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The respiratory system

16.1 Introduction

The energy needed by animals for their normal activities is mainly derived from the oxidative breakdown of foodstuffs— particularly carbohydrates and fats. During this process, which is called internal or cellular respiration, oxygen is utilized by the mitochondria and carbon dioxide is produced. The oxygen needed for this process is ultimately derived from the atmos­phere, while the carbon dioxide produced is eliminated from the body to the atmosphere during external respiration which is the subject of this chapter.

The key process of external respiration is gas exchange between the air deep in the lungs and the blood that perfuses them. In addition to their role in gas exchange, the lungs have a variety of nonrespiratory functions, such as their role in trapping bloodborne particles (e.g. small fragments of blood clots) and the metabolism of a variety of vasoactive substances.

To be able to fully understand the process of breathing and gas exchange the following questions need to be addressed:

  1. What mechanisms are employed to cause air to enter the

  2. How is oxygen taken up in the lungs and carried in the

  3. How is carbon dioxide carried in the blood and eliminated
    from the body via the lungs?

  4. How efficient are the lungs in matching their ventilation
    to their blood flow?

  5. How is the respiratory rhythm generated?

  6. What factors determine the rate and depth of respiration?

  7. What mechanisms prevent the lungs becoming clogged
    with dust from the air?

Gas transport via the blood has been discussed in Chapter 13 and the role of the lungs in acid-base balance is discussed in Chapter 29.

Respiratory physiology employs a large number of standard abbreviations which are often used to calculate respiratory data. The most common are given in Box 16.1 together with the convention for their use.

Box 16.1 The use of symbols in respiratory physiology

Respiratory physiology makes exrensive use of standard symbols

to express concepts concisely and allow

simple algebraic manipu-

lations. The primary variables are giver

as capital letters in italic

(e.g. pressure (P) or volume (V)), while

the location to which they

apply are given by a suffix {e.g. Pao2

is the partial pressure of

oxygen in the arterial blood while Pao

2 is the parrial pressure of

oxygen in the alveolar air). In addition

, rhe prefix, s, is used for

specific, e.g. sRaw is the specific airway



Symbol used

Pressure, partial pressure, or gas tension


Volume of gas


Flow of gas (1 min^1)


Volume of blood


Flow of blood (1 mirr1)


Fractional concentration of dry gas














Mixed venous








End-tidal air (= alveolar gas)








Dead space


Pleural space




Chest wall








^ 326

16 The respiratory system

] 6.2 The application of the gas laws to respiratory physiology

External respiration involves the exchange of oxygen and carbon dioxide between the blood and the air in the lungs. To under­stand the factors that determine their uptake and loss from the lungs, it is useful to have a knowledge of the physical properties of gases. The air we breathe and that in our lungs is a mixture of gases consisting mainly of nitrogen, oxygen, carbon dioxide, and water vapor. Dalton's law of partial pressures states that the total pressure is the sum of the pressures that each of the gases would exert if it were present on its own in the same volume. Thus:

where P, is the total pressure of the gas mixture and PN2, Po2, Pco2, and Ph2O are the partial pressures of nitrogen, oxygen, carbon dioxide, and water vapor, respectively.

^ Boyle's law states that the pressure exerted by a gas is inversely proportional to its volume, so that:

Charles' law states that the volume occupied by a gas is directly related to the absolute temperature (T):

These two laws are combined with Avogadro's law in the ideal gas law, which states that:

where n the number of moles of gas (each mole occupies 22.4 liters at STP) and R is the gas constant (8.31 joules K-'moH).

From the ideal gas law, the pressure and volume of a given mass of gas are related to the absolute temperature by the following relationship:



which makes clear that the volume of gas will depend on both the temperature and the pressure. Thus a volume of gas collected from a subject in a gas sampling bag (known as a Douglas bag) will depend both on the atmospheric pressure and on the room temperature. To be able to compare samples of gas collected at different times, the volume of a gas may be expressed in one of two ways:

  1. As STPD—standard temperature and pressure dry. This
    gives the volume of gas after removal of water vapor at
    standard temperature (273 K or 0 °C) and pressure

  2. As BTPS—body temperature and pressure saturated
    with water vapor, i.e. at 37 °C (310 K) and a water vapor
    pressure of 6.2 kPa (47 mmHg). This would be the volume
    of gas expired from the lungs.

Because a given mass of gas will occupy significantly different volumes at 0 °C and at 37 °C, it is essential to state which stan­dard is being employed. Box 16.2 gives the formulae for con­verting a volume of gas at ambient temperature and pressure (ATP) to BTPS or STPD.

