home
about the project
Medical news
For authors
Licensed books on medicine
<< Previous Next >>

EXTERNAL RESPIRATION AND FUNCTIONS OF THE LUNG RESPIRATORY FUNCTION OF LUNG AND PATHOPHYSIOLOGICAL MECHANISMS OF HYPOXEMIA AND HYPERCAPNIA

The main function of the lungs - the exchange of oxygen and carbon dioxide between the environment and the body - is achieved by a combination of ventilation, pulmonary circulation and diffusion of gases. Acute violations of one, two or all of these mechanisms lead to acute changes in gas exchange.

Pulmonary ventilation. Indicators of pulmonary ventilation include tidal volume (Vt), respiratory rate (f) and minute respiratory volume (VE). The effectiveness of pulmonary ventilation is determined by the amount of alveolar ventilation (VA), i.e. difference between VЕ and minute volume of ventilation of dead space.

Decreased alveolar ventilation may be due to a decrease in VE or an increase in dead space (VR). The determining factor is the value of VT, its relation to the inconsistent value of physiological dead space. The latter includes the anatomical dead space and the volume of inhaled air that ventilates the alveoli, in which the blood flow is absent or significantly reduced. Thus, alveolar ventilation should be considered as ventilation of blood-perfused alveoli. With adequate alveolar ventilation, a certain concentration of gases of the alveolar space is maintained, which ensures normal gas exchange with the blood of the pulmonary capillaries.

Dead space increases when using an anesthesia machine or a respirator, when using long breathing hoses and connectors, impaired gas recirculation. With pulmonary circulation disorders, Vp also increases. A decrease in Vp or an increase in Vp immediately leads to alveolar hypoventilation, and an increase in f does not compensate for this condition.

Alveolar hypoventilation is accompanied by inadequate elimination of CO2 and arterial hypoxemia.

The ratio of ventilation / blood flow. The effectiveness of pulmonary gas exchange largely depends on the distribution of inhaled air over the alveoli in accordance with their blood perfusion. Alveolar ventilation in a person at rest is approximately 4 l / min, and pulmonary blood flow is 5 l / min. Under ideal conditions, 4 air volumes and 5 blood volumes are obtained per unit time of the alveoli, and thus the ventilation / blood flow ratio is 4/5, or 0.8.

Violations of the ventilation / blood flow ratio - the prevalence of ventilation over the bloodstream or blood flow over ventilation - leads to gas exchange disorders. The most significant changes in gas exchange occur with the absolute prevalence of ventilation over the bloodstream (dead space effect) or blood flow over ventilation (the effect of a venoarterial shunt. Under normal conditions, the pulmonary shunt does not exceed 7%. This explains the fact that the saturation of arterial blood with oxygen is less than 100% and equal 97.1%.

An example of a dead space effect is pulmonary embolism. Blood shunting in the lungs occurs with severe lesions of the pulmonary parenchyma, respiratory distress syndrome, massive pneumonia, atelectasis and airway obstruction of any genesis. Both effects lead to arterial hypoxemia and hypercapnia. The shunt effect is accompanied by severe arterial hypoxemia, which is often impossible to eliminate even with high oxygen concentrations.

Gas diffusion. The diffusion capacity of the lungs is the rate at which a gas passes through the alveolar-capillary membrane per unit pressure gradient of this gas. This indicator is different for different gases: for carbon dioxide it is about 20 times more than for oxygen. Therefore, a decrease in the diffusion capacity of the lungs does not lead to the accumulation of carbon dioxide in the blood; the partial pressure of carbon dioxide in arterial blood (PaCO2) is easily balanced with that in the alveoli. The main sign of impaired diffusion capacity of the lungs is arterial hypoxemia.

Causes of violation of gas diffusion through the alveolar-capillary membrane:

• reduction of the diffusion surface (the surface of functioning alveoli in contact with functioning capillaries is normally 90 m2);

• the diffusion distance (the thickness of the layers through which the gas diffuses) can be increased as a result of tissue changes along the diffusion path.

