Oxygen Saturation: Definition, Blood Oxygen Saturation Levels And Its Crucial Importance In Our Body.

Oxygen Saturation Definition

The quantity of oxygen attached to haemoglobin in red blood cells, oxygen saturation, as determined using a pulse oximeter. The abbreviation “SpO2” stands for saturation of peripheral oxygen. In relation to body temperature, pulse rate, respiration rate, and blood pressure, this “fifth vital sign” is a crucial statistic for healthcare experts. The results of the SpO2 test show how effectively oxygen is transported throughout the body.

Blood Oxygen Saturation

The findings of blood oxygen saturation tell a doctor if the blood effectively transfers oxygen. It’s critical to know that enough oxygen is available due to the fact that the brain and heart are both particularly vulnerable to low levels of oxygen.

Although oxygen saturation of blood assesses some characteristics of oxygen delivery, it does not indicate whether the cells are getting it. A blood vessel obstruction or tumour pressure may decrease oxygen content in localised tissue without affecting pulse oximeter values.

The results of noninvasive pulse oxygen saturation measurements (tips of fingers and ear lobes) should be evaluated by someone with a broad medical background.

Thanks to recent technological advancements (rSO2), doctors now have equipment to measure regional oxygen saturation. Monitoring oxygen saturation inside the brain and heart is now possible (cerebral oximetry) (intracardiac oximetry). An arterial blood gas (ABG) evaluation is another approach to determine total blood oxygen saturation.

Oxygen Supply

Oxygen needs to reach the lung’s functioning part for someone to have an adequate oxygen supply. The alveoli may transfer oxygen (O2) via the lungs to the circulatory system (alveolar gas exchange). Hemoglobin levels in the blood must be sufficient to bind oxygen. The heart must also be capable of pumping blood throughout the body. Normal levels of blood oxygen saturation should be anticipated if all of these requirements are satisfied.

Our blood absorbs oxygen in three ways: 

  • There is a modest quantity of unattached oxygen in blood plasma (about 2%).
  • A small fraction of red blood cells (erythrocytes) contain water.
  • Approximately 98 percent bound (reversibly) to haemoglobin


Hemoglobin (Hb) is a polypeptide protein with four components. Hemoglobin consists of a heme molecule and a globin chain, thus the name. Our HBB gene determines how effectively we make haemoglobin. This article provides a thorough explanation of how haemoglobin is made.

Every heme group is composed of one iron ion, which is responsible for binding oxygen. This is why anaemia sufferers should consider taking iron supplementation.

A haemoglobin molecule is said to be saturated when it has been linked to four oxygen molecules. It also develops a brilliant crimson colour. Because it transfers oxygenated blood via the lungs to the heart, arterial blood is a more vibrant shade of red than venous blood.

In the typical human, one gramme of haemoglobin absorbs 1.34 millilitres of oxygen. The typical content of haemoglobin in the blood is 15 grammes per 100 millilitres. This implies that each millilitre of blood may carry around 0.2 millilitres of oxygen linked to haemoglobin.

This is seldom the case in reality. The oxygen-carrying capacity of haemoglobin determines the conditions for perfect oxygen absorption.

Hb Oxygen-Carrying Capacity

Hemoglobin’s oxygen-carrying capacity is a self-explanatory term. However, the subject is not straightforward. This essay will not go into depth on how oxygen attaches to haemoglobin.

A strong arterial oxygen supply is necessary for adequate blood oxygen saturation. The arterial partial pressure of oxygen is the pressure that this gas creates inside the boundaries of an artery (PaO2).In order for O2 to bond to Hb, the PaO2 must be high enough.

PaO2 levels will be normal if the lungs are healthy and there is enough oxygen in the surroundings. The oxygen was absorbed in plasma, not red blood cells. It is measured by arterial partial pressure.

There may be up to 300 million haemoglobin molecules in a single red blood cell. Every heam ion in a haemoglobin molecule has the ability to bond with a molecule of oxygen (two oxygen atoms). Consequently, a single red blood cell is capable of forming bonds with one billion oxygen molecules.

This is almost never the case. Around 10% of the carbon dioxide in the blood is bound to haemoglobin (reversibly). As people become older, their red blood cells become less efficient.

Other factors may reduce hemoglobin’s oxygen-carrying ability include: 

  • Carbon dioxide levels are quite high.
  • Body temperature: “low or high” 
  • Higher blood acidity—high CO2 levels typically cause a low blood pH.
  • Blood diseases that impair the formation or function of red blood cells (such as assickle cell anemia)
  • Blood loss that is severe or ongoing (fewer red blood cells causes less haemoglobin)

Hemoglobin may adhere to acidic hydrogen ions (H+) produced inside the body, which helps to keep the blood pH stable. When hydrogen ion concentrations rise (as they do in acute and chronic renal failure), the molecules of haemoglobin have a diminished oxygen-carrying capability. In addition, H+ ions bond to them more easily, preventing certain oxygen molecules from attaching to the ferrous ions.

Hb molecules become oxyhemoglobin when they are linked to oxygen. Deoxyhemoglobin occurs when haemoglobin is not linked to oxygen. Oxyhemoglobin normally flows from the heart to the tissues through the arteries. It is carried as deoxyhemoglobin to the heart after releasing the bound oxygen. The heart subsequently pumps the blood to the lungs, and the process continues.

How Do Oxygen Saturation Monitors Work?

The most popular kind of oxygen saturation metre is a fingertip model. Ear lobe probes may also be constructed to fit. Probes affixed to the forehead may also be used to monitor brain oxygen saturation.

