Concept 31.1: Respiratory Gas Exchange Depends on Diffusion and Bulk Flow

The respiratory gases that animals must exchange with their environment are oxygen (O₂) and carbon dioxide (CO₂). Cells require O₂ from the environment so they can produce ATP by cellular respiration (see Concept 6.2). Specifically, O₂ is needed in the mitochondria of each cell because electrons removed from food molecules must have a final resting place after they pass through the mitochondrial electron transport chain, releasing the energy that the mitochondria use to make ATP. Their final resting place is in combination with O₂ to make water (see Figure 6.9). This water simply mingles with the rest of the water in an animal’s body.

Cellular respiration also produces CO₂ because CO₂ is one of the products of the citric acid cycle (see Figure 6.8). Unlike the H₂O produced by cellular respiration, CO₂ cannot be allowed simply to accumulate in an animal’s body water. CO₂ is often called a “gaseous acid” because it tends to lower the pH of the body fluids if it accumulates in them. It can also have other disadvantageous effects. For these reasons, as an animal takes in O₂ from its environment and delivers it to cells for use in cellular respiration, the animal must rid itself of the CO₂ made by cellular respiration by releasing the CO₂ into its environment.

Think about the path an O₂ molecule must travel from the atmosphere to a mitochondrion inside a muscle cell in your calf. First the O₂ molecule must travel deep into your lungs. Then it must cross two simple epithelia to enter the blood: first the epithelium forming your lung wall and then the epithelium forming the wall of one of your blood capillaries. The molecule must then travel in the blood from your thorax to your leg to reach a blood capillary next to the muscle cell. Then, to enter the cell, the molecule must cross the wall of the blood capillary (an epithelium) and pass through the cell membrane of the muscle cell. Finally, the molecule must travel through the cell cytoplasm to reach and enter a mitochondrion. CO₂ must make essentially the same journey in reverse.

Notice that some of the steps in an O₂ molecule’s trip from atmosphere to mitochondrion are very short (FIGURE 31.1). The distance across each epithelium is about 1 micrometer (μm) or less. The distance across a cell membrane is roughly 0.01 μm. The distance from the cell membrane of a muscle cell to the deepest mitochondrion inside the cell is likely to be only 5–50 μm.

Figure 31.1: The Path of O₂ Molecules into an Animal’s Body Consists of Long and Short Steps

By contrast, some of the other steps in an O₂ molecule’s trip are extraordinarily long. To pass deep into your lungs from your nostrils, an O₂ molecule needs to travel about 500,000 μm (0.5 meter). To travel in your blood from your lungs to a muscle cell in your calf, the molecule needs to travel about 1,000,000 μm!

Distance is therefore one of the factors we need to take into account as we consider the travels of O₂ and CO₂ molecules between the atmosphere and an animal’s cells.

Also note this important point: active transport mechanisms for O₂ do not exist (see Concept 5.3). This means that energy supplied by ATP cannot be used to drive O₂ through an epithelium or cell membrane. Instead, O₂ always crosses epithelia and cell membranes by diffusion. Much the same can be said for CO₂, although active transport of CO₂ is known in certain cases in the animal kingdom.

The diffusion of gases depends on their partial pressures

Atoms and molecules undergo ceaseless random motions at atomic–molecular scales of space. Diffusion results from these random motions (see Concept 5.2). If a certain type of molecule is more concentrated in one region of a solution than another, it is a simple statistical fact that random motions will carry more molecules away from the concentrated region than they will carry into it. In this way, the random motions cause a net transport of molecules from the concentrated region to neighboring dilute regions. This is the principle by which O₂ and CO₂ diffuse in a solution.

Complexities arise where liquids and gases are in contact. When O₂ is present in a gas mixture, it is simply one of the types of gas molecules. For example, in air 21 percent of the molecules are O₂. When O₂ is present in a liquid, it is in solution. You have seen that when you dissolve sugar in water, the sugar molecules become invisible. O₂ does exactly the same thing. When it dissolves in water, the individual O₂ molecules fit in between water molecules and become invisible (bubbles, regardless of how tiny, are not in solution).