Disturbances of diffusion processes, previously considered to be one of the main causes of hypoxemia (“alveolocapillary block”), are currently considered as factors that do not have great clinical significance in ONE. Limitations of gas diffusion are possible with a decrease in the diffusion surface and changes in the layers through which diffusion passes (thickening of the walls of the alveoli and capillaries, their edema, collapse of the alveoli, filling them with liquid, etc.).

Violations of the regulation of respiration. The rhythm and depth of breathing are regulated by the respiratory center located in the medulla oblongata, the gas composition of arterial blood is of greatest importance in the regulation. An increase in PaCO2 immediately causes an increase in ventilation. Oscillations of RaO2 also lead to changes in respiration, but with the help of impulses going to the medulla oblongata from the carotid and aortic bodies. Chemoreceptors of the medulla oblongata, carotid and aortic bodies are also sensitive to changes in the concentration of H + cerebrospinal fluid and blood. These regulatory mechanisms may be impaired with damage to the central nervous system, the introduction of alkaline solutions, mechanical ventilation in the hyperventilation mode, and an increase in the threshold of excitability of the respiratory center.

Disorders of oxygen transport to tissues. 100 ml of arterial blood contains approximately 20 ml of oxygen. If the cardiac output (MOS) is normal at rest 5 l / min and the oxygen consumption is 250 ml / min, then this means that the tissues extract 50 ml of oxygen from 1 l of circulating blood. With severe physical exertion, oxygen consumption reaches 2500 ml / min, and the MOC increases to 20 l / min, but in this case, the oxygen reserve of the blood remains unused. Tissues take approximately 125 ml of oxygen from 1 liter of circulating blood. An oxygen content in arterial blood of 200 ml / l is sufficient to meet the tissue's oxygen requirements.

However, with apnea, complete obstruction of the respiratory tract, and breathing with an anoxic mixture, the oxygen reserve is depleted very quickly - after a few minutes, consciousness is impaired, and after 4-6 minutes hypoxic cardiac arrest occurs.

Hypoxic hypoxia is characterized by a decrease in all indicators of the oxygen level of arterial blood: partial pressure, saturation and oxygen content.
Its main reason is a decrease or complete cessation of oxygen supply (hypoventilation, apnea). Changes in the chemical properties of hemoglobin (carboxyhemoglobin, methemoglobin) lead to the same type of hypoxia.

Primary circulatory hypoxia occurs due to a decrease in cardiac output (SV) or vascular insufficiency, which leads to a decrease in oxygen delivery to tissues. At the same time, the oxygen parameters of arterial blood are not changed, but PvO2 is significantly reduced.

Anemic hypoxia, usually observed with massive blood loss, is combined with circulatory failure. A hemoglobin concentration below 100 g / l leads to a violation of the oxygen transport system of the blood. Hemoglobin levels below 50 g / l, hematocrit (Ht) below 0.20 pose a great threat to the patient's life, even if MOS is not reduced. The main distinguishing feature of anemic hypoxia is a decrease in the oxygen content in arterial blood with normal PaO2 and SaO2.

The combination of all three forms of hypoxia - hypoxic, circulatory and anemic - is possible if the development of ONE occurs against the background of cardiovascular failure and acute blood loss.

Histotoxic hypoxia is less common and is characterized by the inability of tissues to utilize oxygen (for example, with cyanide poisoning). All three forms of hypoxia (except histotoxic) equally cause venous hypoxia, which is a reliable indicator of a decrease in PO2 in tissues. The partial pressure of oxygen in mixed venous blood is an important indicator of hypoxia. A PvO2 level of 30 mmHg is defined as critical.

