Oxyhemoglobin dissociation graph. Oxyhemoglobin dissociation curve. Shift of the oxyhemoglobin dissociation curve
There is a relationship between the amount of oxyhemoglobin in the blood and the oxygen tension in it. It has been established that when there is no oxygen in the blood (Po2 = 0), then there is no oxyhemoglobin; it is contained in the form of reduced hemoglobin. If PO2 = 1O mm Hg. Art., 5.5% of hemoglobin will turn into the H1O2 form, and when the Po2 content reaches 20 mm Hg. Art., then oxyhemoglobin will already be 25%, at a level of 40 mm Hg. Art. will be 74%, and at a level of 100 mm Hg. Art. in the form of Hb02 there will be about 92% 02. Thus, the greater the oxygen tension in the blood, the more oxyhemoglobin there will be.
But this relationship is not linear, it is expressed by an S-shaped curve.
Oxyhemoglobin dissociation curve. There are two parts of this curve to pay attention to. Top part a curve that approaches horizontal in shape is important for the formation of oxyhemoglobin in the capillaries of the lungs. Venous blood enters here, in which Po2 = 40 mm Hg. Art. (In alveolar air Po2 = 100 mm Hg), therefore oxygen very quickly passes into the blood and the HbO2 level increases to 92%.
But Po2 in the alveolar air will not always be 100 mm Hg. Art. As you rise to altitude and decrease alveolar ventilation, it will decrease. However, despite this, the amount of hemoglobin in the form of HbO2 will be quite high for a long time (up to a certain limit) and hemoglobin will perform the function of oxygen transport (this part of the curve is compared with a graphical representation of the battery charging process).
The middle part of the curve, which is close to vertical in shape, is important for the processes occurring in the capillaries of tissues. At Po2 40 mm Hg. Art. (5.3 kPa) only 72% is accounted for by HbO2, but the more intensely the organ works, the less it will be
in the blood PO2 and more will be dissociated into Hb and oxygen. So, the oxygenation of cells that are working intensively will increase. This part of the curve can be compared to a graphical representation of the battery discharging process.
An indicator characterizing the intensity of oxygen use by tissues is the difference in the amount of HbO2 in the blood that flows in and out - arteriovenous difference
(AVR02).
The slope of the oxyhemoglobin dissociation curve in human blood is not constant and can change under certain conditions. The nature of the dissociation curve is reflected by an indicator called “half-saturation voltage” (P50). P50 is the voltage 02 at which the saturation of hemoglobin with oxygen is 50%. Normally, Pso of arterial blood is about 26 mmHg. Art. (3.5 kPa).
Configuration of Hb02 dissociation curve. due to the chemical affinity of hemoglobin for O2 and other external factors, changing the nature of the curve. Such factors include temperature, pH, CO2, and the concentration of 2,3-DPG in the erythrocyte.
The shape of the oxyhemoglobin dissociation curve largely depends on the concentration of H + in the blood. As pH decreases, the curve shifts to the right, which indicates a decrease in the affinity of Hb to 02. With increasing pH, the affinity of Hb increases to 02 and the curve shifts to the left. The effect of pH on the affinity of Hb for O2 is called the Bohr effect. The Bohr effect plays a certain role in the gas transport function of the blood: the formation of a large amount of CO2 in tissues increases the release of oxygen by reducing the affinity of Hb for it. When CO2 is released in the lungs, blood pH decreases and oxygenation improves. COG also affects the dissociation of HbO2. Moreover, the mentioned effect is due not only to H2CO3, but also to the direct influence of the PCO2 level on hemoglobin.
When the temperature decreases, the release of O2 by oxyhemoglobin slows down, and when it increases, this process accelerates. The shift of the curve to the right is also facilitated by an increase in the content of 2,3-DPG in erythrocytes. The content of this substance in the erythrocyte increases with anemia, promotes the supply of O2 to the tissues and partially compensates for the decrease in the level of KEK. Thus, the lack of oxygen reserves in the body is compensated by a sharp increase in its use from the blood, an increase in ABPO2. With intensive tissue work, when more CO2 and H + are formed and the temperature rises, conditions are created to improve the delivery of oxygen to cells.
Oxyhemoglobin dissociation curve
The normal oxyhemoglobin dissociation curve is shown in Fig. 1.7. At its initial point, when P a O 2 = 0, hemoglobin does not contain oxygen and SaO 2 is also zero. As PaO2 increases, hemoglobin begins to quickly become saturated with oxygen, turning into oxyhemoglobin: a small increase in oxygen tension is sufficient for a significant increase in HbO2 content. At 40 mm Hg. Art. the HbO 2 content already reaches 75%. Then the slope of the curve becomes more and more flat. In this section of the curve, hemoglobin is less willing to accept oxygen, and to saturate the remaining 25% of Hb it is necessary to raise PaO 2 from 40 to 150 mm Hg. However, under natural conditions, arterial blood hemoglobin is never completely saturated with oxygen, because when breathing atmospheric air, P a O > does not exceed 100 mm Hg (see earlier)
Normal P level A ABOUT 2 (92-98 mmHg) correspondsS a O 2 94-98%. It is possible to achieve complete saturation of hemoglobin with oxygen only by increasing the oxygen content in the inhaled gas.
