Final answer:
The hydrostatic weighing procedure directly measures body density by assessing the volume of water displaced when a subject is submerged, and then calculating density with this volume and the subject's mass.
Step-by-step explanation:
The hydrostatic weighing procedure directly measures body density. This technique involves submerging a subject in water to determine their body density. The steps typically include weighing the individual in air and then, while completely submerged, with lungs emptied and holding a metal weight. Corrections are made for residual air in the lungs and the weight of the metal object. This corrected submerged weight, combined with their weight in air and possibly skinfold measurements, is used to calculate the person's percentage of body fat.
Hydrostatic weighing utilizes the principle of Archimedes, whereby the volume of water displaced by the submerged subject is equal to the volume of the subject. Once the volume is known, body density can be calculated by dividing the mass of the subject by this volume. Hence, options like blood pressure, heart rate, and lung capacity are not what hydrostatic weighing directly measures.