Gas exchange

Gas exchange is the uptake of oxygen and the loss (or elimination or excretion) of carbon dioxide. It refers to two major steps in the overall oxygen consumption (or carbon dioxide excretion) by the whole animal. These two steps are the movement of the respiratory medium (containing oxygen) past the site of gas exchange, known as ventilation, and the diffusion of oxygen across the gas-exchange surface into the animal. The final step in the diffusion of oxygen into an animal always involves diffusion from a liquid into a liquid, even in air breathers. Both ventilation and diffusion depend on the design of the structures as well as the way in which the systems and structures work. The additional steps in the whole respiratory system are the internal counterparts to ventilation and diffusion. These are perfusion, or blood flow, and diffusion from the blood to the tissues.

Basically, gas exchange takes place at a respiratory surface where the source of oxygen, the respiratory medium, is brought into contact with the surface. Oxygen diffuses into the animal, carbon dioxide diffuses out, and the spent or used respiratory medium is removed. The movement of the respiratory medium is termed ventilation. Once oxygen is in the animal, it is transported to the site of oxygen utilization, the tissues. Carbon dioxide, on the other hand, must be transported from the tissues to the site of gas exchange for excretion in gaseous form.

Gas-Exchange Organs

Animals have three basic types of gas-exchange organs: skin, invaginations (inpocketings of the epithelium), and evaginations (outpocketings of the epithelium). All three show modifications to improve the conditions of gas exchange. Skin always permits gas exchange unless it is coated with some material that limits diffusion. The skin of a snake or a turtle is so coated and permits very little gas exchange. The skin of a worm or an octopus, on the other hand, is quite thin and permits gas exchange quite freely. Invaginations of the external epithelium are basically what lungs and insect tracheae are, but in a highly modified condition. Evaginations of the skin are represented by the gills of aquatic animals; even when inside a cavity, as are fish and crab gills, they are still evaginations.

There is only one way that animals take up oxygen from the external medium, regardless of whether that medium is air or water. Gas must passively diffuse across the membrane that separates the animal from its environment. That membrane is a type of tissue called epithelium and is similar in nature and structure to the tissue that lines other body surfaces. The different types of epithelia are classified according to their locations and functions; those lining gills, lungs, and certain other organs of gas exchange are all known as respiratory epithelia. The respiratory epithelium not only separates the internal and external fluids but also represents a barrier to the movement of materials such as gas.

Gas Diffusion

Diffusion of a gas across a membrane occurs according to the laws of physics. The driving force for gas diffusion is the difference in the partial pressure of the gas across the membrane. A high external partial pressure and low internal partial pressure will provide a large difference and will enhance diffusion. Oxygen makes up 20.9 percent of the air, so 20.9 percent of the atmospheric pressure at sea level (14.72 pounds per square inch) is attributable to oxygen (3.08 pounds per square inch). At higher altitudes, atmospheric pressure and partial pressure of oxygen are reduced.

The other factors that determine the diffusion of a gas are the thickness of the membrane across which it diffuses, the membrane’s total surface area, and the nature or composition of the membrane. Obviously, a thick membrane will retard diffusion of gas because the gas must move across a greater distance. The distance the gas must diffuse is known as the diffusion distance. Additionally, the total surface area of the membrane available for diffusion has a direct effect on the rate of diffusion from one place to another. The greater the surface area, the greater the quantity of gas that can diffuse in a given time. Finally, the composition of the membrane is of critical importance in determining the diffusion of a gas. The nature of the membrane is referred to as the permeability of the membrane to the gas in question. The greater the permeability, the more easily gas diffuses. A membrane with a layer of minerals (calcium, for example) on the cells will not be as permeable as one without such a layer.

An important point to note is that gases diffuse according to the difference in the partial pressure of the gas and not according to the concentration of the gas in the liquid. Several scientists have proved this by constructing artificial systems with two dissimilar fluids separated by a membrane. The movement of oxygen is always from high partial pressure to low and not from a high concentration to low. The reason is that pressure is a measure of molecular energy, but concentration of a gas in a liquid depends on the amount of that gas that can dissolve in the liquid—its solubility.

A Fluid-Fluid Boundary

All gas exchange occurs across a fluid-fluid boundary—that is, from one liquid to another—even in air breathers. The explanation for this is that all respiratory epithelia are moist and are kept so by the cells that line the surface. If the surface were to dry out, the permeability to gases (and other materials) would be substantially reduced. Thus, in air breathers, oxygen must first dissolve in the thin fluid layer lining the surface before diffusion across the epithelium takes place.

Organs of gas exchange work as do radiators, except that a gas is exchanged instead of heat. In this system, there are two liquids, one the source and the other the sink for the transferred material, the gas. The source is the external supply, and the sink is the blood or other internal fluid. Both fluids are contained in vessels or tubes that channel and direct it, with a thin layer of epithelium between the two. In the most efficient transfer systems, both the source and the sink flow, and they flow in opposite directions. If they did not flow, then the two fluids would simply come to an equilibrium, with oxygen partial pressure the same in both of the fluids. By moving in opposite directions, each is renewed, and the difference between them is always maximized. This type of exchange system is a “countercurrent system,” because the two fluids flow in opposite directions. The most efficient of the gas-exchange organs function this way.

