7-7-3. Gas Exchange across Respiratory Surfaces
The structure of the lung maximizes its surface area to increase gas diffusion. Because of the enormous number of alveoli (approximately 300 million in each human lung), the surface area of the lung is very large (75 m2). Having such a large surface area increases the amount of gas that can diffuse into and out of the lungs.
Basic Principles of Gas Exchange
Gas exchange during respiration occurs primarily through diffusion. Diffusion is a process in which transport is driven by a concentration gradient. Gas molecules move from a region of high concentration to a region of low concentration. Blood that is low in oxygen concentration and high in carbon dioxide concentration undergoes gas exchange with air in the lungs. The air in the lungs has a higher concentration of oxygen than that of oxygen-depleted blood and a lower concentration of carbon dioxide. This concentration gradient allows for gas exchange during respiration.
Lung Volumes and Capacities
Different animals have different lung capacities based on their activities. Cheetahs have evolved a much higher lung capacity than humans; it helps provide oxygen to all the muscles in the body and allows them to run very fast. Elephants also have a high lung capacity. In this case, it is not because they run fast but because they have a large body and must be able to take up oxygen in accordance with their body size.
Human lung size is determined by genetics, gender, and height. At maximal capacity, an average lung can hold almost six liters of air, but lungs do not usually operate at maximal capacity. Air in the lungs is measured in terms of
Table 1. Lung Volumes and Capacities (Avg Adult Male)
The volume in the lung can be divided into four units: tidal volume, expiratory reserve volume, inspiratory reserve volume, and residual volume.
Capacities are measurements of two or more volumes. The
Lung volumes are measured by a technique called
Respiratory therapists or respiratory practitioners evaluate and treat patients with lung and cardiovascular diseases. They work as part of a medical team to develop treatment plans for patients. Respiratory therapists may treat premature babies with underdeveloped lungs, patients with chronic conditions such as asthma, or older patients suffering from lung disease such as emphysema and chronic obstructive pulmonary disease (COPD). They may operate advanced equipment such as compressed gas delivery systems, ventilators, blood gas analyzers, and resuscitators. Specialized programs to become a respiratory therapist generally lead to a bachelor’s degree with a respiratory therapist specialty. Because of a growing aging population, career opportunities as a respiratory therapist are expected to remain strong.
Gas Pressure and Respiration
The respiratory process can be better understood by examining the properties of gases. Gases move freely, but gas particles are constantly hitting the walls of their vessel, thereby producing gas pressure.
Air is a mixture of gases, primarily nitrogen (N2; 78.6 percent), oxygen (O2; 20.9 percent), water vapor (H2O; 0.5 percent), and carbon dioxide (CO2; 0.04 percent). Each gas component of that mixture exerts a pressure. The pressure for an individual gas in the mixture is the partial pressure of that gas. Approximately 21 percent of atmospheric gas is oxygen. Carbon dioxide, however, is found in relatively small amounts, 0.04 percent. The partial pressure for oxygen is much greater than that of carbon dioxide. The partial pressure of any gas can be calculated by:
P = (Patm) x (percent content in mixture)
Patm, the atmospheric pressure, is the sum of all of the partial pressures of the atmospheric gases added together,
Patm = PN2 + PO2 + PH2O + PCO2
The pressure of the atmosphere at sea level is 760 mm Hg. Therefore, the partial pressure of oxygen is:
PO2 = (760 mm Hg) (0.21) = 160 mm Hg
and for carbon dioxide:
PCO2 = (760 mm Hg) (0.0004) = 0.3 mm Hg
At high altitudes, Patm decreases but concentration does not change; the partial pressure decrease is due to the reduction in Patm.
When the air mixture reaches the lung, it has been humidified. The pressure of the water vapor in the lung does not change the pressure of the air, but it must be included in the partial pressure equation. For this calculation, the water pressure (47 mm Hg) is subtracted from the atmospheric pressure:
760 mm Hg − 47 mm Hg = 713 mm Hg
and the partial pressure of oxygen is:
(760 mm Hg − 47 mm Hg) × 0.21 = 150 mm Hg.
These pressures determine the gas exchange, or the flow of gas, in the system. Oxygen and carbon dioxide will flow according to their pressure gradient from high to low. Therefore, understanding the partial pressure of each gas will aid in understanding how gases move in the respiratory system.
