger due to exercise-induced hypertrophy.

Chambers and Circulation

Four chambers: Each side has one atrium and one ventricle. Chambers involved: - Right Atrium: receives deoxygenated blood. - Right Ventricle: pumps blood to lungs. - Left Atrium: receives oxygenated blood. - Left Ventricle: pumps blood to systemic circulation.
Two circuits: - Pulmonary Circuit (to/from lungs) for gas exchange. - Systemic Circuit (to/from body).

Structure of the Heart Wall

Composed of three layers: - Epicardium: outermost layer, also part of the pericardium. - Myocardium: thickest layer, composed of heart muscle responsible for pumping. - Endocardium: innermost layer, lines chambers and covers valves.

Septa and Valves

Interatrial Septum: divides atria; contains the fossa ovalis (remnant of fetal foramen ovale).
Interventricular Septum: thicker, dividing ventricles.
Atrioventricular Valves: Tricuspid (right) and Mitral (left).
Semilunar Valves: pulmonary and aortic valves, preventing backflow into ventricles.

Coronary Circulation

Coronary arteries supply myocardium; the great cardiac vein drains heart.
Blockage leads to myocardial infarction (heart attack).

19.2 Cardiac Muscle and Electrical Activity

Learning Objectives

Describe the structure of cardiac muscle.
Identify components of the electrical conducting system.
Compare ion movement effects in cardiac cells.
Identify ECG characteristics relative to the cardiac cycle.

Structure of Cardiac Muscle

Cardiomyocytes: smaller than skeletal muscle, branched, with a single nucleus.
Intercalated discs: provide connections between muscle cells, aiding synchronized contractions.
Mitochondria are abundant to support energy needs.
Cardiac muscle maintains metabolic activity via aerobic respiration, primarily utilizing lipids and carbohydrates.

Cardiac Conduction System

Components: Sinoatrial (SA) node, Atrioventricular (AV) node, Atrioventricular bundle, Bundle branches, Purkinje fibers.
SA Node: primary pacemaker, initiates electrical impulse for contraction.
AV Node: a pause allows atria to contract before ventricles.
Impulses conducted via bundle branches to Purkinje fibers triggering ventricular contraction.

Electrocardiogram (ECG)

Records electrical activity of the heart through electrodes.
Main components: P wave (atrial depolarization), QRS complex (ventricular depolarization), T wave (ventricular repolarization).
PR interval: time from atrial depolarization to ventricular depolarization.

19.3 Cardiac Cycle

Learning Objectives

Describe the relationship between blood pressure and flow.
Summarize cardiac cycle events.
Compare atrial and ventricular systole and diastole.

Phases of the Cardiac Cycle

Systole: contraction phase. - Atrial systole pumps blood into ventricles. - Ventricular systole pumps blood into arteries.
Diastole: relaxation phase. - Atria fill with blood from veins; ventricles fill through open atrioventricular valves.

Heart Sounds

S1 (lub): Closing of AV valves.
S2 (dub): Closing of semilunar valves.
S3 and S4 may indicate pathologies.

19.4 Cardiac Physiology

Learning Objectives

Relate heart rate to cardiac output.
Describe exercise effects on heart rate.
Identify cardiovascular centers regulating heart function.

Cardiac Output (CO)

Formula: $CO = HR imes SV$
Normal range: 4.0–8.0 L/min for resting individuals.
Stroke Volume (SV): volume of blood pumped per contraction.
Ejection fraction: portion of blood ejected from heart with each contraction.

Factors Influencing Heart Rate

Cardiac output and heart rate influenced by external stimuli (nervous impulses, hormones).
Neural centers in the medulla regulate heart rate.
Positive vs. Negative factors for contractility depending on external signals and ionic balance.

19.5 Development of the Heart

Learning Objectives

Describe embryological development of heart structures.
Identify five regions of the fetal heart.

Development Timeline

Heart begins functioning and pumping blood by Day 21-22 of development.
Forms from mesoderm, developing into a primitive heart tube with five distinguishable sections.
Key structures forming: Truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, sinus venosus.
Divides atria and ventricles by the end of Week 5, with valves forming shortly thereafter.