Like liquids, gases flow from regions of high pressure to regions of lower pressure. Moreover, since the partial pressure of a gas is a direct measure of its molar concentration, a gas will diffuse from a region of high partial pressure to one of a lower partial pressure even though the total pressure in the gas phase is uniform. The rate of diffusion under these circumstances is inversely proportional to the molecular weight of the gas, i.e. the greater the molecular mass the slower the rate of diffusion. This is known as Graham's law. However, since the molecular weights of oxygen, carbon dioxide, and nitrogen are 32, 44, and 28 Da, respectively, all three gases diffuse in the gas phase at very similar rates.

Solubility of gases

To reach the cells where it is required, O2 must first dissolve in the aqueous lining of the lung. The amount of oxygen that is dissolved is proportional to its partial pressure in the gas phase. This is ^ Henry's law. which can be written as:

V = s.P

where s is the solubility coefficient (ml I-1 kPa~' or ml I-1 mmHg-1), V is the volume of dissolved gas in a liter of the liquid phase, and P is the partial pressure of the gas under con­sideration. For oxygen at body temperature (37 °C), s is 0.225mll-1kPa-1 (0.03 ml H mmHg"1) . Thus the amount (V) of oxygen dissolved in 1 liter of water or plasma when Po2 is 13.33 kPa (100 mmHg) is:

V= 0.225 X13.33 = 3 ml

Similar calculations can be made for carbon dioxide, where s = 5.1 ml H kPa-1 (0.68 ml I-1 mmHg4), and for nitrogen, where s = 0.112 ml I-1 kPa-1 (0.015 ml I"1 mmHg-1). Note that this relationship only applies to dissolved gas. Where the gas enters into chemical combination the total amount in the liquid phase is the sum oj that chemically bound plus that in physical solution.

In respiratory physiology the concentration of dissolved gases is usually given as their partial pressures, even when they are present in a solution with no gas phase (e.g. in the arterial blood). The partial pressure of a gas can be converted readily

16.2 Application of the gas laws to respiratory physiology

i,2 Conversion of gas volumes to BTPS and STPD

Conversion from ambient temperature and pressure to BTPS

As air is breathed in, it is heated and humidified. This leads to an increase in its volume, which can be calculated from the universal gas law:

from which the following formula can be derived:


Vgxps is the volume at body temperature and pressure saturated;

^ V/sjp is the volume of air inhaled at ambient temperature and pressure;

TA is the ambient temperature;

PB is the barometric pressure; and

Ph2o is the water vapor pressure of the ambient air.

The numerical constants 37 and 6 are the body temperature in degrees Celsius and the saturated water vapor pressure in kPa (equivalent to

47 mmHg).

Thus, for a liter of room air inhaled when the temperature is 20°C, the barometric pressure is 100 kPa (c. 750 mmHg), and the water vapor

pressure is 2 kPa (c. 17 mmHg), the change in the volume of the thorax will be:

In other words, the expansion of the chest will be about 10 per cent more than the volume of gas inhaled.

^ Conversion from ambient tempera I lire and pressure to S'fPD

The volume of oxygen absorbed or carbon dioxide exhaled is expressed as STPD because, in this case, the need is to express the number of

moles of gas exchanged. The volume of one mole of gas is 22.4 liters at STP (0 °C and 101 kPa (760 mmHg)). In this case the conversion uses

the formula:

Thus 1 liter of humidified oxygen at the same ambient temperature and pressure as the previous example would occupy a volume at STPD of:

273 100-2

y = 1 x x = 0.89 litres.

ST 298 101

In the examples given above, the air has been assumed to be saturated with water vapor. This is normally the case for gas samples taken from a spirometer. For normal ambienr air, the percentage saturation of the air (the relative humidity) must be taken into account. To do this, simply multiply the saturated water vapor pressure for the ambient temperature by the relative humidity expressed as a fraction (70 per cent humidity is 0.7, etc.).

to the equivalent molar concentration using Avogadro's law. Since at STP 1 mole of CO, occupies 22.4 liters, this cor-

Thus when carbon dioxide has a partial pressure of 5.33 kPa responds to: (40 mmHg) each liter of plasma will dissolve:


16 The respiratory system

Diffusion of dissolved gases

When oxygen, for example, is taken up by the blood it must first dissolve in the aqueous phase that lines the lungs and then diffuse across the alveolar membrane into the blood. The rates at which oxygen and carbon dioxide diffuse from the aqueous lining of the alveoli to the blood is governed by Fick's law of diffusion (see Chapter 15, Box 15.5). The extreme thinness of the alveolar membranes and their large area helps to optimize the diffusion of the respiratory gases. If the alveolar membranes become thickened by disease, the diffusion of the respiratory gases is adversely affected.