Light is sent through the tissue of a fingertip to measure oxygen saturation. The probe emits light on one side, which goes through the tissue to the other. The amount of light that has been absorbed is calculated. The greater the absorption of light, the higher the haemoglobin content in the blood and the greater the tissue thickness.

Unbound haemoglobin absorbs less light than oxyhemoglobin. This is likewise the situation with carbon monoxide-bound haemoglobin (carboxyhemoglobin). Carbon monoxide intoxication cannot be diagnosed using a pulse oximeter.

Two light sources are required to monitor blood oxygen saturation levels:

  • the colour red (shorter wavelength) 
  • Infrared radiation (longer wavelength)

Oxyhemoglobin takes more infrared light than red light. Deoxyhemoglobin absorbs more red light than infrared.

The ratio of red and infrared light absorption is calculated using an oxygen saturation instrument. Light in the infrared spectrum is absorbed substantially more than red light in a person with 100% SpO2.

All oxygen saturation metres must be calibrated before being used as medical instruments. This guarantees that the fundamental computation is as precise as feasible.It is calibrated exclusively on the basis of light intake and O2 saturation level, not the length between the probe’s two sides. Light must travel through more tissue when passing through a chubby finger. However, there’s a reason why oximeters are called pulse oximeters.

Light absorption in pulsing tissue-arteries–is the sole thing these gadgets look at. They work out how much light is absorbed in pulsing tissue. They may also evaluate tissue density and adjust the overall value based on this information. In patients having low blood pressure or inadequate circulation to the extremities, measuring pulsing tissue yields erroneous findings. Low-pulsating as well as non-pulsating tissue are difficult to discriminate with the probe.

The oxygen saturation perfusion indicator is a measure of pulse force and is another element of a pulse oximeter described in this issue. The most powerful signal is 20 percent, while the weakest is 0.02 percent. The non-invasive (not within the body) peripheral perfusion index is now being studied to see whether it may predict problems in large groups of patients.

Oxygen Saturation Levels

The fraction of oxygen bound to haemoglobin is indicated by blood oxygen saturation values. A SpO2 reading of more than 95% is considered normal.

When various environmental (and internal) variables interact, fingertip oxygen saturation levels might become erroneous.

The readings of an oximeter placed on a cold finger with poor perfusion may be erroneously low. A skilled nurse or physician may determine the reading is inaccurate by observing the patient’s skin tone. Perfusion levels are also affected by low blood pressure.

Gel nails do not seem to interfere with a pulse oximeter’s infrared signal, but older nail paints can—the deeper the colour, the greater the signal disturbance.

Chronic obstructive pulmonary disease (COPD), for example, may result in persistent SpO2 values of less than 90%. Oxygen saturation should be maintained at or above 90% in those who have COVID symptoms.

It is difficult to have a saturation level of oxygen greater than 100 percent.

Normal Oxygen Saturation Levels

Between 95 and 100 percent oxygen saturation is considered normal. In healthy people, the average SpO2 is 98 percent.

One gramme of haemoglobin has an adequate oxygen-carrying capacity (OCC) of 1.34 ml. In 100 millilitres of blood, a healthy individual contains roughly 15 grammes of haemoglobin. If we test the Sp02, we can calculate the amount of oxygen in 100 mL of this individual’s blood. If the percentage is 98 percent, then:

15 g (Hb) x 0.98 (%) x 1.34 (OCC) = 19.7 ml O2/100 ml

A person suffering from carbon monoxide overdose or high levels of H+ or CO2 may have a similar consequence. A pulse oximeter does not distinguish between gases and solely measures bound haemoglobin (not bound oxygen).

The oxygen saturation level of someone with persistent anaemia is typically normal. The haemoglobin in these few cells can connect to oxygen despite the fact that they contain fewer red blood cells. We can only see (on paper) that anything is amiss with haemoglobin levels.

A low Hb of 7 g/dl indicates that 98 percent is unreliable:

7 g (Hb) x 0.98 (%) x 1.34 (OCC) = 9.2 ml O2/dl

This is less than half of the healthy subject’s result.

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Low Levels of Saturation

Low saturation levels may mean a variety of things:

  • The sensor is broken.
  • The individual is cold.
  • The individual is trembling or shivering.
  • The individual has nail polish on.
  • A patient is deficient in oxygen (low levels of oxygen).

This is a delayed reaction when the SpO2 level begins to decline. A sensor may take up to three minutes to detect hypoxia. This time-scale is substantially shorter in young children, often just a few seconds.

This is because our blood has more excess oxygen than our cells require (in comparison to their lung and circulation capacity, infants and young children need more oxygen). Blood has sufficient oxygen to sustain a healthy individual for around three minutes.

A quick decline in SpO2 values implies that all the available oxygen in the blood has been used up. This happens when they are suffocating, suddenly cease breathing, or have lost a significant amount of blood in a confined space. Even with adequate oxygen flow, significant blood loss prevents the heart from pumping adequate blood to the lungs.

Therefore, experienced paramedics and operating room staff recognise that the pulse oximeter must be utilised in combination with extensive patient knowledge and other measurements. This information includes the four vital signs of body temperature, pulse rate, respiration rate, and blood pressure.


  • Hafen BB, Sharma S. (Updated 2020). Oxygen Saturation. Treasure Island (FL), StatPearls Publishing. Retrieved from: https://www.ncbi.nlm.nih.gov/books/NBK525974/
  • Tintinalli JE, Ma J, Yealy D, Meckler GD, Stapczynski S, Cline DM, Thomas SH. (2020). Tintinalli’s Emergency Medicine: A Comprehensive Study Guide, 9th New Yord, McGraw-Hill.
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