Chapter Objectives

Identify and describe the interior and exterior parts of the human heart, including chambers, valves, and associated blood vessels.
Describe the path of blood through both the pulmonary and systemic circuits, detailing oxygenation events and carbon dioxide removal.
Describe the size, shape, and location of the heart in relation to other thoracic structures, emphasizing its protective surroundings.
Compare cardiac muscle to skeletal and smooth muscle in terms of structure, function, and energy metabolism.
Explain the cardiac conduction system, including the role of pacemaker cells and the propagation of electrical impulses.
Describe the process and purpose of an electrocardiogram (ECG), outlining its clinical significance in diagnosing cardiac abnormalities.
Explain the cardiac cycle, including phases, heart sounds, and the importance of diastole and systole.
Calculate cardiac output using established formulas and understand its clinical implications.
Describe the effects of exercise on cardiac output and heart rate and the physiological adaptations involved.
Name the centers of the brain that control heart rate, such as the medulla oblongata, and describe their functions in regulating autonomic nervous system responses.
Identify other factors affecting heart rate, including hormonal influences and physical fitness levels.
Describe fetal heart development stages, including major structural changes and their functional significance.

The Heart as a Pump

The heart functions primarily as a pump that develops pressure to eject blood into major vessels such as the aorta and pulmonary trunk, distributing blood to vital organs and tissues throughout the body.
Although the term "pump" suggests a simple mechanical device, the heart is actually a highly organized and sophisticated muscular organ made up of specialized cardiac muscle cells.
The term "heart" comes from the Latin word "kardia"; thus, the field of medicine that studies the heart is termed cardiology, with trained professionals known as cardiologists specializing in diagnosing and treating cardiovascular diseases.

Figure 19.1

The human heart depicted visually as a powerful, intricate engine, emphasizing its muscular pump function and vital role in sustaining life by ensuring continuous blood flow and delivering oxygen and nutrients.

19.1 Heart Anatomy

Learning Objectives

Describe the location and position of the heart within the thoracic cavity, including its orientation and relation to the mediastinum and diaphragm.
Describe the internal and external anatomy of the heart, detailing the four chambers, septa, valves, and associated major vessels.
Identify the three tissue layers of the heart wall: epicardium, myocardium, and endocardium, and relate their functions.
Relate the structure of the heart to its function as a dynamic pump, with each chamber playing a specific role in blood circulation.
Compare systemic circulation to pulmonary circulation in terms of blood flow, pressure, and function.
Identify the veins and arteries of the coronary circulation system, understanding their role in nourishing heart tissue and the consequences of blockages.
Trace the pathway of oxygenated and deoxygenated blood through the chambers of the heart, including valve functioning.

Importance of the Heart

Average heart contractions occur at a rate of 75 times per minute, culminating in roughly 108,000 contractions per day. Over a typical lifespan of 75 years, this amounts to nearly 39 million contractions.
Each major pumping chamber is responsible for ejecting approximately 70 mL of blood per contraction, resulting in a total of about 5.25 liters per minute circulating throughout the system — translating to about 14,000 liters per day or around 10 million liters over a year, roughly equivalent to 2.6 million gallons of blood.

Location of the Heart

The heart is situated within the thoracic cavity, medially positioned between the lungs, nestled within the mediastinum, which serves as a critical protective compartment.
It is kept separate from surrounding structures by a tough, fibrous sac known as the pericardium. This pericardial membrane offers support and prevents over-expansion while allowing for necessary movement during cardiac cycles.
The pericardial cavity containing serous fluid surrounds the heart; this fluid acts as a lubricant to minimize friction as the heart beats.
The base of the heart can be located at the level of the 3rd costal cartilage, while the apex forms at the left side of the sternum, typically situated between the 4th and 5th ribs.
The cardiac notch on the medial surface of the left lung provides an impression that allows room for the heart, highlighting its spatial significance and relation to surrounding anatomy.