Two other types of gas-exchange systems are based on fluids flowing in directions other than perfectly opposite to each other. Birds have a respiratory system in which blood and air do not flow in opposite directions; rather, the blood flows perpendicularly to the direction of the air flow. This is referred to as crosscurrent flow. While not as efficient as countercurrent flow, it provides for an acceptably high level of efficiency. A system in which the respiratory medium is not channeled, but the blood is instead in vessels, is known as a mixed volume system. The mammalian lung functions in this way; the air is pumped into sacs that are lined with tiny blood vessels, but the air does not flow.

Ventilation

Ventilation of gas-exchange organs is accomplished by a pumping mechanism that brings the respiratory medium to and across the gas-exchange surface. Both water and air breathers use ventilatory pumps, but water breathers must move a much heavier and denser medium than air breathers. Pumping mechanisms may be located at the inflowing end of the system or at the outflowing end. The former are positive pressure pumps that push respiratory medium, and the latter are negative pressure pumps that pull respiratory medium into the cavity. Negative pressure is used in mammalian lungs, insect tracheas, and crab gills, while positive pressure is used in some fish that push water from the mouth into the gill chamber.

There are two basic patterns of flow of the respiratory medium through gas-exchange organs: one-way and tidal. One-way flow is found in fish, crabs, clams, and a number of other aquatic animals. Interestingly enough, the bird respiratory system also uses one-way flow. In one-way flow systems, the medium is always moving and passes over the gas exchange surface only once. In tidal-flow systems, the respiratory medium moves in and out (like the tide) through the same passages and tubes. The mammalian and insect respiratory systems both utilize tidal ventilation. The respiratory medium is not always moving, and when it is exhaled, there is some amount remaining in the cavity. The remaining respiratory medium will contain more carbon dioxide and less oxygen than fresh respiratory medium, with which it will mix upon inhalation.

Measuring Gas Exchange

The total amount of oxygen used by an animal is a gross measure of gas exchange known as oxygen uptake or oxygen consumption. Its counterpart for carbon dioxide is carbon dioxide excretion. Oxygen uptake is expressed as the amount of oxygen used per minute per kilogram of animal mass. Carbon dioxide excretion is expressed in the same terms. In theory, oxygen uptake and carbon dioxide excretion will be numerically the same, but in live animals, there are several circumstances that cause the two to differ. Measurement of both rates is accomplished in similar ways. One method is the use of a respirometer and involves placing an animal in a sealed container and measuring the rate at which oxygen is depleted or carbon dioxide produced by an animal. Alternatively, the respiratory medium, either air or water, is pumped through the respirometer, and the oxygen or carbon dioxide measured in the inflowing and outflowing medium; the difference will be the amount used by the animal. The flow rate of the air or water must also be known for the calculations. The use of a respirometer is preferable but may not be practical for large animals, such as horses.

In the case of animals too large to use a respirometer, oxygen uptake or carbon dioxide excretion is determined by measuring the rate of flow of the respiratory medium through the gas exchange organ and measuring the oxygen in the inspired and expired air or water. The result is the ventilation rate and the amount of oxygen extraction, the product of which is the oxygen consumption rate. This measurement is straightforward in animals, with one opening for inspired and another for expired respiratory medium, such as a fish or a crab. In animals that inhale and exhale through the same organ, however, there are complications that make the measurements more difficult. Still, it is possible to measure the flow of air in and out of a lung and to collect at least some of the gas and measure either the oxygen or carbon dioxide in that air.

There is an additional advantage to the latter technique, measuring ventilation and the oxygen in the water or air. That advantage is that another measure of gas exchange is provided in these measurements. The difference between the amount of oxygen in inspired and expired water or air is the extraction (the amount taken out) and the efficiency of the gas exchange organ. The efficiency is usually the percentage of oxygen taken out of the respiratory medium (the amount removed divided by the amount in inspired air or water). There are numerous factors that affect extraction, and measuring the efficiency provides one piece of information.

Studies of gas exchange encompass all levels of organization of animals, from the cellular to the whole animal. One of the most important levels concerns the structure of the organ and the parts of the organ. For this, it is necessary to see the spatial relationships among the parts, measure distances and areas, and count structures. The surface area, the volume, the number of structures or substructures, and the diffusion distances must all be measured. The results describe the morphology and morphometrics of the organ. Both whole, intact animals, and preserved specimens are used to make these measurements. The techniques are those used in surgery and dissection, and the results are critical to an understanding of the basic function of the respiratory organ. The electron microscope has been a powerful tool in this regard, permitting the accurate measurement of cellular-level distances, such as the diffusion distance.

