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NAUI Master Scuba Diver 112 Diving Physiology from blood centralizing is the dive reflex, described later in this chapter. Blood centralization, along with other effects of immersion, slightly reduces your lung volume. Negative pressure breathing makes it slightly harder to breathe. Those effects do not impair efficiency of your respiratory system to any significant degree. In fact, your lung diffusion capacity goes up slightly when you are immersed. Head-out immersion is even used to treat certain renal and cardiac problems because of the beneficial effects of blood centralization, among others. The increased need to urinate (increased diuresis) from centralization is often prescribed to help reduce the edema of pregnancy. Immersion is often good water therapy. Next, a look at problems that can occur if you allow diving to interfere with your physiology. These problems can usually be avoided with proper diving technique, although it goes without saying that anyone can be unlucky. Get proper training and certification in first aid and CPR before inflicting yourself on divers in need. Practice all your diving and first aid skills often. Be as well prepared as you can. RESPIRATORY PROBLEMS Breathing Resistance Excess breathing resistance has been implicated in some diving accidents. Improperly tuned regulators and excessively small bore (diameter) snorkels can increase the workload on your respiratory system. Increased workload is not all bad, as long as your system is in shape for it, but some workloads are too much. Breathing resistance can increase with gas density, which increases with depth. Some regulators have excessively high breathing resistance deeper than 30 m (100 ft) during periods when your workload makes your ventilatory requirement high. Effects: Your body compensates for high breathing resistance by reducing ventilation - easily demonstrated by trying to breathe through a narrow tube. Reduced ventilation can contribute to CO2 build-up, discussed next. High breathing resistance is fatiguing and can create air hunger. First Aid: Slow your activity and ascend. Prevention: Avoid rapid, quick breathing patterns that increase turbulence, which can increase breathing resistance. Breathe normally. Keep your regulator well tuned with regular service. Plan your dive, and use appropriate gear for your depth and conditions you will encounter at depth. Carbon Dioxide Toxicity Normally, your body keeps your arterial CO2, almost without exception, within 3 mmHg (millimeters of mercury) during both rest and exercise, a tight range. You produce more CO2 when you exercise but your body does not normally allow levels to rise. Your breathing rate automatically increases to “blow off” the excess. You know that you breathe more when you exercise. Some divers, we don’t know just why, don’t increase ventilation enough (hypoventilation). Other divers deliberately breathe too slowly believing it is relaxing, or skip breathe, which is pausing for a time after each breath, to conserve tank air. In these cases, CO2 may not be removed in a normal fashion, and carbon dioxide levels may rise (hypercapnia). Most of the elevation of CO2 is accounted for by increased work of breathing at depth, mentioned above in “Breathing Resistance.” To a smaller extent, CO2 may rise due to high oxygen partial pressure in the breathing mixture, found in gas mixtures other than air or during hyperbaric oxygen treatment. High oxygen pressure decreases ventilation in some situations; your body has enough oxygen and does not need to breathe as much. Your CO2 production continues as usual but you’re not breathing enough to get rid of it as fast. Respiration does not seem to slow from nitrogen narcosis, as popularly believed, even if the rest of your body is slowed and stuporous. Your breathing mixture itself may contain a higher than normal level of CO2. Two examples are in


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