CPR Connection

Understanding the significance of appropriate CPR is vital; the CPR Technique involves applying firm pressure on the sternum between the fourth and ninth thoracic vertebrae (T4-T9) to maintain blood circulation if the heart stops.
Current life-saving recommendations dictate compressing the sternum to a depth of at least 5 cm and maintaining a rhythm of 100 compressions per minute for optimal effectiveness in resuscitation efforts.

Size and Shape

The heart's shape resembles a pinecone, characterized by a broad top tapering to an apex, which is not just an anatomical uniqueness but also relates to its functional dynamics in pumping blood.
Typical heart measurements for adults are approximately:
  - Length: 12 cm (5 in)
  - Width: 8 cm (3.5 in)
  - Thickness: 6 cm (2.5 in)
On average, a female heart weighs about 250-300 grams (9-11 oz), while a male heart weighs around 300-350 grams (11-12 oz).
Athletic hearts, in response to consistent exercise, may manifest as larger due to physiological adaptations resulting in hypertrophy, enhancing their pumping capacity.

Chambers and Circulation

The heart comprises four distinct chambers, where each side presents one atrium and one ventricle dedicated to specific tasks:
  - Right Atrium: Receives deoxygenated blood from the body.
  - Right Ventricle: Pumps blood to the lungs for oxygenation.
  - Left Atrium: Receives oxygenated blood from the lungs.
  - Left Ventricle: Pumps oxygen-rich blood into systemic circulation to nourish tissues.
The heart operates through two primary circuits:
- Pulmonary Circuit: Focused on transporting deoxygenated blood to the lungs and returning oxygenated blood to the heart.
- Systemic Circuit: Distributes oxygen-rich blood from the heart to the rest of the body.

Structure of the Heart Wall

The heart wall is composed of three integral layers:
  - Epicardium: Outermost layer which also forms part of the pericardial sac.
  - Myocardium: The thickest layer, consisting of cardiac muscle responsible for the contractile function of the heart.
  - Endocardium: The innermost lining that covers the heart chambers and valves, composed of endothelial tissue ensuring a smooth surface for blood flow.

Septa and Valves

Interatrial Septum: A wall that divides the atria and features the fossa ovalis, a remnant of the fetal foramen ovale, indicating a crucial developmental history.
Interventricular Septum: A thicker muscular wall that separates the ventricles, allowing for efficient pressure generation during contraction.
Atrioventricular Valves: The right tricuspid valve and the left mitral valve regulate blood flow between atria and ventricles.
Semilunar Valves: Including the pulmonary and aortic valves, prevent the backflow of blood into the ventricles during diastole.

Coronary Circulation

The coronary arteries supply oxygenated blood to the myocardium, highlighting the heart muscle's need for its own blood supply to function effectively.
The great cardiac vein collects deoxygenated blood, draining it from the heart muscle back into the systemic circulation.
Blockages in these coronary vessels can lead to serious conditions such as myocardial infarction, commonly known as a heart attack, necessitating proper cardiovascular health management.

19.2 Cardiac Muscle and Electrical Activity

Learning Objectives

Describe the unique structure of cardiac muscle and its role in heart function.
Identify components of the electrical conducting system that coordinate heart contractions.
Compare the effects of ion movement in cardiac cells and the resulting actions.
Identify key characteristics of the ECG relative to the cardiac cycle phases.

Structure of Cardiac Muscle

Cardiomyocytes: These heart muscle cells are smaller than their skeletal counterparts, branched, and typically contain a single central nucleus, particularly distinct from multi-nucleated skeletal muscle.
Intercalated Discs: Specialized structures that connect individual cardiomyocytes, facilitating synchronized contraction across the heart muscle.
Mitochondrial abundance within cardiomyocytes underscores their high energy demand, as the heart continuously requires substantial ATP for muscle contractions.
Cardiac muscle primarily operates through aerobic respiration, utilizing lipids and carbohydrates to meet metabolic requirements, in contrast to anaerobic pathways often associated with skeletal muscle fatiguing processes.

