The anatomical dead space is included. Chursin V.V

Minute ventilation is the total amount of air that enters and leaves the airways and lungs in one minute, which is equal to the tidal volume times the respiratory rate. Normally, the tidal volume is approximately 500 ml, and the respiratory rate is 12 times per minute.

Thus, the normal ventilation minute volume averages about 6 liters. With a decrease in minute ventilation to 1.5 liters and a decrease in respiratory rate to 2-4 in 1 min, a person can live only for a very short time, unless he develops a strong inhibition of metabolic processes, as happens with deep hypothermia.

The respiratory rate sometimes increases to 40-50 breaths per minute, and the tidal volume can reach a value close to the vital capacity of the lungs (about 4500-5000 ml in young healthy men). However, at a high respiratory rate, a person usually cannot maintain a tidal volume above 40% of vital capacity (VC) for several minutes or hours.

Alveolar ventilation

The main function of the pulmonary ventilation system is the constant renewal of air in the alveoli, where it comes into close contact with the blood in the pulmonary capillaries. The rate at which newly introduced air reaches the specified area of ​​contact is called alveolar ventilation. During normal, quiet ventilation, the tidal volume fills the airways up to the terminal bronchioles, and only a small part of the inhaled air travels all the way and comes into contact with the alveoli. New portions of air overcome a short distance from the terminal bronchioles to the alveoli by diffusion. Diffusion is due to the movement of molecules, with the molecules of each gas moving at high speed among other molecules. The speed of movement of molecules in the inhaled air is so great, and the distance from the terminal bronchioles to the alveoli is so small that gases overcome this remaining distance in a matter of fractions of a second.

Dead space

Usually, at least 30% of the air inhaled by a person never reaches the alveoli. This air is called dead space air because it is useless for the gas exchange process. Normal dead space in a young man with a tidal volume of 500 ml is approximately 150 ml (about 1 ml per 1 pound of body weight), or approximately 30 % respiratory volume.

The volume of the respiratory tract that conducts inhaled air to the site of gas exchange is called anatomical dead space. Sometimes, however, some of the alveoli do not function due to insufficient blood flow to the pulmonary capillaries. From a functional point of view, these alveoli without capillary perfusion are considered as pathological dead spaces.

Given the alveolar (pathological) dead space, the total dead space is called physiologically dead space. In a healthy person, the anatomical and physiological dead space are almost the same in volume, since all the alveoli are functioning. However, in individuals with poorly perfused alveoli, total (or physiologic) dead space may exceed 60% of the tidal volume.

More on Dead Space Ventilation:

1. Features of ventilation in newborns and young children Indications for ventilatory support and basic principles of mechanical ventilation in newborns and children

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The airways, lung parenchyma, pleura, musculoskeletal skeleton of the chest and diaphragm constitute a single working organ, through which lung ventilation.

Ventilation call the process of updating the gas composition of the alveolar air, ensuring the supply of oxygen to them and the removal of excess carbon dioxide.

The intensity of ventilation is determined inspiratory depth and frequency breathing.
The most informative indicator of lung ventilation is minute volume of breathing, defined as the product of tidal volume times the number of breaths per minute.
In an adult male in a calm state, the minute volume of breathing is 6-10 l / min,
during operation - from 30 to 100 l / min.
The frequency of respiratory movements at rest is 12-16 per 1 min.
To assess the potential of athletes and persons of special professions, a sample with arbitrary maximum ventilation of the lungs is used, which in these people can reach 180 l / min.

Ventilation of different parts of the lungs

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Different parts of the human lungs are ventilated differently, depending on the position of the body.. When a person is upright, the lower sections of the lungs are ventilated better than the upper ones. If a person lies on his back, then the difference in ventilation of the apical and lower parts of the lungs disappears, however, while the rear (dorsal) their areas begin to ventilate better than the front (ventral). In the supine position, the lung located below is better ventilated. The uneven ventilation of the upper and lower parts of the lung in the vertical position of a person is due to the fact that transpulmonary pressure(pressure difference in the lungs and pleural cavity) as a force that determines the volume of the lungs and its changes, these sections of the lung are not the same. Since the lungs are weighty, the transpulmonary pressure is less at their base than at their apex. In this regard, the lower parts of the lungs at the end of a quiet exhalation are more squeezed, however, when inhaling, they straighten out better than the tops. This also explains the more intensive ventilation of the lung sections that are below, if a person lies on his back or on his side.

