A fibrous sac called the pericardium surrounds the heart. This sac has two thin layers with fluid between them. This fluid reduces friction as the two layers rub against each other when the heart beats. Normally, this sac is thin and flexible, but repeated inflammation can cause it to become stiff and thick. When this happens, the heart can’t stretch properly as it beats. This can prevent the heart from filling up with as much blood as it needs. The lack of blood can cause increased pressure in the heart, a condition called constrictive pericarditis.