Rice. 1.7. Oxyhemoglobin dissociation curve
When choosing a pulse oximeter, you usually test it on yourself. If the monitor showsS P O 2 = 100% (and such optimistic models are quite common), think about whether it’s worth buying. A non-smoker should test a pulse oximeter, since after smoking a cigarette, up to 8-10% of blood hemoglobin is converted into carboxy-hemoglobin. In this case, the pulse oximeter overestimates S a O 2 , and the model may be unfairly compromised.
The dependence of S a O 2 on P a O 2 for each patient can be described by empirical formulas (Hill equation, Kelman, Severinghouse algorithms, etc.), which take into account temperature, pH and other factors. These formulas in various modifications are usually introduced into modern automatic devices for monitoring acid base and blood gas composition (Radiometer, AVL, Instrumentation Laboratories, etc.), which calculate hemoglobin saturation by oxygen tension in the blood. Actually, the oxyhemoglobin dissociation curve itself is a graphical expression of these equations. A simpler indicator of the position of the dissociation curve is the P 50 index; it is equal to the oxygen tension in the blood at which hemoglobin saturation is 50% (Fig. 1.7).
Normal value R 50 equal to 27 mm Hg. Art. Its decrease corresponds to a shift of the curve to the left, and its increase corresponds to a shift to the right.
After hemoglobin is completely saturated with oxygen, a further increase in PaO 2 is accompanied by only a slight increase in C a O 2 due to physically dissolved oxygen. Therefore, increasing the oxygen concentration in the inhaled or injected gas (FiO 2) beyond the level sufficient to completely saturate the hemoglobin capacity (S a O 2 = 99-100%) is rarely justified.
Passing through the capillaries, arterial blood gives up part of the oxygen it contains to the tissues and turns into venous blood (P V O 2 = 40 mm Hg, S V O 2 = 75%). Thus, only about 25% of the oxygen supply of arterial blood is involved in gas exchange, and saturation and desaturation of hemoglobin occur in the flat part of the dissociation curve.
Pathology respiratory system leads to impaired blood oxygenation in the lungs with the development of arterial hypoxemia, the degree of which is quantified by a pulse oximeter. Under these conditions, the supply of oxygen to tissues is carried out in an “emergency” mode, on a steep section of the curve, where a slight drop in P and O 2 is sufficient to separate the required amount of oxygen from oxyhemoglobin. The emergency mode consists in reducing the voltage and, consequently, the oxygen content in the tissues, as evidenced by the low oxygen tension in the venous blood.
Hemoglobin, as a transport protein, is designed to solve two problems: absorb oxygen in the lungs and transfer it to tissues. These tasks are opposite in nature, but are performed by the same substance, therefore the desire of hemoglobin to bind to oxygen (the affinity of hemoglobin for oxygen) must be sufficient - to ensure oxygenation of blood in the lungs, but not excessive - so as not to disrupt the process of oxygen release in the periphery . The normal position of the oxyhemoglobin dissociation curve precisely corresponds to the optimal readiness of hemoglobin to carry out both tasks. But under certain conditions, the balance between hemoglobin’s desire to attach oxygen and its willingness to give it away is disrupted. Graphically, this is expressed by a shift of the dissociation curve to the right or left (Fig. 1.8).
With acidosis (respiratory or metabolic), hyperthermia and an increase in the concentration of 2,3-diphosphoglycerate (2,3-DPG) in erythrocytes, the affinity of hemoglobin for oxygen decreases and the HbO 2 dissociation curve shifts to the right. In this case, the saturation of hemoglobin with oxygen in the lungs worsens (decrease in SpO 2 with the same PaO 2), but the separation of oxygen from oxyhemoglobin in the capillaries is facilitated.
If gas exchange in the lungs is not impaired, then even a significant shift of the dissociation curve to the right is accompanied by a very slight decrease in SPO 2, since events in the lungs occur on the flat portion of the curve. In tissues, oxygen tension increases. In terms of oxygen homeostasis, this is a generally safe situation. Some experts even believe that during normal lung function, acidosis contributes to the supply of oxygen to tissues.
