What determines the amount of dissociation of oxyhemoglobin. Oxyhemoglobin: concept, formation mechanism, dissociation curve and its shifts. Formation and breakdown of oxyhemoglobin

The shape of the dissociation curve of HbO 2 is mainly due to the reactivity of hemoglobin, however, the affinity of blood for oxygen can change under the influence of other factors, as a rule, 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.

The effect of temperature. The equilibrium of the oxygenation reaction of hemoglobin (as well as most chemical reactions in general) depends on temperature. As the temperature decreases, the slope of the oxyhemoglobin dissociation curve increases, and as it rises, it decreases. In warm-blooded animals, this effect occurs only when hypothermic or febrile.

Influence of pH and pco2

The shape of the oxyhemoglobin dissociation curve largely depends on the content of H + ions in the blood. With a decrease in pH, i.e. acidification of the blood, the affinity of hemoglobin for oxygen decreases and the slope of the oxyhemoglobin dissociation curve decreases. The effect 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 tension in it of CO 2 (PCO 2): 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 a change in pH. Obviously, carbon dioxide itself has a specific effect on the dissociation of oxyhemoglobin.

The biological meaning of the Bohr effect. The Bohr effect is of some importance both for the absorption of oxygen in the lungs and for its release in the tissues (although the importance of this effect should not be exaggerated). Consider first the processes occurring in the lungs. O 2 absorption occurs simultaneously with the release of CO 2, therefore, as hemoglobin is saturated with oxygen, the oxyhemoglobin dissociation curve shifts to the left. As venous blood (RO 2 = 40 mm Hg; PCO 2 = 46 mm Hg), saturated with oxygen, turns into arterial blood (RO 2 = 95 mm Hg, PCO 2 = 40 mm Hg), the affinity of hemoglobin for oxygen is constantly increasing. As a result, although the transfer of oxygen is carried out by diffusion, the rate of this diffusion increases somewhat. Thus, the Bohr effect promotes oxygen binding 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 oxyhemoglobin dissociation curve 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 content of oxyhemoglobin, and as a result, oxygen enters the tissues at a relatively high PO 2 in the capillary. Thus, in this case, too, the Bohr effect promotes oxygen exchange.

Influence of pathological factors. In a number of pathological conditions, changes are observed in the processes of oxygen transfer by the blood. So, there are diseases (for example, some types of anemia), which are accompanied by shifts in the oxyhemoglobin dissociation curve to the right (less often to the left). The reasons for these shifts are not entirely 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. Among these compounds, 2,3-diphosphoglycerate has the greatest effect. The affinity of hemoglobin for oxygen also depends on the content of cations in red blood cells. It is also necessary to note the influence of pathological pH shifts: with an increase in pH (alkalosis), oxygen uptake in the lungs is facilitated due to the Bohr effect, but its release in 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 in CO poisoning.

Features of the dissociation curve of oxyhemoglobin in the fetus. In the placenta, as in any other organ, gas exchange is carried out by diffusion. At the same time, special attention should be paid to the difference in the affinity of the blood of the mother and fetus for oxygen. When analyzed under the same conditions, the slope of the oxyhemoglobin dissociation curve in the fetal blood is somewhat greater than in the mother's blood, however in vivo this difference is almost entirely negated by the Bohr effect (fetal blood pH is somewhat lower than maternal blood pH). In this regard, the difference in the affinity of the blood of the mother and fetus for oxygen practically does not affect the gas exchange in the placenta. This situation is most favorable for the exchange of gases, which becomes apparent if we take into account the differences in the concentration of hemoglobin in the blood of the mother and fetus, since the hemoglobin content 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 diffusion of gases, the affinity of the mother's blood for oxygen as a result of the intake of CO 2 decreases, and the affinity of the blood of the fetus increases. Due to this dual effect of the Bohr effect, the rate of oxygen exchange increases.

Curves dissociation of oxyhemoglobin valid for normal blood with average values. However, there are a number of factors that can shift this curve to one side 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 on average by 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 the same distance to the left.

