The Heart: Structure, Blood Flow, and Valves

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The Heart and Circulatory Pathways

Overview of Heart Function

The heart acts as a dual pump, propelling blood through two major circuits: the pulmonary circuit and the systemic circuit. Each side of the heart serves a distinct role in oxygenating blood and distributing it throughout the body.

Pulmonary Circuit: The right side of the heart pumps oxygen-poor, carbon dioxide-rich blood to the lungs for gas exchange.
Systemic Circuit: The left side of the heart pumps oxygen-rich, carbon dioxide-poor blood to all body tissues.

Diagram of pulmonary and systemic circulation

Pathway of Blood Through the Heart

Blood flows through the heart in a specific sequence, ensuring efficient oxygenation and nutrient delivery. Equal volumes of blood are pumped to both circuits, but the systemic circuit faces higher resistance, reflected in the thicker wall of the left ventricle.

Blood enters the right atrium from the superior and inferior vena cava.
Passes through the tricuspid valve into the right ventricle.
Right ventricle pumps blood through the pulmonary semilunar valve into the pulmonary trunk and arteries, leading to the lungs.
Oxygenated blood returns via pulmonary veins to the left atrium.
Passes through the bicuspid (mitral) valve into the left ventricle.
Left ventricle pumps blood through the aortic semilunar valve into the aorta and systemic circulation.

Frontal section of the heart showing chambers and valves

Heart Chambers and Major Vessels

Atria: The Receiving Chambers

The atria are the upper chambers of the heart, responsible for receiving blood returning to the heart.

Right Atrium: Receives blood from the superior and inferior vena cava (systemic veins).
Left Atrium: Receives blood from the right and left pulmonary veins (lungs).

Ventricles: The Discharging Chambers

The ventricles are the lower chambers, responsible for pumping blood out of the heart.

Right Ventricle: Pumps blood into the pulmonary trunk (to the lungs).
Left Ventricle: Pumps blood into the aorta (to the systemic circulation).

Heart Valves and Their Function

Atrioventricular (AV) Valves

AV valves prevent backflow of blood into the atria when the ventricles contract. They are anchored by chordae tendineae to papillary muscles, ensuring one-way flow.

Tricuspid Valve: Located between the right atrium and right ventricle.
Mitral (Bicuspid) Valve: Located between the left atrium and left ventricle.

Mechanism of AV valve opening and closing

Semilunar (SL) Valves

SL valves prevent backflow into the ventricles when the ventricles relax. They open and close in response to pressure changes during the cardiac cycle.

Pulmonary Semilunar Valve: Between the right ventricle and pulmonary trunk.
Aortic Semilunar Valve: Between the left ventricle and aorta.

Mechanism of semilunar valve opening and closing

Structural Differences Between Ventricles

Ventricular Wall Thickness

The left ventricle has a much thicker wall than the right ventricle, reflecting the greater force needed to pump blood through the high-resistance systemic circuit compared to the low-resistance pulmonary circuit.

Cross-section showing left and right ventricular wall thickness

Summary Table: Heart Valves and Their Locations

Valve	Location	Function
Tricuspid (Right AV)	Between right atrium and right ventricle	Prevents backflow into right atrium
Mitral (Left AV/Bicuspid)	Between left atrium and left ventricle	Prevents backflow into left atrium
Pulmonary Semilunar	Between right ventricle and pulmonary trunk	Prevents backflow into right ventricle
Aortic Semilunar	Between left ventricle and aorta	Prevents backflow into left ventricle

Key Concepts and Additional Information

Chordae Tendineae: Tendinous cords that anchor AV valve cusps to papillary muscles, preventing valve prolapse.
Papillary Muscles: Contract to tighten chordae tendineae during ventricular contraction.
Fibrous Skeleton of the Heart: Dense connective tissue that supports valves and electrically isolates atria from ventricles.

Example: Cardiac Cycle and Valve Operation

During ventricular contraction (systole), AV valves close to prevent backflow into the atria, while SL valves open to allow blood ejection. During ventricular relaxation (diastole), SL valves close to prevent arterial blood from re-entering the ventricles, and AV valves open to allow ventricular filling.

Equations

The cardiac output (CO) is a key measure of heart function:

Where:

CO: Cardiac Output (mL/min)
HR: Heart Rate (beats/min)
SV: Stroke Volume (mL/beat)

Visual Summary

Diagram of pulmonary and systemic circulation Frontal section of the heart showing chambers and valves *Additional info: The above notes integrate anatomical and physiological context to provide a comprehensive overview of the heart's structure and function, suitable for ANP college students preparing for exams.*