Final answer:
The first Korotkoff sound represents the systolic pressure, occurring as blood begins to flow through the compressed artery when measuring blood pressure using a sphygmomanometer and stethoscope.
Step-by-step explanation:
The first Korotkoff sound heard through the stethoscope signifies the return of blood flow to the arm and indicates the systolic pressure. This sound arises due to the turbulent blood flow through the partially compressed brachial artery when the cuff's pressure is just below the systolic blood pressure. As more air is released from the cuff and it continues to relax, the artery opens more fully, allowing for free blood flow, and eventually, all sounds disappear. The moment the last Korotkoff sound is heard corresponds to the diastolic pressure being recorded.