Oxygen and Carbon Dioxide Transport
A The partial pressure of oxygen (Po2) decreases from 159.6 mm Hg in dry air at sea level to approximately 3 to 23 mm Hg in the mitochondria of the cell (Table 7-1).
TABLE 7-1
| Location | Partial Pressure (mm Hg) | Reason for Change |
| Dry atmospheric air | 159.6 | |
| Conducting airways | 149.6 | Addition of H2O vapor |
| End-expiratory gas | 114 | Mixing of dead-space gas with alveolar gas |
| Ideal alveolar gas | 101 | Addition of CO2 |
| Arterial blood | 97 | Intrapulmonary shunting |
| Mean systemic capillary | 40 | O2 diffusion into cell |
| Cellular cytoplasm | <40 | O2 diffusion into mitochondria |
| Mitochondria | 3-23 | Metabolic rate |
B Four body systems are responsible for the movement of oxygen from the atmosphere to the mitochondria:
C Specifics regarding movement of oxygen from the atmosphere into the blood are detailed in Chapter 2.
A Approximately 90% of the oxygen consumed is used as the final electron acceptor of the electron transport chain in the mitochondria of the cell.
B The actual reaction produces H2O:
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C Without the presence of O2, aerobic metabolism is stopped, whereas anaerobic metabolism continues, resulting in the production of lactic acid.
D The reaction of O2 with H+ to form H2O allows the formation of the high-energy phosphate group adenosine triphosphate (ATP).
E The yield of ATP molecules from aerobic metabolism significantly exceeds that from anaerobic metabolism:
| Aerobic Metabolism | Anaerobic Metabolism |
| Glucose | Glucose |
| ↓ | ↓ |
| Pyruvic acid | Pyruvic acid |
| ↓ | ↓ |
| CO2 + H2O + 38 moles of ATP | Lactic acid + 2 moles of ATP |
F Mitochondrial Po2 values of <2 mm Hg inhibit aerobic metabolism.
III Carriage of Oxygen in the Blood
A Oxygen is carried in two distinct compartments in the blood:
B Volume physically dissolved in plasma
1. According to the Bunsen solubility coefficient for oxygen, 0.023 ml of oxygen can be dissolved in 1 ml of plasma for every 760 mm Hg Po2.
2. Simplifying the above factor to the number of milliliters of oxygen per milliliter of plasma per mm Hg Po2 equals:
3. Because the oxygen content normally is expressed in volumes percent, multiplying 0.00003 ml of oxygen per milliliter of plasma by 100 results in the common factor expressing the quantity of O2 dissolved in plasma:
4. Thus, multiplying the Po2 of blood by 0.003 will yield the number of milliliters of oxygen physically dissolved in every 100 ml of blood (vol%):
C Hemoglobin: Structure and capacity to react with various substances:
1. Composition of the normal hemoglobin molecule:
a. Four porphyrin rings, called hemes, each with a central iron atom (Figure 7-1).
b. Four polypeptide chains: two α chains and two β chains, called the globin portion of the molecule.
c. Each chain is twisted and folded into a basket in which a heme is located.
d. Each iron atom of the heme is bonded via four covalent bonds to the porphyrin ring and via one covalent bond to the globin portion of the molecule. One bond is available to combine with oxygen (Figure 7-2).
e. The four chains are held together by chemical bonds between unlike chains (e.g., α to β and β to α).
f. The hemoglobin molecule undergoes structural changes when it reacts with oxygen.
g. The total molecule contracts when it combines with oxygen and expands when oxygen is released.
h. The site of carbon dioxide attachment is the amino group (R-NH2) on the porphyrin rings (R represents the rest of the molecule).
i. The terminal imidazole (R-NH) groups also are available to buffer H+ (see Chapter 15).
(1) The importance of hemoglobin as a buffer is second only to that of the HCO3−/H2CO3 buffer system.
(2) The buffering capacity of the hemoglobin molecule depends on attachment of oxygen to the iron portion of the heme.
(3) Oxygenated hemoglobin (oxyhemoglobin) is a stronger acid but weaker buffer than unoxygenated hemoglobin (deoxyhemoglobin).
(4) Thus the buffering capacity of venous blood is greater than that of arterial blood.
2. The molecular weight of hemoglobin is approximately 64,500 g.
3. Because oxygen attaches to each of the four iron atoms in the hemoglobin molecule, 4 gram molecular weights (GMWs) or 4 moles of oxygen combine with 64,500 g of hemoglobin (1 mole).
(5)4. One mole of oxygen can combine maximally with 16,126 g of hemoglobin.
5. Because 1 GMW of oxygen at standard temperature and pressure occupies 22.4 L:
(6)6. Thus at 100% saturation, 1.34 ml of oxygen can combine with each gram of hemoglobin.
7. The actual volume of oxygen carried attached to hemoglobin is equal to:
8. As hemoglobin combines with oxygen to form HbO2, the complex takes on a negative charge, and as a result it forms a salt with K+, or KHbO2.
9. When O2 is released at the tissue level, the K+ is also released, and the Hb buffers H+, forming HHb (reduced hemoglobin).
1. The total oxygen content of blood is equal to the volume of oxygen physically dissolved in plasma plus the amount chemically combined with hemoglobin (Figure 7-3).
2. Mathematically this statement is equal to:
E Table 7-2 illustrates the effects of Po2, Hb, and Sao2 on oxygen content.
TABLE 7-2
Effect of PO2, Hb, and Sao2 on Oxygen Content (CaO2)*
| PO2 (mm Hg) | Hb (g%) | Sao2 (%) | CaO2 (vol%) |