Respiratory dead space

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At the end of exhalation, the volume of gases in the lungs is equal to the sum of the residual volume and the expiratory reserve volume, i.e. is the so-called (FOE). At the end of inspiration, this volume increases by the value of the tidal volume, i.e. the volume of air that enters the lungs during inhalation and is removed from them during exhalation.

The air entering the lungs during inhalation fills the airways, and part of it reaches the alveoli, where it mixes with the alveolar air. The rest, usually a smaller part, remains in the respiratory tract, in which the exchange of gases between the air contained in them and the blood does not occur, i.e. in the so-called dead space.

Respiratory dead space - the volume of the respiratory tract in which gas exchange processes between air and blood do not occur.
Distinguish between anatomical and physiological (or functional) dead space.

Anatomical respiratory measures your space represents the volume of the airways, starting from the openings of the nose and mouth and ending with the respiratory bronchioles of the lung.

Under functional(physiological) dead space understand all those parts of the respiratory system in which gas exchange does not occur. The functional dead space, in contrast to the anatomical one, includes not only the airways, but also the alveoli, which are ventilated, but not perfused by blood. In such alveoli, gas exchange is impossible, although their ventilation does occur.

In a middle-aged person, the volume of anatomical dead space is 140-150 ml, or about 1/3 of the tidal volume during quiet breathing. In the alveoli at the end of a calm expiration there is about 2500 ml of air (functional residual capacity), therefore, with each calm breath, only 1/7 of the alveolar air is renewed.

The essence of ventilation

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Thus, ventilation provides intake of outside air into the lungs and parts of it into the alveoli and removal instead of it gas mixtures(exhaled air), consisting of alveolar air and that part of the outside air that fills the dead space at the end of inhalation and is removed first at the beginning of exhalation. Since the alveolar air contains less oxygen and more carbon dioxide than the outside air, the essence of lung ventilation is reduced to delivery of oxygen to the alveoli(compensating for the loss of oxygen passing from the alveoli into the blood of the pulmonary capillaries) and removal of carbon dioxide(entering the alveoli from the blood of the pulmonary capillaries). Between the level of tissue metabolism (the rate of consumption of oxygen by tissues and the formation of carbon dioxide in them) and ventilation of the lungs, there is a relationship close to direct proportionality. Correspondence of pulmonary and, most importantly, alveolar ventilation to the level of metabolism is provided by the system of regulation of external respiration and manifests itself in the form of an increase in the minute volume of respiration (both due to an increase in respiratory volume and respiratory rate) with an increase in the rate of oxygen consumption and the formation of carbon dioxide in tissues.

Lung ventilation occurs, thanks to the active physiological process(respiratory movements), which causes the mechanical movement of air masses along the tracheobronchial tract by volumetric flows. In contrast to the convective movement of gases from the environment into the bronchial space, further gas transport(the transfer of oxygen from the bronchioles to the alveoli and, accordingly, carbon dioxide from the alveoli to the bronchioles) is carried out mainly by diffusion.

Therefore, there is a distinction "pulmonary ventilation" and "alveolar ventilation".

Alveolar ventilation

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Alveolar ventilation cannot be explained only by the convective air currents in the lungs created by active inspiration. The total volume of the trachea and the first 16 generations of bronchi and bronchioles is 175 ml, the next three (17-19) generations of bronchioles - another 200 ml. If all this space, in which there is almost no gas exchange, were "washed" by convective flows of outside air, then the respiratory dead space would have to be almost 400 ml. If the inhaled air enters the alveoli through the alveolar ducts and sacs (the volume of which is 1300 ml) also by convective currents, then atmospheric oxygen can reach the alveoli only with an inhalation volume of at least 1500 ml, while the usual tidal volume in a person is 400-500 ml.

Under conditions of calm breathing (respiratory rate 15 a.m., inhalation duration 2 s, average inspiratory volume velocity 250 ml/s), during inhalation (tidal volume 500 ml) the outside air fills all conductive (volume 175 ml) and transitional (volume 200 ml) zones of the bronchial tree. Only a small part of it (less than 1/3) enters the alveolar passages, the volume of which is several times greater than this part of the respiratory volume. With such an inhalation, the linear velocity of the inhaled air flow in the trachea and main bronchi is approximately 100 cm/s. In connection with the successive division of the bronchi into ever smaller ones in diameter, with a simultaneous increase in their number and the total lumen of each subsequent generation, the movement of inhaled air through them slows down. At the border of the conducting and transitional zones of the tracheobronchial tract, the linear flow velocity is only about 1 cm/s, in the respiratory bronchioles it decreases to 0.2 cm/s, and in the alveolar ducts and sacs to 0.02 cm/s.

