Final answer:
Cavitation is the mechanical effect defined by the compression and expansion of small gas bubbles in the blood and tissue due to ultrasound, used in medical therapies and processes like sonication.
Step-by-step explanation:
The mechanical effect caused by the compression and expansion of small gas bubbles in the blood and tissue from ultrasound is known as cavitation. This phenomenon occurs when ultrasound waves produce rapid localized pressure changes, leading to the formation and collapse of vapor cavities within a fluid. In medical therapy, cavitation can be intentionally induced through ultrasound to damage or lyse cells, which is useful in procedures such as shattering gallstones or destroying cancerous tissues. Additionally, cavitation plays a role in the disruption of cellular structures during sonication, a process that uses high-frequency ultrasound waves to lyse cells for the release of their contents for further analysis.