Archimedes’ Principle states: ‘any object, wholly or partially immersed in liquid, is buoyed up by a force equal to the weight of liquid displaced’. A diver is an object immersed in water and is therefore affected by this principle. It determines the effort the diver must make to dive. If a diver weighs less than the weight of water he or she displaces, the diver will tend to float to the surface – i.e. he or she has positive buoyancy, which makes descent difficult. If the diver weighs more than the weight of water he or she displaces, the diver has negative buoyancy, which will assist descent and make ascent more difficult.

A diver can change buoyancy in several ways. If the diver wears a weight belt, he or she increases weight by a significant amount and displaces only a little more water and, as a result, will decrease buoyancy. If the diver displaces more water, he or she will increase buoyancy. This can be achieved by retaining more air in the lungs. It can also be achieved by inflating the diver’s buoyancy compensator device (BCD) – a device used to control buoyancy. It has an air space that the diver can inflate or deflate to make him positively, negatively or neutrally buoyant, as needed.

An interesting combination of the effects of Boyle’s Law and Archimedes’ Principle is shown by the changes in buoyancy experienced by a diver wearing BCD or a compressible suit. If slightly positively buoyant at the surface with air in the BCD, the diver will experience some difficulty in descending. As the diver descends he or she will pass through a zone where he or she is neutrally buoyant and, if the diver descends further, he or she will become negatively buoyant. The increased pressure reduces the volume of gas in the BCD or suit, the volume of fluid displaced and, consequently, the diver’s buoyancy.

The weight of a scuba cylinder decreases as gas is consumed from it, and this will lead to an increase in buoyancy. An empty cylinder can weigh 1 to 2 kg less than a full one, depending on the initial pressure and the size and type of the cylinder (e.g. steel, alloy).

Immersion creates a condition resembling the gravity-free state experienced by astronauts. In air, a standing person has a pressure gradient in the circulation where the hydrostatic pressure is greatest at the feet and least at the head. For an immersed diver, the hydrostatic gradients in the circulatory system are almost exactly counterbalanced by the ambient water pressure. This reduces the volume of pooled blood in the leg veins. In addition, peripheral vasoconstriction will occur in response to any cold stress. These changes result in an increase in central blood volume, leading to diuresis and subsequent haemoconcentration and decreased plasma volume.

The effect of haemoconcentration on normal dives is not major except that it gives divers a physiological excuse for well-developed thirst and sometimes the need to urinate. Urine production rates of more than 300 mL/hour cause problems for divers trying to keep their dry suit dry, unless it is fitted with a relief outlet.

The other effect of increased central blood volume is on cardiac performance. There is an increase in cardiac output as a result of increased stroke volume. Immersion alone, or in combination with various other factors associated with the diving environment, can precipitate cardiovascular dysfunction in susceptible individuals. This is discussed in Chapter 39.