Oxygen's journey through the body
Oxygen has to be taken in, carried, and then actually used. Follow it through all three stages, because a person can be limited at any one of them.
Three stages, three ways to be limited
Fitness is often reported as a single number, but the journey behind that number has three distinct stages: oxygen has to get out of the air and into the blood, be carried to the tissue that needs it, and then be extracted by the working muscle. A limit at any one of them caps the whole system.
That is why two people with the same VO2max can be limited for completely different reasons, and why an assessment uses more than one sensor. The steps below are grouped by stage, so it is clear which part of the cycle each measurement is watching.
Delivery
Getting oxygen out of the air and into the blood. This is the stage a metabolic analyzer measures directly, breath by breath.
Inhalation
Oxygen enters the body through the nose or mouth as we take a breath.
Nasal cavity and trachea
The air passes through the nasal cavity and trachea, which filter and warm it on the way down.
Lungs
In the alveoli, oxygen crosses into the bloodstream and carbon dioxide crosses out. This is the gas exchange, measured by indirect calorimetry, that makes VO2max measurable rather than estimated.
Transport
Moving oxygenated blood to the tissue that needs it. How hard this stage is working is what a heart rate monitor reports.
Pulmonary veins
Oxygen-rich blood is carried from the lungs back to the heart.
Left atrium and left ventricle
The oxygenated blood enters the left atrium, then the left ventricle, which does the pumping.
Aorta
From the left ventricle the blood is pushed into the aorta, the largest artery in the body.
Systemic circulation
The blood is distributed to every organ and tissue, and under load preferentially to the working muscle.
Extraction
What the working muscle actually takes up. Neither breath nor heart rate can see this stage; a tissue saturation sensor can.
Capillaries
In the capillary beds oxygen is handed to the cells, and carbon dioxide is collected for removal.
Exchange for CO2
Oxygen diffuses into the cell to be used; carbon dioxide diffuses out into the bloodstream.
Veins and vena cava
Carbon dioxide-rich blood returns through the veins to the right atrium and right ventricle.
Pulmonary artery
From the right ventricle the deoxygenated blood is pumped back to the lungs.
Exhalation
Carbon dioxide leaves the body on the breath, and the amount of it, next to the oxygen consumed, is what tells you which fuel was burned.
Which sensor measures which step
One sensor can only watch one stage, so an assessment uses several and the app puts them on a single timeline. That is what turns a number into an explanation: you can see whether the ceiling sat at the lungs, in the circulation, or at the muscle.
The workload matters too. Where the app controls the ergometer it sets the resistance itself, so what the subject was asked to do is recorded alongside what their body did with it.
| Property | What is measured | Sensor |
|---|---|---|
| Inhalation to gas exchange | Oxygen consumed and carbon dioxide produced, breath by breath | Metabolic analyzer: VO2 Master Analyzer or Calibre |
| Transport through the circulation | How hard the cardiovascular system is working to move it | Heart rate monitor: Polar Verity Sense |
| Extraction at the capillaries | How much oxygen the working muscle actually takes up | Tissue saturation sensor: Train.Red FYER |
| Workload driving all of it | The power the subject is producing, and the resistance set for them | Ergometer: Wattbike Atom or Proton, or Assault Airbike |
Measure every step of the cycle
Gas exchange, cardiovascular response and muscle oxygen extraction, in one assessment.
SplendoHealth
Guided from setup to report
SplendoMonitor connects over Bluetooth to metabolic analyzers, heart rate and tissue saturation sensors, and ergometers from different manufacturers, then stitches their signals into a single assessment with one timeline and one report.
