Respiratory System: Function & Physiology

Instructor: Meredith Mikell
The human respiratory system is a crucial facet of a vital, healthy body. In this lesson, we will explore its function and physiology. At the end, you can test your knowledge with a brief quiz.

Take A Breath

Breathe in. Now breathe out. Congratulations! You just successfully operated the respiratory system, your body's mechanism for gas exchange. Thankfully, you do not need to consciously do this; your body automatically breathes. When we think of the respiratory system, we typically think only of the lungs, expanding and relaxing with each breath. There is actually much more to the respiratory system than just the lungs, though they do essentially serve as the Grand Central Station of breathing.


All living things undergo some form of gas exchange. For plants, this involves the intake of carbon dioxide and the release of oxygen as waste during photosynthesis. The process of cellular respiration works in the opposite manner to photosynthesis: oxygen is taken in by cells and carbon dioxide is released as the waste product.

Nearly all living cells undergo respiration across the cell membranes, but we large, multi-cellular animals require a more complex system of gas exchange. This is why we have entire organs dedicated to maximizing how much oxygen we can breathe in and carbon dioxide we breathe out. The respiratory system ensures that enough oxygen is taken in. But it is the cardiovascular system, which includes the heart, arteries, veins, and capillaries, that delivers it to all of the body's many cells, and removes the carbon dioxide waste. These two systems work very closely together, almost as one system.

Physiology & Process

The process of respiration begins at the nose and mouth, where air enters your body. You inhale, and air travels down the back of the throat, the larynx, and into the trachea, a tube that runs down the neck and into the chest cavity, where it then splits into two tubes called bronchii. This is a kind of fork in the road where air is directed into right and left lungs. The bronchii branch outward into smaller passages called bronchioles, kind of like tiny twigs on a tree branch; conveniently, this entire framework is called the bronchial tree. The air then enters tiny sacs called alveoli, which are covered in a web of tiny blood vessels. This is where the first exchange of oxygen and carbon dioxide takes place.

Parts of the human respiratory system.
bronchial tree

Red blood cells contain iron atoms that bind to oxygen kind of like a magnet to a paperclip. The red blood cells serve as cargo trucks for the oxygen, and deliver it throughout the body. First though, the oxygen-rich blood leaves the alveoli through the pulmonary vein and returns to the heart. The heart acts as a routing station, directing the oxygenated blood outward to the body through arteries. Just like the tree branch-like bronchial tubes and bronchioles in the lungs, blood leaves arteries and enters the many tiny, branching capillaries that fill just about every inside surface of your body! This is where the oxygen leaves the red blood cells and is delivered into the many tissue cells, including muscle tissue, intestinal tissue, and skin tissue.

Oxygenated blood flows from arteries into capillaries, where gas exchange occurs. Deoxygenated blood, with carbon dioxide, moves from the capillaries into the veins.

The tissue cells undergo the process of cellular respiration, using up the oxygen and releasing carbon dioxide as a waste product. This is where the second gas exchange occurs. The blood cells pick up the carbon dioxide and travel out of the capillaries, and into veins, which direct blood back to the heart.

Red blood cells deliver oxygen to tissue cells and pick up the carbon dioxide that the cells release.
blood cell respiration

This oxygen-poor blood enters the heart and is routed back to the lungs through the pulmonary arteries, where it drops off the carbon dioxide in the alveoli, and picks up oxygen that was just drawn in with a new breath. When you exhale, the carbon dioxide then moves up the bronchioles, into the bronchii, up the trachea, and out your nose and mouth!

Can you believe that all of this happens during one breath?

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