Rice. 1.8. Shift of the oxyhemoglobin dissociation curve
A different picture is observed in severe respiratory pathology, when blood flows from the lungs with a low oxygen tension, corresponding to a steep section of the HbO 2 dissociation curve. If the curve is shifted to the right, SpO 2 may be much lower than in the normal position of the curve. This circumstance is an additional blow to the supply of oxygen to tissues and an important contribution to the development of hypoxia. Thus, with initial arterial hypoxemia (low P a O 2 level) metabolic acidosis, hypercapnia and hyperthermia can significantly reduce hemoglobin saturation (SpO 2) and, consequently, the oxygen content in arterial blood.
Alkalosis (respiratory or metabolic), hypothermia and a decrease in the concentration of 2,3-DPG increase the affinity of hemoglobin for oxygen, and the HbO 2 dissociation curve shifts to the left. Under these conditions, hemoglobin greedily absorbs oxygen in the lungs (SP O2 increases at the same P a O 2) and reluctantly gives it to the tissues. It is believed that the shift of the dissociation curve to the left Always adversely affects tissue oxygenation, because a small increase in the content (but not tension) of oxygen in arterial blood does not compensate for the subsequent reluctance of oxyhemoglobin to share oxygen with tissues in the periphery. Perhaps, only newborns do not suffer from the left position of the HbO 2 dissociation curve. But this is a separate topic.
Inconsistency of relationships between P A ABOUT 2 AndS P O 2 may make it difficult to understand pulse oximetry data: it is not always known which dissociation curve hemoglobin follows at the moment.
About dyshemoglobins, dyes and nail polish
Another circumstance that affects the pulse oximeter readings is the presence of additional hemoglobin fractions in the blood. These include dishemoglobins (carboxy- and methemoglobin), as well as fetal hemoglobin.
Normally, the content of carboxyhemoglobin (COHb) in the blood is low (1-3%) and does not affect the SpO 2 value. However, in cases of carbon monoxide poisoning or in patients with recent flame burns, carboxyhemoglobin can account for tens of percent of the total hemoglobin. SONNY absorbs light in almost the same way as HCL, so instead of saturating hemoglobin with oxygen, the pulse oximeter in such patients shows the sum of the percentage concentrations of SONNY and HCL. For example, if S..O2 = 65% and SONY = 25%, the pulse oximeter will display an SpO2 value close to 90%.
With carboxyhemoglobinemia, the pulse oximeter overestimates the degree of oxygen saturation of hemoglobin.
Methemoglobinemia occurs as a result of the action of methemoglobin-forming substances on hemoglobin. These include not only certain poisons, but also some medications, in particular sodium nitroprusside or sulfalene meglumine. MetHb absorbs red and infrared light in the same way as hemoglobin, which is 85% oxygenated.
With moderate methemoglobinemia, the pulse oximeter underestimatesS P O 2 , and with severe methemoglobinemia it shows a value close to 85%, which is almost independent of fluctuations S a O 2 .
Fetal hemoglobin (HbF) is found in the red blood cells of the fetus and in children of the first year of life. In low concentrations (up to 5%), it can also be detected in women in the first trimester of pregnancy. HbF differs from adult hemoglobin (which is designated “HbA” [from English, adult]) by its significantly greater affinity for oxygen. And this is not surprising. The oxygen tension in oxygenated blood flowing through the umbilical vein from the placenta to the fetus is only 30 mmHg. Art., and only the position of the dissociation curve of fetal oxyhemoglobin shifted to the left provides SaO 2 = 75%. The fetal metabolism is tuned to low oxygen tension in the tissues, and the increase in metabolism after birth is compensated by an increase in P a O 2 and S a O 2 during the transition to breathing atmospheric air.
Fetal hemoglobin differs from adult hemoglobin only in the amino acid composition of two globin chains, which makes HbF less sensitive to changes in the concentration of 2,3-DPG, which explains the high affinity of fetal hemoglobin for oxygen.
How does a pulse oximeter react to the presence of fetal hemoglobin in the blood? Almost nothing. The value of SPO 2 in newborns corresponds to the true value of SaO 2, because the heme groups HbF and HbA, which determine the light-absorbing properties of hemoglobin, are identical, and the globin molecules are colorless and do not affect the measurement. Features of pulse oximetry in neonatology relate mainly to the interpretation of monitoring data. In particular, it is necessary to take into account the high affinity of fetal hemoglobin for oxygen and the significant difference in the normal values of oxygen homeostasis parameters in a newborn and an adult.
A few weeks before the birth date, the synthesis of adult hemoglobin begins in the fetal red blood cells, and by the time the child is born, the HbA content reaches 15-25%. Due to the sharp predominance of HbF, the oxyhemoglobin dissociation curve in a newborn is shifted to the left (P 50 = 19-22 mm Hg). A week after the baby is born, HbF gradually begins to be replaced by HbA.
