Mechanism of Carbon Dioxide Removal
Carbon dioxide is produced in cells during active metabolism. As a result, its partial pressure ($pCO_2$) in tissue fluid reaches peak levels of 60 mm Hg. In incoming arterial blood, this value is significantly lower at 40 mm Hg. This gradient drives carbon dioxide unidirectionally from the tissues into the capillary lumen. After gas exchange, outgoing venous blood has a $pCO_2$ of 46–48 mm Hg.
Carbon dioxide transport occurs in several stages. Only a small fraction of the gas dissolves directly and physically in the plasma. The bulk enters the erythrocytes, where hydration reaction occurs: $CO_2$ combines with water molecules to form carbonic acid ($H_2CO_3$).
This chemical reaction is catalyzed by a specific enzyme, carbonic anhydrase. Crucially, this enzyme is located exclusively within erythrocytes (absent in blood plasma). Carbonic anhydrase accelerates the process 15,000-fold. Consequently, thanks to erythrocytes, blood can carry three times more dissolved carbon dioxide than it could through plasma transport alone.
Blood-Tissue Gas Exchange: Oxygen Delivery
The fourth phase of respiration involves oxygen delivery to cells. In actively metabolizing tissues, the partial pressure of oxygen ($pO_2$) drops almost to zero. To ensure arterial blood unloads oxygen as completely as possible, auxiliary physiological mechanisms are activated.
The primary factor is the Bohr effect. This states that the accumulation of acidic metabolic products promotes oxyhemoglobin dissociation, facilitating oxygen release. These "acidic" products include:
- Carbon dioxide ($CO_2$).
- Lactic acid (lactate).
Additionally, tissue metabolites (such as histamine) act on the vascular wall, causing additional reserve capillaries to open. Temperature also plays a significant role: higher temperatures in working tissues enhance oxygen transfer from blood to cells (shifting the oxyhemoglobin dissociation curve to the right). Consequently, the oxygen extraction ratio (OER), which is 40% at rest, rises to 60% during physical exertion.
Muscle Oxygen Supply and the Role of Myoglobin
During intense muscular contraction, a physiological paradox arises: the tissue's demand for oxygen spikes, yet delivery conditions paradoxically worsen due to mechanical compression of muscle capillaries by contracting fibers. The solution is a short-term cellular oxygen reserve protein: myoglobin.
Myoglobin acts as an emergency reserve supporting aerobic processes, making it critical for the rhythmically contracting myocardium. Myoglobin has a significantly higher oxygen affinity than hemoglobin, determining its unique saturation and unloading dynamics:
- Saturation: At a $pO_2$ of just 4 mm Hg, half of the molecules convert to oxymyoglobin. At a $pO_2$ of 40 mm Hg, myoglobin is fully saturated (while hemoglobin is still undergoing saturation at this pressure). In the lungs (at $100\text{ mm Hg}$), both proteins are 100% saturated.
- Release during hypoxia: When tissue oxygen tension drops to 10 mm Hg, oxyhemoglobin actively dissociates, unloading up to 90% of its oxygen. Under these same severe conditions, oxymyoglobin still retains up to 80% of its reserve, releasing it only during profound, critical pressure drops.