Gas Exchange across the Alveoli
In the body, oxygen is used by cells of the body’s tissues and carbon dioxide is produced as a waste product. The ratio of carbon dioxide production to oxygen consumption is the
The RQ is used to calculate the partial pressure of oxygen in the alveolar spaces within the lung, the
alveolar PO2 = inspired PO2 - (alveolar PO2 / RQ)
With an RQ of 0.8 and a PO2 in the alveoli of 40 mm Hg, the alveolar PO2 is equal to:
alveolar PO2 = 150 mm Hg − (40 mm Hg / 0.8) = 100 mm Hg
Notice that this pressure is less than the external air. Therefore, the oxygen will flow from the inspired air in the lung (PO2 = 150 mm Hg) into the bloodstream (PO2 = 100 mm Hg) (Figure 2).
In the lungs, oxygen diffuses out of the alveoli and into the capillaries surrounding the alveoli. Oxygen (about 98 percent) binds reversibly to the respiratory pigment hemoglobin found in red blood cells (RBCs). RBCs carry oxygen to the tissues where oxygen dissociates from the hemoglobin and diffuses into the cells of the tissues. More specifically, alveolar PO2 is higher in the alveoli (PALVO2 = 100 mm Hg) than blood PO2 (40 mm Hg) in the capillaries. Because this pressure gradient exists, oxygen diffuses down its pressure gradient, moving out of the alveoli and entering the blood of the capillaries where O2 binds to hemoglobin. At the same time, alveolar PCO2 is lower (PALVO2 = 40 mm Hg) than blood (PCO2 = 45 mm Hg). CO2 diffuses down its pressure gradient, moving out of the capillaries and entering the alveoli.
Oxygen and carbon dioxide move independently of each other; they diffuse down their own pressure gradients. As blood leaves the lungs through the pulmonary veins, the PO2 = 100 mm Hg, whereas the
Which of the following statements is false?
In short, the change in partial pressure from the alveoli to the capillaries drives the oxygen into the tissues and the carbon dioxide into the blood from the tissues. The blood is then transported to the lungs where differences in pressure in the alveoli result in the movement of carbon dioxide out of the blood into the lungs, and oxygen into the blood.
Link to Learning
Watch this video to learn how to carry out spirometry.
The lungs can hold a large volume of air, but they are not usually filled to maximal capacity. Lung volume measurements include tidal volume, expiratory reserve volume, inspiratory reserve volume, and residual volume. The sum of these equals the total lung capacity. Gas movement into or out of the lungs is dependent on the pressure of the gas. Air is a mixture of gases; therefore, the partial pressure of each gas can be calculated to determine how the gas will flow in the lung. The difference between the partial pressure of the gas in the air drives oxygen into the tissues and carbon dioxide out of the body.
Figure 2. Which of the following statements is false?
Figure 2. C
The inspiratory reserve volume measures the ________.
Of the following, which does not explain why the partial pressure of oxygen is lower in the lung than in the external air?
The total lung capacity is calculated using which of the following formulas?
What does FEV1/FVC measure? What factors may affect FEV1/FVC?
FEV1/FVC measures the forced expiratory volume in one second in relation to the total forced vital capacity (the total amount of air that is exhaled from the lung from a maximal inhalation). This ratio changes with alterations in lung function that arise from diseases such as fibrosis, asthma, and COPD.
What is the reason for having residual volume in the lung?
If all the air in the lung were exhaled, then opening the alveoli for the next inspiration would be very difficult. This is because the tissues would stick together.
How can a decrease in the percent of oxygen in the air affect the movement of oxygen in the body?
Oxygen moves from the lung to the bloodstream to the tissues according to the pressure gradient. This is measured as the partial pressure of oxygen. If the amount of oxygen drops in the inspired air, there would be reduced partial pressure. This would decrease the driving force that moves the oxygen into the blood and into the tissues. PO2 is also reduced at high elevations: PO2 at high elevations is lower than at sea level because the total atmospheric pressure is less than atmospheric pressure at sea level.
If a patient has increased resistance in his or her lungs, how can this detected by a doctor? What does this mean?
A doctor can detect a restrictive disease using spirometry. By detecting the rate at which air can be expelled from the lung, a diagnosis of fibrosis or another restrictive disease can be made.
expiratory reserve volume (ERV)
forced expiratory volume (FEV)
functional residual capacity (FRC)
inspiratory capacity (IC)
inspiratory reserve volume (IRV)
residual volume (RV)
respiratory quotient (RQ)
tidal volume (TV)
total lung capacity (TLC)
vital capacity (VC)