Blood plasma buffer solutions. Blood buffer systems
When there are shifts in the content of H+ ions in the blood and other environments of the body (both with an increase and a decrease in their quantity), the fast-acting and powerful chemical buffer systems of plasma and red blood cells are first triggered ( hemoglobin, bicarbonate, phosphate, protein). The hemoglobin buffer system is the main buffer of red blood cells and accounts for about 75% of the total buffer capacity of the blood. Hemoglobin, like other proteins, is an ampholyte, that is, the hemoglobin buffer system consists of an acidic component (oxygenated Hb, i.e. HbO2) and a basic component (non-oxygenated, i.e. reduced Hb). It has been shown that hemoglobin is a weaker acid (about 70 times) than oxyhemoglobin. In addition, Hb maintains a constant pH by binding CO2 and transferring it from the tissue to the lungs and then to the external environment. The bicarbonate (hydrocarbonate) buffer system is the main buffer of blood plasma and extracellular fluid and accounts for approximately 15% of the total buffer capacity of the blood. It is represented in the extracellular environment by carbonic acid (H2CO3) and sodium bicarbonate (NaHCO3). The concentration of hydrogen ions in this buffer is = K [H2C03 / NaHC03 = 1/20, where K is the dissociation constant of carbonic acid. This buffer system ensures, on the one hand, the formation of NaHC03, on the other, the formation of carbonic acid (H+ + HCO3 - H2CO3) and the decomposition of the latter (H2CO3 - H20 + CO2) under the influence of the enzyme carbonic anhydrase to H20 and CO2. Carbon dioxide is removed by the lungs when you exhale, but there is no pH shift. This buffer system prevents pH shifts when strong acids and bases are introduced into the biological environment as a result of their conversion either into weak acids or into weak bases. Hydrocarbonate buffer systems are an important indicator of WWTP. This is an open type system, which is associated with the function of both the external respiration system and the kidneys and skin. The phosphate buffer system is represented by mono- and disubstituted sodium phosphate (NaH2P04 and Na2HP04). The first compound behaves like a weak acid, the second - like a weak base. Acids formed in the body and entering the blood interact with Na2HP04, and bases - with NaH2P04. As a result, the blood pH remains unchanged. Phosphates play a buffering role mainly in the intracellular environment (especially kidney tubule cells) and maintain the initial state of the bicarbonate buffer. The protein buffer system acts as an intracellular buffer system. Possessing ampholytic properties, they behave as bases in an acidic environment, and as acids in an alkaline environment. The protein buffer system consists of a weakly dissociating protein with acidic properties (COOH protein) and a protein complexed with strong bases (COONa protein). This buffer system also helps prevent blood pH shifts. Later (after a few minutes and hours), physiological (organ and systemic) mechanisms compensate for and eliminate shifts in the CBS (carried out by the lungs - with exhaled air, kidneys - with urine, skin - with sweat, liver and other organs digestive tract- with feces).
Maintaining a constant internal environment is a necessary condition normal exchange substances. The most important indicators characterizing the constancy of the internal environment include acid-base balance, that is, the relationship between the amount of cations and anions in the tissues of the body, which is expressed by pH indicators. In mammals, blood plasma has a slightly alkaline reaction and remains within the range of 7.30-7.45.
The state of acid-base balance is influenced by the intake and formation in the body of both acidic products (organic acids are formed from proteins and fats, and also appear as products of interstitial metabolism in tissues) and alkaline substances (formed from plant foods rich in alkaline salts of organic acids and alkaline earth salts, metabolic products - ammonia, amines, basic salts of phosphoric acid). Acidic and alkaline products are also formed during various pathological processes.
Internal environment of living organisms.
Circulating blood is a suspension of living cells in a liquid medium, chemical properties which are very important for their life. In humans, the normal range of blood pH fluctuations is 7.37-7.44 with an average value of 7.4. Blood buffer systems are composed of plasma and blood cell buffer systems and are represented by the following systems:
- bicarbonate (hydrogen carbonate) buffer system;
- phosphate buffer system;
- protein buffer system;
- hemoglobin buffer system
- red blood cells
In addition to these systems, the respiratory and urinary systems are also actively involved.
