Heart and Cardiovascular System Anatomy Lesson

Lesson Overview

• Heart Location and Orientation
• Layers of the Heart and Pericardium
• Chambers of the Heart
• Heart Valves
• Blood Flow Through the Heart
• Cardiac Conduction System
• Cardiac Cycle Phases
• Cardiac Output
• Electrocardiogram (ECG) Waves

The human heart is a muscular organ that pumps blood throughout the body. It lies in the center of the chest (thoracic cavity), slightly to the left, and supplies oxygen and nutrients to tissues while removing waste products. 

Understanding the heart's anatomy – its chambers, valves, layers, and electrical system – is key to grasping how blood circulates and how the cardiovascular system functions as a whole. This lesson provides a structured overview of heart anatomy and core concepts underlying heart function.

Heart Location and Orientation

The heart is located in the mediastinum, the central compartment of the chest, between the lungs. It sits behind the sternum and above the diaphragm, with about two-thirds of its mass positioned to the left of the body's midline. 

The heart's pointed apex (the tip of the left ventricle) angles downward toward the left hip. The broader base (top part of the heart) faces upward toward the right shoulder and is where the major blood vessels attach. This slight leftward orientation explains why the heartbeat is often felt more strongly on the left side of the chest.

Layers of the Heart and Pericardium

The heart is enclosed in a protective sac called the pericardium. The outer fibrous pericardium is a tough connective tissue layer that anchors the heart in place and prevents overstretching. Inside it is the serous pericardium, a double-layered membrane with a parietal layer (lining the fibrous sac) and a visceral layer (adhering to the heart's surface). 

The visceral layer of the serous pericardium is also the epicardium, the outermost layer of the heart wall. Between the parietal and visceral layers is the pericardial cavity, a thin space filled with lubricating fluid that reduces friction as the heart beats.

The heart wall itself has three layers:

• Epicardium: outer layer (same as the visceral pericardium), a thin protective layer.
  
• Myocardium: middle layer of cardiac muscle tissue. This thick, muscular layer provides the force for pumping. The myocardium is especially thick in the left ventricle, which must pump blood to the entire body.
  
• Endocardium: inner layer, a smooth lining of the chambers and valves. It minimizes friction for blood flowing inside the heart.

However, if too much fluid builds up in the pericardial cavity (a condition called cardiac tamponade), it can compress the heart and impair its filling.

Chambers of the Heart

The heart has four chambers that collect and pump blood in a coordinated way. The two upper chambers, called atria (singular: atrium), receive blood entering the heart. The two lower chambers, called ventricles, pump blood out of the heart:

• Right Atrium (RA): Receives deoxygenated (oxygen-poor) blood returning from the body via the superior and inferior vena cava.
  
• Right Ventricle (RV): Receives blood from the RA and pumps this deoxygenated blood to the lungs via the pulmonary artery. The RV's muscular wall is thinner than the LV's, as pumping to the nearby lungs requires less force.
  
• Left Atrium (LA): Receives oxygenated blood from the lungs through four pulmonary veins.
  
• Left Ventricle (LV): Receives blood from the LA and pumps oxygenated blood out to the entire body through the aorta. The LV has the thickest wall of myocardium, reflecting the high force needed to propel blood through the systemic circulation.

A muscular septum divides the heart into left and right sides, preventing mixing of oxygenated and deoxygenated blood. Functionally, the right side of the heart is the pump for the pulmonary circuit (sending blood to the lungs for oxygenation), and the left side is the pump for the systemic circuit (sending oxygen-rich blood to all body tissues).

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Heart Valves

Four one-way valves within the heart keep blood moving in the correct direction and prevent backflow. They open and close in response to pressure changes as the chambers contract and relax:

• Atrioventricular (AV) valves: These valves are between the atria and ventricles. The right AV valve, or tricuspid valve, has three cusps and separates the right atrium from the right ventricle. The left AV valve, also called the bicuspid (mitral) valve, has two cusps and separates the left atrium from the left ventricle. When the atria contract, the AV valves open to allow blood into the ventricles. When the ventricles contract, pressure forces the AV valves shut, preventing backflow into the atria. To reinforce this closure, thin fibers called chordae tendineae anchor the valve cusps to small papillary muscles on the ventricle walls; this mechanism prevents the valve flaps from inverting during ventricular contraction.
  
• Semilunar valves: These valves are located at the exits of the ventricles, where blood leaves the heart. Each has three pocket-like cusps. The pulmonary valve is between the right ventricle and the pulmonary trunk (leading to the lungs), and the aortic valve is between the left ventricle and the aorta. During ventricular contraction, the semilunar valves open to let blood be ejected into the pulmonary artery and aorta. When the ventricles relax, some blood briefly flows backward, filling the semilunar valve cusps and snapping them shut. This prevents blood from re-entering the ventricles from the arteries.

Proper functioning of the valves ensures efficient, unidirectional flow of blood through the heart.

