NAUI Master Scuba Diver 108 Diving Physiology hinged ribs (figure 4-2). If you want to inhale more, as during exercise, a sigh, in distress, or to speak or sing, your brain sends more signals to some of your neck muscles (part of your accessory muscles) and more of your rib muscles. Your enlarging chest cavity pulls on your pleura, which pulls on your lungs and enlarges them. Air flows into your lungs. To exhale, you relax your diaphragm and inspiratory muscles around your rib cage. Elastic recoil of all your structures pushes on your lungs, pushing air out again. You can add force to your exhalation by contracting your abdominal wall and the expiratory muscles of your chest (your internal intercostals). Sometimes, something will irritate your diaphragm, making it suddenly and involuntarily contract and yank on your lungs. Air rushes in. Your glottis snaps shut to stop this mistake, creating the abrupt sound of a hiccup. The volume of air that you breathe in and breathe out is called tidal volume, because like the tide, it comes in and goes out (figure 4-3). Your tidal volume at rest averages only about half a liter (one pint). You don’t need to breathe much just sitting there. The biggest breath you can exhale after your biggest inhale is your vital capacity. Vital capacity is usually determined by your size and age; bigger people usually have higher vital capacity, and capacity usually decreases with aging. Additional air that you can inhale after a normal inspiration, your inspiratory reserve, varies from person to person, averaging around three liters (quarts). After breathing a normal breath out, you could still breathe out another liter or so of air, called your expiratory reserve. When you exercise, you increase your tidal volume, using more and more of your inspiratory and expiratory reserves, and so a greater portion of your vital capacity. Even after forcefully expelling all air possible, just over a liter (quart) still remains. This is your residual volume, which keeps your lungs from collapsing. More on this is in the section “Thoracic Squeeze.” How much and how deeply you breathe is regulated by how much carbon dioxide is in your bloodstream, how much oxygen, your blood pH, by reflexes in your lung and chest wall, and through control by your brain. Of all the various inputs, CO2 is the most influential. Normally, you probably breathe around 12-16 breaths per minute at rest. Several things increase your breathing rate. Rising production of CO2 with exercise can increase your ventilation by several times your resting rate. Increased breathing rate removes CO2 more rapidly, which reduces your blood CO2 level back to normal. Your net level does not normally rise at all, even during heavy exercise. (Low oxygen in your breathing mixture Ribs Expand and Contract Diaphragm Contracts and Relaxes FIGURE 4-2. MECHANICS OF BREATHING Vital Capacity (6.5 liters) Dead Air Space (0.15 liters) Total Volume (9-9.5 liters) Tidal Volume (1.5 liters) Residual Volume (1-1.5 liters) FIGURE 4-3. LUNG CAPACITIES
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