Nitric oxide is a gas your body produces in tiny amounts that acts as a signaling molecule, telling blood vessels to relax, immune cells to attack invaders, and nerve cells to communicate. The “cycle” part refers to the way your body continuously generates, uses, and recycles the raw materials for nitric oxide through two overlapping pathways. One pathway runs on an amino acid called L-arginine; the other pulls nitric oxide from the nitrate and nitrite in the food you eat and the saliva you swallow. Both feed into the same downstream effects, and understanding how they work reveals why nitric oxide touches nearly every organ system you have.
Two Pathways, One Molecule
Your body makes nitric oxide through two distinct routes. The first and most studied is the enzymatic pathway. A family of enzymes called nitric oxide synthases takes L-arginine, an amino acid circulating in your blood, and converts it into nitric oxide plus a byproduct called L-citrulline. The conversion happens in two steps: L-arginine is first oxidized to an intermediate compound, and that intermediate is then oxidized again to release nitric oxide and L-citrulline.1PubMed. Characteristics of the nitric oxide synthase-catalyzed conversion of arginine to N-hydroxyarginine, the first oxygenation step in the enzymic synthesis of nitric oxide The L-citrulline does not go to waste. Your kidneys recycle much of it back into L-arginine, which can feed into the process again. That recycling loop is a big part of why it is called a “cycle” rather than a one-way street.
The second route is the nitrate-nitrite-nitric oxide pathway. When you eat nitrate-rich foods like leafy greens or beets, your body absorbs the nitrate and concentrates it in your saliva. Bacteria living on the back of your tongue then reduce that nitrate to nitrite. You swallow the nitrite, and in the acidic environment of your stomach and elsewhere in your tissues, that nitrite is converted to nitric oxide through non-enzymatic chemistry.2Nature Reviews Drug Discovery. The nitrate–nitrite–nitric oxide pathway in physiology and therapeutics This pathway does not need enzymes to produce nitric oxide and becomes especially important when oxygen levels drop, a situation where the enzymatic pathway slows down.
What Nitric Oxide Does Once It Is Made
The signature job of nitric oxide is relaxing blood vessels. When the lining of a blood vessel (the endothelium) releases nitric oxide, it diffuses into the smooth muscle cells wrapped around the vessel. Inside those cells, nitric oxide activates an enzyme that produces a messenger molecule called cGMP. The rising cGMP levels trigger a chain of events that ultimately lower the calcium concentration inside the muscle cell, causing it to relax.3PubMed. Molecular mechanism of cGMP-mediated smooth muscle relaxation The muscle loosens, the vessel widens, and blood flows more freely. Research has also shown that nitric oxide and cGMP open potassium channels in vascular smooth muscle, which further drives relaxation by changing the electrical state of the cell.4PubMed. Nitric oxide and cGMP cause vasorelaxation by activation of a charybdotoxin-sensitive K channel by cGMP-dependent protein kinase
This mechanism is not an occasional event. It is happening constantly, all day, in arteries throughout your body. Nitric oxide acts as a baseline restraint on blood pressure, keeping vessels in a partially relaxed state at all times. When researchers blocked the enzymatic production of nitric oxide in healthy volunteers, blood pressure climbed by roughly 30 mmHg once the body’s other regulatory reflexes were also controlled for.5PubMed. Contribution of endothelial nitric oxide to blood pressure in humans That is a massive swing, roughly the difference between normal blood pressure and a hypertensive crisis. It confirms that nitric oxide is not a minor contributor to vascular tone; it is one of the strongest forces keeping your blood pressure in check.
Blood Pressure and What Goes Wrong
Given that nitric oxide continuously holds blood pressure down, it makes sense that losing nitric oxide production is a direct route to hypertension. Research over the past four decades has identified the loss of nitric oxide production, often called endothelial dysfunction, as one of the earliest detectable events in the development of high blood pressure.6PubMed. Nitric oxide deficiency is a primary driver of hypertension People with essential hypertension show measurable defects in the nitric oxide system, with impaired relaxation responses in their blood vessels compared to people with normal pressure.7PubMed. Role of endothelium-derived nitric oxide in the abnormal endothelium-dependent vascular relaxation of patients with essential hypertension
This is not just a consequence of high blood pressure; evidence suggests it is actually upstream of it. The endothelium starts producing less nitric oxide before blood pressure numbers rise. That framing matters because it shifts the focus from treating the elevated number to preserving the system that prevents the number from rising in the first place.