Intravenous administration of dyes. Some dyes used for diagnostic purposes are capable of changing the light-absorbing properties of blood precisely in the frequency range used in pulse oximetry (strong absorption of light with a wavelength of 660 nm). These substances include methylene blue (methylene blue) and, to a lesser extent, indocyanine. Their intravenous administration is accompanied by a rapid and pronounced decrease in the SpO 2 value, which lasts 5-10 minutes. A simple test for the correct installation of an intravenous catheter is based on this: if immediately after the injection of the dye there is a sharp decrease in saturation, the catheter is in the vein.
Nail polish usually does not interfere with pulse oximeter readings. In some cases, it is able to reduce the signals of both LEDs, but this does not affect the SpO 2 calculation. However, there are reports that blue varnish can selectively attenuate the emission of one of the LEDs (660 nm), which leads to an artifactual underestimation of SpO 2. This should be kept in mind when working with patients who enter the operating room in full war paint.
PPG amplitude
The photoplethysmogram is not only the starting material for calculating SpO 2: it also has its own diagnostic value. The PPG amplitude reflects the volumetric pulsation of arterioles and, therefore, characterizes peripheral blood flow. Nice models Pulse oximeters are capable of detecting even sharply weakened pulsations, when the value of peripheral blood flow reaches only 4-5% of normal. Of course, the photoplethysmogram is not suitable for quantification blood supply to the periphery, but it allows you to make a fairly accurate impression about local blood flow. There is no need to neglect this possibility, especially since the method is non-invasive and the duration of its use is not limited.
Displaying PPG on the display is not provided in all models of pulse oximeters. Don't forget about this when choosing a monitor.
In clinical conditions, the amplitude of the PPG can change tens of times, therefore, on the display, the teeth of the curve in some cases do not fit on the screen, and in others they decrease to such an extent that they become indistinguishable. To ensure that the PPG always has a form convenient for analysis and a standard height, it is subject to automatic scaling (autoscaling); this procedure is performed every time steadfast amplitude change. As a result, even in a deplorable state of peripheral blood flow, the curve on the display may have a normal appearance and from its form it is difficult to suspect something was wrong. IN software Some monitors contain a set of standard scales, and a new scale is selected automatically only in cases where the peaks of the curve go beyond the display or merge with the isoline. This method of presenting data is convenient in that it allows one to track changes in the PPG amplitude within given ranges.
To prevent loss of information about the real PPG amplitude, a special indicator is provided on the display of some models. As a rule, this is a bar whose height reflects the true magnitude of the peaks of the curve. The maximum column height is characteristic of normal peripheral blood flow; if the blood supply is disrupted, the column decreases. In the future, when considering the amplitude of the PPG, we will keep in mind the readings of this particular indicator.
Another, more convenient, but rare method of displaying PPG deserves special mention. After the initial automatic scaling, the doctor manually selects a more successful, from his point of view, constant scale and observes changes in the shape and height of the photoplethysmogram over time. This is how, for example, monitors from DATEX and BRUEL & KJAER work. Pulse oximeters from DATEX, in addition, provide a numerical parameter (it is called the “amplitude factor”), reflecting the real volume of arterial pulsations. Monitors with such a display organization allow you to monitor situations when the PPG amplitude exceeds the norm. The diagnostic value of this function is given in the section on the clinical aspects of the method.
The photoplethysmogram is very similar in shape to the blood pressure curve, but, unlike the latter, it characterizes fluctuations volume microvessels.
PPG amplitude depends on microvascular tone and stroke volume of the heart.
This is why changes in the photoplethysmogram do not always correspond to changes in blood pressure. At arterial hypotension caused by vasodilators, the waveform on the pulse oximeter screen may have a high amplitude. Conversely, a decrease in PPG waves during vasoconstriction is sometimes observed against the background arterial hypertension.
The microvessels of the finger tissues are richly innervated by fibers of the sympathetic system and contain a large number of receptors for "floating" catecholamines. Therefore, activation of the sympathetic system, infusion of alpha 1 -adrenergic agonists, beta 2 -adrenergic blockers, angiotensin and other vasoconstrictor drugs is accompanied by a decrease in PPG amplitude. It must be remembered that the data obtained from pulse oximetry, due to the specific regulation of digital blood flow, is not always suitable for judging blood supply internal organs. An example of such a discrepancy is cold vasoconstriction.
The second factor on which the shape of the photoplethysmographic curve depends is the stroke volume of the heart, which determines the filling of the pulse wave. Its direct influence on the amplitude of individual PPG waves is clearly visible on the pulse oximeter screen with a paradoxical or alternating pulse. In addition, the effect of cardiac output on the shape of PPG may be indirect, since its decrease is often accompanied by peripheral vasoconstriction.
