Carbon dioxide is primarily a vasodilator. In most blood vessels throughout the body, rising CO2 levels cause vessel walls to relax and widen, increasing blood flow. This effect is especially pronounced in the brain, where each 1 mmHg increase in arterial CO2 boosts cerebral blood flow by 2 to 4%. However, CO2 does cause vasoconstriction in one important location: the lungs. This opposite behavior in pulmonary vessels serves a specific purpose, and understanding both responses explains why CO2’s vascular effects come up so often in medicine.
How CO2 Relaxes Blood Vessels
When CO2 levels rise in the blood, it dissolves into surrounding tissue and produces hydrogen ions, making the local environment more acidic. This drop in pH activates potassium channels on the cells lining blood vessel walls. The resulting electrical shift (hyperpolarization) reduces calcium inside these cells, and since calcium is what drives muscle contraction, the vessel wall relaxes and the vessel dilates.
That pH-driven pathway isn’t the only mechanism at work. Endothelial cells, the thin layer lining the inside of blood vessels, ramp up their production of nitric oxide when CO2 rises. Nitric oxide is one of the body’s most potent natural vasodilators. Studies using drugs that block nitric oxide production have confirmed that this signaling molecule is a key mediator of CO2-induced vasodilation, particularly in the brain. Other signaling molecules, including certain prostaglandins and hydrogen sulfide, also appear to contribute, though nitric oxide plays the leading role.
There is also evidence that CO2 itself, not just the pH change it produces, directly influences blood vessel behavior. Research published in Nature Communications found that surplus CO2 can block certain neurovascular responses independently of changes in brain tissue pH, suggesting the CO2 molecule carries its own signaling weight.
The Brain Is Especially Sensitive
Nowhere in the body is CO2’s vasodilatory effect more important than in the brain. Cerebral blood flow tracks arterial CO2 levels in a predictable, sigmoidal curve. Within the normal range, every 1 mmHg rise in arterial CO2 increases brain blood flow by about 3 to 4%. In the other direction, every 1 mmHg drop reduces flow by 2 to 3%, bottoming out once CO2 falls about 10 to 15 mmHg below normal.
This sensitivity exists because the brain has an enormous, constant demand for oxygen and glucose. CO2 is the primary byproduct of that metabolism, so it serves as a built-in signal: areas of the brain working harder produce more CO2, which dilates local vessels and delivers more blood precisely where it’s needed. The response has a ceiling, though. Once CO2 climbs about 10 to 20 mmHg above normal resting levels, the vessels are essentially fully dilated and can’t open further.
What Happens When CO2 Drops Too Low
The flip side of this system explains why hyperventilation makes you feel lightheaded. Breathing too fast or too deeply blows off CO2 faster than your body produces it, dropping arterial CO2 levels. This triggers vasoconstriction in the brain, reducing blood flow and oxygen delivery. The resulting symptoms are a familiar cluster: dizziness, lightheadedness, blurred vision, tingling in the hands and feet, weakness, and in some cases fainting. These symptoms overlap substantially with those of orthostatic intolerance, which is the feeling of being unable to tolerate standing upright, and research has shown that hypocapnia (low CO2) and cerebral hypoperfusion are directly linked in people who experience that condition.
This is why the classic remedy for hyperventilation, breathing into a paper bag, works. It forces you to re-inhale some of the CO2 you just exhaled, raising arterial levels back toward normal and reversing the cerebral vasoconstriction.
The Lung Exception: CO2 Causes Constriction
Pulmonary blood vessels behave in the opposite way from the rest of the body. In the lungs, elevated CO2 enhances vasoconstriction rather than vasodilation. This is part of a broader mechanism called hypoxic pulmonary vasoconstriction, where blood vessels in poorly ventilated areas of the lung tighten up to redirect blood toward regions that are getting more oxygen.
CO2 potentiates this response. When alveolar CO2 is elevated in a particular lung region, it signals that ventilation there is inadequate, and the local vessels constrict to shunt blood elsewhere. In animal studies, this CO2-enhanced constriction reduced perfusion to hypoxic lung regions and improved overall oxygenation. Conversely, the absence of CO2 in alveolar gas blunts the constriction response. This opposite behavior is not a quirk. It’s an elegant design: systemic vasodilation delivers more blood to metabolically active tissues, while pulmonary vasoconstriction steers blood away from lungs that can’t oxygenate it.
A Rare Brain Region That Bucks the Trend
Even within the brain, not every blood vessel dilates in response to CO2. Research published in eLife identified a small brainstem region called the retrotrapezoid nucleus (RTN), a key area for sensing CO2 levels and regulating breathing, where arterioles actually constrict by about 11% when exposed to high CO2. Meanwhile, vessels in the cortex and other brainstem regions dilated by 4 to 7% under the same conditions.
The constriction in this region depends on a specific receptor on smooth muscle cells that triggers vessel tightening when nearby support cells release a signaling molecule in response to CO2. When researchers genetically removed this receptor, the RTN vessels stopped constricting and instead dilated like vessels everywhere else in the brain. This localized constriction may help fine-tune how the brain’s breathing control centers detect CO2 changes, by limiting blood flow that would otherwise wash away the CO2 signal they need to sense.
Systemic Effects Beyond the Brain
CO2’s vasodilatory effect extends throughout the systemic circulation. Elevated CO2 (hypercapnia) decreases total systemic vascular resistance, meaning blood vessels across the body relax. This drop in resistance lowers blood pressure, which in turn triggers compensatory responses: baroreceptors sense the pressure drop and prompt the release of stress hormones like norepinephrine, and the kidneys activate the renin-angiotensin system to retain fluid and restore pressure. High CO2 also has a direct depressant effect on cardiac muscle, but this is partially offset by the stimulant effect of those same catecholamines.
Medical Uses of CO2 as a Vasodilator
The vasodilatory properties of CO2 have been put to clinical use. Carbogen, a gas mixture of 5% CO2 and 95% oxygen, has been used to treat sudden hearing loss. The rationale is straightforward: the CO2 component dilates blood vessels in the inner ear, while the high oxygen concentration maximizes oxygen delivery to damaged hair cells. This combination increases both blood flow and oxygen partial pressure in the cochlea. Carbogen has also been explored in oncology research as a way to improve oxygen delivery to tumors, making radiation therapy more effective, since well-oxygenated tumor cells are more vulnerable to radiation than hypoxic ones.