Measuring Partial Pressure Difference

Measuring the partial pressure difference between the inside and outside of the animal is critical because of the role that this pressure difference has in gas exchange. Partial pressure of oxygen may be measured in two ways: in the intact animal or in a sample removed from the animal and injected into an instrument. The instrument most commonly used for measuring oxygen or carbon dioxide partial pressure is an electrode that changes electrical output when oxygen diffuses across an artificial membrane into a salt solution. Some of these electrons have been miniaturized and are only four millimeters across, and they will fit in a syringe needle. Still, it is difficult to use one of these in an intact mammal. The other way to measure partial pressure of oxygen or carbon dioxide on either side of the respiratory epithelium is to withdraw a sample of the air or water on the outside or the blood from the vessels on the inside. This procedure may be routine (in animals such as fish and crabs) or somewhat difficult (as in a mammal). A small tube is threaded into the lung to withdraw the air sample.

Measuring Ventilation

The movement of the respiratory medium, ventilation, is an important measure in determining the rate at which oxygen is brought to the respiratory surface. The blood flow (perfusion) on the inside is the counterpart to ventilation and is equally important. Ventilation can be measured either indirectly (meaning it is calculated) or directly. Indirect determinations require measuring other functions and then calculating ventilation based on known equations. If the rate of oxygen uptake and the extraction are measured, for example, then ventilation can be calculated.

Direct measures of ventilation use an electronic sensing device to determine the flow of water or air at the site of intake or outflow of respiratory medium on the animal. A human subject can simply breathe into such an electronic or mechanical device. Nonhuman mammals are more difficult and frequently require indirect techniques.

Direct measures may be the flow rate of the respiratory medium, the frequency of breathing, the hydrostatic pressure in the respiratory chamber, or a change in shape and size of the respiratory chamber. Any of these measures can be used to monitor routine respiratory function, but all are needed to assess gas exchange completely and accurately.

It is also necessary to know the general pattern of water or air movement at the respiratory surface. To do so often requires some invasive technique and the use of an indicator, such as a dye, in the respiratory medium. The movement of the medium can then be visualized to determine the pattern. In some animals, video cameras can be used to photograph flow patterns of dyed medium, particularly water.

Uses of Gas Exchange Study

Gas exchange is studied by researchers and health practitioners to assess basic function and determine the source and nature of limitations of the systems of the body. These two areas may seem quite different at first; one is applied research, and the other is considered basic research. Both, however, have the same bases and use the same equations and principles. Only the subjects or conditions differ.

One of the clinical applications, or contexts, in which gas exchange is studied is in respiratory distress or pulmonary (lung) disease. In these cases, the respiratory epithelium may become inflamed and thickened. This will increase the diffusion distance and retard, or limit, oxygen uptake and carbon dioxide release at the lung. Secretion of mucus by a respiratory epithelium may have a similar result for the same reasons. Mucus secretion occurs in several diseases and also takes place in fish gills when irritated by noxious chemicals in the water.

Gas exchange is also studied in diverse animals to understand evolutionary trends and pressures. Animals that live at high altitudes, for example, are constantly faced with low oxygen pressure in the air, and therefore some adjustment must be made by the animal. Scientists study the respiratory systems of these animals to determine if one of the other factors that affect gas exchange, such as diffusion distance or total surface area, is altered to compensate for the lower pressure difference. Animals, notably humans, living at higher altitudes are at risk for hypobaric hypoxia, a condition that occurs at high altitudes when there is not enough oxygen in the body's tissues. Studies show that hypobaric hypoxia can cause a range of issues, as the body must attempt to regulate gas exchange and raise oxygen levels. Potential side effects include altitude sickness or acute mountain sickness, both of which can cause headaches, vomiting, dizziness, restlessness, and cognitive impairment. Other physiological side effects can occur, such as biochemical alterations of the blood-brain barrier. However, each body can respond differently, and some studies have shown that, on an individual basis, the potential for the body to regulate gas exchange exists.

All animals have similar basic physiological needs, including a need for oxygen to fuel the conversion of food materials into energy and other substances. Many animals have unique or specific forms or structures enabling them to survive in a particular habitat. Some of these forms affect the respiratory system—as in the differences between the respiratory surface in land animals compared with similar species that live in water. Scientists have compared the gas-exchange systems in aquatic and terrestrial species to learn more about evolutionary processes.

Principal Terms

Diffusion: The passive movement of a gas across a membrane from a region of high pressure to one of low pressure

Epithelium: A thin layer of cells that lines a body surface, such as the lining of the lungs or the intestines

Partial Pressure: That part of the atmospheric pressure caused by only a single gas of many in a mixture; it is determined by how much of the gas is present in the mixture

Permeability: The tendency, in this case of a membrane, to permit the movement of a gas across that membrane

Respiratory Medium: The water or air that contains the oxygen used by an animal to carry out biochemical reactions

Ventilation: The movement of the respiratory medium to and across the site of gas exchange

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