What happens when O₂ diffuses from a gas into an aqueous (watery) solution—so that the O₂ molecules switch from being in a gaseous to a dissolved state—or vice versa? How can we predict whether this diffusion will be from the gas to the liquid or in the opposite direction? Chemists have discovered that if you measure a property called partial pressure, you can solve this problem. O₂ always diffuses from where its partial pressure is high to where its partial pressure is low. Partial pressure is easily defined in a gas phase (not so easily in a liquid phase): it’s simply the portion of the total pressure exerted by any particular gas, such as O₂, in a mixture of gases.

To clarify the relationship between concentration and partial pressure, suppose we have a gas phase in contact with a liquid phase, as in FIGURE 31.2. Within a gas phase, the concentration and partial pressure of O₂ are proportional to each other. O₂ always diffuses from where its concentration is high to where its concentration is low, for the reasons already discussed. Because concentration and partial pressure are proportional, O₂ also diffuses from where its partial pressure is high to where its partial pressure is low. Within a liquid phase, the same statements are true. Concentration and partial pressure are proportional, and O₂ always diffuses from places where both are high to places where both are low.

Figure 31.2: A Gas Always Diffuses from High Partial Pressure to Low Partial Pressure The concepts illustrated here apply to any gas. The gas is assumed to be in solution in the liquid phase. Within a gas phase or within a liquid phase, the partial pressure and concentration of a gas are proportional. Thus as the gas diffuses from high to low partial pressure, it also diffuses from high to low concentration. Concentration, however, is not a guide to diffusion across an interface between a gas and a liquid phase. Diffusion occurs from high to low partial pressure, but this does not necessarily mean the gas moves from where its concentration is high to where its concentration is low.

However, when O₂ diffuses between a gas phase and a liquid phase, its direction of diffusion cannot be predicted from its concentrations in the two phases. When diffusing from one phase to the other, O₂ moves from where its partial pressure is high to where it is low, even though this direction may be opposite to what concentrations would suggest (the partial pressures have the advantage that they take into account the effects of the solubility of O₂ in water, which the concentrations do not take properly into account). The same principles apply to all other gases, such as CO₂ and N₂.

The law of diffusion for gases—often called Fick’s law—is expressed by the equation

where

• D, the diffusion coefficient, depends on the diffusion medium (air or water), the temperature, and the particular gas that’s diffusing
• A is the cross-sectional area across which the gas is diffusing
• P₁ and P₂ are the partial pressures of the gas at the two locations
• L is the distance (diffusion path length) between the two locations

This equation applies to the diffusion of all gases, and it applies within a gas phase, within a liquid phase, and between gas and liquid phases. Partial pressures are expressed in units of pressure. Thus, the standard unit for partial pressure is the pascal (Pa). Alternative units in common use are atmospheres (atm) and millimeters of mercury (mm Hg), the latter referring to the height of a mercury column the pressure can support (1 atm = 760 mm Hg = 101.3 kilopascals).

Diffusion can be very effective but only over short distances

Let’s ask the simple question, Can diffusion move lots of O₂ (or CO₂) from one place to another in a short period of time? The answer is both yes and no. This distinction is one of the most important concepts to understand about diffusion. As we’ll see, the answer depends on the distance involved.

Notice that the distance between two locations, L, appears in the denominator of the diffusion equation. The rate of diffusion between two locations increases as the distance between the locations decreases.

Suppose O₂ on the outside of a cell is diffusing across the cell membrane into the cell. The distance, L, for this diffusion is very short—about 0.01 μm. Thus this diffusion will be very fast. In fact, if you were to arrange to have a high O₂ concentration outside a cell and a low concentration inside, diffusion would move enough O₂ into the cell in just 0.1 microsecond to halve the concentration difference between outside and inside! This explains why all cells depend simply on diffusion for O₂ to pass into them through their cell membranes.