In addition to the area and thickness of the alveolar mem­branes, the rate at which the respiratory gases diffuse will depend on their solubility and their concentration gradient. The importance of solubility in determining the rate of diffusion is evident with carbon dioxide. This gas is about 20 times more soluble in the alveolar membranes than oxygen. Although it has a concentration gradient one-tenth that of oxygen, it diffuses from the blood to the alveolar air about twice as fast as oxygen diffuses from the alveoli to the blood.

The composition of the expired air

The expired air contains less oxygen and more carbon dioxide than the inspired air. Standard values for the partial pressures of the gases present in expired and alveolar air are given in Table 16.1. Note that although nitrogen is not exchanged with the blood, its partial pressure changes as it becomes diluted by the water vapor and carbon dioxide from the lungs.

The ratio of the carbon dioxide produced divided by the oxygen uptake is called the respiratory exchange ratio or respiratory quotient (RQ). Under normal resting conditions the value of RQ varies according to the type of food being metabolized to produce ATP. It ranges from 0.7 when fats are the principal sub­strate to 1.0 for carbohydrates. Usually RQ is around 0.75-0.8 as both carbohydrates and fats are metabolized. During starva­tion protein becomes an important source of energy and RQ has a value of about 0.8 (see Chapter 24 for further details).

Table 16.1 Standard values for respiratory gases





Inspired air (kPa)










% total





Expired air (kPa)










% total'





Alveolar air (kPa)










% total






  1. The volume of a given quantity of gas depends both on the tem­
    perature and pressure, and is governed by the ideal gas law. In respir­
    atory physiology the volume of a gas is expressed as standard
    temperature and pressure dry (STPD) or as body temperature and
    pressure saturated with water vapor (BTPS).

  2. The amount of a gas in solution is proportional to its partial pressure
    in the gas phase (Henry's law). Its rate of diffusion is governed by
    Fick's law.

3- The expired air has less oxygen and more carbon dioxide than room air. The ratio of the amount of carbon dioxide expired divided by the amount of oxygen taken up is known as the respiratory exchange ratio and is an index of the foodstuffs being metabolized.

^ 16.3 The structure of the respiratory tree

The lungs are the principal organs of the respiratory system. They form the surface over which oxygen is absorbed and carbon dioxide is excreted. As the lungs are situated in the chest, air from the atmosphere must pass through the nose or mouth and enter the airways before it can be directed to the respiratory surface where gas exchange occurs.

During quiet breathing air is normally taken in via the nose but during heavy exercise air is taken in via the mouth, which offers much less resistance to air flow. Although the nasal passages offer a high resistance to air flow, they are able to trap particles of airborne dust and they help to moisten and warm the air during its passage to the lungs. After entering the nose or mouth, the air passes through the pharynx to the larynx. Like the nose, the larynx is a significant source of resistance to the flow of air and this property is exploited in vocalization.

The trachea links the larynx to the lungs. In an adult it is about 1.8 cm in diameter and 12 cm in length. It is the first component of the respiratory tree—the branching set of tubes that link the respiratory surface to the atmosphere.

In the upper chest the trachea branches to form the two main bronchi—one for each lung. In turn, the bronchi branch to give rise to two smaller branches on the left and three on the right, corresponding to the lobes of the lung. (The right lung has three lobes while the left has two.) A diagram of the arrangement of the trachea and lungs within the chest is shown in Fig. 16.1.

Within each lobe, the bronchi divide into two smaller branches and these smaller branches also divide into two, and so on until the final branches reach the respiratory surface. In all, there are 23 generations of airways between the alveoli and the atmosphere. The tubes that form the fourth to the sixteenth generations are called bronchioles. The sixteenth generation that links the bronchi­oles to the respiratory surface are known as the terminal bronchioles. The terminal bronchioles branch to form the first generation of the respiratory bronchioles, which branch to form the alveolar ducts from which the principal gas exchange structures arise. These are the alveolar sacs, which consist of two or more alveoli.

16.3 The structure of the respiratory tree


Fig. 16.1 The disposition of the lungs within

the thorax.

The first 16 branches of the airways play no significant part in gas exchange and are known as conducting airways. The respir­atory bronchioles, alveolar ducts, and alveoli comprise the tran­sitional and respiratory airways, which provide a total area for gas exchange of about 60—80 m2 in an adult.