A decrease in PPG amplitude is a sign of peripheral vasoconstriction and/or a decrease in stroke volume, while an increase in amplitude indicates the opposite. Vascular tone is the main factor determining the height of the photoplethysmogram waves.
Unfortunately, pulse oximetry in its modern version does not allow differentiating vasoconstriction from a decrease in stroke volume. The fundamental possibility of such differentiation, based on mathematical analysis of the pulse wave shape, exists, but has not yet been implemented in serial monitors.
FIG form
The PPG waveform is individual, but its full clinical interpretation is not yet available. On the descending limb of each wave there is a noticeable notch - a dicrotic incisura - which corresponds to the closure of the aortic valve. The incisura is followed by an additional peak - a dicrotic tooth (Fig. 1.9). Clarity of the image of the incisor and prong on the displays different models pulse oximeters are not the same, and often they are represented by a barely noticeable wave.
With severe arterial hypertension or aortic insufficiency, the dicrotic wave can be very high (Fig. 1.10) and the pulse oximeter interprets it as an independent pulse wave. As a result, the heart rate is artifactually overestimated.
In each case when pulse oximetry data indicate severe tachycardia, be sure to pay attention to the PPG shape and count the pulse manually. When working with a pulse oximeter that does not display PPG on the display, correction of tachycardia must be preceded by checking the pulse rate. The presence of a high dicrotic wave is a typical cause of discrepancies between pulse oximeter and ECG monitor readings, therefore such artifacts are not typical for models that use the C-lock principle.
Rice. 1.9. PPG wave as a reflection of arteriolar pulsation
Sometimes, in the intervals between PPG peaks, additional oscillations are observed - venous waves (about their origin and role, see “Artifacts and their sources,” p. 22).
Pulse oximetry allows you to continuously monitor the most important function of the lungs - the saturation of hemoglobin in the blood with oxygen. With all the undoubted usefulness of this information, we must not forget that SpO 2 is only one of many parameters used to describe oxygen homeostasis. We hope that the above fragmentary information from physiology is enough to understand how difficult the interpretation of this indicator can become when it is taken out of the clinical and physiological context. However, pulse oximetry is the most common, and in many cases the only available method for determining oxygenation.
Rice. 1.10. High dicrotic wave simulating a PPG wave
Monitoring the amplitude of the photoplethysmogram is a simple and non-invasive method for an indicative assessment of peripheral arterial blood flow. If the reason for the change in PPG lies on the surface, the conclusion made by the doctor will help to take the right measures in a timely manner and monitor their effectiveness. However, if there is complex disorders blood circulation, when the PPG amplitude is formed under the influence of several factors at once, it loses its independent diagnostic value and becomes only an additional argument in differential diagnosis.
In the next chapter we'll show you how to get the most out of these parameters.
Practical application of pulse oximeters
Some practical tips
Before starting work, try to position the device so that its existence is not threatened. The power cord and sensor cable should not dangle under the feet of personnel: sooner rather than later, the monitor will end up on the floor, and in such cases the free repair guarantee does not apply. The smartest thing to do is to find and adapt a convenient permanent place for it.
Train medical personnel to use the top panel of the monitor as a storage area for ampoules, vials, laryngoscopes, or tracheal catheter containers. Some models are equipped with a special stand that allows you to tilt the device to improve viewing of the display. Resort to her help also because no one can put anything on an inclined plane.
LCD monitors should be positioned to provide maximum viewing at eye height standing man It is advisable to buy monitors with adjustable display brightness.
If the pulse oximeter responds to interference from electrical equipment (and this is easy to check yourself), try to place the sensor cable as far as possible from electrical cables. Electrical defibrillation Pulse oximeter safe.
In a patient with motor restlessness or convulsive syndrome Use an ear or flexible Y-sensor. In any case, the sensor must be visible to staff, so it is better to buy pulse oximeters that do not react to ambient light. It's a shame to remove from under the blanket the fragments of what was once called a sensor and cost several hundred dollars.
It’s hard not to quote the advice given by DATEX:
Treat the sensor with the same care as you would your own watch or glasses.
If the sensor is broken and there is no spare one, do not connect a sensor from another manufacturer to the monitor, even if it has the same plug. This is a typical mistake, fraught with a wide variety of - and always bad - consequences, burns, equipment breakdown, sharp decline measurement accuracy, etc. There is a large, but still limited number of connector types in the world, and therefore different companies are sometimes simply forced to use the same plugs. Without relying on the common sense of doctors, companies provide appropriate warnings in the manuals for monitors, stick them on the sensor cable and even publish them in professional journals, but sometimes adventurism turns out to be stronger. Some large companies, such as NELLCOR-PURITAN BENNET, sell their sensors along with their proprietary calibration technology to other pulse oximeter manufacturers, but this is always disclosed in the documentation.
