Lungs and Breathing

Your lungs do something remarkable every few seconds: they pull oxygen from the air and transfer it into your blood, then send carbon dioxide back the other way. They do this automatically, thousands of times a day, in a structure compact enough to sit inside your chest. Understanding how that system works is the foundation for understanding almost every respiratory symptom a doctor investigates.

The structure of the respiratory system The respiratory system includes the nose, mouth, throat, voice box, windpipe, and lungs. Its job is to move air in and out of the body and to exchange gases between the air and the blood. When you breathe in, air enters through your nose or mouth. The nose warms and filters the air before it continues. It then passes through the throat and voice box, down the windpipe, and into two main airways called bronchi. These divide into smaller and smaller tubes (bronchioles) inside each lung, like the branches of a tree. The walls of the airways are lined with mucus and tiny hair like structures called cilia. Mucus traps particles, dust, and some microorganisms before they can reach the delicate lung tissue. Cilia beat in coordinated waves, moving mucus upward and away from the smaller airways. Coughing is another layer of protection: it can clear irritants or mucus that the cilia cannot remove silently. Gas exchange At the end of the smallest airways are tiny air sacs called alveoli. These are where gas exchange takes place. The walls of the alveoli are extremely thin and are surrounded by a dense network of tiny blood vessels (capillaries). This arrangement allows gases to cross easily by diffusion: the movement of molecules from where they are more concentrated to where they are less concentrated. Oxygen moves from inside the air sac, where its concentration is high, into the blood in the surrounding vessels, where it is lower. Carbon dioxide does the opposite. It moves from the blood, where it has built up as a waste product of the body's energy processes, into the air sac, where it can be breathed out. This exchange happens continuously with every breath, across hundreds of millions of alveoli in both lungs. Breathing mechanics Breathing is controlled automatically. The brain monitors the balance of oxygen and carbon dioxide in the blood and adjusts breathing rate and depth in response. The diaphragm does most of the work. It is a dome shaped muscle that sits just below the lungs. When it contracts, it flattens slightly, increasing the space inside the chest and drawing air in. When it relaxes, air moves out passively. Muscles in the chest wall assist, particularly during exercise or when breathing becomes effortful. During exercise, muscles use more oxygen and produce more carbon dioxide. The brain detects this change and signals the respiratory system to breathe faster and more deeply, keeping levels within a working range. The lungs and circulation are closely linked. Blood returning to the lungs from the rest of the body carries carbon dioxide. The lungs remove it and load the blood with fresh oxygen. The heart then sends that oxygen rich blood back to every organ and tissue in the body. Examining the chest Understanding the structure of the respiratory system is what makes a chest examination meaningful. When a doctor examines a patient's chest, they follow a systematic sequence. They observe whether breathing is laboured, whether the chest moves equally on both sides, and whether the breathing rate is within a normal range. They feel the chest to assess expansion. They percuss the chest wall to detect changes in sound: a hollow note suggests air filled lung, while a dull note can indicate fluid or solid material where air should be. They listen with a stethoscope for the quality of breath sounds and any added noises, such as wheeze or crackle. Each finding means something only because it represents a change from what normal looks like. A doctor who understands normal lung anatomy can interpret abnormal findings. Without that understanding, examination produces observations with no interpretation attached. Doctors also measure oxygen saturation, the percentage of oxygen your blood is carrying, using a small clip on device called a pulse oximeter. Inspecting the hands and face can also give information. Blue discolouration of the lips or fingertips and clubbing of the nails may be visible in certain chronic respiratory or circulatory conditions. Symptoms and what they represent Normal breathing is what makes changed breathing interpretable. A cough, wheeze, breathlessness, or change in oxygen level only means something relative to a baseline. That baseline is a clear airway, healthy alveoli, a working diaphragm, and effective gas exchange at rest and during physical activity. When something disrupts that system, the symptoms reflect where and how. A wheeze suggests narrowed airways. Breathlessness on effort suggests reduced capacity for gas exchange under demand. A productive cough suggests the mucus clearance system is dealing with something it cannot remove silently. Later lessons explore each of those symptoms in turn. This lesson gives you the foundation for understanding all of them.

The lungs exchange gases at the alveoli: oxygen enters the blood with each breath, and carbon dioxide leaves it.