Your Mouth Bacteria Are Part of the System
One of the more surprising aspects of the nitric oxide cycle is that the bacteria on your tongue are not bystanders. They are essential partners. Oral bacteria that live without much oxygen specialize in converting the nitrate concentrated in your saliva into nitrite, which is the precursor your body needs for the dietary nitric oxide pathway. Without these bacteria, the nitrate-nitrite-nitric oxide pathway essentially shuts down.8PubMed Central. Antiseptic mouthwash, the nitrate-nitrite-nitric oxide pathway, and hospital mortality: a hypothesis generating review
This has led to real concern about antiseptic mouthwash. A cross-sectional study found that people who used over-the-counter mouthwash twice a day or more had significantly lower serum nitrite levels compared to those who used it less often.9Frontiers in Oral Health. Association of over-the-counter mouthwash use with markers of nitric oxide metabolism, inflammation, and endothelial function—a cross-sectional study Separately, a longitudinal study found that twice-daily mouthwash users had nearly double the rate of physician-diagnosed hypertension compared to non-users, and even once-daily use was linked to a meaningful rise in systolic blood pressure over time.10PubMed Central. Over-the-counter mouthwash use, nitric oxide and hypertension risk The proposed mechanism is straightforward: kill the bacteria that convert nitrate to nitrite, and you lose a meaningful chunk of your daily nitric oxide production.
This does not mean you should never use mouthwash. It does mean that chronic, heavy use of bactericidal mouthwash deserves a conversation with your dentist, especially if you already have blood pressure concerns. Targeted oral hygiene that does not carpet-bomb every bacterial species may preserve this pathway while still protecting your gums.
The Immune System Uses Nitric Oxide as a Weapon
Your immune cells do not just use nitric oxide for signaling. They weaponize it. When macrophages detect an invader, they ramp up production of nitric oxide through a specific form of the enzyme called inducible nitric oxide synthase. The high concentrations of nitric oxide produced this way are directly toxic to bacteria and other pathogens.11PubMed Central. Nitric Oxide in Macrophage Immunometabolism: Hiding in Plain Sight When the amino acid supply runs low during inflammation, the same enzyme can also generate superoxide and peroxynitrite, both of which add to the antimicrobial firepower.12PubMed. Superoxide and peroxynitrite generation from inducible nitric oxide synthase in macrophages
Interestingly, the nitric oxide produced by activated macrophages also acts as a built-in brake on inflammation. It chemically modifies a protein involved in driving the inflammatory response, creating a negative feedback loop that prevents the immune reaction from spiraling out of control.13Nature Communications. Myeloid cell-derived inducible nitric oxide synthase suppresses M1 macrophage polarization So nitric oxide simultaneously kills pathogens and limits the collateral damage from the inflammatory response. That dual role helps explain why nitric oxide deficiency can show up as both increased susceptibility to infections and excessive, poorly controlled inflammation.
Nitric Oxide in the Brain
The brain has its own dedicated form of nitric oxide synthase, and its role there is quite different from the vascular system. In the brain, nitric oxide acts as a retrograde messenger at synapses. That means it is produced in the receiving cell and travels backward to the sending cell, adjusting how much neurotransmitter gets released in the future. This backward signaling is involved in both excitatory and inhibitory synaptic connections and plays a role in experience-dependent changes in the cerebral cortex.14PubMed Central. The role of nitric oxide in pre-synaptic plasticity and homeostasis
Blocking nitric oxide production in animal experiments impairs certain types of learning and memory, particularly spatial navigation tasks, while leaving other types of memory intact.15PubMed. Altered synaptic plasticity and memory formation in nitric oxide synthase inhibitor-treated rats The selectivity is noteworthy: nitric oxide is not required for all memory, but it seems to be critical for the kind of memory that involves forming a mental map of your environment. This finding connects nitric oxide to the broader phenomenon of synaptic plasticity, which is the cellular process believed to underpin learning.
Why Aging Shrinks the Nitric Oxide Supply
Advancing age is the single biggest risk factor for cardiovascular disease, and a major reason is that the endothelium’s ability to produce nitric oxide declines over time. Aging blood vessels show reduced dilation in response to stimuli that should trigger nitric oxide release, and the primary culprit appears to be increased oxidative stress that destroys nitric oxide before it can do its job.16PubMed Central. Aging and vascular endothelial function in humans
The machinery also gets sluggish. In aging arteries, the enzyme that produces nitric oxide does not get properly activated in response to blood flow, leading to significantly less nitric oxide production when it is needed most. Experiments comparing young and old aortas found that the nitric oxide production response to increases in blood flow was attenuated by roughly two-thirds in older tissue. Older animals also showed substantially greater arterial stiffness, a change that closely mirrors what happens when nitric oxide production is pharmacologically blocked in younger animals.17PubMed. Impaired shear stress-induced nitric oxide production through decreased NOS phosphorylation contributes to age-related vascular stiffness In other words, an aging artery and a young artery deprived of nitric oxide look functionally similar. This reinforces the idea that declining nitric oxide availability is a central mechanism behind age-related vascular stiffening, not just a side effect of it.