In addition to pH changes other factors are also known that can shift the dissociation curve. To 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 the metabolic conditions, is present in the blood in different concentrations.

Increasing the supply of oxygen to tissues in cases where carbon dioxide and hydrogen ions shift the dissociation curve of oxyhemoglobin. Bohr effect. The shift of 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, which is expressed in accelerating the release of oxygen from the blood in the tissues and increasing blood oxygenation in the lungs. This is called the Bohr effect and is explained as follows.

When passing blood carbon dioxide diffuses through the tissue from tissue cells into the blood. As a result, Po2 increases in the blood, and then the concentration of carbonic acid (H2CO3) and hydrogen ions increases. These changes shift the oxyhemoglobin dissociation curve to the right and down, reducing the affinity of oxygen for hemoglobin, and as a result, the release of oxygen into the tissues increases.

At diffusion of carbon dioxide reverse processes occur from the blood to the alveoli - as a result, Pco2 and the concentration of hydrogen ions decrease in the blood, shifting the oxyhemoglobin dissociation curve to the left and up. At the same time, the amount of oxygen that binds to hemoglobin at any existing level of alveolar Po2 increases significantly, which increases the transport of oxygen to the tissues.

Shift oxyhemoglobin dissociation curve under the influence of diphosphoglycerate. The normal content of DFG 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 to the right even more.

In the presence such a concentration of DPG oxygen in tissues is released when Po2 exceeds the normal level by 10 mm Hg. Art., therefore, in some cases, such a mechanism involving DFG 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 exercise. During exercise, certain factors cause a significant shift in the oxyhemoglobin dissociation curve to the right, so active, physical muscle fibers 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 working time muscle temperature often rises 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 at a sufficiently high level of Po2 (40 mmHg) even in cases where 70% of oxygen has already been released from it. The shift of the curve to the other side shows that an additional amount of oxygen from the alveolar air has joined in the lungs.

44) What is "oxyhemoglobin dissociation curve"?

The dissociation curve between the percentage of hemoglobin saturation with oxygen (plotted on the standard chart along the vertical axis - the "Y" axis - since Sa02 is the dependent variable) and the environmental POX (plotted along the horizontal axis - the "X" axis - since POX is the independent variable).

When the RH increases, the percentage of hemoglobin that is saturated with oxygen gradually increases. The shape of the curve significantly affects the absorption of oxygen by hemoglobin in the lungs and its release in peripheral tissues. In addition, certain physiological conditions, as well as diseases, can shift the oxyhemoglobin dissociation curve to the left (i.e., increase the affinity of Hb for oxygen) or to the right (i.e., decrease the affinity of Hb for oxygen).

45) What level of PO2 is associated with hemoglobin oxygen saturation of 10%, 30%, 50%, 70% and 90% (at normal temperature 37°C and pH 7.40)? What is P50?

Analysis of the normal curve of dissociation of oxyhemoglobin shows that SO2 in 10, 30, 50, 70 and 90% approximately correspond to the value of ROg 10; 19; 26.6; 37 and 58 mmHg The PO2 value that results in SO2 = 50% is called P50 and is commonly used as a measure of the affinity of hemoglobin for oxygen (normal value 26.6 mmHg). Large P50 values ​​are observed when the oxyhemoglobin curve shifts to the right, decreasing the affinity of hemoglobin for oxygen (a beneficial effect). On the contrary, smaller P 50 values ​​are observed when the oxyhemoglobin curve shifts to the left, increasing the affinity of Hb for oxygen (adverse effect).

46) What most influences the oxyhemoglobin dissociation curve? When does this curve shift to the right or to the left? What is the effect of this shift on the release of oxygen in the tissues?

