Oxygen-Hemoglobin Dissociation Curve | How pH, CO and CO2 Affect it

Author: Scott A. Sheffield MS

Last update:

The Oxygen-Hemoglobin Dissociation curve describes the oxygen saturation of hemoglobin against the partial pressure of O2 in the blood. It is affected by several influences like an acidic or alkaline environment, CO and CO2.

  • As O2 enters the vial of blood, the plasma pO2 increases and more O2 binds with hemoglobin.

pO2

Hb + O2 <———–>HbO2

  • The reaction also causes the color of the red blood cells (RBCs) in the vial to change from purple to red.
An animation demonstrating the concentration of oxygen in the blood plasma affects oxygen-hemoglobin (O2-Hb) saturation.
An animation demonstrating the concentration of oxygen in the blood plasma affects oxygen-hemoglobin (O2-Hb) saturation.
An animation demonstrating the concentration of oxygen in the blood plasma affects oxygen-hemoglobin (O2-Hb) saturation.
An animation demonstrating the concentration of oxygen in the blood plasma affects oxygen-hemoglobin (O2-Hb) saturation.
An animation demonstrating the concentration of oxygen in the blood plasma affects oxygen-hemoglobin (O2-Hb) saturation.
An animation demonstrating the concentration of oxygen in the blood plasma affects oxygen-hemoglobin (O2-Hb) saturation.
An animation demonstrating the concentration of oxygen in the blood plasma affects oxygen-hemoglobin (O2-Hb) saturation.
An animation demonstrating the concentration of oxygen in the blood plasma affects oxygen-hemoglobin (O2-Hb) saturation.
An animation demonstrating the concentration of oxygen in the blood plasma affects oxygen-hemoglobin (O2-Hb) saturation.
An animation demonstrating the concentration of oxygen in the blood plasma affects oxygen-hemoglobin (O2-Hb) saturation.
An animation demonstrating the concentration of oxygen in the blood plasma affects oxygen-hemoglobin (O2-Hb) saturation.
The concentration of oxygen in the blood plasma affects oxygen-hemoglobin saturation.
  • As the pO2 approaches 100 torrs (or mmHg), the hemoglobin molecules become nearly fully saturated.

Hb + 1 O2 = 25% saturation

Hb + 2 O2 = 50% saturation

Hb + 3 O2 = 75% saturation

Hb + 4 O2 = 100% saturation

The O2-Hb relationship is sigmoidal (or s-shaped) and not linear. (see the image below)
The upper end of the curve is flatter than the lower end.

Struggling to remember all the new information? Find out how you can memorize more efficiently using the technique of active recall.

This indicates that the first three O2 bind to hemoglobin molecules at relatively low pO2 (= 0 – 40 torr). In contrast, adding the 4th O2 to hemoglobin moleculess requires a relatively high pO2 (= 40 – 100 torr).

How pH affects the Oxygen-Hemoglobin Dissociation Curve:

 

As blood plasma pH decreases (= becomes more acidic), H+ ions increasingly bind to hemoglobin amino acids, which lessens hemoglobin’s affinity for O2. This is referred to as the Bohr effect.

The situation reverses as plasma pH increase (= becomes more alkaline; basic).

Normal pH Blood:

The normal ph as demonstrated in a graph

Acidic Blood:

A low (= acidic) blood plasma pH of 7.2 causes the O2-Hb saturation curve to shift about 15% to the right of normal (= pH 7.4).

An animation that demonstrates the low blood plasma
An animation that demonstrates the low blood plasma
An animation that demonstrates the low blood plasma
An animation that demonstrates the low blood plasma
An animation that demonstrates the low blood plasma
An animation that demonstrates the low blood plasma
An animation that demonstrates the low blood plasma
An animation that demonstrates the low blood plasma
An animation that demonstrates the low blood plasma
An animation that demonstrates the low blood plasma
An animation that demonstrates the low blood plasma
An illustration that demonstrates the low blood plasma ph effect on oxygen-hemoglobin dissociation curve
A low blood plasma pH of 7.2

Alkaline Blood:

In contrast, an elevated (= alkaline or basic) blood plasma pH of 7.6 causes the O2-Hb saturation curve to shift about 15% to the left of normal.

Need to learn the anatomy of the respiratory system, too? Look no further than these respiratory system quizzes and worksheets.

An animation that demonstrates elevated blood plasma
An animation that demonstrates elevated blood plasma
An animation that demonstrates elevated blood plasma
An animation that demonstrates elevated blood plasma
An animation that demonstrates elevated blood plasma
An animation that demonstrates elevated blood plasma
An animation that demonstrates elevated blood plasma
An animation that demonstrates elevated blood plasma
An animation that demonstrates elevated blood plasma
An animation that demonstrates elevated blood plasma
An animation that demonstrates elevated blood plasma
An animation that demonstrates elevated blood plasma effect on oxygen-hemoglobin dissociation curve
An elevated blood plasma pH of 7.6

How CO Affects Oxy-Hemoglobin Saturation:

 

Each of hemoglobin’s four heme groups can also bind to carbon monoxide (CO). If this occurs, O2 cannot bind and carbon monoxide poisoning results.