In patients with severe peripheral circulatory disorders, try moving the sensor to an adjacent finger or other hand. Try warming your hand with a heating pad or massaging it. In some cases, it is possible to improve local blood flow with the help of nitroglycerin ointment applied in a thin layer to the earlobe or finger.
A more reliable signal in conditions of impaired peripheral blood flow can be obtained from an ear sensor.
It is not advisable to place the sensor on the same arm that is used to measure blood pressure, as this results in unnecessary alarm activation each time the cuff is inflated 1 . In patients with atherosclerotic or other arterial damage upper limbs the sensor should be installed on the arm where the PPG amplitude is higher. After catheterization of the radial artery, a decrease in the PPG amplitude in this arm is possible.
1 On multifunction monitors during non-invasive automatic measurement blood pressure The pulse oximeter alarm is turned off.
When the human body cools, the PPG amplitude at the periphery often decreases sharply. This condition is observed in patients at the end of long operations. Their thermoregulation is suppressed, heat loss is increased, and the temperature in the operating room is far from comfortable, and the patient is covered with one sterile sheet. This must be kept in mind when performing pulse oximetry in the early postoperative period.
Pulse oximetry, like any other monitoring method, should be used only when needed. The pulse oximeter is rarely turned on unnecessarily, but people often forget to turn it off when there is no longer a need. It must be remembered that the service life of the device (in to a greater extent this applies to sensors) depends on the total operating time.
pulse oximetry monitoring end-tidal carbon dioxide concentration ( capnography) percutaneous monitoring... World”, 1985. Shurygin I.A. “ Monitoringbreathing"P-sk, 2000...
The reaction between oxygen and hemoglobin obeys the law of mass action. This means that the ratio between the amounts of hemoglobin and oxyhemoglobin depends on the content of physically dissolved O 2 in the blood; the latter, according to the Henry-Dalton law, is proportional to the O 2 voltage. The percentage of oxyhemoglobin from the total hemoglobin content is called the oxygen saturation (SO 2) of hemoglobin. If hemoglobin is completely deoxygenated, then SO 2 = 0%; if all hemoglobin has turned into oxyhemoglobin, then SO 2 = 100%. In accordance with the law of mass action, the saturation of hemoglobin with oxygen depends on the O 2 tension. Graphically, this dependence is reflected by the so-called oxyhemoglobin dissociation curve. This curve is S-shaped. The location of the oxyhemoglobin dissociation curve depends on a number of factors (see below). The simplest indicator characterizing the location of this curve is the so-called half-saturation voltage (50%), i.e. such an O 2 voltage at which the saturation of hemoglobin with oxygen is 50%. Normally (at pH=7.4 and t=37°C), the half-saturation of arterial blood is about 26 mmHg. (3.46 kPa).
The reasons for the S-shape of the oxyhemoglobin dissociation curve are not completely clear. If each hemoglobin molecule attached only one O 2 molecule, then the kinetics of this reaction would be graphically described by a hyperbola. It is precisely this hyperbolic dissociation curve that is characteristic, for example, of the reaction of the combination of oxygen with the red muscle pigment myoglobin, similar to the reaction of oxygenation of hemoglobin. The structure of myoglobin is similar to the structure of one of the four subunits of hemoglobin, therefore molecular weights these two substances have a 1:4 ratio. Since myoglobin contains only one pigment group, one myoglobin molecule can attach only one O 2 molecule. Based on the completely plausible assumption that the S-shaped shape of the HbO 2 dissociation curve is due to the binding of four O 2 molecules by one hemoglobin molecule, Eder put forward the so-called intermediate compounds hypothesis. According to this hypothesis, the addition of four O 2 molecules to hemoglobin occurs in several stages, and each of these stages affects the equilibrium of the next reaction. Thus, the reaction of the oxygen compound with hemoglobin is described by four equilibrium constants, which explains the sigmoid shape of the oxyhemoglobin dissociation curve.
At the same time, another explanation is possible, according to which there are two forms of hemoglobin - oxygenated and deoxygenated, transforming into one another as a result of conformational rearrangements. If we assume that the equilibrium parameters of oxygenation reactions for these two forms of hemoglobin are different, then from the standpoint of this hypothesis we can explain the S-shaped shape of the HbO 2 dissociation curve
Biological meaning of the shape of the oxyhemoglobin dissociation curve. The configuration of the oxyhemoglobin dissociation curve is important from the point of view of oxygen transport in the blood. During the absorption of oxygen in the lungs, the O2 tension in the blood approaches that in the alveoli. In young people, arterial blood PO 2 is about 95 mm Hg. (12.6 kPa). At this voltage, hemoglobin oxygen saturation is approximately 97%. With age (and to an even greater extent with lung diseases), the O2 tension in arterial blood can decrease significantly, however, since the oxyhemoglobin dissociation curve on the right side is almost horizontal, the oxygen saturation of the blood does not decrease much. So, even when PO 2 in arterial blood drops to 60 mm Hg. (8.0 kPa) hemoglobin oxygen saturation is 90%. Thus, due to the fact that the areas high voltage oxygen corresponds to the horizontal section of the oxyhemoglobin dissociation curve, a significant decrease in arterial blood oxygen saturation is prevented.