The structure of the airways

The trachea and the primary bronchi are held open by C-shaped rings of cartilage. In the smaller bronchi this role is taken by over-

lapping plates of cartilage. The bronchioles, which are less than 1 mm diameter, have no cartilage and are easily collapsed when the pressure outside the lung exceeds the pressure in the airways, which happens during a forced expiration (see p. 338). The struc­ture of the bronchi and bronchioles is illustrated in Fig. 16.2.

Smooth muscle is found in the walls of all the airways, including the alveolar ducts (but not in the walls of the alveoli themselves). In the terminal bronchioles, the smooth muscle accounts for much of the thickness of the wall. The outermost

Fig. 16.2 The structure of (a) the bronchi and (b) the bronchioles. Note that the bronchus has a thicker epithelium and lamina propria compared to the bronchiole. It also has plates of cartilage while the bronchioles have a higher proportion of smooth muscle.


16 The respiratory system

Fig. 16.3 Diagrammatic representation of the layers separating the alveolar air space from the blood in the pulmonary capillaries.

part of the bronchiolar wall—the adventitial layer—is composed of dense connective tissue, including elastic fibers.

Much of the respiratoty ttee is lined with a ciliated columnar epithelium rhat contains many goblet cells. Beneath the epithelial layer there are numerous submucosal glands that discharge their secretions into the bronchial lumen. The cilia beat continuously, and slowly move the mucus secreted by the goblet cells and sub­mucosal glands towards the mouth, where it can be coughed up ot swallowed. This arrangement is known as the mucociliary escalator, which plays an important role in the removal of inhaled particles (Section 16.8). The epithelium of the bronchioles also contains nonciliated cells that are probably secretory in function.

The site of gas exchange is the alveolar—capillary unit. There are about 300 million alveoli in the adult lung and each is almost com­pletely enveloped by pulmonary capillaries. Estimates suggest that there are about 1000 pulmonary capillaries for each alveolus. This provides a huge area for gas exchange by diffusion. The walls of rhe alveoli consist of a thin epithelial layer that covers the pulmonary capillaries (Fig. 16.3). The alveolar epithelium consists of two types of cell, called the alveolar type I and type II cells. The type I cells are squamous epithelial cells, while the type II cells ate thicker and produce the fluid layer that lines the alveoli. The type II cells also synthesize and secrete pulmonary surfactant (Section 16.4). The cell membranes of the alveolar epithelial cells and pulmonary capillary endothelial cells are in close apposition and the pulmonary blood is separated from the alveolat air by as little as 0.5 yU,m.

Interspersed between the capillaries in the walls of the alveoli are the elastic and collagen fibers that form the connective tissue of the lung. This connective tissue links the alveoli together to form

Fig. 16.4 The arrangement of the respiratory muscles of the human chest. Note that the angle of the external intercostal muscles allows them to lift the rib cage when they shorten, so expanding the chest. The internal intercostal muscles act to lower the rib cage. The contraction of the accessory muscles acts to lift the rib cage while contraction of rhe abdominal muscles tends to force the diaphragm upwards into the chest, assisting expiration.

the lung parenchyma, which is sponge-like in appearance. Neigh­boring alveoli are interconnected by small air passages called the pores of Kohn.

The structure of the chest wall

The lungs are not capable of inflating themselves; this is achieved by changing the dimensions of the chest wall by means of the respiratory muscles (Section 16.4). The principal respiratory muscles are the diaphragm and the internal and external intercostal muscles. In addition, some other muscles which are not involved during normal quiet breathing may be called upon during exer­cise. These are the accessory muscles, which assist inspiration, and the abdominal muscles, which assist in expiration (Fig. 16.4).

The chest wall is lined by a membrane called the parietal pleura (Fig. 16.5). This is separared from the visceral pleura, which covers the lungs, by a thin layer of liquid that serves to lubricate the surfaces of the pleutal membranes as they move during respiration. The volume of fluid is about 10 ml in total. It is an ulttafilttate of plasma and is normally drained by the lymphatic system that lies beneath the visceral pleura. The pleural membranes themselves consist of two layers of colla­genous and elastic connective tissue that are joined at the roots of the lungs. Beneath the visceral pleura lies the limiting mem­brane of the lung itself which, together with the visceral pleura, limits the expansion of the lungs. The lungs are separated from the chest wall only by the pleural membranes and, in health, they occupy almost all of the cavity of the chest.

^ 16.4 The mechanics of breathing
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