As shown in the animation below, carbon monoxide association with hemoglobin is directly related to the plasma partial pressure of CO (= pCO).
In this simulation, pCO is allowed to increase to from 0.0 – 0.4 mmHg while the pCO2 is maintained at 40 mmHg, which is normal.
At pCO = 0.4 mmHg, the hemoglobin is almost fully saturated with CO. This pressure is approximately 250 X less than the pO2 needed to fully saturate hemoglobin with O2.

An animation demonstrating carbon monoxide association with hemoglobin is directly related to the plasma partial pressure of CO
An animation demonstrating carbon monoxide association with hemoglobin is directly related to the plasma partial pressure of CO
An animation demonstrating carbon monoxide association with hemoglobin is directly related to the plasma partial pressure of CO
An animation demonstrating carbon monoxide association with hemoglobin is directly related to the plasma partial pressure of CO
An animation demonstrating carbon monoxide association with hemoglobin is directly related to the plasma partial pressure of CO
An animation demonstrating carbon monoxide association with hemoglobin is directly related to the plasma partial pressure of CO
An animation demonstrating carbon monoxide association with hemoglobin is directly related to the plasma partial pressure of CO
An animation demonstrating carbon monoxide association with hemoglobin is directly related to the plasma partial pressure of CO
An animation demonstrating carbon monoxide association with hemoglobin is directly related to the plasma partial pressure of CO
Carbon monoxide association with hemoglobin is directly related to the plasma partial pressure of CO (= pCO).

These data indicate that heme has a much greater affinity for CO than for O2. Therefore, if an individual breathes in a relatively small amount of CO, it will saturate the hemoglobin and prevent O2 from binding. As a result, O2 cannot be distributed as needed to the body’s tissues.

Fully saturated hemoglobin

Struggling to remember all the new information? Discover how the technique of active recall can help you store and retrieve information as efficiently as possible.

How pCO2 Affects Oxy-Hemoglobin Dissociation Curve:

 

The animations show how the concentration of carbon dioxide in the plasma (partial pressure of CO2 or pCO2) affects oxygen-hemoglobin dissociation curve (O2-Hb saturation).

 

As the graphs reveal, high pCO2 has the same effect on the O2-Hb dissociation curve as low plasma pH and low pCO2 has the same effect as high plasma pH (= Bohr effect).

Need to learn respiratory system anatomy? Look no further than these interactive quizzes and worksheets

High partial pressure of CO2
High partial pressure of CO2
High partial pressure of CO2
High partial pressure of CO2
High partial pressure of CO2
High partial pressure of CO2
High partial pressure of CO2
High partial pressure of CO2
High partial pressure of CO2
High partial pressure of CO2
High partial pressure of CO2
High partial pressure of CO2
High partial pressure of CO2
Low partial pressure of CO2
Low partial pressure of CO2
Low partial pressure of CO2
Low partial pressure of CO2
Low partial pressure of CO2
Low partial pressure of CO2
Low partial pressure of CO2
Low partial pressure of CO2
Low partial pressure of CO2
Low partial pressure of CO2
Low partial pressure of CO2
Low partial pressure of CO2
Low partial pressure of CO2
  • High pCO2 lessens hemoglobin’s affinity for O2 in two ways.
  • First, carbon dioxide is converted to H+ and bicarbonate ion in red blood cells via the enzyme carbonic anhydrase.

Carbon Dioxide Converted

  • The H+ ions bind to hemoglobin amino acids, and the alteration makes it more difficult for O2 to also associate.
  • Secondly, some of the carbon dioxide binds directly to hemoglobin amino acids. This also causes alteration to the hemoglobin that make it more difficult for O2 to bind.

Review:

An animation that demonstrates elevated blood plasma effect on oxygen-hemoglobin dissociation curve

An illustration that demonstrates the low blood plasma ph effect on oxygen-hemoglobin dissociation curve

 

 

Related Articles
The featured image-- a myoglobin and hemoglobin molecule.
Oxygen Myoglobin Dissociation Curve

If an isolated muscle fiber has been placed in a vial of deoxygenated blood. Inside the muscle fiber, each molecule of myoglobin can bind one O2. The O2-Mb bond is reversible, and the direction of the reaction is dependent on the concentration of O2 (partial pressure or pO2) in the surrounding fluids.
The globin molecule and an expanded view of the polyphyrin ring
Myoglobin Structure and Function

Myoglobin binds and stores oxygen inside of skeletal and cardiac muscle cells. The heme group consists of ferrous iron and a surrounding porphyrin ring. Two histidine molecules are associated with the heme.
A red blood cell with hemoglobin molecule inside
Hemoglobin Molecule – Structure & Function

Hemoglobin structure in a detailed, interactive and illustrated tutorial (globin, alpha and beta subunits, porphyrin, heme groups,…etc) with colorful and simple animations about its states (relaxed & tensed).