The main factors that change the position of the oxyhemoglobin dissociation curve include the type of hemoglobin (eg, fetal, adult, etc.), body temperature, PCO2, blood pH, erythrocyte 2,3-diphosphoglycerate (2,3-DPG) levels, and concentration of carboxyhemoglobin. A shift of the curve to the right indicates a reduced affinity of hemoglobin for oxygen, which facilitates its release in tissues; this effect is observed in the presence of abnormal hemoglobin (eg, type E, Seattle, Kansas), hyperthermia, hypercapnia, acidosis, and elevated levels of 2,3-DPG. Conversely, a shift of the curve to the left indicates an increased oxygen affinity, which reduces the release of oxygen in the tissues. This effect is observed in the presence of large amounts of fetal hemoglobin (and other types, including Yakenia, Chesapeake, Ranier hemoglobin), hypothermia, hypocapnia, alkalosis, reduced levels of 2,3-DPG, and high concentrations of carboxyhemoglobin.

47) Explain in more detail the features of the type of the oxyhemoglobin dissociation curve and its physiological significance.

The dissociation curve in a solution of pure hemoglobin has the form of a hyperbola and an S-shaped character, if the erythrocytes are not damaged due to the interaction of hemoglobin with 2,3-DPG and other factors that are involved in controlling the properties of hemoglobin. The initial section of the curve is steep, showing a large change in hemoglobin oxygen saturation with moderate changes in PO2 (eg, SO increases from 10 to 70% when RO increases from 10 to 37 mmHg).

The end section of the curve, on the contrary, is flat and reflects small changes in hemoglobin saturation with a significant change in RH (for example, SO2 increases from 70.0 to 97.5% when RH increases from 37 to 100 mmHg). The steep part of the curve facilitates the separation of oxygen from hemoglobin in peripheral tissues in the presence of a slightly reduced PO2. On the other hand, the flat part of the oxyhemoglobin curve facilitates adequate oxygen binding by hemoglobin in the lungs, even with a significant decrease in PO2 or in pulmonary diseases. The properties of hemoglobin provide almost the maximum binding of oxygen in the lungs and the release of this gas in the tissues.

48) Explain the role of hemoglobin in maintaining a relatively constant PO2 in peripheral tissues during significant changes in alveolar PO2 (caused by changes in PO2 and lung diseases).

The relatively flat upper portion of the oxyhemoglobin dissociation curve explains the small change in percentage of arterial oxygen saturation when alveolar OR decreases to 60 mmHg. (SO2 = 89%) or increases to 500 mmHg. (SO2 - 99%); these values ​​represent respectively a decrease of only 8% and an increase of only 2% relative to the normal 97% saturation. Since approximately 5 ml of oxygen per dl of blood is removed from the tissues, the corresponding SO2 values ​​in mixed venous blood are 77% (PO2 = 43 mmHg) and 67% (PO2 =

35 mm Hg). Consequently, PO2 in tissues does not change by more than a few millimeters of mercury, despite a significant change in alveolar PO2.

49) Compare the amount of oxygen released by Hb under normal conditions with its amount when the oxyhemoglobin dissociation curve is shifted in any direction.

Shifting the oxyhemoglobin dissociation curve in any direction can significantly change the amount of oxygen released in tissues. For example, at a blood pH of 7.20, about 30% more oxygen is released than at a blood pH of 7.60 (these pH values ​​correspond to a right and left shift of the curves compared to the normal oxyhemoglobin dissociation curve at pH 7. ,40).

50) How does the oxyhemoglobin dissociation curve shift during exercise? How is offset determined?

During exercise, there is a shift to the right of the oxyhemoglobin dissociation curve in capillary blood, which delivers oxygen to the muscles. The main factors responsible for this shift are: 1) an increase in muscle temperature of about 3°C; 2) a higher level of PCO2 in the tissue caused by increased production of carbon dioxide; 3) acidemia caused by a high PCO2 and a lower plasma bicarbonate concentration ([HCO3"]p) (a reaction to lactic acidosis); 4) the release of phosphate compounds from the muscles. Therefore, from 75 to 85% of the oxygen transported by hemoglobin can be released into tissue as opposed to only 25% secreted in a normal resting state.