When Nitric Oxide Turns Harmful
Nitric oxide is not always beneficial. When it encounters superoxide, another reactive molecule that cells produce in greater amounts under stress, the two combine almost instantly to form peroxynitrite. This compound is aggressively oxidizing and damages proteins, fats, and DNA. It has been implicated in stroke, heart attack, heart failure, diabetes, chronic inflammatory diseases, and neurodegenerative conditions.18PubMed Central. Nitric oxide and peroxynitrite in health and disease
The reaction between nitric oxide and superoxide is so fast that when superoxide levels rise, nitric oxide gets diverted away from its normal signaling role and funneled into producing peroxynitrite instead. This creates a double problem: you lose nitric oxide’s protective effects and gain a destructive oxidant at the same time.19PubMed. The superoxide radical switch in the biology of nitric oxide and peroxynitrite This “superoxide switch” is a key reason why antioxidant status matters for cardiovascular health. Antioxidants like vitamin C can reduce superoxide levels, which protects nitric oxide from being scavenged and helps preserve endothelium-dependent vessel dilation.20Free Radical Biology and Medicine. The role of natural antioxidants in preserving the biological activity of endothelium-derived nitric oxide Pomegranate juice, rich in flavonoid antioxidants, has shown a strong capacity to protect nitric oxide from superoxide-mediated destruction in laboratory experiments.21Nitric Oxide. Pomegranate juice protects nitric oxide against oxidative destruction and enhances the biological actions of nitric oxide
The Dietary Angle and Citrulline vs. Arginine
Because L-arginine is the raw material for enzymatic nitric oxide production, it seems logical that eating more arginine would boost nitric oxide. The reality is more complicated. When you take L-arginine by mouth, roughly 70% of it gets broken down in the gut and liver before it ever reaches your bloodstream.22PubMed Central. Supplemental Citrulline Is More Efficient Than Arginine in Increasing Systemic Arginine Availability in Mice L-citrulline, by contrast, passes through the gut almost entirely intact and then gets converted to L-arginine by the kidneys. In a human pharmacokinetic study, citrulline supplements raised plasma arginine levels more effectively than arginine itself and increased urinary markers of nitric oxide metabolism, including nitrate and cGMP.23PubMed Central. Pharmacokinetic and pharmacodynamic properties of oral L-citrulline and L-arginine: impact on nitric oxide metabolism
For nitrate-based support, the most studied dietary source is beetroot juice. A study in healthy adults found that supplementation with dietary nitrate (via beetroot juice) reduced the oxygen cost of moderate exercise by about 19% and extended time to exhaustion during high-intensity exercise by roughly a minute and a half compared to a placebo.24PubMed. Dietary nitrate supplementation reduces the O2 cost of low-intensity exercise and enhances tolerance to high-intensity exercise in humans Later work confirmed the reduction in whole-body oxygen consumption, finding about a 3% decrease during exercise at moderate intensity, but showed this was not due to improved efficiency at the level of individual mitochondria.25PubMed Central. Beetroot juice supplementation reduces whole body oxygen consumption but does not improve indices of mitochondrial efficiency in human skeletal muscle The mechanism is still being sorted out, but the practical takeaway for recreational athletes is that nitrate-rich foods before exercise can meaningfully improve performance.
How Nitric Oxide Keeps Your Blood From Clotting Unnecessarily
Beyond blood vessels and immune cells, nitric oxide plays a constant role in preventing unwanted blood clots. It inhibits platelets from sticking to vessel walls and to each other, and it can even break apart platelet clumps that have already formed.26PubMed. Nitric oxide and platelet function: implications for neonatology This anti-clotting effect works through the same cGMP-based signaling used in blood vessel relaxation, which means the same molecule simultaneously widens vessels and keeps the blood inside them flowing smoothly.27PubMed. Nitric oxide-mediated fibrinogen deposition prevents platelet adhesion and activation When endothelial function declines with age or disease and nitric oxide production drops, you lose both the vessel-widening and the anti-clotting effects at the same time, compounding cardiovascular risk.
