Human Body: Respiratory System (2nd Ed, 1980)

[Music] For you to stay alive, there must be a steady flow of oxygen into your body and a steady flow of carbon dioxide out. This exchange of gases is the work of your respiratory system. The energy that moves you, that keeps your body functioning, that keeps your trillions of cells alive, that keeps you alive, is released by oxygen from the food you eat. There's more than enough oxygen in the air around you. Most of it comes from green plants, a waste product of photosynthesis. You draw oxygen into your body by breathing. The process of breathing involves the work of your lungs and the flat sheet of muscle under them, the diaphragm. Breathing begins with signals from your brain. You breathe in when clusters of cells in the medela of your brain send nerve impulses to the muscles attached to your ribs and to the diaphragm. The rib muscles contract, lifting the ribs up and out. The diaphragm contracts and flattens. These two movements increase the space inside your chest and stretch your lungs. Air pressure in your lungs then drops below the pressure of the air around you. So, air rushes in. This brings about 10% more air into your lungs than there was before. you have breathed in. Nerve cells in the lining of your lungs sense this stretched condition. They fire impulses back to the medela and the cell clusters there switch off the impulses that contracted your rib muscles and diaphragm. These muscles relax. Your elastic lungs recoil a little and about 10% of the air in them is pushed back into the air around you and you have breathed out. And so about 10% at a time, the air inside your lungs changes. When you're at rest, the cycle repeats about 15 times a minute. Now let's see how the oxygen you breathe in gets to your cells and the carbon dioxide gets out. The journey begins through your nose and mouth. Air enters carrying its oxygen load. The moist inner surfaces of your nose and mouth moisten the air and the warm blood that flows through the capillaries that line these surfaces heats the air. The surfaces are also covered with a sticky mucus that traps some of the tiny particles suspended in the air. Other particles drop out of the air as it loses speed. The passageway at the back of your nose and mouth divides into two tubes. One, the esophagus, is part of your digestive system. It leads to your stomach. The other is the trachea. It's part of your respiratory system and directs the air you breathe in toward your lungs. The entrance to the trachea is guarded by a muscular flap, the epiglatus. As long as you're breathing, this flap stays open. But as soon as you start to swallow, it automatically closes, covers the end of the trachea and usually prevents anything except air from entering the trachea and getting to your lungs. The trachea is a soft tube, but it's supported and held open by rings of cartilage. Here you can see where the trachea separates from the esophagus [Music] and how the rings of cartilage give the trachea its shape. The trachea is lined with tiny hairs, psyia, that are surrounded by mucus. These psyia vibrate, waving about 20 times a second, sweeping more particles out of the air and trapping them in the mucus. The waving motion of the psyia carries the particles back up towards the throat where they are swallowed and disposed of by the digestive system. The air that has been cleaned, warmed, and moistened continues down the trachea or windpipe as it's often called. The trachea then divides into two smaller tubes called bronchi. Each bronchial tube divides into smaller tubes that enter the lungs. Inside your lungs, these tubes divide and subdivide until they are microscopic in size. Now they're called bronchioles. Air that you've breathed in travels through these small tubes to grapeike clusters of air sacks at their ends. The alvei. The alvei make up the inner surface of your lungs. There are about 300 million of them with a combined surface area of about 70 m, some 600 square ft. These tiny air sacks provide the large surface area that the trillions of cells of your body need to get their supply of oxygen from the air. Each air sack or alveis is covered by a thin film of fluid and patrolled by white blood cells fagasittes. Fagasites engulf and remove intruding matter. This intruding matter along with mucus that also traps the dirt from the air is passed back up the airways by the movement of psyia then eliminated from your respiratory system. The oxygen in the air that has entered your lungs now dissolves into the fluid that covers each alveus. The dissolved oxygen passes through the cells that form the thin walls of the alvei and then passes through the equally thin walls of the capillaries that line the inside surface of each alveis. This movement of oxygen is a process called diffusion and it works this way. When the pressure of the gas on one side of the cell membrane is higher than on the other, the gas will move through it until there is the same pressure on both sides. But if the gas that gets through is constantly removed, the pressure never equalizes and the gas keeps diffusing. And this is what happens in your lungs. Oxygen that passes through your alvei into your capillaries is constantly being carried away by your blood. When oxygen enters your blood, nearly all of it is transported by the hemoglobin in the red blood cells. Hemoglobin is a complex iron-based molecule that turns red when it unites chemically with oxygen. This is what gives blood its red color. Your blood flows through a web of tiny capillaries carrying its load of oxygen. The capillaries merge into tiny veins that merge into larger ones that take oxygenated blood back to your heart. [Music] Now, your heart pumps the blood with its oxygen into arteries that carry the blood to all parts of your body, eventually reaching capillaries so small in diameter that only one red blood cell at a time can move through them. [Music] Now, a difference in pressure again moves the oxygen, this time out of the blood. It passes through the thin capillary walls into the fluid surrounding the cells and then into the cells themselves. And so the first big job of your respiratory system has been accomplished. Oxygen from the air has reached your cells. In your cells, the oxygen participates in chemical reactions that release energy from the nutrients there. One of the waste products of these reactions is carbon dioxide. And now the second big task begins, removing that from your body. First, the carbon dioxide passes out of the cells and through the walls of the capillaries into the blood. Much of this gas unites with chemicals in the hemoglobin and stays in the red blood cells. But more than half of the carbon dioxide is transported by the plasma, the watery portion of the blood. Your circulating blood carries the carbon dioxide to your heart and then through it to your lungs. The blood flows back into the capillaries that surround the air sacks, the alvei that make up your lungs. Now carbon dioxide leaves your blood passing out through the walls of the capillaries and the walls of the alvei. The carbon dioxide enters the air inside the tiny alvei and then squeezed from there into the quarter million bronchioles. The air retraces its path up the bronchi to the trachea. You breathe it out and the second big job of your respiratory system is done. The amount of carbon dioxide in your blood controls the rate and depth of your breathing because nerve cells in the medulla of your brain are sensitive to your blood's carbon dioxide content. When the level of carbon dioxide rises, nerve cells send impulses to your rib muscles and diaphragm. the rate and depth of your breathing increase. Increasing the rate at which carbon dioxide can be eliminated from your body. The level of carbon dioxide in your blood also controls how fast your heart beats and therefore how fast blood is moved through your circulatory system. When the increased activity of your heart and lungs has lowered the level of carbon dioxide sufficiently, then the rate of nerve impulses is slowed and so is the pace of your heart and lungs. Just before the air leaves your trachea, it passes the vocal cords. Two fibrous strands in your larynx that stretch across the passageway. Air moving past your vocal cords can make them vibrate and produce sounds. The muscles that tighten and relax these cords can change the pitch of the sound they make. This sound can be changed even more when you move your tongue, lips, and jaw and change the shape of your mouth. All this makes speech and song. The carbon dioxide you breathe into the air is recycled by plants. And so the cycle continues. You breathe, you speak, you sing, and the work of your respiratory system goes on. Hey, hey, hey.