Encyclopedic YouTube
1 / 3
✪ Lesson 1 - pH - acid alkaline acid is within the power of everyone
✪ Buffer solutions and the Henderson-Hasselbach equation
✪ Analysis of acid-base balance is normal and its interpretation
Subtitles
Bicarbonate buffer system
One of the most powerful and at the same time the most controlled system of extracellular fluid and blood, which accounts for about 53% of the total buffer capacity of the blood. It is a conjugate acid-base pair consisting of a carbonic acid molecule H 2 CO 3, which is a source of proton, and a bicarbonate anion HCO 3 −, which acts as a proton acceptor:
H 2 C O 3 ⇄ H C O 3 − + H + (\displaystyle (\mathsf (H_(2)CO_(3)\rightleftarrows HCO_(3)^(-)+H^(+)))) Due to the fact that the concentration of sodium bicarbonate in the blood significantly exceeds the concentration of H 2 CO 3, the buffer capacity of this system will be significantly higher for acid. In other words, the bicarbonate buffer system is especially effective at compensating for the effects of substances that increase blood acidity. These substances primarily include lactic acid, the excess of which is formed as a result of intensive physical activity. The hydrocarbonate system most “quickly” responds to changes in blood pHPhosphate buffer system
In the blood, the capacity of the phosphate buffer system is small (about 2% of the total buffer capacity), due to the low content of phosphates in the blood. The phosphate buffer plays a significant role in maintaining physiological pH values in intracellular fluids and urine.
The buffer is formed by inorganic phosphates. The role of an acid in this system is played by a monosubstituted phosphate (NaH 2 PO 4), and the role of a conjugate base is played by a disubstituted phosphate (Na 2 HPO 4). At pH 7.4, the ratio [HPO 4 2- /H 2 PO 4 - ] is equal to 10 p H − p K a , o r t o I I = 1 , 55 (\displaystyle 10^(pH-pK_(a,orto)^(II))=1.55) since at a temperature of 25 + 273.15 K pK a, ortho II = 7.21, while the average charge of the orthophosphoric acid anion< q >=((-2)*3+(-1)*2)/5=-1.4 units of positron charge.
The buffer properties of the system with an increase in the content of hydrogen ions in the blood are realized due to their binding with HPO 4 2- ions with the formation of H 2 PO 4 -:
H + + H P O 4 2 − → H 2 P O 4 − (\displaystyle (\mathsf (H^(+)+HPO_(4)^(2-)\rightarrow H_(2)PO_(4)^(-)) ))and with an excess of OH- ions - due to their binding with H 2 PO 4 - ions:
H 2 P O 4 − + O H − ⇄ H P O 4 2 − + H 2 O (\displaystyle (\mathsf (H_(2)PO_(4)^(-)+OH^(-)\rightleftarrows HPO_(4)^( 2-)+H_(2)O)))The phosphate buffer system of the blood is closely related to the bicarbonate buffer system.
Protein buffer system
Compared to other buffer systems, it is less important for maintaining acid-base balance (7-10% of buffer capacity)
The main part of blood plasma proteins (about 90%) are albumins and globulins. The isoelectric points of these proteins (the number of cationic and anionic groups is the same, the charge of the protein molecule is zero) lie in a slightly acidic environment at pH 4.9-6.3, therefore, under physiological conditions at pH 7.4, the proteins are predominantly in the “protein-base” forms " and "protein-salt".
The concentration of hydrogen ions in the blood, which is defined as blood pH, is one of the parameters of homeostasis; fluctuations are normally possible within a very narrow range from 7.35 to 7.45. It is worth noting that a shift in pH beyond the specified limits leads to the development of acidosis (shift to the acidic side) or alkolosis (to the alkaline side). The body is able to maintain vital functions if the blood pH does not go beyond 7.0-7.8. Unlike blood, the parameters of the acid-base state for various organs and tissues fluctuate within wider limits. For example, the pH of gastric juice is normally 2.0, the prostate is 4.5, and in osteoblasts the environment is alkaline, and the pH value reaches 8.5.
