Do We Exhale Carbon Dioxide
Humans inhale oxygen when they breathe. But what happens to the carbon dioxide we exhale? Do we actually exhale it or just let it escape through our open mouth? And how does that affect us? The answer isn’t as obvious as you might think.
You probably don’t pay much attention to your breath unless there’s something wrong with it — like if you have a cold or are suffering from asthma or allergies. So, why would breathing be important? It helps regulate body temperature, keeps cells healthy and allows for efficient gas exchange between the air inside your body and the outside environment. In short, it has several functions.
The first thing to know about breathing is that we all do it whether we realize it or not. When you wake up in the morning, take a deep breath of air. You’ll notice that your chest expands. This expansion indicates that you’ve taken an inhalation. As soon as you become aware of this, you start exhalation by pursing your lips and blowing out the air. Your diaphragm contracts and pushes the air down through your windpipe into your lungs where it gets reabsorbed into your bloodstream. Once in the bloodstream, the oxygen-rich hemoglobin molecules bind to the oxygen molecules and carry them throughout the body. This process takes place so quickly that most people don’t even notice it.
A few things happen during each breath cycle. First, the diaphragm moves downward while the muscles surrounding the trachea (windpipe) contract. Second, the lungs expand. Third, the diaphragm relaxes. Fourth, the lungs fill with fresh, oxygen-rich blood. Finally, the new blood flows back into the heart via small arteries called capillaries. The newly oxygenized blood then makes its way throughout the body.
The lungs are made of tiny, thin-walled alveoli filled with fluid. Alveoli are little sacs that cover the outer surface of the lung tissue. Each one is connected to the others by narrow tubes called bronchioles. These bronchioles branch off repeatedly until they reach smaller and smaller airways known as bronchi. At the end of these branches are tubes called bronchial passages. Bronchial passages connect directly to the trachea. The diameter of each passage increases gradually from one section of the passage to another. Eventually, the largest portion of the passage connects directly to the trachea.
Each alveolus contains millions of microscopic air sacs covered with a mucous membrane. Inside each alveolus there is a layer of liquid filled with water and electrolytes. Surrounding the liquid are two types of fibers: collagen and elastic. Collagen surrounds the liquid and gives it strength. Elastic fibers surround the collagen and keep the walls of the alveolus flexible. There are also smooth muscle fibers in the walls. Together, they help control the movement of the liquids within the alveolus.
As we mentioned before, carbon dioxide is part of every living cell and plays an essential role in regulating cellular metabolism. However, the presence of carbon dioxide in the atmosphere is toxic at high levels. For example, too much carbon dioxide can cause hypercapnia which results in excessive amounts of carbonic acid in the blood. High concentrations of carbonic acid irritate nerve endings causing headaches, nausea, weakness and confusion. Even lower concentrations of carbonic acid can cause fatigue and drowsiness. Therefore, organisms need some mechanism to remove excess carbon dioxide from their bodies.
When carbon dioxide enters the body, it combines with the watery fluids in the lungs’ air sacs. This combination forms carbonic acid. The carbonic acid dissolves the carbon dioxide and releases hydrogen ions (protons). Hydrogen ions flow across the membranes of specialized proteins, called ion pumps, located on the exterior surfaces of red blood cells. Ion pumps drive the sodium potassium ATPase system which actively transports three different kinds of electrolyte cations (positively charged atoms): sodium, potassium and calcium across the cell membrane. On the other side of the cell membrane, the cations combine with hydroxide ions to form stable salt compounds. The result is a net transfer of positively charged sodium, potassium and calcium ions into the extracellular space (space outside the cell) and negatively charged chloride ions into the intracellular space (the space inside the cell).