51) What is the value of PO2 and the percentage of saturation of hemoglobin with oxygen in arterial and mixed venous blood in healthy people?

Arterial oxygen saturation is approximately 97%, because in healthy people the PaC > 2 is approximately 95 mm Hg. The SO2 and PO2 values ​​in mixed venous blood (derived from the pulmonary artery and representing normal venous blood returning from peripheral tissues) are 75% and 40 mmHg, respectively.

52) What is the Bohr effect? What is the significance of this effect?

The Bohr effect consists in a shift in the oxyhemoglobin dissociation curve caused by changes in PCO2 in the blood. This process is of great physiological importance because it facilitates the binding of oxygen by hemoglobin in the lungs, as well as its release in tissues. The transition of carbon dioxide from the blood into the alveoli of the lungs reduces the level of carbonic acid in the blood and its. Therefore, the upward shift of the curve, which also shifts it to the left, is due to the loss of carbon dioxide and thereby increases the uptake of oxygen by hemoglobin as the blood passes through the lungs. The opposite occurs in peripheral tissues, when an increase in carbon dioxide in the blood shifts the curve to the right. This shift of the oxyhemoglobin dissociation curve to the right provides more oxygen release in the tissues.

53) How does hemoglobin help to maintain a relatively constant PO2 in peripheral tissues in conditions accompanied by increased tissue oxygen demand (for example, during heavy physical exertion)?

The steep lower part of the oxyhemoglobin dissociation curve is characterized by a large change in the percentage saturation of Hb with oxygen with a relatively slight decrease in PO2. This part of the dissociation curve facilitates the release of oxygen into the tissues, especially during exercise. In the normal state, SvO 2 is 75%, and PvO 2 is 40 mm Hg, but during heavy muscle exercise, SvO 2 reaches 30%, and PvO 2 is 20 mm Hg. Thus, during exercise, SvO 2 is reduced by 45% (from 75 to 30%). A relatively small decrease in PO 2 in tissues - from 40 to 20 mm Hg. accompanied by the release of an additional 9 ml of oxygen per 1 dl of blood (i.e. 15 g Hb 1.34 ml O 2 0.45 = 9 ml). Thus, hemoglobin helps to maintain a relatively constant PO 2 in tissues when PaO 2 is markedly reduced and when tissue oxygen demand increases substantially; this property is often thought to result from the "buffering" effect of Hb on tissue oxygen partial pressure.

Gas exchange is carried out by simple diffusion according to Fick's law:

gas diffusion is directly proportional to its partial pressure gradient and the barrier area, and inversely proportional to the barrier thickness:

v = TO S (R 1 - R 2 ) / L ,

Where v- diffusion rate; TO - diffusion coefficient; S - barrier area; R- partial pressure O 2 (R 1 - in aveloli, R 2 - in pulmonary capillaries) or CO 2 ; L - barrier thickness.

2. Gas exchange in the lungs (between alveolar gas and blood)

9.3. Gas transport by blood

Oxygenated arterial blood from the lungs enters the heart and is carried throughout the body through the vessels of the systemic circulation. The O 2 tension in the arteries of the systemic circulation is somewhat lower than in the arterial blood of the pulmonary capillaries. This is due to the fact that, firstly, there is constant mixing of blood from well and poorly ventilated areas of the lungs, and secondly, part of the blood through arteriovenular shunts can be transferred from the veins to the arteries of the systemic circulation, bypassing the lungs. O 2 tension in arterial blood undergoes age-related changes: in young healthy people it is 95 mm Hg. Art., by the age of 40 decreases to 80 mm Hg. Art., by the age of 70 - up to 70 mm Hg. Art. The tension of CO 2 in the arteries of the systemic circulation in young people is 40 mm Hg. Art. and changes little with age. O 2 and CO 2 in the blood are in two states: chemically bound and dissolved. The content of these gases in the blood is a constant value.

transport of oxygen. IN arterial blood content O 2 is 18-20 vol.%, and in the venous - 12 vol.%. The amount of O 2 physically dissolved in the blood is only 0.3 vol.%; consequently, practically all O 2 is transported by the blood in the form of a chemical compound with hemoglobin.

