Transport pathways
Oxygen delivery is carried out by two main pathways:
- Chemical binding. This is the primary pathway. Oxygen reacts with hemoglobin to form oxyhemoglobin ($HbO_2$).
- Physical dissolution. Only an insignificant fraction of the gas is transported by this pathway.
Oxygen capacity
Blood oxygen capacity refers to the maximum volume of oxygen that can be bound by hemoglobin in a specific volume of blood.
Normally, 100 ml of blood contains 16–17 g of hemoglobin. Since each gram of hemoglobin can bind 1.36 cm³ of oxygen, 100 ml of blood carries 18–20 ml of $O_2$. Accordingly, 1 liter of blood carries 180–200 ml of oxygen.
Oxygenation process
The binding of oxygen to hemoglobin is called oxygenation. An important chemical feature of this process is that iron in the hemoglobin molecule is not oxidized—it remains divalent.
The strength of the formed bond directly depends on the partial pressure of oxygen ($pO_2$). At high $pO_2$ values, the bond is strong, while with its decrease, it weakens, allowing hemoglobin to release oxygen to the tissues.
Oxygen-hemoglobin dissociation curve
The dependence of the percentage of oxyhemoglobin on $pO_2$ is illustrated by the dissociation curve. Key points on it include:
- $pO_2$ = 60 mm Hg: corresponds to the plateau of the curve, when the blood is almost completely saturated with oxygen.
- $pO_2$ = 40 mm Hg: observed in muscle capillaries. A steep decline of the curve begins from this point, reflecting active oxygen release (dissociation).
The shape of the curve is influenced by physicochemical factors. Shift of the curve to the right (decreased oxygen affinity) occurs with:
- an increase in $pCO_2$;
- an increase in temperature;
- acidification of the environment (increase in $H^+$ ion concentration);
- an increase in the level of 2,3-diphosphoglycerate (2,3-DPG) in erythrocytes.
Shift of the curve to the left (increased affinity) is observed when these parameters decrease (e.g., in the lungs).
Tissue oxygen consumption
The rate of oxygen release varies depending on the body's needs.
- At rest: blood retains 65–70% of oxyhemoglobin, delivering only 30% of $O_2$ to tissues.
- During physical exertion: in working muscles, $pO_2$ drops to 20 mm Hg. Blood oxygen saturation drops to 25%, and 75–80% of $O_2$ is released to tissues.
Additionally, hemoglobin's affinity for oxygen during muscle work is reduced by: an increase in $pCO_2$, the accumulation of lactic acid (acidosis), and the action of the 2,3-DPG metabolite. All this promotes maximum oxygen release where it is needed most.
Briefly: answers to topical questions
What is the Bohr effect and how does it affect oxygen delivery to tissues?
The Bohr effect is a mechanism by which the accumulation of acidic metabolic products promotes oxyhemoglobin dissociation and oxygen release.
The effect of various states on oxygen transport:
- Increase in carbon dioxide and lactate — the dissociation curve shifts to the right, which significantly increases the ability of oxyhemoglobin to release oxygen to tissues.
- Decrease in carbon dioxide (alkalosis) — the dissociation curve shifts to the left, hemoglobin's affinity for oxygen increases, and oxygen delivery to tissues is severely hindered.
Which pathological forms of hemoglobin are unable to participate in oxygen transport?
According to sources, forms of hemoglobin that disrupt oxygen transport or release include:
- Carboxyhemoglobin (HbCO) — formed by the reaction of hemoglobin with carbon monoxide (CO); this is a very stable compound. Hemoglobin bound to CO loses its ability to carry O₂.
- Methemoglobin (MetHb) — formed by the oxidation of heme iron to Fe³⁺. It tightly binds O₂ and hinders its dissociation (release to tissues), leading to impaired gas exchange.
For sulfhemoglobin, sources only indicate that it is a pathological form of hemoglobin whose presence is associated with central cyanosis; the claim that it is unable to carry O₂ is not elaborated in sources.
Class and exam questions
Does the valence of iron change when binding oxygen?
No, the process is called oxygenation, not oxidation. Iron in the heme remains divalent.
What does a shift of the dissociation curve to the right mean?
A shift to the right means a decrease in hemoglobin's affinity for oxygen, meaning hemoglobin releases oxygen to tissues more easily.
What is the oxygen capacity of blood?
This is the maximum volume of oxygen carried by a specific volume of blood. On average, for 100 ml of blood, it is 18–20 ml.
What else to study on this topic
- The role of 2,3-diphosphoglycerate in erythrocytes
- The effect of acidosis on oxyhemoglobin dissociation
- Differences between oxygenation processes in the lungs and tissues
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