Table 2 presents the volume, percentage, and partial pressures of gases in dry, ambient air at sea level. The partial pressure of oxygen equals 20.93% of the total 760 mm Hg pressure exerted by air, or 159 mmHg (0.2093 x 760 mm Hg); the random movement of the minute quantity of carbon dioxide exerts a pressure of only 0.2 mm Hg (0.0003 x 760 mmHg), while nitrogen molecules exert a pressure that raises the mercury in a manometer about 600 mm (0.7904 x 760 mmHg). The letter P before the gas symbol denotes partial pressure. For sea level ambient air: PO2 = 159 mmHg; PCO2 = 0.2 mmHg; PN2 = 600 mmHg.
Air entering the nose and mouth passes down the respiratory tract; it becomes completely saturated with water vapor, that slightly dilutes the inspired air mixture. At body temperature, for example, the pressure of water molecules in humidified air equals 47 mm Hg; this leaves 713 mmHg (760 - 47) as the total pressure exerted by the inspired dry air molecules at sea level. Consequently, the effective Po2 in tracheal air decreases by about 10 mmHg from its ambient value of 159 mm Hg to 149 mmHg (0.2093 x [760 - 47 mmHg]). Humidification has little effect on the inspired PCO2 because of carbon dioxide's almost negligible concentration in inspired air.
Alveolar air composition differs considerably from the incoming breath of moist ambient air because carbon dioxide continually enters the alveoli from the blood, whereas oxygen leaves the lungs for transport throughout the body. Table 3 shows that alveolar air contains approximately 14.5% oxygen, 5.5% carbon dioxide, and 80.0% nitrogen.
After subtracting vapor pressure in moist alveolar gas, the average alveolar Po2 equals 103 mmHg (0.145 x [760 - 47 mmHg]) and 39 mmHg (0.055 x [760 - 47 mmHg]) for PCO2. These values represent the average pressures exerted by oxygen and carbon dioxide molecules against the alveolar side of the respiratory membrane. They do not exist as physiologic constants, but vary slightly with the phase of the ventilatory cycle and adequacy of ventilation in various lung segments.
Gas Exchange in the Body
The exchange of gases between the lungs and blood, and their movement at the tissue level, takes place entirely passively by diffusion.
Gas Exchange in Lungs
The first step in oxygen transport involves oxygen transfer from oxygen alveoli into the blood. The alveolar Po2 equals 100 mmHg, which is less than the Po2 of ambient air. Three main reasons for the dilution of oxygen in inspired air include:
Water vapor saturates relatively dry inspired air
Oxygen is continually removed from alveolar air
Carbon dioxide is continually added to alveolar air
The pressure of oxygen molecules in alveolar air averages about 60 mm Hg higher than the Po2 in venous blood entering the pulmonary capillaries. Consequently, oxygen diffuses through the alveolar membrane into the blood. Carbon dioxide exists under slightly greater pressure in returning venous blood than in the alveoli causing diffusion of carbon dioxide from the blood into the lungs. Although only a small pressure gradient of 6 mm Hg exists for carbon dioxide diffusion compared with oxygen, adequate carbon dioxide transfer occurs rapidly because of carbon dioxide's high solubility. Nitrogen, an inert gas in metabolism, remains essentially unchanged in alveolar-capillary gas.
Gas Exchange in the Tissues
In the tissues, where energy metabolism consumes oxygen at a rate almost equal to carbon dioxide production, gas pressures can differ considerably from arterial blood. At rest, the average Po2 in the muscle's extracellular fluid rarely drops below 40 mmHg, while cellular PCO2 averages about 46 mmHg. In contrast, heavy exercise can reduce the pressure of oxygen molecules in muscle tissue to 3 mmHg, whereas the pressure of carbon dioxide approaches 90 mmHg. The pressure differential between gases in plasma and tissues establishes the gradients for diffusion – oxygen leaves capillary blood and diffuses toward metabolizing cells, while carbon dioxide flows from the cell to the blood. Blood then enters the veins and returns to the heart for delivery to the lungs. Diffusion rapidly begins once again as venous blood enters the lung's dense capillary network.
Oxygen and Carbon Dioxide Transport
Oxygen Transport in the Blood
The blood transports oxygen in two ways:
In physical solution – dissolved in the fluid portion of the blood.
