The mitral and tricuspid valves close after the ventricles are filled with blood because the
A.
Chordae tendineae relax, which allows the valves to close.
B.
Increased pressure in the ventricles pushes the valves to close.
C.
Trabeculae carneae contract, which pulls the valves closed.
D.
Reduced pressure in the atria creates a negative pressure that pulls the valves closed.
The Correct Answer and Explanation is:
The correct answer is B. Increased pressure in the ventricles pushes the valves to close.
Explanation:
The mitral and tricuspid valves are part of the heart’s atrioventricular (AV) valve system, which separates the atria from the ventricles. Their primary function is to ensure that blood flows in one direction—from the atria to the ventricles—without regurgitating back into the atria.
Valve Closure Mechanism:
- Ventricular Filling:
- During diastole (the heart’s relaxation phase), blood fills the ventricles from the atria. As the ventricles fill, the pressure within them rises.
- Increasing Ventricular Pressure:
- Once the ventricles are filled with blood and the pressure starts to build up, this increased pressure is the key factor in closing the AV valves. The mitral valve (left AV valve) and the tricuspid valve (right AV valve) are positioned between the atria and ventricles. When the ventricles contract (systole), the pressure inside the ventricles increases significantly.
- Valve Closure:
- As the ventricular pressure exceeds the pressure in the atria, it forces the cusps of the mitral and tricuspid valves to close. This prevents blood from flowing backward into the atria and ensures that it moves forward into the pulmonary artery and aorta. The closure of these valves is crucial for the effective pumping of blood out of the ventricles and maintaining the unidirectional flow of blood through the heart.
Why Other Options Are Incorrect:
- A. Chordae tendineae relax, which allows the valves to close:
- The chordae tendineae are connective tissue strands that anchor the valve cusps to the papillary muscles in the ventricles. They help to keep the valves closed under pressure, rather than causing them to close.
- C. Trabeculae carneae contract, which pulls the valves closed:
- Trabeculae carneae are muscular ridges in the ventricles and do not play a direct role in closing the AV valves. Their contraction helps to strengthen the ventricular contraction but does not directly close the valves.
- D. Reduced pressure in the atria creates a negative pressure that pulls the valves closed:
- The closing of the valves is primarily driven by the pressure dynamics within the ventricles rather than a negative pressure in the atria.
Thus, the increased pressure in the ventricles effectively forces the mitral and tricuspid valves to close, ensuring proper cardiac function and efficient blood flow.