The steep slope of the middle section of the oxyhemoglobin dissociation curve indicates very favorable conditions for the release of oxygen to tissues. When the local demand for oxygen changes, it should be released in sufficient quantities in the absence of significant changes in PO 2 in arterial blood. At rest, PO 2 at the venous end of the capillary is approximately 40 mmHg. (5.3 kPa), which corresponds to approximately 73% saturation. If, as a result of an increase in oxygen consumption, its tension in the venous blood drops by only 5 mm Hg. (0.7 kPa), then the saturation of hemoglobin with oxygen decreases by at least 7%; the O2 released in this case can be immediately used for metabolic processes.
Curves dissociation of oxyhemoglobin valid for normal blood with average scores. However, there are a number of factors that can shift this curve in one direction or the other. The figure shows that with some acidification of the blood with a decrease in pH from the normal level of 7.4 to 7.2, the dissociation curve shifts by an average of 15% to the right, and an increase in the pH level from the normal level of 7.4 to 7.6 shifts the curve by this amount the same distance to the left.
In addition to pH changes Other factors are also known that can shift the dissociation curve. Let's name three, the action of which shifts the curve to the right: (1) an increase in the concentration of carbon dioxide; (2) increased blood temperature; (3) an increase in the concentration of 2,3-diphosphoglycerate, a metabolically important phosphate, which, depending on metabolic conditions, is present in the blood in different concentrations.
Increased oxygen supply to tissues in cases where carbon dioxide and hydrogen ions shift the dissociation curve of oxyhemoglobin. Bohr effect. A shift in the oxyhemoglobin dissociation curve in response to an increase in the content of carbon dioxide and hydrogen ions in the blood has a significant effect, expressed in accelerating the release of oxygen from the blood into the tissues and increasing blood oxygenation in the lungs. This is called the Bohr effect and is explained as follows.
When blood passes Through tissue, carbon dioxide diffuses from tissue cells into the blood. As a result, Po2 increases in the blood, and then concentrations carbonic acid(H2CO3) and hydrogen ions. These changes shift the oxyhemoglobin dissociation curve to the right and downward, decreasing the affinity of oxygen for hemoglobin, and as a result, the release of oxygen into the tissue increases.
At carbon dioxide diffusion processes in the opposite direction occur from the blood to the alveoli - as a result, Pco2 and the concentration of hydrogen ions in the blood decrease, shifting the oxyhemoglobin dissociation curve to the left and up. At the same time, the amount of oxygen binding to hemoglobin significantly increases at any existing level of alveolar Po2, which increases the transport of oxygen to the tissues.
Shift oxyhemoglobin dissociation curve under the influence of diphosphoglycerate. The normal content of DPG in the blood causes a constant slight shift of the oxyhemoglobin dissociation curve to the right. In the case of a hypoxic state lasting more than several hours, the concentration of DPG in the blood increases significantly, and the oxyhemoglobin dissociation curve shifts even more to the right.
In the presence such a concentration of DPG oxygen released in tissues when Po2 exceeds normal level by 10 mm Hg. Art., therefore, in some cases, such a mechanism with the participation of DPG may be important for adaptation to hypoxia, especially if the cause of hypoxia is a decrease in blood flow in the tissue.
Curve shift dissociation during physical activity . During physical activity, certain factors cause a significant shift of the oxyhemoglobin dissociation curve to the right, so active muscle fibers performing physical work receive additional oxygen. In turn, working muscles release large amounts of carbon dioxide; this, together with the action of some other acids released by the muscles, increases the concentration of hydrogen ions in the blood of the muscle capillaries.
In addition, in work time muscle temperature often increases by 2-3°C, which can further increase oxygen delivery to muscle fibers. All these factors cause a significant shift in the dissociation curve of oxyhemoglobin in the blood of muscle capillaries to the right. A shift to the right means the release of oxygen by hemoglobin in the muscle with sufficient high level Po2 (40 mmHg) even in cases when 70% of oxygen has already been released from it. A shift of the curve to the other side shows that additional oxygen from the alveolar air has been added to the lungs.
The shape of the HbO 2 dissociation curve is determined mainly by the reactivity of hemoglobin, but the affinity of blood for oxygen can change under the influence of other factors, usually leading to an increase or decrease in the slope of the dissociation curve without changing its S-shape. This effect is exerted by temperature, pH, CO 2 tension and some other factors, the role of which increases in pathological conditions.