By contrast, suppose O₂ present in body fluids near your lungs is diffusing through your body fluids to a muscle cell in your calf. For this diffusion, L is much longer—about 1 meter. This diffusion therefore will be slow—far slower, in fact, than you might ever have imagined. If you had a high O₂ concentration near your lungs and a low concentration in your calf muscle, diffusion would require more than 30 years to halve the concentration difference! Diffusion would obviously be totally useless for getting O₂ to your calf muscle.

We see, then, that diffusion can be very effective for moving O₂ over short distances but not over long distances. The founder of modern respiratory physiology, August Krogh, calculated an important rule of thumb for understanding diffusion. He analyzed a system in which O₂ had to diffuse through tissue to reach mitochondria that were carrying out cellular respiration at an average rate for animals, and he asked how thick the tissue could be. His answer was about 0.5 millimeter. Today, physiologists still use Krogh’s rule of thumb and view 0.5 mm as the greatest distance over which O₂ can diffuse fast enough through tissue or tissue fluids to meet metabolic needs.

Gas transport in animals often occurs by alternating diffusion and bulk flow

Returning to Figure 31.1, we can see that diffusion is suitable for moving O₂ through the short steps in the path from the atmosphere to a muscle cell. It is not suitable, however, for the long steps.

This limitation explains why animals have often evolved processes of bulk flow to transport O₂ over long distances. Bulk flow refers to a flow of matter from one place to another. Winds and water currents are forms of bulk flow in the physical world. When you breathe, you create a bulk flow of air into and out of your lungs. The beating of your heart causes a bulk flow of blood through your blood vessels. Bulk flow can carry O₂ over long distances at high rates.

Diffusion and bulk flow alternate in humans and other mammals (FIGURE 31.3). O₂ is carried by bulk flow over the long distance from the atmosphere to the depths of the lungs. Then it diffuses across the thin epithelia in the lungs to pass into the blood. O₂ is then again carried by bulk flow, in the circulating blood, to reach the legs. Finally, O₂ diffuses out of blood in the legs to cross a blood-capillary epithelium, cross a cell membrane, and finally reach a mitochondrion.

Figure 31.3: O₂ Transport into an Animal Often Occurs by Alternating Bulk Flow and Diffusion This alternation is clear if you read from top to bottom. Compare this figure with Figure 31.1.

Breathing is the transport of O2 and CO2 between the outside environment and gas exchange membranes

The gas exchange membranes of an animal are thin layers of tissue—usually comprising only one or two simple epithelia—where the respiratory gases move between the animal’s environmental medium—air or water—and its internal tissues. Our own gas exchange membranes, for example, are the membranes deep in our lungs where O₂ is removed from the air and enters our blood and where CO₂ leaves our blood and enters the air. All animals have gas exchange membranes of some type. O₂ moves into the internal tissues from the layer of environmental medium immediately next to the gas exchange membranes, and CO₂ moves from the internal tissues into that layer.

How does O₂ from the outside world get to the layer of environmental medium next to the gas exchange membranes? And how does CO₂ move from that layer to the outside world? These questions are the subject of the study of breathing.

Breathing—also called external expiration—is the process by which O₂ and CO₂ are transported between the layer of environmental medium next to the gas exchange membranes and the outside world. Ventilation refers to bulk flow of air or water between the gas exchange membranes and the outside world. Breathing often occurs by ventilation. This is true in humans and most other vertebrates, for example. However, breathing can occur entirely by diffusion if the distances to be covered are short. Many tiny insects, such as gnats, are believed to breathe by diffusion.

Physiologists categorize breathing organs into two fundamental types: lungs and gills (FIGURE 31.4). Lungs are invaginated (folded inward) into the body, and their passages contain the environmental medium. Gills are evaginated (folded outward) from the body and are surrounded by the environmental medium. In general, air-breathing animals have lungs and water-breathing animals have gills, although exceptions occur.