The value of the dissociation curve of oxyhemoglobin (НbО2). Oxygen in the blood is present in two forms - physically dissolved and chemically associated with hemoglobin. The relationship between PO2 and SO2 is graphically expressed as an oxyhemoglobin (CDO) dissociation curve having an S-shape. This form of BWW corresponds to the optimal conditions for the saturation of blood with oxygen in the lungs and the release of oxygen from the blood in the tissues. At PO2 equal to 100 mmHg, only 0.3 ml of oxygen was dissolved in 100 ml of water. In the alveoli, PO2 is about 100 mmHg. 2.9 ml of oxygen is physically dissolved in 1 liter of blood. Most oxygen is transported in a hemoglobin-bound state. 1 g of hemoglobin, fully saturated with oxygen, binds 1.34 ml of oxygen. If the concentration of hemoglobin in the blood is 150 g / l, then the content of chemically bound oxygen is 150 g / l x1.34 ml / g = 201 ml / l. This value is called the oxygen capacity of the blood (KEK). Since the oxygen content in mixed venous blood (CvO2) is 150 ml / l, then 1 l of blood passing through the lungs must attach 50 ml of oxygen to turn it into arterial. Accordingly, 1 liter of blood passing through the tissues of the body leaves 50 ml of oxygen in them. Only about 3 ml of oxygen per 1 liter of blood is carried in a dissolved state.

Displacement of BWW is the most important physiological mechanism for the transport of oxygen in the body. The circulation of blood from the lungs to tissues and from tissues to the lungs is due to changes that affect the affinity of oxygen to hemoglobin. At the tissue level, due to a decrease in pH, this affinity decreases (the Bohr effect), which improves oxygen delivery. In the blood of pulmonary capillaries, the affinity of hemoglobin for oxygen increases due to a decrease in РСО2 and an increase in pH compared with similar parameters of venous blood, which leads to an increase in the saturation of arterial blood with oxygen.

Under normal conditions, 50% SO2 is achieved with a PO2 of about 27 mmHg. This value is denoted by P50 and characterizes the overall position of the BWW. An increase in P50 (for example, up to 30–32 mm Hg) corresponds to a shift in the BWW to the right and indicates a decrease in the interaction of hemoglobin and oxygen. With a decrease in P50 (up to 25–20 mm Hg), a shift in BWW to the left is observed, which indicates an increase in the affinity between hemoglobin and oxygen. Due to the S-shaped form of BWW, with a rather significant decrease in the fractional concentration of oxygen in the inhaled air (IFC) to 0.15, instead of 0.21, oxygen transfer is not significantly disturbed. With a decrease in RaO2 to 60 mm Hg SaO2 decreases to about 90% of the level, and cyanosis does not develop. However, a further fall in PaO2 is accompanied by a more rapid fall in SaO2 and the oxygen content in arterial blood. When RaO2 falls to 40 mmHg Sa02 is reduced to 70%, which corresponds to PO2 and SO2 in mixed venous blood.

The mechanisms described are not the only ones. Intracellular organic phosphate - 2,3-diphosphoglycerate (2,3-DPH) - enters the hemoglobin molecule, changing its affinity for oxygen. An increase in the level of 2,3-DPH in red blood cells reduces the affinity of hemoglobin for oxygen, and a decrease in the concentration of 2,3-DPH leads to an increase in the affinity for oxygen. Some syndromes are accompanied by pronounced changes in the level of 2,3-DFG. For example, in chronic hypoxia, the content of 2,3-DPG in erythrocytes increases and, accordingly, the affinity of hemoglobin for oxygen decreases, which gives an advantage in supplying the tissues with the latter. Massive transfusions of canned blood can impair the release of oxygen in tissues.

Thus, factors leading to an increase in the affinity of hemoglobin for oxygen and a shift in BWW to the left include an increase in pH, a decrease in PCO2, concentrations of 2,3-DPH and inorganic phosphate, and a decrease in body temperature. Conversely, a decrease in pH, an increase in PCO2, a concentration of 2,3-DPH and inorganic phosphate, and an increase in body temperature lead to a decrease in the affinity of hemoglobin for oxygen and a shift in BWW to the right.

In the table. 1.1 shows the normal functional indicators of the lungs.

Table 1.1.