In addition to transporting electrolytes, ion pumps work against the pull of gravity to transport glucose across the membrane into the bloodstream. Glucose is an organic compound containing six carbon bonds. Since glucose molecules contain no charge, they can cross the cell wall only by using energy provided by the hydrolysis of ATP (adenosine triphosphate), a molecule composed of three phosphate groups linked together by covalent bonds. A specific enzyme catalyzes the breakdown of ATP. During this reaction, an intermediate product is formed consisting of a free phosphoric acid group, one ribose sugar unit and one adenine base. The phosphoric acid breaks apart to release inorganic phosphorus and ADP (adenosine diphosphate). Phosphorylating enzymes convert ADP into two simpler molecules of AMP (adenosine monophosphate) plus inorganic phosphate. One of the phosphorylated amino acids is serine. Serine is used to make protein and nucleotides such as DNA. Another phosphorylated amino acid is threonine. Threonine is used to build carbohydrates. Other sugars found in plants are called polysaccharides. They’re larger than simple sugars but still fit into the spaces along the sides of the glucose molecules. Polysaccharides consist of long chains of repeating units called monosaccharides. Monosaccharides include fructose, galactose, glucose, mannose, rhamnose, xylose and many more. Some sugars may appear in multiple forms because they exist naturally as both acids and alcohols.
Once glucose crosses over into the bloodstream, it binds tightly to the oxygen-carrying hemoglobin in red blood cells. Hemoglobin molecules attach to each other, forming the basic structure of red blood cells. Without hemoglobin, oxygen wouldn’t be able to cross the cell membrane and deliver oxygen to tissues. Oxygen-hemoglobin complexes travel through the blood stream and enter various cells in the body. Cells use the oxygen-hemoglobin complex to produce metabolic waste products such as carbon dioxide. Once inside the cell, the oxygen-hemoglobin complexes release the oxygen atoms to carry out specific biochemical reactions. The carbon dioxide remains trapped inside the cell. If the oxygen level falls below a certain threshold, the cell dies.
Now that we understand how oxygen works, let’s consider CO2. Although it is produced by respiration, it doesn’t dissolve well in watery solutions. Instead, it tends to collect in bubbles. Since the concentration of carbon dioxide in the air is 0.04 percent, the total amount of carbon dioxide in Earth’s atmosphere is about 5 percent. Most of the carbon dioxide comes from human activities including burning fossil fuels and chopping down trees.
While humans exhale carbon dioxide when they breathe, animals such as birds and insects absorb carbon dioxide from the air. Plants also absorb carbon dioxide into their leaves and stems. Photosynthesis uses sunlight and carbon dioxide to create carbohydrate building blocks called sugars. Green vegetation converts solar radiation into chemical energy in photosynthetic pigments embedded in chloroplasts. Chloroplasts contain stacks of membranes stacked atop one another. Membranes bound together by light-sensitive proteins act as windows that allow visible wavelengths of light to pass through. Light-harvesting antennae attached to the membranes capture photons of light.
During photosynthesis, green plants consume atmospheric carbon dioxide and water to generate carbohydrate molecules. These molecules serve as food sources for bacteria, fungi, protozoa and animal life. Animals obtain carbon dioxide by consuming plants. When prey consumes plant matter, it absorbs the carbon dioxide. When predators eat prey, they ingest the digested material which includes undigested carbon dioxide.
While carbon dioxide is vital to life on Earth, scientists aren’t sure exactly what happens to carbon dioxide once it’s absorbed by plants. Is it stored away indefinitely? Or does it get converted into other chemicals? What happens when carbon becomes concentrated? Are we contributing to global warming? Will carbon dioxide eventually disappear from the atmosphere? To find out, read How Carbon Works.
Learn more about carbon dioxide and related topics on the next page.
For centuries, scientists believed that mammals exhaled carbon dioxide. That view changed in 1817 when Antoine-Laurent Lavoisier proved that respiration produces carbon dioxide without being consumed. He did so by placing frogs, dogs, pigeons and guinea pigs inside glass chambers and supplying them with food and water. After observing the animals for several days, he concluded that they breathed in oxygen and exhaled carbon dioxide. Lavoisier’s discovery led Charles Darwin to conclude that animals evolved to survive on oxygen and excrete carbon dioxide. Today, biologists refer to this phenomenon as “respiration.”
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