Chapter 4- Diving Physiology Diving Physiology 107 you, discussed in the section on drowning. The front of your larynx is the bump that sticks out, looking like a small, swallowed apple, or Adam’s apple. The angle of the bump is smaller in males than females (about 90° in males, 120° in females), so it looks more pointed and prominent. From your larynx, your trachea continues down into your chest where it divides into two bronchi that begin each of your lungs. Your bronchi divide into smaller and smaller tubes called bronchioles. Tiny waving hairs dot the insides of your bronchioles, and all but your smallest bronchioles are lined with smooth muscle. The muscular lining can squeeze or relax to regulate how much air can pass. Other cells lining your bronchioles swell with mucus until they look like glistening little cups, so are called goblet cells. The mucus traps foreign particles like a small, sticky escalator, and your waving hairs move the escalator up to your throat, so you can clear the particles. Irritating or allergy-producing stimuli can make you secrete too much mucus or make your bronchiole muscles go into spasm, reducing the amount of air you can breathe in a given time. When spasms happen too much and too often, you have asthma. When you have excessive secretions, your air passages can stuff up, creating respiratory contraindications to diving. Both are discussed in the section “Fitness for Diving.” Your bronchioles divide again and again, like the roots of a tree. After about the 16th division, little hollow sacs called alveoli (singular: alveolus) begin to bud off. Alveoli are your body’s air-exchanging units. Your bronchioles continue branching up to approximately 23 times until the smallest are hair-thin. At these smallest airways, your alveoli cluster like berries on the vine, about 300 million in each lung. Your alveoli, like those of other mammals, are far smaller than the alveoli of “lower” vertebrates, which gives your alveoli a much larger relative surface area for gas exchange. Your alveolar walls are all only one cell thick. Surrounding each alveolus is a network of tiny blood vessels called capillaries, where you transfer dissolved oxygen and carbon dioxide between your lungs and bloodstream. The walls of your capillaries are also only one cell thick, so gas transfers easily. The purpose of all your bronchial tree divisions is to put a lot of gas transfer tissue in a compact area. If you could mash your lungs out flat, they would each measure between 70 and 80 square meters (750 to 860 square feet). A thin, double membrane called the pleura surrounds your lungs. One pleural membrane covers and attaches to your lungs, and the other attaches to your chest wall, anchoring your lungs to your chest wall. The tiny layer between the membranes is your pleural cavity. Pleural fluid pumping through your pleural cavity makes a suction between the membranes, holding them together, which holds open your lungs. Damage to your pleura can allow air to enter your pleural cavity, breaking the suction that holds your lungs open. Your lungs may collapse, discussed in the section on lung barotrauma. Fluid in your pleural cavity lubricates the membranes so they slide easily past one another as you breathe. If your pleura becomes inflamed, such as during pneumonia, the layers don’t slide but grate painfully with each breath, a condition called pleurisy. To inhale a normal breath, your brain tells your diaphragm muscle and some of your rib cage muscles (external intercostals) to contract and pull on your Esophagus Lungs Epiglottis Trachea Bronchus Alveoli FIGURE 4-1. RESPIRATORY ANATOMY
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