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CHAPTER 3 — Underwater Physiology and Diving Disorders 3-47 body as a whole is far from saturation. If enough time elapses at depth, all tissues will become equally saturated, as shown in lower diagram. 3‑9.3.1.1 Saturation of Tissues. The sequence of events in the process of saturation can be illustrated by consid ering what happens in the body of a diver taken rapidly from the surface to a depth of 100 fsw ( Figure 3 ­ 16 ). To simplify matters, we can say that the partial pressure of nitrogen in his blood and tissues on leaving the surface is roughly 0.8 ata. When the diver reaches 100 fsw, the alveolar nitrogen pressure in his lungs will be about 0.8 × 4 ata = 3.2 ata, while the blood and tissues remain temporarily at 0.8 ata. The partial pressure difference or gradient between the al ­ veolar air and the blood and tissues is thus 3.2 minus 0.8, or 2.4 ata. This gradient is the driving force that makes the molecules of nitrogen move by diffusion from one place to another. Consider the following 10 events and factors in the diver at 100 fsw: 1. As blood passes through the alveolar capillaries, nitrogen molecules move from the alveolar air into the blood. By the time the blood leaves the lungs, it has reached equilibrium with the new alveolar nitrogen pressure. It now has a nitrogen tension Venous Return Lung Capillary Bed Right Heart Pump Venous Return Left Heart Pump Arterial Supply Arterial Supply Lung Capillary Bed Right Heart Pump Left Heart Pump SATURATION OF TISSUES A BC A B C Figure 3-16. Saturation of Tissues. Shading in diagram indicates saturation with nitrogen or helium under increased pressure. Blood becomes saturated on passing through lungs, and tissues are saturated in turn via blood. Those with a large supply (as in A above) are saturated much more rapidly than those with poor blood supply (C) or an unusually large capacity for gas, as fatty tissues have for nitrogen. In very abrupt ascent from depth, bubbles may form in arterial blood or in “fast” tissue (A) even through the body as a whole is far from saturation. If enough time elapses at depth, all tissues will become equally saturated, as shown in lower diagram.

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