Normal lung function tests

[Comroe J. et al., 1961] 1

1Data are for a healthy person (body surface 1.7 m2) at rest in a prone position while breathing air. Pulmonary volumes and ventilation are given according to BTPS, diffusion capacity of the lungs is given according to STPD.
<< Previous Next >>
= Skip to textbook content =

EXTERNAL RESPIRATION AND FUNCTIONS OF THE LUNG RESPIRATORY FUNCTION OF LUNG AND PATHOPHYSIOLOGICAL MECHANISMS OF HYPOXEMIA AND HYPERCAPNIA

  1. Respiratory function of the lungs and pathophysiological mechanisms of hypoxemia and hypercapnia
    The main function of the lungs - the exchange of oxygen and carbon dioxide between the environment and the body - is achieved by a combination of ventilation, pulmonary circulation and diffusion of gases. Acute violations of one, two or all of these mechanisms lead to acute changes in gas exchange. Pulmonary ventilation. Indicators of pulmonary ventilation include tidal volume (Ut), respiratory rate (f), and minute volume
  2. External respiration and lung function
    External respiration and function
  3. Respiratory function
    The respiratory function of the lungs is provided by three main processes: 1 by air transport (ventilation); 2) blood circulation in the lungs (perfusion); 3) gas exchange through the alveolo-capillary membrane (diffusion). Ventilation Ventilation of the lungs is called the process of updating the gas composition of the alveolar air, providing oxygen and removing excess carbon dioxide
  4. Assessment of respiratory function of respiratory failure
    The study of the functions of external respiration (HFD) along with the study of the gas composition of arterial blood makes it possible to objectively assess the severity and sometimes the nature of the pathological process that underlies the development of respiratory failure. Using routine methods provides information on the size of pulmonary volumes and capacities, the volumetric rate of air flow and the condition
  5. Metabolic lung function
    The lungs perform not only the function of gas exchange between blood and air, but also a variety of non-respiratory functions of a mechanical and metabolic nature. The most important non-respiratory functions of the lungs include: 1) protective - the lungs retain harmful mechanical and toxic products from the atmosphere; 90% of particles larger than 2 microns in diameter are retained in the lungs and removed. Slime
  6. LESS RESPONSIBLE FUNCTIONS
    Until the 60s, there was an opinion that the role of the lungs is limited only by gas exchange function. Only later it was proved that the lungs, in addition to their main function of gas exchange, play a large role in exogenous and endogenous defense of the body. They provide purification of air and blood from harmful impurities, carry out detoxification, inhibition and deposition of many biologically active substances.
  7. Non-respiratory lung function
    Until the 60s, there was an opinion that the role of the lungs is limited only by gas exchange function. Only later it was proved that the lungs, in addition to their main function of gas exchange, play a large role in exogenous and endogenous defense of the body. They provide purification of air and blood from harmful impurities, carry out detoxification, inhibition and deposition of many biologically active substances.
  8. Non-respiratory lung function
    Filtration and reservoir function A. Filtration. All venous blood enters the pulmonary capillaries from a large circle of blood circulation, which allows them to act as a filter for various particles entering the bloodstream. The high content of heparin and plasminogen activator in the lungs facilitates the cleavage of retained fibrin fragments. Although the average diameter of the pulmonary capillaries is 7 microns,
  9. Effect of anesthesia and surgery on lung function
    General anesthesia leads to a decrease in pulmonary volumes and a change in ventilation-perfusion ratios. As a rule, general anesthetics remove the regulatory effect of hypoxia and hypercapnia on the respiratory center. Patients with a compromised respiratory system are highly likely to develop atelectasis in the postoperative period. Postoperative pain exacerbates pulmonary disorders
  10. Typical disorders of gas exchange function of the lungs.
