La Place's Law

Imagine blood flowing through a blood vessel which has a certain radius and a certain wall thickness. The blood vessel wall is stretched as a result of the difference between the blood pressure inside the vessel and the surrounding pressure outside the vessel. La Place's law describes the relationship between the transmural pressure difference and the tension, radius, and thickness of the vessel wall. Obviously, the higher the pressure difference the more tension there will be. On the other hand, the thicker the wall the less tension there is. Also, the larger the radius the more tension there is. These three rules culminate into one equation:

T = ( P * R ) / M

Where T is the tension in the walls, P is the pressure difference across the wall, R is the radius of the cylinder, and M is the thickness of the wall. An example of LaPlace Law is Dilated cardiomyopathy. In this condition heart becomes greatly distended and the radius (R) of ventricle increases. Therefore to create the same pressure (P) during ejection of the blood much larger wall tention (T) has be developed by the cardiac muscle. Thus dilated heart requires more energy to pump the same amount of blood as compared to the heart of normal size. The new surgical procedure, called ventricular remodeling, uses LaPlace principle to improve the function of dilated, failing hearts.

Imagine yourself blowing a balloon. The harder you blow the higher the air pressure inside the balloon and the higher the pressure difference between the outside and inside of the balloon become. Since the pressure difference rises, the tension in the rubber walls of the balloon also rises, and this is what causes the balloon to stretch. Now imagine you are blowing a balloon which is made of much thicker rubber. Now you will notice that the balloon is harder to inflate because more pressure difference is required to raise the tension in the walls of the balloon.