Chapter 4- Diving Physiology Diving Physiology 111 means oval.) This opening receives much attention by divers because bubbles can use it to get into your arterial circulation. Normally this opening is only present before you are born because you don’t use your lungs much then. Shortly after you are born, a flap usually closes forever to separate the two upper chambers of your heart. Blood going to your lungs and body remains separate. If the flap does not close completely, you have a patent foramen ovale. An amount of blood can pass from the right side of your heart directly to the left, bypassing your lungs, and go back to your body. Sometimes the flap is closed, so that little or no blood goes through under ordinary conditions, but opens if pushed sufficiently. A large amount of blood, and any decompression bubbles it contains, may shunt during a maneuver. This is commonly called the Valsalva maneuver, though it is technically incorrect. The Valsalva, as used in diving, consists of forcibly exhaling against a closed nose and mouth that divers sometimes use to “pop” their ears, some with more vigor than others (described more in the section on ear barotrauma). The maneuver increases internal pressure and can push open a partial PFO. An estimated 10% to 30% of adults retain some degree of patency in their foramen ovale. Even though some bubbles pass into arterial circulation through a PFO, it is not known how much this contributes to decompression sickness. A hole connecting the lower chambers of your heart (your ventricles) called a ventricular septal defect can also shunt blood. Some work indicates that these septal defects may increase severity of a DCS hit but not the likelihood of occurrence. Diving medical authorities and others debate the relevance of PFO to your diving health. Immersion Effects on Respiration and Circulation During immersion, blood centralizes (comes to your central thorax area) for several reasons. Most centralization is due simply to buoyancy – blood just floats up there, as it does in astronauts during the microgravity of space flights. It is not greater water pressure on your lower body that pushes blood upward. External water pressure matches but does not exceed your vascular pressure, so that water immersion negates the normal blood pooling you get in your lower limbs in air due to gravity. Your blood centralizes even if you are upside down in the water – water pressure does not push blood into your feet. Centralization also occurs when you are horizontal, a common diving position. Further centralization occurs from cold. When you are cold, blood shifts from your limbs to your core to reduce heat loss through your limbs. Another phenomenon contributing to centralization, although only slightly, is negative pressure breathing. The second stage of your regulator delivers air at the same pressure as water surrounding your mouth. When you are upright in the water, your mouth is higher than your chest, so pressure at your mouth is slightly less than pressure at your chest. A slight breathing resistance develops due to this “negative pressure.” You need the opposite condition in order to breathe – you need the pressure in your lungs to be lower than the pressure at your mouth, which you accomplish by expanding your chest. When you breathe in, the drop in pressure in your chest draws air into your lungs and also draws more blood into your chest. When you add a negative pressure breathing situation, slightly more blood volume is drawn into your thorax. The harder your regulator is to “draw” from, the greater the effect. With too much negative pressure, you cannot breathe in at all. This can occur when you try to breathe in from a snorkel longer than a meter (three or four feet long). If you turn upside down, your second stage delivers air at higher pressure than that on your chest, since it only knows about the pressure at your mouth. A slight positive pressure breathing situation results, reversing the chain of effects, and draws a small volume out of your thorax. None of this is normally dangerous or consequential. It is just another interesting event during immersion. Blood centralization has several interesting effects. One is to increase blood flow into your heart. Your heart senses the extra fluid and secretes a chemical that signals your kidneys to get rid of some fluid to normalize things. That is why you often need to urinate when entering the water. It is a physiologic phenomenon. Another effect
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