Question

The "lub," or first heart sound, is produced by closing of a. the aortic semilunar valve. b. the pulmonary semilunar valve. c. the tricuspid valve. d. the bicuspid valve. e. both AV valves.

Short answer

The "lub" sound or the first heart sound is produced by the closing of both atrioventricular (AV) valves, which include the tricuspid and bicuspid valves. These valves prevent the backflow of blood from the ventricles to the atria and ensure that blood moves forward to the lungs and the rest of the body. Thus, the correct answer is option e. both AV valves.

Step-by-step solution

The first step is to have a general understanding of the structure and functions of the heart. In a nutshell, the heart has four chambers: left and right atria (top chambers) and left and right ventricles (bottom chambers). Blood comes to the right atrium from the body, gets pumped into the right ventricle, and then goes to the lungs to receive oxygen. From the lungs, the blood returns to the left atrium, gets pumped into the left ventricle, and then is sent out to the rest of the body. #Step 2: Review the valves in the heart#

The heart has four valves that regulate the flow of blood between these chambers and the major arteries and veins. The tricuspid valve is located between the right atrium and right ventricle, whereas the bicuspid or mitral valve separates the left atrium and left ventricle. These are called atrioventricular (AV) valves. The other two valves are semilunar valves. The pulmonary semilunar valve is between the right ventricle and pulmonary artery, while the aortic semilunar valve is between the left ventricle and the aorta. #Step 3: Determine which valves closing produces the "lub" sound#

The "lub" sound or the first heart sound is produced when the atrioventricular (AV) valves, the tricuspid and bicuspid valves, close. The closure of these valves prevents the backflow of blood from the ventricles to the atria, ensuring that blood moves forward to the lungs and the rest of the body. #Step 4: Identify the correct answer choice#

After analyzing the function and structure of the heart valves, we can now determine the correct answer. In this case, the "lub" sound, or the first heart sound, is produced by the closing of both AV valves. So the correct answer is option e. both AV valves.

Key concepts

Heart Structure

Understanding the heart's structure is vital to comprehending how it functions efficiently. The heart is a muscular organ, roughly the size of a closed fist, located in the chest. It acts as a pump to circulate blood throughout the body. It is divided into four chambers:

• The right atrium: Receives deoxygenated blood from the body through the superior and inferior vena cavae.
• The right ventricle: Pumps this blood into the pulmonary arteries, leading to the lungs where gas exchange occurs.
• The left atrium: Collects oxygenated blood from the lungs via the pulmonary veins.
• The left ventricle: The strongest chamber, it pumps oxygen-rich blood into the aorta distributing it to the entire body.

Each atrium and ventricle is identified by their location on either the left or right side of the heart. The septum, a wall of muscle, separates the left and right sides, preventing the mixing of oxygen-rich and oxygen-poor blood.
The heart's unique structure allows it to perform its critical function of maintaining a steady blood supply, essential for survival.

Heart Valves

Heart valves play a crucial role in directing blood flow through the heart's chambers and major blood vessels. There are four main valves in the human heart:

• Atrioventricular (AV) Valves: Include the tricuspid valve between the right atrium and right ventricle, and the mitral (or bicuspid) valve between the left atrium and left ventricle. They ensure that blood flows from the atria to the ventricles.
• Semilunar Valves: Comprise the pulmonary valve between the right ventricle and the pulmonary artery, and the aortic valve between the left ventricle and the aorta. These valves control blood flow out of the heart and prevent backflow.

When the heart muscle contracts, these valves open and close, tightly regulating blood flow and ensuring it moves in the correct direction.
Valve malfunction can lead to heart inefficiency or diseases like stenosis or regurgitation, stressing the importance of healthy heart valves.

Cardiac Cycle

The cardiac cycle describes the sequence of events in a single heartbeat. It is composed of two main phases: systole and diastole.

• Systole: Occurs when heart muscle contracts. During this phase, blood is pumped out of the ventricles. Pressure in the ventricles rises, forcing the AV valves (tricuspid and bicuspid) to close producing the "lub" sound, while semilunar valves open allowing blood to flow to the lungs and body.
• Diastole: This is the relaxation phase. Blood fills the atria, pushing the valves open passively to refill the ventricles in preparation for the next cycle. During this time the heart's muscle tissue gets oxygenated blood flow.

This cycle ensures the heart effectively pumps blood continuously, supporting the body's metabolic demands. Each complete cardiac cycle typically lasts under a second, around 0.8 seconds, demonstrating the heart's efficiency and capability of sustaining life non-stop.