The Law of LaPlace explains
O why stretching of the heart increase the stroke volume
• the different factors that add to resistance of fluid flow
• the difference between resistance and compliance in small arterioles
• why an enlarged section of a blood vessel is more likely to burst than a narrow section of the same vessel
• why stretching of the heart increases heart rate
The Law of LaPlace states that the tension or stress within a vessel wall is directly proportional to the amount of pressure within the vessel and the radius of the vessel. Therefore, when the heart is stretched due to increased blood volume, the tension within the heart wall increases, which in turn increases the force of contraction and stroke volume.
Resistance to fluid flow is affected by factors such as the diameter of the vessel, viscosity of the fluid, and length of the vessel. Resistance is the opposition to flow, while compliance is the ability of a vessel to expand or contract in response to changes in pressure.
An enlarged section of a blood vessel has a greater radius and therefore more tension within the vessel wall. This increased tension can cause weakness and ultimately rupture, whereas a narrow section has less tension and is less likely to burst.
Stretching of the heart does not increase heart rate. However, it can increase contractility and stroke volume, which may initially lead to an increased heart rate as the body compensates for the new flow demands.
1. The Law of Laplace explains why stretching of the heart increases stroke volume. According to this law, the tension in the wall of a hollow structure, such as the heart, is directly proportional to its radius. When the heart is stretched, the radius increases, which leads to an increase in tension within the heart muscle. As a result, the muscle fibers generate a stronger force during contraction, allowing the heart to eject a greater volume of blood with each heartbeat (increased stroke volume).
2. There are several factors that contribute to resistance in fluid flow:
- Viscosity: The thickness or stickiness of a fluid affects how easily it flows. Higher viscosity increases resistance.
- Length of the vessel: Longer vessels offer more resistance to fluid flow compared to shorter ones.
- Radius of the vessel: The narrower the vessel's radius, the higher the resistance to fluid flow. According to the Law of Poiseuille, resistance is inversely proportional to the fourth power of the vessel radius.
3. In small arterioles, resistance and compliance have distinct meanings:
- Resistance refers to the opposition that blood encounters as it flows through the small arterioles. Resistance is mainly determined by the diameter or radius of the arterioles. A smaller radius increases resistance and hampers blood flow.
- Compliance refers to the ability of the arterioles to stretch or expand in response to changes in pressure. Compliance allows the arterioles to accommodate changes in blood volume and maintain a steady pressure. Compliance is primarily dependent on the elasticity of the arterioles' walls.
4. An enlarged section of a blood vessel is more likely to burst than a narrow section of the same vessel due to increased wall tension. According to the Law of Laplace, the tension in the wall of a vessel is directly proportional to its radius. When a vessel has a larger radius, the tension in the vessel wall increases. If the wall tension exceeds the wall's strength, it can lead to vessel rupture or bursting. Therefore, an enlarged section of a blood vessel is more susceptible to bursting.
5. Stretching of the heart does not directly increase heart rate. However, stretching of the heart can indirectly influence heart rate through the Frank-Starling mechanism. When the heart is stretched, the muscle fibers generate a stronger force during contraction (as explained by the Law of Laplace) and also stretch the cardiac muscle cells. This stretch triggers the release of neurotransmitters that enhance the heart's automaticity, leading to an increase in heart rate. In other words, stretching of the heart enhances its ability to contract forcefully and stimulates the heart's intrinsic pacemaker, resulting in an elevated heart rate.