Laboratory Exercise 35 Heart Structure Answers

Laboratory Exercise 35: Heart Structure – Complete Answers and Study Guide

Laboratory Exercise 35 provides students with a hands-on opportunity to examine the anatomical structure of the mammalian heart, typically using a sheep heart or pig heart specimen. This exercise is designed to help you understand the external and internal anatomy of the heart, identify the major chambers, valves, and blood vessels, and comprehend how the heart functions as a pump. This thorough look will walk you through the key structures you should identify and explain their functions in detail.

Introduction to Heart Structure in the Laboratory

The heart is a muscular organ approximately the size of a closed fist, located in the thoracic cavity between the lungs. In Laboratory Exercise 35, you will examine a preserved heart specimen to observe both external features and internal structures. Understanding the heart's anatomy is fundamental to comprehending how blood circulates throughout the body and how the heart maintains life-sustaining blood flow.

The heart consists of four main chambers: two upper chambers called atria and two lower chambers called ventricles. It also contains four valves that ensure blood flows in one direction, along with numerous blood vessels that enter and exit the heart. By the end of this laboratory exercise, you should be able to identify all major structures and explain their roles in cardiac function.

External Anatomy of the Heart

When you first examine the heart specimen externally, several key features become immediately apparent. The heart is covered by a thin, transparent membrane called the pericardium, which protects the organ and anchors it within the thoracic cavity. Carefully remove this membrane to expose the underlying muscle tissue.

The Four Chambers

The external view reveals the four chambers of the heart:

Right Atrium: Located on the upper right portion of the heart (when viewed from the anterior perspective). This chamber appears somewhat triangular and receives deoxygenated blood from the body.
Left Atrium: Located on the upper left portion of the heart. This chamber receives oxygenated blood returning from the lungs.
Right Ventricle: The lower right chamber that pumps deoxygenated blood to the lungs.
Left Ventricle: The lower left chamber that pumps oxygenated blood to the entire body. This chamber has the thickest walls because it must generate the greatest force.

The right and left sides of the heart are separated externally by a groove called the interventricular sulcus (for the ventricles) and the coronary sulcus (for the atria). These grooves contain blood vessels that supply the heart muscle itself.

Major Blood Vessels

Several large blood vessels are visible on the external surface of the heart:

Superior Vena Cava: Enters the right atrium from above, bringing deoxygenated blood from the head, neck, and upper limbs.
Inferior Vena Cava: Enters the right atrium from below, bringing deoxygenated blood from the lower body.
Pulmonary Trunk: Exits the right ventricle and divides into the left and right pulmonary arteries that carry blood to the lungs.
Pulmonary Veins: Four veins (two from each lung) that enter the left atrium, carrying oxygenated blood.
Aorta: The largest artery in the body, exiting the left ventricle to distribute oxygenated blood to all parts of the body. The aorta has a characteristic arch that is visible externally.

The coronary arteries branch from the aorta just above the aortic valve and supply blood to the heart muscle itself. These arteries are often visible in the grooves on the heart's surface.

Internal Anatomy of the Heart

When you cut open the heart (or examine a pre-dissected specimen), you can observe the internal structures in detail. The interior of the heart reveals the chambers, valves, and the paths that blood takes through the organ.

The Atria

The atria serve as receiving chambers for blood. The right atrium has relatively thin walls because it only needs to pump blood the short distance to the right ventricle. Inside, you may observe the opening of the coronary sinus, a vein that drains blood from the heart muscle itself.

The left atrium also has thin walls and receives blood from the four pulmonary veins. Its interior is relatively smooth compared to the ventricles.

The Ventricles

The ventricles are the pumping chambers of the heart, and their structure reflects their different functions:

The right ventricle has moderately thick walls and pumps blood to the lungs, which are nearby and offer relatively low resistance.
The left ventricle has very thick walls (approximately three times thicker than the right ventricle) because it must generate enough pressure to pump blood throughout the entire body. When you examine a cross-section, you will clearly see this difference in wall thickness.

