When external pressure exceeds normal capillary pressure of 25 mm Hg, blood flow in the capillary beds is decreased. When the external pressure surpasses arteriole pressure, blood flow to the area is impaired. Ischemia occurs when the pressure exceeds 50 mm Hg and blood flow is completely blocked. Pressure from the bony prominence is transmitted from the surface of the body to the underlying bone, and all underlying tissues are compressed.

768 Pressure Ulcer

Pressure ulcers caused by shearing or friction result when one tissue layer slides over another. Shearing results in stretching and angulating of blood vessels, causing injury and thrombosis to the area. These injuries commonly occur when the head of the bed is elevated, causing the torso to slide downward.

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