Combined with hemoglobin – in loose combination with the iron-protein hemoglobin molecule in the red blood cell
Oxygen Transport in Physical Solution
Oxygen does not dissolve readily in fluids. At an alveolar Po2 of 100 mm Hg, only about 0.3 mL of gaseous oxygen dissolves in the plasma of each 100 mL of blood (3 mL of oxygen per liter of blood). Because the average adult’s total blood volume equals about 5 liters, 15 mL of oxygen dissolve for transport in the fluid portion of the blood (3 mL per L x 5 = 15 mL). This amount of oxygen could sustain life for only about four seconds. Viewed from a different perspective, the body would need to circulate 80 liters of blood each minute just to supply the resting oxygen requirements if oxygen were transported only in physical solution. This represents a blood flow two times higher than the maximum ever recorded for an exercising human!
Oxygen Combined With Hemoglobin (Hb)
The blood of many animal species contains a metallic compound to augment its oxygen-carrying capacity. In humans, the iron-containing protein pigment hemoglobin constitutes the main component of the body’s 25 trillion red blood cells. Hemoglobin increases the blood’s oxygen-carrying capacity 65 to 70 times above that normally dissolved in plasma. Thus, for each liter of blood, hemoglobin temporarily “captures” about 197 mL of oxygen. Each of the four iron atoms in a hemoglobin molecule loosely binds one molecule of oxygen to form oxyhemoglobin in the reversible oxygenation reaction:
Hb + 4 O2 ––––––––> Hb4O8
This reaction requires no enzymes. The partial pressure of oxygen in solution solely determines the oxygenation of hemoglobin to oxyhemoglobin.
Oxygen-Carrying Capacity of Hemoglobin
In men, each 100 mL of blood contains approximately 15 to 16 g of hemoglobin. The value averages 5 to 10% less for women, or about 14 g per 100 mL of blood. Sex difference in hemoglobin concentration contributes to the lower aerobic capacity of women, even after adjusting for differences in body mass and fat.
Each gram of hemoglobin can combine loosely with 1.34 mL of oxygen. Thus, oxygen-carrying capacity can be calculated by knowing blood’s hemoglobin concentration as follows:
Blood’s oxygen capacity = Hb (g•100 mL-1 blood) x Oxygen capacity of Hb (1.34 mL)
For example, if the blood’s hemoglobin concentration equals 15, then approximately 20 mL of oxygen (15 g per 100 mL x 1.34 mL = 20.1) would be carried with the hemoglobin in each 100 mL of blood if hemoglobin achieved full oxygen saturation (i.e., if all Hb existed as Hb408).
Po2 and Hemoglobin Saturation
Thus far, the discussion of blood’s oxygen-carrying capacity assumes that hemoglobin achieves full saturation with oxygen when exposed to alveolar gas.
Figure 4 shows the relationship between percent hemoglobin saturation (left vertical axis) at various Po2s under normal resting physiologic conditions (arterial pH 7.4, 37°C) and the effects of changes in pH (Figure 6B) and temperature (Figure 6C) on hemoglobin’s affinity for oxygen. Percent saturation of hemoglobin computes as follows:
Percent saturation = (Total O2 combined with Hb (Oxygen carrying capacity of Hb) x 100
This curve, termed the "oxyhemoglobin dissociation curve", quantifies the amount of oxygen carried in each 100 mL of normal blood in relation to plasma Po2 (right axis). For example, at a Po2 of 90 mm Hg the normal complement of hemoglobin in 100 mL of blood is about 19 mL of oxygen; at 40 mm Hg the oxygen quantity falls to 15 mL, and 6.5 mL at a Po2 of 20 mm Hg.
The Bohr Effect
Figures 5B and 5C show that increases in acidity (H+ concentration and CO2) or temperature cause the oxyhemoglobin dissociation curve to shift downward to the right (to enhance unloading of oxygen), particularly in the Po2 range of 20 to 50 mm Hg. This phenomenon, known as the Bohr effect (named after its discoverer physiologist Christian Bohr), results from alterations in hemoglobin’s molecular structure.
Bohr Effect becomes important in vigorous exercise, as increased metabolic heat and acidity in tissues augments oxygen release. For example, at a Po2 of 20 mm Hg and normal body temperature (37°C), %O2 saturation of hemoglobin equals 35%. At the same Po2, with body temperature increased to 43°C (like at the end of a marathon run), hemoglobin’s saturation decreases to about 23%. Thus, more oxygen unloads from hemoglobin for use in cellular metabolism. Similar effects take place with increased acidity.
Download 3.19 Mb.
Share with your friends:
The database is protected by copyright ©sckool.org 2020