Hemoglobin - red blood pigment contained in erythrocytes; consists of 4 identical groups - gems. Heme is a protoporphyrin with a ferrous ion at its center, which plays an important role in O 2 transport. Each heme attaches 1 O 2 molecule to itself, one hemoglobin molecule binds 4 O 2 molecules, a reversible bond occurs, while the valence of iron does not change. This is called hemoglobin oxygenation. The reduced hemoglobin (Hb) becomes oxidized - HbO 2 (oxyhemoglobin).

The maximum amount of oxygen that can be bound in 100 ml of blood when hemoglobin is completely saturated with oxygen is called oxygen capacity of the blood. It is for-and occurs by diffusion.

46. ​​Transport of gases (O 2 , CO 2) by blood. Factors affecting the formation and dissociation of oxyhemoglobin. The oxygen capacity of the blood. Oxygemometry. Gas exchange between blood and tissues.

Transport of oxygen in the blood. Oxyhemoglobin dissociation curve, its characteristics. Factors affecting the formation and dissociation of oxyhemoglobin.

Almost all liquids can contain some amount of physically dissolved gases. The content of dissolved gas in a liquid depends on its partial pressure.

Although the content of O 2 and CO 2 in the blood in a physically dissolved state is relatively small, this state plays a significant role in the life of the organism. In order to contact certain substances, the respiratory gases must first be delivered to them in a physically dissolved form. Thus, during diffusion into tissues or blood, each O or CO molecule is in a state of physical dissolution for a certain time.

Most of the oxygen is carried in the blood in the form of a chemical compound with hemoglobin. 1 mole of hemoglobin can bind up to 4 moles of oxygen, and 1 gram of hemoglobin can bind 1.39 ml of oxygen. When analyzing the gas composition of the blood, a slightly lower value is obtained (1.34 - 1.36 ml of O 2 per 1 g of Hb). This is due to the fact that a small part of hemoglobin is in an inactive form. Thus, approximately, we can assume that in vivo 1 g of Hb binds 1.34 ml of O 2 (the so-called Hüfner number).

Based on the Hüfner number, it is possible, knowing the hemoglobin content, to calculate the oxygen capacity of the blood: [O 2 ] max = 1.34 ml O 2 per 1 g of Hb; 150 g Hb per 1 liter of blood = 0.20 l O 2 per 1 liter of blood. However, such an oxygen content in the blood can only be achieved if the blood is in contact with a gas mixture with a high oxygen content (PO 2 = 300 mm Hg), therefore, under natural conditions, hemoglobin is not completely oxygenated.

The reaction that reflects the combination of oxygen with hemoglobin obeys the law of mass action. This means that the ratio between the amount of hemoglobin and oxyhemoglobin depends on the content of physically dissolved O 2 in the blood; the latter is proportional to the voltage O 2 . The percentage of oxyhemoglobin to total hemoglobin is called hemoglobin oxygen saturation. In accordance with the law of mass action, the saturation of hemoglobin with oxygen depends on the voltage O 2 . Graphically, this dependence is reflected by the so-called oxyhemoglobin dissociation curve. This curve is S-shaped.

The simplest indicator characterizing the location of this curve is the so-called half-saturation voltage PO 2, i.e. such a voltage of O 2 at which the saturation of hemoglobin with oxygen is 50%. Normally, RO 2 of arterial blood is about 26 mm Hg.

The configuration of the oxyhemoglobin dissociation curve is essential for the transport of oxygen in the blood. In the process of oxygen absorption in the lungs, the O 2 tension in the blood approaches the partial pressure of this gas in the alveoli. In young people, arterial blood RO 2 is about 95 mm Hg. At this voltage, hemoglobin saturation with oxygen is approximately 97%. With age (and even more so with lung disease), arterial O 2 tension can decrease significantly, however, since the oxyhemoglobin dissociation curve on the right side is almost horizontal, oxygen saturation does not decrease much. So, even with a drop in RO 2 in arterial blood to 60 mm Hg. saturation of hemoglobin with oxygen is 90%. Thus, due to the fact that the region of high oxygen tensions corresponds to the horizontal section of the oxyhemoglobin dissociation curve, the saturation of arterial blood with oxygen remains at a high level even with significant shifts in RO 2 .