The Backup System in Low Oxygen
When oxygen levels fall, either because of altitude, localized poor circulation, or a blocked artery, the enzymatic pathway for nitric oxide production weakens because the enzyme needs oxygen to work. This is precisely when nitric oxide is needed most, to dilate vessels and restore blood flow. The nitrate-nitrite pathway fills this gap. Hemoglobin in red blood cells acts as a nitrite reductase under low-oxygen conditions, converting circulating nitrite back into nitric oxide along the oxygen gradient in tissues.28PubMed. Nitrite as a vascular endocrine nitric oxide reservoir that contributes to hypoxic signaling, cytoprotection, and vasodilation Blood vessels and red blood cells both increase their conversion of nitrite to nitric oxide under acidic and low-oxygen conditions, with the help of an enzyme called xanthine oxidoreductase.29PubMed Central. Mechanisms underlying erythrocyte and endothelial nitrite reduction to NO in hypoxia: role for Xanthine Oxidoreductase and eNOS
Even in the brain, astrocytes (support cells for neurons) dramatically increase their nitric oxide production from nitrite during oxygen deprivation. This local nitric oxide release helps regulate cerebral blood flow, directing more blood toward oxygen-starved regions.30Cell Reports. Astrocyte mitochondria are oxygen sensors that regulate cerebral blood flow The whole arrangement works like a built-in emergency system: the worse the oxygen shortage, the more aggressively the backup pathway generates nitric oxide to compensate.
Drugs That Exploit the Nitric Oxide Pathway
Nitroglycerin, used for chest pain since the 1800s, works by releasing nitric oxide in blood vessel walls, dilating the coronary arteries and reducing the heart’s workload.31PubMed Central. The Role of Nitroglycerin and Other Nitrogen Oxides in Cardiovascular Therapeutics It was prescribed for decades before anyone knew nitric oxide was involved; the mechanism only became clear in the late 1980s.
Inhaled nitric oxide is used in hospitals to treat pulmonary hypertension, especially in newborns. Because the gas is delivered directly to the lungs and then quickly neutralized by hemoglobin, it dilates only the blood vessels in ventilated portions of the lung, improving oxygen pickup without lowering blood pressure everywhere else.32PubMed Central. Inhaled nitric oxide
Erectile dysfunction drugs like sildenafil work at the other end of the cycle. They do not increase nitric oxide production; instead, they block the enzyme that breaks down cGMP in the smooth muscle of the penis. During arousal, nerves and endothelial cells release nitric oxide locally, which raises cGMP levels and relaxes the smooth muscle to allow blood flow. By preventing cGMP from being degraded, these drugs amplify and prolong the effect of whatever nitric oxide the body produces on its own.33PubMed. Mechanisms of action of PDE5 inhibition in erectile dysfunction 34PubMed. Phosphodiesterase 5 mechanisms and therapeutic applications This is why these medications require sexual stimulation to work: without the initial nitric oxide release, there is no cGMP to preserve.
Sunlight, Skin, and a Nitric Oxide Reservoir You Did Not Know About
Your skin stores significant quantities of nitric oxide precursors, particularly nitrite and compounds called S-nitrosothiols, and ultraviolet A light can release nitric oxide from these stores directly into the bloodstream. In a study of healthy volunteers, whole-body UVA exposure caused systolic and diastolic blood pressure to drop by about 11% within 30 minutes, with corresponding increases in blood flow and vessel dilation. The blood pressure decrease correlated strongly with the rise in nitrosated compounds detected in the blood.35PubMed. Whole body UVA irradiation lowers systemic blood pressure by release of nitric oxide from intracutaneous photolabile nitric oxide derivates This mechanism is completely independent of vitamin D production, which requires UVB light rather than UVA, and it may help explain the epidemiological observation that blood pressure tends to be lower in sunnier months and at lower latitudes. Your skin, in this sense, acts as a large-area nitric oxide reservoir that sunlight can tap into.
Protecting Your Stomach Lining
The nitrate-nitrite-nitric oxide pathway has an underappreciated role in the gut. In the stomach, nitric oxide helps maintain blood flow to the mucosal lining, inhibits inflammatory white blood cells from sticking to blood vessel walls, and helps repair damage caused by painkillers like diclofenac. Animal studies show that dietary nitrate and the nitric oxide produced from it in the stomach actively protect against mucosal injury.36PubMed. Dietary nitrate increases gastric mucosal blood flow and mucosal defense 37PubMed Central. Role of nitric oxide in the gastrointestinal tract This finding flips the long-held assumption that dietary nitrate is mostly a food-safety concern. In the stomach, the nitrate from your salad is not just harmless; it may be actively protective.