The Details of CO2 Transport and CO2 Tolerance for Freedivers
Authors: Jaap
DOI / Source: freedivewire.com
Date: 10 March 2019
Reading level: Beginner
Why This Matters for Freedivers
Most divers think CO2 tolerance is “mental toughness,” but your blood chemistry and blood volume are a huge part of the story too. Understanding where CO2 sits in your body helps you train more intelligently, get fewer surprise contractions, and avoid the two big mistakes: over-hyperventilating or pushing statics so hard you flirt with blackout.
Synopsis
CO2 tolerance is often talked about like it’s one single thing: “I can handle discomfort.” But CO2 buildup is a whole system, and the way your body moves and stores CO2 changes what you feel during a breath-hold.
This guide breaks CO2 transport into three main “containers”:
1) CO2 bound to hemoglobin (carbaminohemoglobin).
CO2 can attach to hemoglobin in red blood cells. A useful trick here is that hemoglobin that has already dropped off its oxygen can pick up CO2 more easily. Then, when that blood reaches the lungs and hemoglobin loads oxygen again, it lets go of CO2. This helps shuttle CO2 from tissues to lungs efficiently.
2) CO2 turned into bicarbonate (HCO₃⁻).
Most CO2 doesn’t stay as a gas. Inside red blood cells there’s an enzyme (carbonic anhydrase) that rapidly converts CO2 into carbonic acid, which splits into bicarbonate and hydrogen ions. The bicarbonate moves into the plasma, and the red blood cell’s buffering helps stop your blood from becoming dangerously acidic. In simple terms: red blood cells don’t just carry oxygen — they also run the chemistry that lets you store and transport huge amounts of CO2 safely.
3) CO2 dissolved as a gas in the blood.
A smaller fraction stays dissolved. This is important because dissolved CO2 can cross into the fluid around the brain and reach the central chemoreceptors — the sensors that contribute to the urge-to-breathe and (often) the start of contractions. That leads to a useful idea: if dissolved CO2 is what “talks to” the brain sensors most directly, then your experience of CO2 might be influenced by how much CO2 ends up in the dissolved form versus being “parked” as bicarbonate or carried on hemoglobin.
The guide then makes a helpful distinction between two kinds of CO2 tolerance:
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“Physical” CO2 tolerance: how much CO2 your body can store and handle chemically. This is linked to things like blood volume and red blood cell capacity (more “space” to buffer and transport CO2). That’s why endurance athletes often seem naturally strong at CO2 tolerance: their blood volume and transport systems are highly trained.
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“Mental” CO2 tolerance: how quickly your brain’s alarm system reacts. With repeated exposure, the warning bells may ring later, so you can stay calm longer before the struggle phase feels intense.
Finally, it offers a practical training message: you can work on mental tolerance with CO2-style breath-hold routines and slow-breathing practices, but you shouldn’t ignore the “hardware” side — general fitness that supports blood volume and oxygen transport can quietly improve CO2 handling too.
Abstract
This article explains CO2 tolerance in freediving through the physiology of CO2 transport and storage. CO2 is carried in three main forms: bound to hemoglobin, converted to bicarbonate (with red blood cells and carbonic anhydrase playing a key role), and dissolved in blood. The dissolved fraction is emphasized as especially relevant to central chemoreceptor stimulation and the urge to breathe. The article proposes two contributors to CO2 tolerance: “physical” tolerance linked to blood volume and red blood cell capacity, and “mental” tolerance linked to desensitization of chemoreceptor-driven alarm responses through repeated exposure and specific breathing/breath-hold training.