The steep slope of the middle section of the oxyhemoglobin dissociation curve indicates a favorable situation for the return of oxygen to the tissues. At rest, RO 2 in the region of the venous end of the capillary is approximately 40 mm Hg, 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, then the saturation of hemoglobin with oxygen decreases by 75%: the O 2 released during this can be immediately used for metabolic processes.

Despite the fact that the configuration of the oxyhemoglobin dissociation curve is mainly due to the chemical properties of hemoglobin, there are a number of other factors that affect the blood's affinity for oxygen. Typically, all of these factors will shift the curve, increasing or decreasing its slope, but not changing its S-shape. These factors include temperature, pH, CO 2 tension and some other factors, the role of which increases in pathological conditions.

The equilibrium of the hemoglobin oxygenation reaction depends on temperature. As the temperature decreases, the slope of the oxyhemoglobin dissociation curve increases, and as it rises, it decreases. In warm-blooded animals, this effect occurs only when hypothermic or febrile.

The shape of the oxyhemoglobin dissociation curve largely depends on the content of H + ions in the blood. With a decrease in pH, i.e. acidification of the blood, the affinity of hemoglobin for oxygen decreases, and the dissociation curve of oxyhemoglobin is called the Bohr effect.

Blood pH is closely related to CO 2 tension (PCO 2): 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 flattening of the HbO 2 dissociation curve. This dependence is also called the Bohr effect, although such a quantitative analysis showed that the effect of CO 2 on the shape of the oxyhemoglobin dissociation curve cannot be explained only by a change in pH. Obviously, carbon dioxide itself has a "specific effect" on the dissociation of oxyhemoglobin.

In a number of pathological conditions, changes in the process of oxygen transport by the blood are observed. So, 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 these shifts have not been fully elucidated. It is known that the shape and location of the oxyhemoglobin dissociation curve are strongly influenced by some organophosphorus compounds, the content of which in erythrocytes may change during pathology. The main such compound is 2,3-diphosphoglycerate - (2,3 - DFG). The affinity of hemoglobin for oxygen also depends on the content of cations in red blood cells. It is also necessary to note the influence of pathological pH shifts: in alkalosis, oxygen uptake in the lungs increases as a result of the Bohr effect, but its return to the tissues becomes more difficult; and with acidosis, the reverse picture is observed. Finally, a significant shift of the curve to the left occurs with carbon monoxide poisoning.

CO transport in blood. forms of transport. The value of carbonic anhydrase.

Carbon dioxide, the end product of oxidative metabolic processes in cells, is transported with the blood to the lungs and removed through them into the external environment. Like oxygen, CO 2 can be transported both in a physically dissolved form and as part of chemical compounds. Chemical reactions of CO 2 binding are somewhat more complicated than oxygen addition reactions. This is due to the fact that the mechanisms responsible for the transport of CO 2 must simultaneously ensure the maintenance of the constancy of the acid-base balance of the blood and thus the internal environment of the body as a whole.

The tension of CO 2 in the arterial blood entering the tissue capillaries is 40 mm Hg. In the cells located near these capillaries, the tension of CO 2 is much higher, since this substance is constantly formed as a result of metabolism. In this regard, physically dissolved CO 2 is transferred along the voltage gradient from the tissues to the capillaries. Here, a certain amount of carbon dioxide remains in a state of physical dissolution, but most of the CO 2 undergoes a series of chemical transformations. First of all, the hydration of CO 2 molecules occurs with the formation of carbonic acid.

