Anatomy of the Heart Lesson: Structure, Function & More

Lesson Overview

• Location and Overall Structure
• Heart Chambers and Blood Flow
• Key Differences Between Chambers:
• Heart Valves and One-Way Flow
• Cardiac Conduction System
• Fetal Circulation and Adaptations
• Heart Wall and Layers

The human heart is a fist-sized muscular organ located in the chest between the lungs. It functions as the central component of the circulatory system, responsible for pumping blood throughout the body. The heart ensures that oxygen and nutrients are delivered to tissues while waste products like carbon dioxide are removed. 

The heart's anatomy is critical to understanding how it efficiently circulates blood, and this lesson will explore its structure, function, and the pathway of blood through its chambers and valves.

Location and Overall Structure

The heart's position within the thoracic cavity and its structure form the foundation of its function. Understanding its location helps clarify how it interacts with other organs.

• Mediastinum Position: The heart is located in the mediastinum, the central compartment of the chest, between the lungs. It sits behind the sternum and is tilted slightly to the left. This leftward tilt explains why the heartbeat is typically felt on the left side of the chest.
  
• Protective Pericardium: The heart is encased in a double-layered sac known as the pericardium, which serves to protect the heart and prevent friction during its contractions. The outer layer, the fibrous pericardium, provides a tough outer covering, while the inner serous pericardium consists of two layers: the parietal and visceral layers. The visceral layer is also called the epicardium and is in direct contact with the heart's surface. Between these layers is the pericardial cavity, which contains lubricating fluid that reduces friction.
  
• External Anatomy: Externally, the heart's four chambers are delineated by grooves. The atrial-ventricular sulcus marks the separation between the atria and the ventricles, while the interventricular sulcus divides the left and right ventricles. These grooves reflect the internal partitions and help orient the heart's structure.
  
• Double-Pump System: The heart operates as a double pump, with the right side handling pulmonary circulation (blood to the lungs) and the left side managing systemic circulation (blood to the rest of the body). This setup ensures efficient oxygenation of the body and optimal distribution of oxygenated blood.

Heart Chambers and Blood Flow

The heart consists of four chambers – two atria and two ventricles – which work together to circulate blood through the body. The pathway of blood through these chambers is critical for understanding the heart's function.

Blood Flow Through the Right Side:

1. Right Atrium (RA): Oxygen-poor blood returns from the body through the superior vena cava (upper body) and inferior vena cava (lower body) into the right atrium. The RA holds this blood and prepares to send it into the right ventricle.
   
2. Right Ventricle (RV): The tricuspid valve, located between the RA and RV, opens to allow blood flow into the RV. Once the RV contracts, it sends blood through the pulmonary valve into the pulmonary artery, which carries the blood to the lungs for oxygenation.

Blood Flow Through the Left Side:

3. Left Atrium (LA): After blood is oxygenated in the lungs, it returns to the heart through the pulmonary veins into the left atrium. This oxygen-rich blood is then directed toward the left ventricle.
   
4. Left Ventricle (LV): The mitral (bicuspid) valve allows blood to pass from the LA to the LV. The LV, which is the most muscular chamber, then contracts, sending the oxygen-rich blood through the aortic valve into the aorta, from where it is distributed to the entire body.

This continuous loop ensures that deoxygenated blood is sent to the lungs for oxygenation, while oxygenated blood is pumped throughout the body to nourish tissues.

Take This Quiz:

Anatomy of the Heart quiz

Key Differences Between Chambers:

• Atria vs. Ventricles: The atria are thin-walled chambers responsible for receiving blood from the veins and passing it to the ventricles. The ventricles are thicker and more muscular because they must generate higher pressure to pump blood to the lungs (right ventricle) or the entire body (left ventricle).
  
• Left vs. Right Ventricles: The left ventricle has a much thicker muscular wall compared to the right because it must pump blood to the entire body, which requires more force. The right ventricle only pumps blood a short distance to the lungs, which requires less pressure.

Heart Valves and One-Way Flow

The heart has four main valves that control the direction of blood flow and prevent backflow. These valves open and close in response to pressure changes during heart contractions.

The Four Valves:

1. Tricuspid Valve: This valve is located between the right atrium and right ventricle. It prevents blood from flowing back into the atrium when the ventricle contracts.
   
