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What Does The Bronchi Do

by Lyndon Langley
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What Does The Bronchi Do

What Does The Bronchi Do

Your lungs are important organs that help you breathe and stay alive. They’re also complex structures with lots going on inside them. Your lungs have three main parts — a pair of primary lobules, secondary lobules and pleura (a thin membrane surrounding each lung). Inside these areas is a network of tubes, which we call bronchi. These bronchi lead to the bronchioles, where they eventually merge with other smaller branches into larger tubes known as trachea. The trachea connect to the windpipe (trachaea) which leads outside your body so air can get inside. We inhale oxygen through our nose or mouth while exhaling carbon dioxide from our lungs.
Primary lobule – This area contains two types of cells that form a spongy structure within the lungs. Alveolar capillary units are made up of type I epithelial cells, which cover the surface of the lung tissue and make it porous. Type II epithelial cells produce surfactant, a substance that keeps the liquid part of your blood from sticking to the walls of tiny blood vessels. Surfactant prevents the formation of thick mucus layers at the interfaces between your airways and fluids, such as water.
Secondary lobule – The second largest section of the lungs is this lobe. It has many more type I and type II epithelial cells than the primary lobule. Its outer layer of tissue consists mostly of collagen fibers, which support the inner layer of fibroblasts. Fibroblasts create new collagen proteins for structural purposes. In addition, there are small numbers of smooth muscle cells here. Smooth muscles contract when exposed to various stimuli, including nervous signals. When they do, the smooth muscle cells tighten the tissues around the bronchi and bronchioles. This helps push the airways open during breathing.
Pleura – The third section of the lungs is the pleural cavity. Pleuritis is an inflammation of the pleura. If you ever suffer one of those you will feel sharp pains radiating from your chest. Your doctor may prescribe medication to treat the pain associated with pleuritis.
Bronchi – All of the large airways start out as branching tubes attached to the primary lobules. There are different kinds of bronchi depending on their location. For example, some branch off directly from the primary lobules. Others branch off from the secondary lobules. Still others come together to form bronchioles. While all bronchi have air-filled passageways, only about half of them actually transport air. The rest are just empty space.
When you take a breath, the diaphragm moves down. As it does, the stomach contracts and pushes air down into the lower portion of the bronchi. From there, the air makes its way into the bronchioles, which then send the air down into the alveolae. The alveoli take the air inside and distribute it throughout the bloodstream and lymphatic system. Oxygen then diffuses across the membranes of red blood cells and enters your tissues. Carbon dioxide leaves your tissues and goes back into your lungs by diffusion. Once the process starts, it takes about 20 seconds for the entire flow to occur.
Inhaled particles go into your lungs’ air passages, but most never reach the actual lung tissue. Instead, they become trapped in the mucus lining of your throat, upper respiratory tract, or cilia within your nostrils. Cilia are microscopic hairlike projections that line the nasal cavities. Their job is to sweep foreign matter from your nose and throat before it reaches your lungs. Mucus acts like a filter and traps particles as they travel toward the lungs.
Once the particles enter your airways, they encounter several barriers. First, there’s the epiglottis, a flap that covers your larynx to protect it from food or drink. Second, there are the soft palate and the pharynx. Third, there are the cilia that line your nostrils and sinuses. Finally, there are the bronchial cartilage and internal elastic ligaments. Together they keep the airways from collapsing completely shut. But if the pressure builds too high, the bronchioles could burst. That’s why the bronchii aren’t fully closed even after taking a deep breath.
On the next page we’ll look at how the bronchi get air into the lungs.
Air Movement Through the Lungs

While you breathe, the diaphragm rises. As it does, the abdominal contents expand. This causes the stomach to fill with air. Because the stomach is normally surrounded by air, it expands outwardly when filled with air. However, the stomach doesn’t rise very high because it must remain near your rib cage. It stays close to the thorax, which contains the heart and lungs. So, instead of pushing the air straight down into the lower portions of the bronchi, the stomach sends it inward toward the heart and lungs.
As the stomach fills with air, the air pressure increases inside it. This causes the stomach to squeeze the airtight seal between the lungs and the diaphragm. Because the stomach can no longer expand further, the force of expansion is transferred to the diaphragm. The diaphragm remains below the level of the stomach, allowing it to act as a piston pushing the air downward. Air continues to move down until it reaches the lowest point in your bronchial tree.
From there, the air passes over the top of the bronchial tree. Near the bottom of the tree, the air encounters a ring of cartilage known as the bronchial valve. This valve ensures that air flows down the correct path. The bronchial valve opens wide enough to let the air continue moving downward. However, it closes again quickly once the air exits the valve. The closing allows the airway to return to its normal size.
After passing over the bronchial valve, the air travels along the bronchioles. Each bronchiole ends in an alveolus. An alveolus is a rounded pouch that looks like a honeycomb. Some people refer to these pouches as “air sacs.” Within the alveolus, the air mixes with fluid and becomes oxygenated. Then it spreads out through the pulmonary capillaries. The capillaries allow oxygen to diffuse across the membranes of red blood cells. The oxygenated blood then gets distributed to every part of the body via the systemic circulation. Carbon dioxide returns to the lungs by diffusion and is exhaled.
Next time we’ll learn what happens to the air after it leaves your lungs.
The average adult breathes 20 cubic feet (0.6 m3) of air per day. To put that number in perspective, consider that the Empire State Building holds approximately 1 million cubic feet of air (300,000 m3), or almost 4% of the total amount of air in the atmosphere.
How does the human body use all of the air it consumes? About 70 percent of the air taken in goes to fuel combustion; 15 percent is used for respiration, and the remaining 15 percent is expelled from the body as waste. Humans don’t need much extra oxygen to live. After burning fuels, the body uses up the oxygen left behind in the combustion products. During exercise, however, the body needs extra oxygen to burn fat as fuel.

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