Regulation of the acid-base state in the blood is carried out through special buffer systems that respond to changes in pH quickly enough, through the respiratory system and kidneys, as well as the digestive canal and skin, through which acidic and alkaline products are eliminated. It takes about 1-3 minutes for the lungs to change the pH of the blood (due to a decrease or increase in the rate of respiration and the removal of carbon dioxide), and for the kidneys about 10-20 hours.
Thus, blood buffer systems are the most quickly responsive mechanism for regulating blood pH. Buffer systems include blood plasma proteins, hemoglobin, bicarbonate and phosphate buffers.
Protein buffer. The ability of blood plasma proteins to play the role of a buffer is determined by the so-called amphoteric properties, i.e. the ability to exhibit the properties of acids or bases depending on the environment. In an acidic environment, the protein exhibits the properties of a base, the COOH group dissociates, hydrogen ions attach to the NH2 group, and they become negatively charged, and the proteins exhibit basic properties. In an alkaline environment, only the carboxyl group dissociates, and the released hydrogen ions bind to OH– residues and thereby stabilize the acid-base state.
Hemoglobin buffer is one of the most powerful, it contains free, reduced, oxidized hemoglobin, as well as carboxyhemoglobin and the potassium salt of hemoglobin. It is believed that this buffer accounts for about 75% of all buffering properties of blood, and it is based on the ability of the globin part of the molecule to change its conformation and, as a consequence, acidic properties when transitioning from one form to another. Thus, reduced hemoglobin is a weaker acid compared to carbonic acid, while oxidized hemoglobin is a stronger acid. Therefore, when the content of carbonic acid in the blood increases and the pH shifts to the acidic side, a hydrogen ion joins free hemoglobin, resulting in the formation of reduced hemoglobin. In the capillaries of the lungs, carbon dioxide is removed from the blood, the pH shifts to the alkaline side, and oxidized hemoglobin becomes a proton donor, which stabilizes the pH, preventing it from shifting to the alkaline side.
Processes that occur in tissues:<
1. Carbon dioxide, which is released during cellular respiration, enters the blood and binds with water, forming carbonic acid. This acid is very unstable and dissociates in the blood into a hydrogen cation and a bicarbonate anion. Free hydrogen shifts the pH to the acidic side.
2. Under acidic conditions, oxyhemoglobin dissociates, forming free oxygen, which enters the tissues, and the potassium salt of hemoglobin, which remains inside the red blood cells.
3. The carbonic acid anion interacts with the potassium salt of hemoglobin, forming free hemoglobin and the potassium salt of carbonic acid. Such hemoglobin has pronounced alkaline properties and binds free hydrogen ions. Already reduced hemoglobin attaches carbon dioxide and forms carboxyhemoglobin.
4. Thus, the dissociation of oxyhemoglobin is determined by the reaction of the environment, and free hemoglobin formed after the breakdown of oxyhemoglobin is a strong base, it prevents acidification of the blood in the area of tissue capillaries.
Processes that occur in the pulmonary capillaries:
1. Carbon dioxide passes into the alveoli, its concentration in the blood decreases, which enhances the dissociation of carboxyhemoglobin.
2. A large amount of reduced hemoglobin is formed, which attaches oxygen. As the environment becomes alkaline, a hydrogen ion is split off from hemoglobin, which stabilizes the pH, and a potassium ion is added to the hemoglobin itself.
3. From the potassium salt of carbonic acid and free hydrogen ions, carbonic acid is formed, which dissociates into carbon dioxide and water, due to a shift in the equilibrium of the chemical reaction due to a decrease in the concentration of carbon dioxide in the blood.