Cardiac Conduction System

Components: Encompasses the Sinoatrial (SA) node, Atrioventricular (AV) node, Bundle of His (AV bundle), bundle branches, and Purkinje fibers, forming a network critical to the rhythmic beating of the heart.
SA Node: Acts as the primary pacemaker, spontaneously generating electrical impulses that initiate each heartbeat.
AV Node: Provides a necessary delay between atrial and ventricular contractions, ensuring sufficient filling time for the ventricles before ejection of blood.
The impulses propagate through the bundle branches to the Purkinje fibers, prompting coordinated contraction of the ventricles to effectively pump blood out of the heart.

Electrocardiogram (ECG)

An ECG records the electrical activity of the heart via electrodes positioned on the skin, allowing for non-invasive assessment of cardiac function.
Key components include the P wave indicating atrial depolarization, the QRS complex reflecting ventricular depolarization, and the T wave representing ventricular repolarization.
The PR interval on an ECG indicates the time delay between atrial depolarization and ventricular depolarization, which can provide critical insights into conduction abnormalities in clinical assessments.

19.3 Cardiac Cycle

Learning Objectives

Describe the physiological relationship between blood pressure and flow throughout the cardiac cycle.
Summarize key events that occur during the cardiac cycle phases.
Compare the phases of atrial and ventricular systole and diastole in terms of timing and function.

Phases of the Cardiac Cycle

Systole: The contraction phase of the heart.
- Atrial Systole: Here, the atria contract, pushing blood into the ventricles to ensure they are filled prior to ventricular contraction.
- Ventricular Systole: The ventricles contract, ejecting blood into the aorta and pulmonary artery, crucial for systemic and pulmonary circulation, respectively.
Diastole: The relaxation phase following contraction.
- During this phase, the atria fill with blood from the veins, while the ventricles passively fill through open atrioventricular valves, preparing for the next cycle.

Heart Sounds

The heart produces characteristic sounds corresponding to valve closure:
  - S1 (lub): A sound produced by the closing of the AV valves, marking the start of ventricular systole.
  - S2 (dub): Produced by the closure of the semilunar valves, indicating the end of ventricular systole.
  - Additional sounds S3 and S4 may arise under pathological conditions, potentially indicating issues such as heart failure or hypertrophy.

19.4 Cardiac Physiology

Learning Objectives

Relate heart rate to cardiac output effectively.
Describe how exercise influences heart rate and cardiac adaptations.
Identify regulatory cardiovascular centers governing heart function.

Cardiac Output (CO)

Formula: $CO = HR imes SV$ where CO is cardiac output, HR is heart rate, and SV is stroke volume.
Normal ranges for cardiac output fall between 4.0–8.0 L/min for resting individuals, providing important benchmarks for assessing cardiovascular performance.
Stroke Volume (SV) refers to the volume of blood expelled from the ventricle with each contraction, and the ejection fraction gauges the percentage of blood pumped out during ventricular systole.

Factors Influencing Heart Rate

Cardiac output and heart rate can fluctuate based on external stimuli including nervous impulses (sympathetic and parasympathetic inputs) and hormonal regulation (such as adrenaline).
Neural centers located in the medulla oblongata regulate heart rate by integrating signals from various body systems and responding accordingly.
Positive inotropic factors enhance contractility, while negative factors may arise from alterations in ionic balance or feedback mechanisms, indicating complexity in homeostatic heart function regulation.

19.5 Development of the Heart

Learning Objectives

Describe embryological development of heart structures, outlining the critical stages of formation.
Identify five regions of the fetal heart based on their developmental significance.

Development Timeline

The heart commences its functional role by Day 21-22 of embryonic development, originating from mesodermal tissue and evolving into a primitive heart tube.
This initial structure differentiates into five regions: truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and sinus venosus—each with specific anatomical and functional implications.
By the end of Week 5, atrial and ventricular divisions develop to facilitate proper blood circulation, with valvular structures forming shortly thereafter, ensuring efficient blood flow during and after gestation.