    Allocate the following typical violations of the gas exchange function of the lungs? 1. Violation of alveolar ventilation? a) alveolar hypoventilation b) alveolar hyperventilation c) uneven ventilation 2. Impaired lung perfusion. 3. Violation of ventilation-perfusion relationships. 4. Violation of the diffusion ability of the lungs. Mixed
  11. Respiratory function of the nose. The value of nasal breathing for the body
    The respiratory function of the nose is to conduct air (aerodynamics). Breathing is carried out mainly through the respiratory region. When inhaling, part of the air comes out of the paranasal sinuses, which contributes to the warming and moistening of the inhaled air, as well as its diffusion into the olfactory region. When you exhale, air enters the sinuses. About 50% of the resistance of all airways falls on
  12. The effect of mechanical ventilation on pulmonary function
    Many authors have shown that with mechanical ventilation increases the mismatch between the distribution of air and blood flow in the lungs [Zilber A. P., 1986; Butler D.P., 1994; Peters R. M., 1984]. As a result, the volume of physiological dead space and blood shunting increase from right to left, the alveolar-arterial oxygen gradient increases. Violations of ventilation-perfusion relations
  13. Causes of RDS. Metabolic lung function
    This pathology was first described by D. Ashbaugh (1967) as a clinical syndrome consisting of shortness of breath, tachypnea, oxygen refractory cyanosis, loss of lung compliance and diffuse infiltration on a chest radiograph. To indicate acute lesions of the pulmonary parenchyma, concepts such as adult respiratory distress syndrome (RDSV), “shock lung”, and acute pulmonary syndrome are used.
  14. How does lung function change during laparoscopic surgery?
    A distinctive feature of laparoscopy is the creation of pneumoperitoneum by forcing carbon dioxide into the abdominal cavity. Due to the increase in intra-abdominal pressure, the dome of the diaphragm shifts in the cranial direction, which reduces the extensibility of the lungs and increases the peak pressure of the inspiration. Atelectasis, decrease in FOB, violation of ventilation-perfusion relations and intrapulmonary bypass
  15. The effect of mechanical ventilation on some other body functions
    The literature describes some other adverse effects of mechanical ventilation, in particular E. Balzamo et al. (1996) in the experiment showed an increase in the content of the neuropeptide "P" in the vagus, sympathetic and phrenic nerves. Это может серьезно нарушить центральную регуляцию самостоятельного дыхания и затруднить процесс прекращения респираторной поддержки. It is established that with mechanical ventilation
  16. Методы коррекции острой дыхательной недостаточности при остром повреждении легких/остром респираторном дистресс-синдроме с доказанным эффектом на летальность и вентилятор-индуцированное повреждение легких
    •????Вентиляция малыми дыхательными объемами. Применение малых дыхательных объемов позволяет уменьшить проявления волюмотравмы и избежать высоких транспульмональных давлений. По данным крупнейшего мультицентрового рандомизированного контролируемого исследования, проведенного ARDSnet в 41 центре и включившего 861 пациента, использование малых дыхательных объемов (6 мл/кг массы тела) приводит к
  17. The relationship of structure and function. Problems of function localization. The brain as a dynamic system. The mechanisms of systemic integrative activity of the brain. Consciousness, psyche, brain. Unconsciousness
    1. The relationship of structure and function in normal and pathological conditions. Problems of function localization. In the neurological sciences, writes academician N.P. Bekhtereva (1988), there is a peculiar contradiction. On the one hand, in the human brain, not only a very large number of cells and even more connections between them, but, in addition, populations of nerve cells can participate in providing not one but many
  18. БОЛЕЗНИ ЛЕГКИХ. ХРОНИЧЕСКИЕ ДИФФУЗНЫЕ АСТМА. ИНТЕРСТИЦИАЛЬНЫЕ БОЛЕЗНИ ЛЕГКИХ. РАК ВОСПАЛИТЕЛЬНЫЕ ЗАБОЛЕВАНИЯ ЛЕГКИХ. БРОНХИАЛЬНАЯ ЛЕГКОГО
    БОЛЕЗНИ ЛЕГКИХ. ХРОНИЧЕСКИЕ ДИФФУЗНЫЕ АСТМА. ИНТЕРСТИЦИАЛЬНЫЕ БОЛЕЗНИ ЛЕГКИХ. РАК ВОСПАЛИТЕЛЬНЫЕ ЗАБОЛЕВАНИЯ ЛЕГКИХ. БРОНХИАЛЬНАЯ
Medical portal "MedguideBook" © 2014-2019
info@medicine-guidebook.com