First, let's finish the airway path — Page 440, Lesson 688

I want to walk you through a set of definitions that build on the anatomy we've been discussing. These are all key terms you'll need to know as a professional pilot, so let's take them one by one. First, let's finish the airway path. After air passes through the nose and mouth, it goes through the trachea. The trachea then divides into two large airways — the left and right bronchi. The bronchi carry the air into the left and right lungs. From there, they divide into smaller and smaller airways, eventually terminating in the alveoli. Those alveoli are the tiny air sacs where gas exchange happens. Now, a capillary. A capillary is the smallest division of the blood circulation system. These are very thin-walled blood vessels. Their critical job is that oxygen, which has been carried by haemoglobin in the red blood cells, gets close to the body's tissues. At the capillary, the oxygen unlatches from the haemoglobin. Then, the oxygen molecules diffuse — that means they move passively — down a pressure-dependent gradient across the cell walls into the respiring tissues. So oxygen moves from an area of higher pressure in the capillary to an area of lower pressure in the tissue. In exchange, the capillary picks up carbon dioxide and water from those tissues. That deoxygenated blood then passes on into the veins to return to the heart. Next, carbonic acid. Carbon dioxide is produced in the tissues as a result of the oxidation of foodstuffs to provide energy. That carbon dioxide is carried in the blood in two ways: partly in simple solution, but largely in chemical combination as carbonic acid. So carbonic acid is the main chemical form in which carbon dioxide is transported in the bloodstream. Cardiac arrest. This is the state in which the heart ceases to pump blood around the body. No pumping means no circulation, which is obviously a critical emergency. Central vision. This is vision at the fovea. The fovea is a small pit in the centre of the retina. Only at this part of the retina is vision considered 20/20 or, in metric terms, 6/6. That means you can see at 20 feet what a standard eye sees at 20 feet. Central vision gives you the sharpest detail. The cerebellum. This is the second and smaller division of the brain. It is responsible for receiving information from all nerve endings, including the semicircular canals in the inner ear. Those semicircular canals are part of your balance system, so the cerebellum plays a key role in coordination and equilibrium. The cerebrum. This is a part of the forebrain. It contains the cells that perform the functions of memory, learning, and other higher mental powers. So this is where your conscious thought, decision-making, and cognitive processing happen. Finally, chokes. This is the difficulty in breathing experienced as a result of decompression sickness. Decompression sickness occurs when nitrogen bubbles form in the body during rapid depressurisation, and the chokes is the specific symptom of breathing difficulty that can result from that condition. Those are the definitions from this section. Let me know if you have any questions on any of them, or if you'd like to move on.