Pathways of Transport
Oxygen delivery is carried out via two main pathways:
- Chemical Binding. This is the primary pathway. Oxygen reacts with hemoglobin to form oxyhemoglobin ($HbO_2$).
- Physical Dissolution. Only a minor fraction of the gas is transported in this dissolved form.
Oxygen-Carrying Capacity
Oxygen-carrying capacity of blood refers to the maximum volume of oxygen that can be bound by hemoglobin in a given volume of blood.
Normally, 100 mL of blood contains 14–16 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 transports 180–200 mL of oxygen.
The Oxygenation Process
The binding of oxygen to hemoglobin is called oxygenation. An important chemical feature of this process is that the iron in the hemoglobin molecule is not oxidized—it remains in the ferrous state ($Fe^{2+}$).
Prone to dissociation, the strength of the resulting bond depends directly on the partial pressure of oxygen ($pO_2$). At high $pO_2$ values, the bond is strong, whereas as $pO_2$ decreases, the bond weakens, allowing hemoglobin to release oxygen to the tissues.
Oxyhemoglobin Dissociation Curve
The dependence of the percentage of oxyhemoglobin saturation on $pO_2$ is illustrated by the dissociation curve. Key points on the curve include:
- $pO_2$ = 60 mmHg: corresponds to the upper flat plateau of the curve, where blood is almost fully saturated with oxygen.
- $pO_2$ = 40 mmHg: observed in muscle capillaries. This point marks the beginning of the steep descent of the curve, reflecting active oxygen release (dissociation).
Physicochemical factors affect the shape of the curve. Rightward shift (decreased oxygen affinity) occurs with:
- increased $pCO_2$;
- elevated temperature;
- acidification (increased $H^+$ ion concentration);
- increased 2,3-bisphosphoglycerate (2,3-BPG) levels in erythrocytes.
Leftward shift (increased affinity) is observed when these parameters decrease (e.g., in the lungs).
Tissue Oxygen Consumption
The rate of oxygen unloading varies depending on the body's metabolic demands.
- At rest: blood retains 65–70% of oxyhemoglobin, releasing only 30% of $O_2$ to the tissues.
- During physical exertion: in active muscles, $pO_2$ drops to 20 mmHg. Blood oxygen saturation falls to 25%, and 75–80% of $O_2$ is delivered to the tissues.
Additionally, during muscle activity, hemoglobin's affinity for oxygen is further reduced by rising $pCO_2$, accumulation of lactic acid (acidosis), and the action of 2,3-BPG. All of these factors promote maximum oxygen release where it is needed most.