A cough is a three-phase explosion of air that clears your airways in under a second. It starts with a quick inhale, followed by a pressure buildup in your chest that can reach 300 mmHg, and ends with a burst of air rushing out at speeds that can hit 12 liters per second. Every step is coordinated by a reflex arc that runs from nerve endings in your throat and lungs to your brainstem and back.
What Triggers a Cough
The lining of your larynx, trachea, and larger airways is packed with sensory nerve endings that act as irritant detectors. These fall into two main categories. The first type responds primarily to physical touch: a crumb going down the wrong pipe, dust particles, pooled mucus, or even a sudden stretch of the airway wall. The second type is more chemically sensitive, reacting to irritant gases, cigarette smoke, capsaicin (the compound that makes chili peppers hot), and inflammatory signals your own immune cells release.
These nerve endings aren’t single-purpose. They’re polymodal, meaning the same receptor can respond to both mechanical and chemical irritation. Capsaicin, for instance, mainly activates the chemical-sensing fibers, but it can also stimulate the mechanical ones. The receptors in your throat and trachea are especially tuned to physical contact, which is why a stray drop of water can trigger a violent cough. Receptors deeper in the bronchi lean more toward chemical sensitivity, picking up on inhaled fumes or inflammatory molecules.
How the Signal Reaches Your Brain
Once triggered, these nerve endings send electrical signals up the vagus nerve, which runs from your airways to the base of your brain. The signals originate from two clusters of nerve cells near the base of the skull. One cluster handles fast-conducting fibers that detect mechanical stimuli and sudden pH changes, providing the split-second warning you need to avoid inhaling a foreign object. The other handles slower fibers that make up the majority of sensory nerves in the airways and are more involved in sustained irritation and cough sensitivity.
These signals converge at a relay station in the brainstem called the nucleus tractus solitarius. This area doesn’t just receive input from the lungs. It also gets signals from the esophagus, which is why acid reflux can trigger a cough even though the irritation is happening in your digestive tract, not your airways. The brainstem interprets the signals as “something needs to be expelled” and fires off motor commands to the muscles that will produce the cough.
Phase One: The Inhale
The first thing your body does before coughing is take a breath. This isn’t accidental. You need a reservoir of air to generate the blast that follows. The volume you inhale can range from about half a normal breath to half the total capacity of your lungs, depending on how forceful the cough needs to be. This deep inhale also stretches your expiratory muscles, putting them in a lengthened position where they can generate more force, the same principle behind pulling a rubber band back before releasing it.
Phase Two: The Pressure Buildup
Immediately after the inhale, your glottis (the opening between your vocal cords) snaps shut, sealing your windpipe. This closure lasts roughly 0.2 seconds. During that brief window, your abdominal muscles and chest wall muscles contract hard against the sealed airway. Because the air has nowhere to go, pressure inside your chest skyrockets, reaching anywhere from 100 to 300 mmHg. For context, that’s several times higher than normal blood pressure.
The glottis isn’t just a passive gate. By keeping the airway sealed while the muscles contract, it prevents those muscles from shortening too quickly. This creates something closer to an isometric contraction, where the muscles stay at a favorable length and can generate far more force than if the air were already flowing out. It’s the difference between pushing against a locked door (building maximum force) and pushing against an open one (wasting energy on motion).
Phase Three: The Blast
When the glottis is forced open, all that compressed air explodes outward. The initial burst lasts only 30 to 50 milliseconds and can produce flow rates up to 12 liters per second. Studies measuring actual cough velocity in people have found average peak speeds around 13 to 15 meters per second (roughly 30 to 34 miles per hour), though the flow rate within the compressed central airways can be significantly higher.
This burst isn’t just air from your lungs flowing up and out. The high pressure also compresses your central airways, squeezing them narrower. Air rushing through a narrower tube moves faster, the same way water speeds up when you put your thumb over a garden hose. The total blast combines air expelled from deep in the lungs with air displaced by the collapsing walls of the airways themselves, creating a turbulent, high-velocity jet.
How Coughing Actually Clears Mucus
The high-speed airflow does its cleaning work through shear force: the friction of fast-moving air dragging across the sticky mucus layer coating your airways. In the largest airways (the trachea and main bronchi), air velocity during a cough can reach around 100 meters per second, generating shear stress of roughly 100 pascals at the mucus surface. That’s enough force to physically strip mucus off the airway wall or tear chunks of mucus away from a larger mass.
There are two distinct ways this happens. The first is disadhesion, where the force of air overcomes the bond between the mucus and the underlying cell surface, peeling the mucus off like tape from a wall. The second is tearing, where the air breaks the mucus apart internally, ripping pieces away from a larger glob. Both mechanisms depend heavily on the thickness and stickiness of the mucus itself. Thicker, more concentrated mucus (the kind you produce when you’re sick) dissipates more energy internally, making it harder to clear. This is why diseases that produce thick, dehydrated mucus make coughing less effective even though the mechanical force is the same.
Deeper in the lungs, where the airways are much smaller, cough velocity drops to around 10 meters per second and shear stress falls to about 1 pascal or less. That’s not enough to strip mucus effectively. This is why a cough is good at clearing your throat and large airways but poor at cleaning out the smallest branches of the lung, and why conditions affecting those deeper airways often need other strategies to move mucus upward to where a cough can reach it.
The Muscles That Power It
A cough recruits muscles across your entire torso in a coordinated sequence. During the inhale phase, your diaphragm and the scalene muscles in your neck pull air in. During the compression and blast phases, your abdominal muscles (particularly the rectus abdominis and external obliques) and your intercostal muscles between the ribs contract forcefully to squeeze the chest cavity. Research using surface electromyography in healthy adults shows that both inspiratory and expiratory muscles actually fire together during a cough in what’s described as a synergistic antagonistic contraction, with the rib cage doing the majority of the work. This coordinated squeeze is what generates the extreme pressures needed to produce an effective cough.
This is also why people with weak abdominal or chest wall muscles, whether from surgery, neuromuscular disease, or simply aging, often have a weak, ineffective cough. The reflex fires normally, but the muscles can’t generate enough pressure to produce the airflow needed to clear the airways.