Thus, oxyhemoglobin dissociates with the formation of a hydrogen ion, which, on the one hand, shifts the pH to the acidic side, and on the other, promotes the dissociation of carbonic acid with the formation of carbon dioxide, which must pass into the pulmonary alveoli and leave the body with exhaled air.
The bicarbonate buffer is considered next in importance after hemoglobin; it is also associated with the act of respiration. Thus, the blood always contains a fairly large amount of weak carbonic acid and sodium bicarbonate, so the entry of stronger acids into the blood leads to the fact that they interact with sodium bicarbonate to form the corresponding salt and carbonic acid. The latter is quickly broken down by the enzyme carbonic anhydrase into water and carbon dioxide, which are removed from the body.
The entry of alkali into the bloodstream leads to the formation of carbonates - carbonic acid salts and water. The carbonic acid deficiency that occurs in this case can be quickly compensated for by reducing the release of carbon dioxide by the lungs.
The state of the bicarbonate buffer system is assessed by the equilibrium of the following reaction:
H2O + CO2 = H2CO3 = H+ + HCO3
In clinical practice, the following indicators are used to assess the state of the bicarbonate buffer system:
1. Standard bicarbonates. This is the concentration of bicarbonate anion in the blood under standard conditions (partial pressure of carbon dioxide 40 mm Hg, complete saturation of the blood with oxygen, equilibrium with the gas mixture at a temperature of 38 degrees Celsius).
2. Actual bicarbonates - the concentration of bicarbonate anion in the blood at 38 degrees and real values of the partial pressure of carbon dioxide and pH.
3. The ability of blood to bind carbon dioxide is an indicator reflecting the concentration of bicarbonates in plasma. Previously, they were actively determined by the gasometric method, but today the method has lost its significance due to the development of electrochemical methods.
4. Alkaline reserve - the ability of the blood to neutralize acids due to alkaline compounds, was determined by the titration method, today the method has lost its practical significance.
5. Partial pressure of carbon dioxide. The pressure in a gas that is balanced at a temperature of 38 degrees with arterial blood plasma. It depends on the diffusion of carbon dioxide through the alveolar membrane and respiration, and therefore can be disrupted when the permeability of the alveolar membrane changes or the ventilation of the lungs is impaired.
Phosphate buffer system
This system includes sodium hydrogenphosphate and sodium dihydrogenphosphate. Hydrogen phosphate has alkaline properties, while dihydrogen phosphate exhibits the properties of a weak acid. When acid enters the blood, it reacts with a weak base - hydrogen phosphate, free hydrogen ions are bound to form dihydrogen phosphate, and the blood pH is stabilized (there is no shift to the acidic side). If bases enter the blood, their hydroxide anions bind to free hydrogen ions, the source of which is a weak acid - dihydrogen phosphate.
The phosphate buffer system is of greatest importance for regulating the pH of interstitial fluid and urine (in the blood, hemoglobin and bicarbonate buffers are of greater importance). In urine, hydrogen phosphate plays a role in storing sodium bicarbonate. Thus, hydrogen phosphate interacts with carbonic acid, dihydrogen phosphate and bicarbonate (sodium, potassium, calcium and other cations) are formed. Bicarbonate is completely reabsorbed, and the pH of the urine depends on the concentration of dihydrogen phosphate.
Acid-base balance.
Acid-base balance is the ratio of the concentrations of hydrogen (H +) and hydroxyl (OH -) ions in body fluids.
The constancy of the pH of the internal environment of the body is due to the combined action of buffer systems and a number of physiological mechanisms.
1. Blood and tissue buffer systems:
Bicarbonate: NaHCO 3 + H 2 CO 3
Phosphate: NaHPO 4c + NaHPO 4k
Protein: protein-Na + + protein-H +
Hemoglobin: HbK+HbH +
2. Physiological control:
Respiratory function of the lungs
Excretory function of the kidneys
ASR reflects cellular metabolism, gas transport function of the blood, external respiration and water-salt metabolism.