Effect of temperature. The equilibrium of the hemoglobin oxygenation reaction (like most chemical reactions in general) depends on temperature. As the temperature decreases, the slope of the oxyhemoglobin dissociation curve increases, and as it increases, it decreases. In warm-blooded animals, this effect occurs only during hypothermia or a febrile state.
Effect of pH and pco2
The shape of the oxyhemoglobin dissociation curve largely depends on the content of H + ions in the blood. When pH decreases, i.e. acidification of the blood, the affinity of hemoglobin for oxygen decreases and the slope of the oxyhemoglobin dissociation curve decreases. The influence of pH on the nature of the oxyhemoglobin dissociation curve is called the Bohr effect. The pH of the blood is closely related to the level of CO 2 (PCO 2) in it: the higher the PCO 2, the lower the pH. An increase in CO 2 tension in the blood is accompanied by a decrease in the affinity of hemoglobin for oxygen and a decrease in the slope of the HbO 2 dissociation curve. This dependence is also called the Bohr effect, although a detailed quantitative analysis showed that the effect of CO 2 on the shape of the oxyhemoglobin dissociation curve cannot be explained only by changes in pH. Obviously, carbon dioxide itself has a specific effect on the dissociation of oxyhemoglobin.
Biological meaning of the Bohr effect. The Bohr effect has some significance both for the uptake of oxygen in the lungs and for its release in the tissues (although the importance of this effect should not be exaggerated). Let us first consider the processes occurring in the lungs. The absorption of O 2 occurs simultaneously with the release of CO 2, therefore, as hemoglobin is saturated with oxygen, the dissociation curve of oxyhemoglobin shifts to the left. As venous blood (PO 2 = 40 mm Hg; PCO 2 = 46 mm Hg), saturated with oxygen, turns into arterial blood (PO 2 = 95 mm Hg, PCO 2 = 40 mm Hg), the affinity of hemoglobin for oxygen is constantly increasing. As a result, although oxygen is transferred by diffusion, the rate of diffusion increases slightly. Thus, the Bohr effect promotes the binding of oxygen in the lungs.
The Bohr effect is somewhat more important for the transfer of O 2 from capillaries to tissues. Since, simultaneously with the release of oxygen from the blood, CO 2 enters it, the dissociation curve of oxyhemoglobin shifts to the right. All these processes correspond to a shift in the effective dissociation curve. A decrease in the affinity of hemoglobin for oxygen leads to an even greater drop in the oxyhemoglobin content, and as a result, oxygen enters the tissues at a relatively high PO 2 in the capillary. Thus, in this case, the Bohr effect promotes oxygen exchange.
Influence of pathological factors. In a number of pathological conditions, changes in the processes of oxygen transfer in the blood are observed. Thus, there are diseases (for example, some types of anemia) that are accompanied by shifts in the oxyhemoglobin dissociation curve to the right (less often to the left). The reasons for such shifts are not completely clear. It is known that the shape and slope of the oxyhemoglobin dissociation curve are strongly influenced by some phosphorus-containing organic compounds, the concentration of which in erythrocytes may change during pathology. The greatest effect among such compounds is 2,3-Diphosphoglycerate. The affinity of hemoglobin for oxygen also depends on the content of cations in erythrocytes. It is also necessary to note the influence of pathological changes in pH: with an increase in pH (alkalosis), the capture of oxygen in the lungs due to the Bohr effect is facilitated, but its release to the tissues becomes more difficult, and with a decrease in pH (acidosis), the opposite picture is observed. Finally, a significant shift of the oxyhemoglobin dissociation curve to the left occurs during CO poisoning.
Features of the oxyhemoglobin dissociation curve in the fetus. In the placenta, as in any other organ, gas exchange occurs by diffusion. In the same time Special attention One should pay attention to the difference in the affinity of maternal and fetal blood for oxygen. When analyzed under the same conditions, the slope of the oxyhemoglobin dissociation curve in the fetal blood is slightly greater than in the maternal blood, however in vivo this difference is almost entirely negated by the Bohr effect (the pH of fetal blood is slightly lower than the pH of maternal blood). In this regard, the difference in the affinity of the blood of the mother and fetus for oxygen has practically no effect on gas exchange in the placenta. This situation is most favorable for the exchange of gases, which becomes obvious if we take into account the differences in the concentration of hemoglobin in the blood of the mother and fetus, since the content of hemoglobin in the blood of the mother and fetus is different (120 and 180 g/l, respectively). The Bohr effect plays a special role in placental gas exchange. In the process of gas diffusion, the affinity of the mother's blood for oxygen as a result of the intake of CO 2 decreases, and the affinity of the fetal blood increases. Due to this dual influence of the Bohr effect, the rate of oxygen exchange increases.
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