Gas exchange in alveolar sacs is governed by the laws of chemistry
There is much recent discussion about the amount of carbon dioxide gas that is present in the air we breathe. To understand what this means for the efficiency of lung function, we need to review the chemistry of gases. Do not panic. It is easy to understand gas chemistry if you take it a step at a time. Here are 5 tips for understanding gas exchange in human lungs.
Tip #1 Atmospheric gases behave like ideal gases
Mathematical models of the dispersion of gas molecules were formulated to define behavior of ideal gases. The atmospheric gasses, oxygen, nitrogen, carbon dioxide and water vapor at normal air and body temperature generally follow the rules that explain distribution of ideal gases. So, what is an ideal gas?
All mathematical models are based upon a limited set of assumptions. For the mathematical descriptions of ideal gas behavior the list of assumptions includes the following.
- Molecules are very far apart relative to their size
- The number of molecules even in small volumes is so large that statistical treatments are valid
- Molecules collide frequently with each other and with the wall of any container enclosing them
- Collisions between molecules are elastic – they bounce off of each other
- Total energy, kinetic + potential, contained by the gas remains constant
- Molecules are spherical in shape and all contain the same mass
- There are no forces of attraction or repulsion between the gas molecules
- Average kinetic energy of molecules depends on temperature – warmer equals faster moving, higher energy
Tip #2 There is a simple formula for air transfer into lung
The amount of pressure produced by a gas mixture such as Earth’s atmosphere is determined by the number of gas molecules in given volume and the kinetic energy (movement) of those molecules. There are three laws describing the interdependence of gas pressure, gas volume and temperature for ideal gases. They are named Boyle’s Law, Charles’s Law and Gay-Lussac’s Law. Those three laws can be mathematically combined and simply written as
P1V1/T1 = P2V2/T2
The important thing to notice about these equations when they are applied to lung physiology is that all three quantities, P = pressure of the gas, V = volume of the container, and T = temperature of the gases, vary as rib cage muscles work to expand and compress lung alveoli. Alveolar air sacs enlarge increasing the volume available for movement of gas molecules when chest muscles contract. This causes a drop in gas pressure within the lung to below the pressure of ambient air. Gases are also usually warmed above ambient air temperature in the body and this produces an effect on pressure that counters to a degree the drop due to increased volume alone.
Air flows into and out of the lung as gas pressure in the alveolar sacs fluctuates, because the high kinetic energy of gas molecules causes them to expand into any available container that has a lower gas pressure. As lung volume expands, pressure in the air sacs drops below atmospheric pressure and air molecules move into the lung. As lung volume lessens with relaxation of the respiratory muscles, pressure in the air sacs rises above atmospheric pressure and gas molecules leave the lung through the respiratory passages.
Tip #3 Lung O2 and CO2 flow in opposite directions
Pressure differences created by rib cage expansion and contraction move volumes of air into and out of lungs, but why does that bulk movement of air result in the flow of carbon dioxide and oxygen in opposite directions? To explain this another gas law must be introduced, Dalton’s Law of Partial Pressures. This gas law explains best why an increase in carbon dioxide in air may sabotage lung function.
Dalton’s law claims first that when there is a mixture of gases, as in the atmosphere, each gas contributes to the total air pressure in proportion to its percentage in the blend. Nitrogen (N2) is responsible for most of the pressure exerted by the atmosphere. As a result, most tissues in the body including blood are saturated with nitrogen. Carbon dioxide contributes a very small amount to total air pressure. This is good because carbon dioxide is a waste product of body metabolism.
Second, Dalton’s Law of Partial Pressures comes to the less obvious conclusion that each gas in a mixture can be treated as if the other gasses were not present for purposes of predicting flow of its molecules from an area of high concentration to an area of low concentration. Pressure brought by each gas is not influenced by the other gases in the mixture because all of the gas molecules are too far apart to interact with each other.
This means the pressure difference between oxygen in the air and oxygen in the alveoli will determine the direction of flow of oxygen as the lung moves air in and back out. Likewise the difference between pressure of carbon dioxide in the air and the pressure exerted by carbon dioxide in the alveoli will determine the direction of flow of carbon dioxide. Each of these gases inevitably expands away from the area where its own pressure is higher.
Because the body produces carbon dioxide as a waste product that is delivered to lung alveoli, it is important that it leave the alveoli quickly and easily. The low pressure of carbon dioxide in inhaled air facilitates its movement into the air that is pushed out by relaxation of the chest muscles.
The same principle applies for oxygen, but the required path for movement of oxygen molecules is the reverse. The body uses oxygen very rapidly for extracting chemical energy from food. Oxygen in air entering alveoli is quickly captured by blood cells moving through the lung. To maintain an adequate supply of oxygen for the entire body the oxygen pressure of inhaled air must be relatively high.
Tip #4 Air pressure can be quantified
For a mathematical formula to work there must be a numerical value for each of the parameters. Temperature and volume are easy to quantify, but how to quantify atmospheric gas pressure?
YouTube Video: Barometers Measure Atmospheric Pressure by LHS Atkins
By actual measurement at sea level when air temperature is 15°C (59°F) atmospheric pressure is equal to the force needed to drive a column of mercury (Hg) to a height of 760 mm. However, air pressure varies with altitude and can drop at 15,000 feet on a mountain top to about 440 mm Hg. The number of gas molecules is less per unit volume at high altitude but the percentage distribution of component gases does not change.
Air is composed of nitrogen, oxygen, carbon dioxide, an assortment of other minor gas components and small quantities of water vapor. Remember Dalton’s Law of Partial Pressures states that when there is a mixture of gases, as in the atmosphere, each gas contributes to the total gas pressure in proportion to its percentage in the blend. If we assume oxygen is 20% of air, its pressure at sea level calculates to be 152 mm Hg (20% of 760 mm Hg) and at 15,000 feet to be about 88 mm Hg (20% of 760 mm Hg). Given that lungs have a limited capacity for volume expansion, the smaller pressure of oxygen in mountain air results in a lower pressure of oxygen in alveolar air and slower delivery of oxygen to blood.
Tip # 5 Oxygen wanders through water without dissolving
The air sacs of the lung, the alveoli, are lined with a watery fluid through which lung gasses must pass when entering and exiting the red cells of the blood. This means solubility of oxygen and carbon dioxide in water is an important issue.
Oxygen and carbon dioxide are nonpolar molecules and therefore do not actually ‘dissolve’ in water like polar molecules. In fact the presence of carbon dioxide in water at it normal partial pressure in air is extremely low. Moving carbon dioxide through the watery environment of the body requires a complex arrangement of enzyme reactions that involves the bicarbonate buffering system. That process will be described in a future post.
The nonpolar oxygen molecule takes a different path through water. It is able to slip into pockets in the loose lattice of liquid water molecules. The overall size of the pockets between water molecules suits the size and shape of oxygen molecules. Oxygen molecules become trapped in place until the dynamic interaction between water molecules causes the pockets to disappear. Once free, oxygen pressure pushes the molecules into another pocket always moving them away from where oxygen pressure is high to where it is lower. Once oxygen reaches an alveolar cell, it dissolves rapidly through the cell’s nonpolar membrane and on into adjacent red blood cells.
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