In fact, scar tissue can restrict many layers of muscle and connective tissue, which can cause varying degrees of pain or discomfort. Scar tissue is weaker, less elastic and more prone to re-injury than normal, healthy tissue, and can lead to chronic pain if it affects the functioning of other structures--especially nerves and blood vessels.
Negative pressure means the pressure in the pipe/duct/room is less so flow would go into the room. These are all gauge pressures, which can be positive or negative since they are merely measuring a relative pressure compared against some constant pressure, typically atmospheric which is about 14.7 psi.
This type of pressure reading is called gage pressure. p = 50 kPa gage or p = 150 kPa absolute. Whenever atmospheric pressure is used as a reference, the possibility exists that the pressure thus measured can be either positive or negative. Negative gage pressure are also termed as vacuum pressures. Hence, if a gage tapped into a tank indicates a vacuum pressure of 31 kPa, this can also be stated as 70 kPa absolute, or -31 kPa gage, assuming that the atmospheric pressure is 101 kPa absolute.