In blood plasma, this reaction proceeds very slowly; in an erythrocyte, it accelerates by about 10 thousand times. This is due to the action of the enzyme carbonic anhydrase. Since this enzyme is present only in cells, practically all CO 2 molecules involved in the hydration reaction must first enter the erythrocytes.

The next reaction in the chain of chemical transformations of CO 2 is the dissociation of the weak acid H 2 CO 3 into bicarbonate and hydrogen ions.

The accumulation of HCO 3 - in the erythrocyte leads to the fact that a diffusion gradient is created between its internal environment and blood plasma. HCO 3 - ions can move along this gradient only if the equilibrium distribution of electric charges is not disturbed. In this regard, simultaneously with the release of each HCO 3 - ion, either the exit from the erythrocyte of one cation, or the entry of one anion, must occur. Since the erythrocyte membrane is practically impermeable to cations, but rather easily passes small anions, instead of HCO 3 - Cl - ions enter the erythrocyte. This exchange process is called chloride shift.

CO 2 can also be bound by direct attachment to the amino groups of the protein component of hemoglobin. In this case, a so-called carbamin bond is formed.

Hemoglobin associated with CO 2 is called carbohemoglobin.

The dependence of CO 2 on the degree of oxygenation of hemoglobin is called the Haldane effect. This effect is partly due to the different ability of oxyhemoglobin and deoxyhemoglobin to form carbamic bonds.

47. Regulation of breathing. Functional relationship between the processes of breathing, chewing and swallowing. Reserve capacity of the respiratory system.

The binding of oxygen to hemoglobin. Oxygen entering the blood is first dissolved in the blood plasma. At Pu0 100 mm Hg. Art. only 0.3 ml of 02 dissolves in 100 ml of plasma. Although there is not much dissolved oxygen, this form of it plays an important intermediate role in gas exchange. Such oxygen penetrates the erythrocyte membrane along the concentration gradient and first dissolves in its cytoplasm. Only after this, O2 enters into combination with Fe2+ heme and forms compounds that are called oxy-hemoglobin (HbO2). In this case, the valency of iron does not change. Oxyhemoglobin is a low-power compound that is easily broken down in tissues. The direct reaction is called oxygenation, and the reverse process, which occurs in the tissues, is the deoxygenation of hemoglobin (Fig. 83).

Each hemoglobin molecule is capable of attaching four oxygen molecules, which, in terms of 1 g of hemoglobin, means 1.34 ml of 02. Knowing the level of hemoglobin in the blood, it is easy to calculate the blood oxygen capacity (KEK):

KEK \u003d Hb- 1,34.

For example: 15 o 1.34 \u003d 20 (ml) of oxygen is contained in 100 ml of blood. Considering that those same 100 ml of blood contain only 0.3 ml of dissolved 02, we can conclude that the main amount of oxygen transported by the blood is chemically bound to hemoglobin.

Rice. 83.

Association and dissociation of oxyhemoglobin

The intensity of the formation (association) of oxyhemoglobin is due to the partial voltage of 02 in the blood: the higher the level of P0, the more oxyhemoglobin is formed. However, this relationship is not directly proportional. It has the form of an 8-shaped curve, which is more convenient to determine by the rate of dissociation of oxyhemoglobin (Fig. 84). Its 8-shaped character is determined by the fact that with an increase in the number of O2 molecules that attach to each oxyhemoglobin molecule, this process proceeds more actively (autocatalysis). So, if in the absence of oxygen in the blood (P0 = 0) there is no oxyhemoglobin, and if P0 = 10 mm Hg. Art. 10% of hemoglobin passes into oxyhemoglobin, then at P0 = 20 mm Hg. Art. already contains about 30% oxyhemoglobin, and at P0 = 40 mm Hg. Art. - about 80% oxyhemoglobin, at P0 = 100 mm Hg. Art. the blood will contain about 100% oxyhemoglobin.