Figure 31.4: Gills and Lungs Physiologists classify all specialized breathing organs (blue in these diagrams) as either gills or lungs, depending on whether they are (A) evaginated structures (folded outward from the body) that are surrounded by the environmental medium or (B) invaginated structures (folded inward into the body) that contain the environmental medium. The use of the word “lungs” does not always match everyday usage. For example, the tracheal breathing system of an insect (discussed later in this chapter) is a lung because it is invaginated and contains air, but it is rarely called a lung in everyday language. Gills are subdivided into external gills (which project directly into the animal’s environment) and internal gills (which project into a protective gill chamber that is ventilated with water).

When lungs are ventilated, air usually moves into them along lung passages during inhalation and then moves out through the same passages during exhalation. This ventilation is described as tidal because air flow occurs first in one direction and then in the opposite direction in the same airways—in and out like the ocean tides. Humans exemplify tidal ventilation.

In animals with gills, breathing usually involves pumping water in a one-way stream over the gill surfaces. This water movement is the ventilation of the gills, and the ventilation is described as unidirectional.

Air and water are very different respiratory environments

Diffusion takes place much faster through air than water (a fact reflected by the value of D in the diffusion equation). If all factors that affect diffusion are kept constant except that diffusion may be through air or water, the rate of diffusion of O₂ will be about 200,000 times faster through air!

Sea turtle nests that become flooded provide a striking illustration of the difference in diffusion rates through air and water (FIGURE 31.5). Loggerhead sea turtles lay their eggs about 40 centimeters deep on ocean beaches and cover them with sand. Nathan Miller outfitted nests with tiny probes that continually monitored the partial pressure of O₂ in the nests. He found that the eggs received plenty of O₂ by diffusion through the sand from the atmosphere above, provided the spaces between sand grains were full of air (Krogh’s rule does not apply in air). However, if the sand became flooded with water for a long period—so the spaces between sand grains were water-filled—the O₂ partial pressure in the nest could fall to zero, causing the death of many eggs.

Figure 31.5: The O₂ Supply for Sea Turtle Eggs Depends on whether the Sand in which They Are Buried Is Dry or Wet The O₂ partial pressure in a nest of sea turtle eggs depends on how fast the developing turtles use O₂ and how fast O₂ is resupplied by diffusion from the atmosphere. During the first 50 days of development in the nest studied here, the sand was dry and O₂ diffused into the nest as fast as the developing turtles used O₂—keeping the O₂ partial pressure in the nest high. When the sand became soaked with water for 2 days, as the developing turtles used O₂, the O₂ they used was not replaced, and their metabolism reduced the O₂ partial pressure to zero.

In addition to its slow rate of diffusion through water, O₂ also is not very soluble in water. Suppose we have some fresh water in an aquarium and we bubble it continuously with air so that the water and air are at equilibrium with each other (in this state they have the same O₂ partial pressure). At 12°C the air will contain about 200 milliliters (ml) of O₂ in each liter. The water, however, will contain only about 7.7 ml of O₂ in each liter! The solubility of O₂ in water is so low that the amount present per liter is only about 4 percent as high as in air—even when the water is steadily bubbled with air.

The solubility of O₂ in water decreases as water temperature rises. Thus the concentration of O₂ in water tends to decrease as water warms. Fresh water at 24°C, for example, dissolves 6.2 ml of O₂ in each liter, substantially less than the 7.7 ml per liter dissolved at 12°C. Most aquatic animals are poikilothermic (see Concept 29.3). Thus as water warms, their body temperatures and rates of O₂ consumption increase (see Figure 29.6). At the same time, the amount of dissolved O₂ available from each liter of water decreases. Water-breathing animals can find themselves caught in a respiratory trap, in which their O₂ needs increase as O₂ availability decreases. This can weaken them—interfering with growth or reproduction—and even kill them if the mismatch between O₂ needs and O₂ availability becomes too great.

All the animals alive today with the highest metabolic rates are air-breathers. Water is a challenging place to live in terms of getting enough O₂, if an animal is a water-breather.

Now that we have examined the basics of respiratory gas exchange, let’s look at the diversity of structures and processes by which animals obtain O₂ and get rid of CO₂.