Why Do We Need Oxygen?

Why Do We Need Oxygen to Live?

We need oxygen to live because almost every cell in the body runs on energy that is released by burning food with oxygen, a process called aerobic cellular respiration. Without a steady supply, cells cannot make enough ATP (adenosine triphosphate), the small molecule that pays for muscle contraction, nerve signals, heartbeats and the building of new proteins.

The body stores almost no oxygen. Apart from what is held in the lungs, bound to hemoglobin in the blood and to myoglobin in muscle, there is no reserve tank, so oxygen has to be delivered nonstop. That is why we breathe every few seconds for our whole lives, while we can go days without water and weeks without food.

Explained simply

Think of food as fuel and oxygen as the thing that lets the fuel burn. A log will not burn in a sealed jar, and your cells cannot fully burn sugar without oxygen. Breathing keeps bringing in fresh oxygen, and breathing out removes the smoke, which in your body is carbon dioxide.

What Does Oxygen Do Inside Your Cells?

Inside your cells, oxygen acts as the final electron acceptor of the electron transport chain in the mitochondria, where it combines with electrons and hydrogen ions to form water. This last step keeps the whole energy-releasing assembly line moving.

Aerobic respiration breaks down glucose in three main stages, summed up by the equation glucose + 6 O2 → 6 CO2 + 6 H2O + energy:

1. Glycolysis in the cytoplasm splits glucose into two molecules of pyruvate and makes a small amount of ATP. It does not need oxygen.
2. The Krebs cycle (citric acid cycle) in the mitochondrial matrix takes the carbon atoms apart. The carbon leaves as carbon dioxide, and the energy is loaded onto carrier molecules called NADH and FADH2.
3. The electron transport chain on the inner mitochondrial membrane passes electrons from those carriers along a chain of proteins. The flow pumps hydrogen ions across the membrane, and their return through ATP synthase makes most of the cell's ATP.

Oxygen sits at the very end of that chain. If it is missing, electrons have nowhere to go, the chain backs up, and the Krebs cycle stops as well. The cell is then left with glycolysis alone, which yields only 2 ATP per glucose instead of about 30 or more.

How Does Oxygen Get From the Air to Your Cells?

Oxygen reaches your cells in four linked steps: breathing moves air into the lungs, gas exchange loads oxygen into the blood, the circulation carries it around the body, and diffusion delivers it from the capillaries into each cell. Biologists split this into external and internal respiration, and our page on breathing vs respiration explains why breathing and respiration are not the same thing.

• Ventilation: during inhalation the diaphragm pulls air through the airways down to the alveoli.
• External respiration: oxygen diffuses across the thin alveolar wall into the lung capillaries, as described on our gas exchange page.
• Transport: about 98% of that oxygen binds to hemoglobin in red blood cells, as covered under oxygen transport, and the heart pumps it to the tissues.
• Internal respiration: in the tissue capillaries, oxygen leaves the blood and diffuses into cells, where it is used in the mitochondria.

The waste travels the same route in reverse. Carbon dioxide made in the Krebs cycle diffuses into the blood, is carried mostly as bicarbonate, and is breathed out, a journey traced on our carbon dioxide transport page.

Why Do We Breathe Oxygen and Not Nitrogen or Other Gases?

We breathe oxygen and not nitrogen because oxygen is the only common gas in air that eagerly accepts electrons, which is exactly the job the electron transport chain needs done. Nitrogen makes up about 78% of air, but its two atoms are held together by a very strong triple bond, so our cells cannot use it and it passes in and out of the lungs unchanged.

Why not carbon dioxide?

Carbon dioxide is the end product of burning food, not a fuel or an electron acceptor. Its carbon is already fully oxidized, so there is no energy left to release from it. Animals breathe it out, while plants take it in for photosynthesis and release oxygen, which is how the two groups balance each other.

Why O2 and not single O atoms?

In the air we breathe, oxygen exists as O2, two atoms bonded together, because single oxygen atoms are so reactive that they combine with something almost at once. Only O2 is stable enough to make up about 21% of the lower atmosphere and to be carried safely by hemoglobin.

Some microbes live without oxygen and use other electron acceptors such as sulfate or nitrate, but they get far less energy per molecule of food. Large, warm-blooded animals with busy brains need the high energy yield that only oxygen provides.

What Happens When Cells Do Not Get Enough Oxygen?

When cells do not get enough oxygen, they fall back on anaerobic glycolysis, which makes ATP quickly but in small amounts and produces lactate, and if the shortage continues the cells stop working and begin to die. A low oxygen level in the tissues is called hypoxia.

Muscles can run on anaerobic energy for short bursts, such as a sprint, which is one reason they feel tired and burn during hard effort. The brain cannot: it makes up only about 2% of body weight but uses about 20% of the body's oxygen at rest, and it has almost no energy store. Consciousness is lost within seconds of a complete stop in blood flow to the brain, and brain cells start to be damaged after a few minutes without oxygen.

The heart muscle is similarly dependent on a constant oxygen supply, which is why a blocked coronary artery is so serious. Anyone with breathing trouble, bluish lips or confusion needs prompt medical care.

Why Do We Breathe Faster When We Exercise?

We breathe faster when we exercise because working muscles burn more fuel, so they use more oxygen and make more carbon dioxide, and the brain raises the rate and depth of breathing to match. The main signal is the rise in carbon dioxide and acid in the blood, detected by the respiratory chemoreceptors.

The breathing centers in the brainstem then send stronger, faster signals to the diaphragm and the muscles between the ribs, as explained under control of breathing. At the same time, the heart beats faster to move more oxygen-loaded blood. The full set of short-term and training changes is covered on our page about exercise and the respiratory system.