The interior of the ventricles contains papillary muscles that attach to the heart valves via tendinous cords called chordae tendineae. These structures prevent the valves from flipping backward during contraction.

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The Heart Valves

The heart contains four valves that ensure unidirectional blood flow:

Atrioventricular (AV) Valves (between atria and ventricles):

Tricuspid Valve: Located between the right atrium and right ventricle. It has three leaflets or "cusps," which is how it gets its name.
Mitral Valve (also called the bicuspid valve): Located between the left atrium and left ventricle. It has two cusps.

Semilunar Valves (at the exits of the ventricles):

Pulmonary Valve: Located at the exit of the right ventricle, where the pulmonary trunk begins.
Aortic Valve: Located at the exit of the left ventricle, where the aorta begins.

When examining the valves, notice that they open and close passively in response to pressure changes during the cardiac cycle. The AV valves close when the ventricles contract, preventing backflow into the atria. The semilunar valves close when the ventricles relax, preventing backflow from the arteries.

Blood Flow Through the Heart

Understanding the path of blood through the heart is essential for comprehending cardiac function. Here is the complete circuit:

Right Side of the Heart (Pulmonary Circuit):

Deoxygenated blood enters the right atrium through the superior and inferior vena cava.
Blood flows from the right atrium into the right ventricle through the tricuspid valve.
The right ventricle contracts, pumping blood through the pulmonary valve into the pulmonary trunk.
Blood travels to the lungs, where it picks up oxygen and releases carbon dioxide.

Left Side of the Heart (Systemic Circuit):

Oxygenated blood returns to the left atrium through the four pulmonary veins.
Blood flows from the left atrium into the left ventricle through the mitral valve.
The left ventricle contracts, pumping blood through the aortic valve into the aorta.
The aorta distributes oxygenated blood to all parts of the body.

Frequently Asked Questions

What is the function of the pericardium?

The pericardium is a double-layered membrane that surrounds the heart. It protects the heart from infection, anchors it within the thoracic cavity, and prevents the heart from over-expanding when it fills with blood.

Why is the left ventricle wall thicker than the right ventricle?

The left ventricle must generate sufficient pressure to pump blood through the entire body, which requires much more force than pumping blood just to the nearby lungs. This greater workload results in significantly thicker muscular walls.

What is the purpose of the chordae tendineae?

The chordae tendineae are tendinous cords that connect the papillary muscles to the AV valves. They prevent the valves from inverting back into the atria when the ventricles contract, ensuring that blood flows forward rather than leaking backward.

How do the heart valves function?

The valves function passively based on pressure differences. That said, when pressure is greater on the valve's upper side, it opens; when pressure is greater on the lower side, it closes. This automatic opening and closing ensures blood flows in only one direction.

What is the coronary circulation?

The coronary circulation refers to the blood vessels that supply the heart muscle itself. The coronary arteries branch from the aorta and deliver oxygenated blood to the myocardium, while cardiac veins drain blood into the coronary sinus, which empties into the right atrium.

Conclusion

Laboratory Exercise 35 provides an invaluable opportunity to examine the heart's complex anatomy firsthand. By carefully studying both the external and internal structures of the heart, you gain a deeper understanding of how this remarkable organ performs its vital functions.

The key structures to identify include the four chambers (right and left atria, right and left ventricles), the four valves (tricuspid, mitral, pulmonary, and aortic), and the major blood vessels (vena cava, pulmonary trunk, pulmonary veins, and aorta). Understanding the relationship between structure and function—why the left ventricle has thicker walls, how the valves prevent backflow, and why the heart is divided into separate right and left sides—will prepare you for more advanced studies in cardiovascular physiology and medicine.

This knowledge forms the foundation for understanding heart function, diagnosing cardiac disorders, and appreciating the involved design that keeps the human body alive through the continuous circulation of blood.