Thus, the speed of convective air flows that occur during active inspiration and are due to the difference between the air pressure in the environment and the pressure in the alveoli is very small in the distal sections of the tracheobronchial tree, and air enters the alveoli from the alveolar ducts and alveolar sacs by convection with a small linear speed. However, the total cross-sectional area not only of the alveolar passages (thousands cm 2), but also of the respiratory bronchioles that form the transition zone (hundreds of cm 2), is large enough to ensure the diffusion transfer of oxygen from the distal parts of the bronchial tree to the alveoli, and carbon dioxide gas - in the opposite direction.

Due to diffusion, the composition of the air in the airways of the respiratory and transitional zones approaches the composition of the alveolar. Hence, diffusion movement of gases increases the volume of the alveolar and reduces the volume of dead space. In addition to a large diffusion area, this process is also provided by a significant partial pressure gradient: in the inhaled air, the partial pressure of oxygen is 6.7 kPa (50 mm Hg) higher than in the alveoli, and the partial pressure of carbon dioxide in the alveoli is 5.3 kPa (40 mm Hg). Hg) more than in the inhaled air. Within one second, due to diffusion, the concentration of oxygen and carbon dioxide in the alveoli and nearby structures (alveolar sacs and alveolar ducts) almost equalize.

Hence, starting from the 20th generation, alveolar ventilation is provided exclusively by diffusion. Due to the diffusion mechanism of oxygen and carbon dioxide movement, there is no permanent boundary between the dead space and the alveolar space in the lungs. In the airways there is a zone within which the diffusion process occurs, where the partial pressure of oxygen and carbon dioxide varies, respectively, from 20 kPa (150 mm Hg) and 0 kPa in the proximal part of the bronchial tree to 13.3 kPa (100 mm Hg .st.) and 5.3 kPa (40 mm Hg) in its distal part. Thus, along the bronchial tract there is a layer-by-layer unevenness of the air composition from atmospheric to alveolar (Fig. 8.4).

Fig.8.4. Scheme of alveolar ventilation.
"a" - according to obsolete and
"b" - according to modern ideas. MP - dead space;
AP - alveolar space;
T - trachea;
B - bronchi;
DB - respiratory bronchioles;
AH - alveolar passages;
AM - alveolar sacs;
A - alveoli.
Arrows indicate convective air flows, dots indicate the area of ​​diffusion exchange of gases.

This zone shifts depending on the mode of breathing and, first of all, on the rate of inhalation; the greater the inspiratory rate (i.e., as a result, the greater the minute volume of respiration), the more distally along the bronchial tree, convective flows are expressed at a rate that prevails over the diffusion rate. As a result, with an increase in the minute volume of breathing, the dead space increases, and the border between the dead space and the alveolar space shifts in the distal direction.

Hence, the anatomical dead space (if it is determined by the number of generations of the bronchial tree in which diffusion does not yet matter) changes in the same way as the functional dead space - depending on the volume of breathing.

Ventilation denote the exchange of air between the lungs and the atmosphere. A quantitative indicator of lung ventilation is the minute volume of respiration, defined as the amount of air that passes (or is ventilated) through the lungs in 1 minute. At rest, the minute volume of breathing in humans is 6-8 l / min. Only part of the air that ventilates the lungs reaches the alveolar space and is directly involved in gas exchange with the blood. This part of ventilation is called alveolar ventilation. At rest, alveolar ventilation averages 3.5-4.5 l/min. The main function of alveolar ventilation is to maintain the concentration of 02 and CO2 necessary for gas exchange in the air of the alveoli.

Lungs consist of air conducting (Airways) and respiratory zones (alveoli). Airways, starting from the trachea and up to the alveoli, are divided according to the type of dichotomy and form 23 generations of elements of the respiratory tract (Fig. 10.11). In the air-conducting or conductive zones of the lungs (16 generations), there is no gas exchange between air and blood, since in these sections the respiratory tract does not have a vascular network sufficient for this process, and the walls of the respiratory tract, due to their considerable thickness, prevent the exchange of gases through them. This section of the airways is called the anatomical dead space, with an average volume of 175 ml. On fig. 10.12 shows how the air that fills the anatomical dead space at the end of exhalation mixes with “useful”, i.e. atmospheric air and re-enters alveolar space of the lungs.