It is necessary to pay special attention to two sections of the curve: the upper one, which runs almost parallel to the y-axis, and the middle one, which falls sharply down. The configuration of the first section indicates the ability of hemoglobin to actively capture 02 in the lungs, and the second - to easily release it in the tissues. So, in the process of absorption of 02 by blood in the lungs, already at P0a = 60 mm Hg. Art. almost all hemoglobin can attach oxygen (more than 90% of oxyhemoglobin).

Rice. 84. under normal conditions; 2 - for an increase in pH or temperature; WITH- for lowering the pH or temperature; 4 - Р50О2

In mixed venous blood obtained from the right atrium, at P0 of 40 mm Hg. Art. the content of oxyhemoglobin still exceeds 70%. With KEK in 20 ml 1100 ml, it is still about 15 ml 1100 ml blood creates a reserve 02. Starting from a P0 value of 40 mmHg. Art., the curve descends steeply. Due to even a slight decrease in P0 below 40 mm Hg. Art., which occurs in the tissues in the case of their more intensive functioning, the rate of dissociation of oxyhemoglobin increases dramatically. This provides a significant acceleration of the supply of oxygen to the tissues from the previous blood volume. For example, with Ryu, which is 20 mm Hg. Art., oxyhemoglobin remains only 30%. So, tissues from every 100 ml of blood no longer receive 5 ml of oxygen, as under normal conditions, but about 14 ml, that is, almost three times more.

It can be noted that due to this feature of hemoglobin, a person can live high in the mountains, perform intense muscular work and not always die from a lack of 02 with a decrease in the level of hemoglobin in the blood (anemia), difficulty in gas exchange through the membrane (for example, with pneumonia).

Change in the slope of the oxyhemoglobin dissociation curve.

The slope of the curve, i.e., the rate of dissociation of oxyhemoglobin in human blood, is not constant and may change under certain conditions. The rate of dissociation of oxyhemoglobin is due to the chemical affinity of hemoglobin to 02 and some external factors that change the nature of the curve. These factors include temperature, pH, Pro.

The shape of the oxyhemoglobin dissociation curve largely depends on the concentration of H+ ions in the blood. At lowering the pH the curve shifts to the right, which indicates a decrease in the affinity of hemoglobin with 02 and activation of its entry into tissues. pH increase increases the affinity and shifts the curve to the left - the flow of oxygen into the blood increases. The effect of pH on the affinity of hemoglobin with O2 is called the Bohr effect. The Bohr effect in many conditions in normal and pathological conditions plays a significant role in the gas transport function of the blood. The formation of a large amount of CO2 in the tissues contributes to an increase in the return of O2 due to a decrease in the affinity of hemoglobin with O2, and the release of CO2 in the lungs, by reducing the pH of the blood, on the contrary, improves oxygenation. CO2 also affects the oxyhemoglobin dissociation curve.

With a decrease in temperature, the release of 02 by oxy-hemoglobin slows down, and an increase in temperature accelerates this process.

An indicator characterizing the intensity of oxygen use by tissues is the difference in the level of oxyhemoglobin in the blood, inflowing and outflowing (arteriovenous oxygen difference, ABP-02).

Thus, the practical absence of oxygen reserves in the body is compensated by the possibility of a sharp increase in its use from the bloodstream due to an increase in ABP-02. The intensive functioning of tissues, when more CO2, H+ is formed and the temperature rises, creates conditions for increasing the delivery of oxygen to cells.

Carbon monoxide poisoning.

Carbon monoxide (CO) has a greater (about 350 times) affinity for hemoglobin than oxygen. Therefore, even at very low concentrations in the air, and hence in the blood, carboxyhemoglobin compounds (HbCO) are formed. Due to the fact that these compounds are stable, the ability of hemoglobin to bind oxygen is sharply reduced. This is due to the fact that CO binds to the iron molecules in the ham, and in this case, the dissociation curve shifts to the left. As a result, even free hemoglobin molecules interact worse with oxygen.

The dissociation of carboxyhemoglobin occurs very slowly, therefore, in the case of a mild degree of poisoning, the victim must be taken out to fresh air or given oxygen for breathing.