The Heart: Structure, Function, and Physiology – Study Guide

The Heart: Structure, Function, and Physiology

Layers and Structure of the Heart

The heart is composed of several distinct layers, each with unique characteristics and functions essential for cardiac physiology.

• Pericardium: A double-walled sac that surrounds the heart, providing protection and reducing friction. It consists of the fibrous pericardium (outer layer) and the serous pericardium (inner layer, further divided into parietal and visceral layers).

• Heart Wall Layers:

  • Epicardium: The outermost layer, also known as the visceral layer of the serous pericardium.

  • Myocardium: The thick, muscular middle layer responsible for the heart's contractile force.

  • Endocardium: The innermost layer, lining the heart chambers and valves.

• Chambers of the Heart: The heart has four chambers: right atrium, right ventricle, left atrium, and left ventricle.

Example: The myocardium is thickest in the left ventricle, as it must generate enough force to pump blood throughout the systemic circulation.

Blood Flow Through the Heart

Blood follows a specific pathway through the heart, passing through chambers, valves, and vessels in a precise sequence.

1. Deoxygenated blood enters the right atrium via the superior and inferior vena cava.

2. Blood flows through the tricuspid valve into the right ventricle.

3. From the right ventricle, blood is pumped through the pulmonary valve into the pulmonary arteries and to the lungs.

4. Oxygenated blood returns from the lungs via the pulmonary veins to the left atrium.

5. Blood passes through the bicuspid (mitral) valve into the left ventricle.

6. The left ventricle pumps blood through the aortic valve into the aorta and systemic circulation.

Key Valves: Tricuspid, Pulmonary, Mitral (Bicuspid), Aortic

Example: The pulmonary valve prevents backflow of blood from the pulmonary artery into the right ventricle.

Pulmonary and Systemic Circuits

The heart pumps blood through two main circuits:

• Pulmonary Circuit: Carries deoxygenated blood from the right side of the heart to the lungs and returns oxygenated blood to the left side.

• Systemic Circuit: Delivers oxygenated blood from the left side of the heart to the body and returns deoxygenated blood to the right side.

Example: The left ventricle is part of the systemic circuit, pumping blood to all body tissues except the lungs.

Major Vessels Associated with the Heart

Several major vessels are directly connected to the heart:

• Superior and Inferior Vena Cava: Bring deoxygenated blood from the body to the right atrium.

• Pulmonary Arteries: Carry deoxygenated blood from the right ventricle to the lungs.

• Pulmonary Veins: Return oxygenated blood from the lungs to the left atrium.

• Aorta: Distributes oxygenated blood from the left ventricle to the body.

Heart Valves and Their Function

Valves ensure unidirectional blood flow through the heart:

• Atrioventricular (AV) Valves: Tricuspid (right) and Mitral/Bicuspid (left) valves prevent backflow into the atria during ventricular contraction.

• Semilunar Valves: Pulmonary and Aortic valves prevent backflow into the ventricles after contraction.

Example: The mitral valve prevents blood from flowing back into the left atrium when the left ventricle contracts.

Coronary Circulation

The heart muscle receives its own blood supply via the coronary arteries.

• Right and Left Coronary Arteries: Branch from the aorta and supply oxygenated blood to the myocardium.

• Cardiac Veins: Collect deoxygenated blood from the myocardium and return it to the right atrium via the coronary sinus.

Example: Blockage of a coronary artery can lead to a myocardial infarction (heart attack).

Electrical Conduction System of the Heart

The heart's rhythmic contractions are coordinated by its intrinsic conduction system:

• Sinoatrial (SA) Node: The natural pacemaker, located in the upper wall of the right atrium; initiates the heartbeat.

• Atrioventricular (AV) Node: Receives impulses from the SA node and delays them before passing to the ventricles.

• Bundle of His (AV Bundle): Conducts impulses from the AV node to the bundle branches.

• Bundle Branches: Carry impulses through the interventricular septum.

• Purkinje Fibers: Distribute the impulse throughout the ventricles, causing contraction.

Example: Damage to the SA node can result in arrhythmias, requiring artificial pacemakers.

Electrocardiogram (ECG/EKG) Waves

An ECG records the electrical activity of the heart. Each wave represents a specific event:

• P wave: Atrial depolarization (contraction)

• QRS complex: Ventricular depolarization (contraction)

• T wave: Ventricular repolarization (relaxation)

Example: An abnormally long QRS complex may indicate a problem with ventricular conduction.

Cardiac Cycle

The cardiac cycle describes the sequence of events in one heartbeat, including contraction (systole) and relaxation (diastole) of the atria and ventricles.

• Atrial Systole: Atria contract, pushing blood into the ventricles.

• Ventricular Systole: Ventricles contract, ejecting blood into the pulmonary artery and aorta.

• Diastole: Chambers relax and fill with blood.

Example: During ventricular systole, the AV valves close and the semilunar valves open.

Heart Rate and Arrhythmias

Heart rate is the number of beats per minute. Abnormal rates include:

• Tachycardia: Heart rate above the normal range.

• Bradycardia: Heart rate below the normal range.

Example: Bradycardia may be normal in athletes but can indicate pathology if symptomatic.

Cardiac Output

Cardiac output is the volume of blood pumped by each ventricle per minute.

• Formula:

$\text{Cardiac Output (CO)} = \text{Heart Rate (HR)} \times \text{Stroke Volume (SV)}$

• Stroke Volume (SV): The amount of blood ejected by a ventricle with each beat.

• Factors Affecting CO: Heart rate, stroke volume, contractility, preload, afterload.

Example: If HR = 70 bpm and SV = 70 mL, then CO = 4900 mL/min.

Blood Pressure Measurement Sites

Blood pressure is commonly measured at the brachial artery (upper arm) or radial artery (wrist). In emergencies, carotid or femoral arteries may be used.

Regulation of Heart Rate

Heart rate is regulated by the autonomic nervous system:

• Sympathetic Stimulation: Increases heart rate and contractility.

• Parasympathetic Stimulation: Decreases heart rate.

• Other Factors: Hormones (e.g., epinephrine), electrolyte levels, temperature.

Summary Table: Heart Valves and Their Locations

Valve	Location	Function
Tricuspid	Between right atrium and right ventricle	Prevents backflow into right atrium
Pulmonary	Between right ventricle and pulmonary artery	Prevents backflow into right ventricle
Mitral (Bicuspid)	Between left atrium and left ventricle	Prevents backflow into left atrium
Aortic	Between left ventricle and aorta	Prevents backflow into left ventricle

Additional info:

• Understanding the cardiac cycle phases and valve actions is essential for interpreting heart sounds and murmurs.

• Arrhythmias can be diagnosed using ECG patterns and may require pharmacological or electrical intervention.