2. Mitral (Bicuspid) Valve: Found between the left atrium and left ventricle, the mitral valve performs the same function as the tricuspid valve but on the left side of the heart.
   
3. Pulmonary Valve: This semilunar valve is located between the right ventricle and the pulmonary artery. It prevents blood from returning to the right ventricle after it is pumped to the lungs.
   
4. Aortic Valve: Similar to the pulmonary valve, the aortic valve is located between the left ventricle and the aorta, ensuring that blood flows efficiently from the heart to the body.

Valve Function:

• The atrioventricular (AV) valves (tricuspid and mitral) prevent backflow from the ventricles to the atria during ventricular contraction.
  
• The semilunar valves (pulmonary and aortic) prevent backflow from the arteries into the ventricles after the blood has been pumped out.

Valve Structure:

• The AV valves are equipped with chordae tendineae (fibrous cords) that connect the valve flaps to papillary muscles in the ventricles. These structures help prevent the valves from prolapsing into the atria during ventricular contraction.
  
• The semilunar valves do not have chordae tendineae. Instead, they are shaped like half-moons and function based on pressure differences. When the ventricles contract, these valves open; when the ventricles relax, the valves close, preventing blood from flowing backward.

Cardiac Conduction System

The heart's electrical system controls the rhythm of heartbeats and ensures that the atria contract first, followed by the ventricles. This coordination is essential for efficient blood pumping.

The Path of Electrical Impulses:

1. Sinoatrial (SA) Node: Located in the upper part of the right atrium, the SA node acts as the natural pacemaker of the heart. It generates electrical impulses that initiate each heartbeat. These impulses spread through the atria, causing them to contract and push blood into the ventricles.
   
2. Atrioventricular (AV) Node: The electrical signal then travels to the AV node, located at the junction of the atria and ventricles. The AV node briefly delays the impulse, ensuring the atria finish contracting before the ventricles begin to contract.
   
3. Bundle of His and Purkinje Fibers: The electrical impulse travels down the Bundle of His and then through the Purkinje fibers, which distribute the signal throughout the ventricles. This ensures coordinated contraction of the ventricles, allowing blood to be efficiently pumped out of the heart.

Electrical Impulse Summary:

• SA Node → Atria → AV Node → Bundle of His → Purkinje Fibers → Ventricles

This electrical conduction system ensures that the heart beats in a coordinated rhythm. The SA node sets the pace, while the AV node provides a slight delay, and the Purkinje fibers ensure the ventricles contract at the correct time.

Fetal Circulation and Adaptations

In fetal circulation, the heart and blood vessels undergo unique adaptations that allow blood to bypass the lungs, which are not yet functional. These shunts close after birth, and the circulatory system changes to accommodate breathing.

Key Fetal Circulatory Adaptations:

1. Foramen Ovale: This is an opening between the right and left atria that allows oxygen-rich blood from the placenta to bypass the lungs and enter the left atrium.
   
2. Ductus Arteriosus: A vessel connecting the pulmonary artery to the aorta, which allows blood to bypass the lungs and flow directly to the systemic circulation.

Changes After Birth:

• When the baby is born and begins breathing, the lungs expand, and the pressure in the right atrium drops. This causes the foramen ovale to close, and the ductus arteriosus constricts, redirecting blood to the lungs for oxygenation.

Take This Quiz:

quiz questions regarding the heart

Heart Wall and Layers

The heart's wall is made up of three layers: the epicardium, myocardium, and endocardium. Each layer has a specific function in ensuring the heart functions efficiently.

Layers of the Heart:

1. Epicardium: The outermost layer, also known as the visceral pericardium, is a thin, transparent membrane that covers the heart's surface. It contains the coronary arteries.
   
2. Myocardium: The middle layer of the heart, made up of cardiac muscle, is the thickest layer and is responsible for the contraction of the heart.
   
3. Endocardium: The innermost layer, the endocardium lines the heart's chambers and covers the valves. It ensures smooth blood flow and minimizes turbulence.

Function of Each Layer:

• Epicardium: Provides a smooth, protective surface and contains blood vessels.
  
• Myocardium: The muscle layer that contracts to pump blood.
  
• Endocardium: Prevents clotting and maintains smooth blood flow.

Each layer works together to ensure the heart can pump blood effectively and efficiently. Understanding these layers and their functions helps clarify the heart's overall role in the body's circulatory system.