Blood Flow Through the Heart

Blood flows through the heart in a specific sequence, passing through chambers and valves to complete the pulmonary and systemic circuits. Starting from the body and returning to the body, the path is:

1. Body to Right Atrium: Deoxygenated blood from the body enters the right atrium via the superior and inferior vena cavae.
   
2. Right Atrium to Right Ventricle: The RA contracts, pushing blood through the tricuspid valve into the right ventricle.
   
3. Right Ventricle to Lungs: The RV contracts and pumps blood through the pulmonary semilunar valve into the pulmonary arteries towards the lungs.
   
4. Lungs to Left Atrium: In the lung capillaries, the blood releases carbon dioxide and picks up oxygen. Oxygenated blood then returns to the left atrium through the pulmonary veins.
   
5. Left Atrium to Left Ventricle: The LA contracts, sending blood through the mitral (bicuspid) valve into the left ventricle.
   
6. Left Ventricle to Body: The LV contracts and propels blood through the aortic semilunar valve into the aorta. From the aorta, blood is distributed through the systemic arteries to the entire body.
   

Once blood delivers oxygen to the tissues and picks up carbon dioxide, it returns via the veins to the heart's right atrium, and the cycle repeats. This continuous loop ensures that deoxygenated blood is sent to the lungs for reoxygenation and oxygen-rich blood is delivered to the body with each heartbeat.

Cardiac Conduction System

The heart has an internal electrical conduction system that coordinates each heartbeat. This specialized network allows the heart muscle to depolarize (activate) in an orderly sequence, causing the chambers to contract at the right time. The key components of this system, in the order an impulse travels, are:

• Sinoatrial (SA) Node: The heart's natural pacemaker, located in the right atrium near where the superior vena cava enters. The SA node fires electrical impulses at a regular rhythm, initiating each heartbeat. Each impulse spreads quickly through both atria, causing the atrial muscles to depolarize and contract.
  
• Atrioventricular (AV) Node: Located at the junction between the atria and ventricles (within the interatrial septum). The AV node receives the electrical impulse from the atria and delays it slightly, allowing the atria to finish contracting before the impulse moves on to the ventricles.
  
• AV Bundle (Bundle of His) and Bundle Branches: From the AV node, the impulse travels through the AV bundle in the interventricular septum (the only electrical link between atria and ventricles). The AV bundle then splits into the right and left bundle branches, which run down the septum toward the apex of the heart, carrying the signal into both ventricles.
  
• Purkinje Fibers: These fibers spread the impulse rapidly throughout the ventricular walls, causing both ventricles to depolarize and contract nearly simultaneously from the apex upward (pushing blood up and out).

This sequence (SA node → AV node → AV bundle → bundle branches → Purkinje fibers) ensures that the atria contract before the ventricles. The coordinated spread of excitation causes the ventricles to contract from the bottom up, which maximizes the efficient ejection of blood out of the heart.

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Cardiac Cycle Phases

Each heartbeat consists of a repeating cardiac cycle of coordinated contraction and relaxation:

• Systole: The contraction phase of the heart. Usually "systole" refers to ventricular systole, when the ventricles contract to pump blood out. During the ventricular systole, pressure inside the ventricles rises sharply. The AV valves close as soon as the ventricles begin contracting (preventing backflow into the atria), and once the pressure is high enough, the semilunar valves open so blood can be ejected into the aorta and pulmonary artery.
  
• Diastole: The relaxation phase, when the heart muscle relaxes and the chambers refill with blood. During ventricular diastole, the ventricles relax and the pressure within them falls. The semilunar valves snap shut to prevent blood from flowing back from the arteries. Once ventricular pressure drops below atrial pressure, the AV valves open and blood flows from the atria into the ventricles.

Cardiac Output

Cardiac output (CO) is the volume of blood the heart pumps per minute, and it is calculated by multiplying the heart rate (beats per minute) by the stroke volume (blood volume pumped per beat). CO = HR × SV.

At rest, a typical adult's CO is about 5 L/min. During vigorous exercise, it can increase several-fold as heart rate and stroke volume rise to meet the body's needs.

Electrocardiogram (ECG) Waves

An electrocardiogram (ECG) records the heart's electrical activity as a series of waves corresponding to depolarization and repolarization of the atria and ventricles:

• P Wave: Represents atrial depolarization (the electrical impulse spreading over the atria), which causes the atrial muscles to contract.
  
• QRS Complex: Represents ventricular depolarization (electrical activation of the ventricles), which triggers the ventricles to contract. The QRS complex is larger than the P wave because the ventricles have more muscle mass. Note: Atrial repolarization occurs during the QRS period, but it is not seen as a separate wave on the ECG because it is obscured by the stronger ventricular signals.
  
• T Wave: Represents ventricular repolarization (the recovery phase as the ventricular muscle cells reset electrically and prepare for the next beat). The ventricles relax during this electrical "reset."

By analyzing these waves, one can link the ECG trace to the heart's mechanical events: the P wave precedes atrial contraction, the QRS complex precedes ventricular contraction, and the T wave corresponds to ventricular relaxation. Thus, the T wave is the electrical signature of ventricular repolarization.

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