Respiratory bronchioles of the 17th-19th generations are classified as a transitional (transient) zone, in which gas exchange begins in small alveoli (2% of the total number of alveoli). The alveolar ducts and alveolar sacs, which pass directly into the alveoli, form the alveolar space, in the region of which O2 and CO2 gas exchange with blood occurs in the lungs. However, in healthy people, and especially in patients with lung diseases, a part alveolar space can be ventilated, but not participate in gas exchange, since these parts of the lungs are not perfused with blood. The sum of the volumes of such areas of the lung and the anatomical dead space is referred to as physiological dead space. Increase physiological dead space in the lungs leads to an insufficient supply of body tissues with oxygen and an increase in the content of carbon dioxide in the blood, which disrupts gas homeostasis in it.

Anatomical dead space is the volume of the conducting airways (Fig. 1.3 and 1.4). Normally, it is about 150 ml, increasing with a deep breath, as the bronchi are stretched by the lung parenchyma surrounding them. The volume of dead space also depends on the size of the body and posture. There is an approximate rule according to which, in a seated person, it is approximately equal in milliliters to body weight in pounds (1 pound == 453.6 g).

Anatomical dead space volume can be measured using the Fowler method. In this case, the subject breathes through the valve system and the nitrogen content is continuously measured using a high-speed analyzer that takes air from a tube starting at the mouth (Fig. 2.6, L). When, after inhaling 100% Oa, a person exhales, the N2 content gradually increases as dead space air is replaced by alveolar air. At the end of exhalation, an almost constant nitrogen concentration is recorded, which corresponds to pure alveolar air. This section of the curve is often called the alveolar "plateau", although even in healthy people it is not completely horizontal, and in patients with lung lesions it can go up steeply. With this method, the volume of exhaled air is also recorded.

To determine the volume of dead space build a graph linking the content of N 2 with exhaled volume. Then, a vertical line is drawn on this graph so that area A (see Fig. 2.6.5) is equal to area B. The volume of dead space corresponds to the point of intersection of this line with the x-axis. In fact, this method gives the volume of the conducting airways up to the “midpoint” of the transition from dead space to alveolar air.

Rice. 2.6. Measurement of anatomical dead space volume using the fast N2 analyzer according to the Fowler method. A. After inhaling from a container with pure oxygen, the subject exhales, and the concentration of N 2 in the exhaled air first increases, and then remains almost constant (the curve practically reaches a plateau corresponding to pure alveolar air). B. Dependence of concentration on exhaled volume. The volume of dead space is determined by the point of intersection of the abscissa axis with a vertical dotted line drawn in such a way that the areas A and B are equal

Functional dead space

You can also measure dead space Bohr's method. From Fig.2c. Figure 2.5 shows that the exhaled CO2 comes from the alveolar air and not from the dead space air. From here

vt x-fe == va x fa.

Insofar as

v t = v a + v d ,

v a =v t -v d ,

after substitution we get

VT xFE=(VT-VD)-FA,

hence,

Since the partial pressure of a gas is proportional to its content, we write

(Bohr equation),

where A and E refer to alveolar and mixed exhaled air, respectively (see Appendix). With quiet breathing, the ratio of dead space to tidal volume is normally 0.2-0.35. In healthy people, Pco2 in alveolar air and arterial blood are almost the same, so we can write the Bohr equation as follows:

asr2"CO-g ^ CO2

It should be emphasized that the Fowler and Bohr methods measure somewhat different indicators. The first method gives the volume of the conducting airways up to the level where the incoming air during inhalation quickly mixes with the air already in the lungs. This volume depends on the geometry of the rapidly branching airways with an increase in the total cross section (see Fig. 1.5) and reflects the structure of the respiratory system. For this reason it is called anatomical dead space. According to the Bohr method, the volume of those parts of the lungs in which CO2 is not removed from the blood is determined; since this indicator is related to the work of the body, it is called functional(physiological) dead space. In healthy individuals, these volumes are almost the same. However, in patients with lung lesions, the second indicator may significantly exceed the first due to uneven blood flow and ventilation in different parts of the lungs (see Chapter 5).