Normally, blood pH ranges from 7.37 to 7.44, with an average pH value of 7.4.
Buffer systems maintain a constant pH when acidic and basic (OH -) products are supplied. The buffering effect is explained by the binding of free H + and OH - ions by the buffer components and their conversion into the undissociated form of a weak acid or water.
The body's buffer systems consist of weak acids and their salts with strong bases.
To eliminate the pH shift, different times are required:
Buffer systems – 30 sec
Breathing control – 1 – 3 min
Excretory function of the kidneys – 10 – 20 hours.
Buffer systems only correct pH shifts. Physiological mechanisms also restore buffer capacity.
Bicarbonate buffer system.
The bicarbonate buffer accounts for about 10% of the total buffer capacity of the blood.
The bicarbonate buffer consists of carbonic acid, which acts as a proton donor, and a bicarbonate ion, which acts as a proton acceptor.
H 2 CO 3 is a weak acid, difficult to dissociate
H 2 CO 3 H + +
NaHCO 3 - a salt of a weak acid and a strong base dissociates completely:
NaНСО 3 Na + +
Mechanism of buffer action
1. When acidic products enter the blood, hydrogen ions interact with bicarbonate ions, and weakly dissociating carbonic acid is formed:
H + + NaHCO 3 Na + + H 2 CO 3
The ratio H 2 CO 3 / NaHCO 3 is restored, the pH does not change (the concentration of NaHCO 3 decreases slightly).
The lungs ensure the removal of carbon dioxide.
2. When bases enter the blood from tissues, OH - ions interact with weak carbonic acid (OH - ions interact with H + from the buffer, forming H 2 O)
H 2 CO 3 + OH - H 2 O +
The pH remains the same and increases. The excess enhances the dissociation of H 2 CO 3, the consumption of H + is replenished by increasing the dissociation of H 2 CO 3.
At normal value Blood pH, the concentration of bicarbonate ions in the blood plasma exceeds the concentration of carbon dioxide by about 20 times:
Phosphate buffer system
Buffer components:
Na 2 HPO 4c – salt – disubstituted phosphate
NaH 2 PO 4k – weak acid – monosubstituted phosphate
Ratio 
The phosphate buffer system accounts for 1% of the buffer capacity of the blood.
The mechanism of action of the buffer.
1. When acidic metabolic products enter the blood, H + ions bind to a disubstituted phosphate ion, an acidic monosubstituted ion is formed, the excess of which is removed by the kidneys in the urine:
The phosphate buffer acts when the pH changes in the range from 6.1 to 7.7. In the blood, the maximum phosphate buffer capacity appears at 7.2.
Buffer systems are compounds that counteract sudden changes in the concentration of H + ions. Any buffer system is an acid-base pair: a weak base (anion, A –) and a weak acid (H-Anion, H-A). They minimize shifts in the number of H + ions due to their binding to the anion and inclusion in a poorly dissociating compound - a weak acid. Therefore, the total number of H + ions does not change as noticeably as it could be.
There are three buffer systems of body fluids - bicarbonate, phosphate, protein(including hemoglobin).They take effect instantly and after a few minutes their effect reaches its maximum possible.
Phosphate buffer system
The phosphate buffer system makes up about 2% of the total buffer capacity of the blood and up to 50% of the buffer capacity of the urine. It is formed by hydrogen phosphate (HPO 4 2–) and dihydrogen phosphate (H 2 PO 4 –). Dihydrogen phosphate weakly dissociates and behaves like a weak acid; hydrogen phosphate has alkaline properties. Normally, the ratio of HPO 4 2– to H 2 PO 4 – is 4: 1.
When acids (H + ions) interact with disubstituted phosphate (HPO 4 2‑), dihydrogen phosphate (H 2 PO 4 –) is formed:
Removal of H+ ions with phosphate buffer
As a result, the concentration of H + ions decreases.
When bases enter the blood (excess OH – ‑ groups), they are neutralized by H + ions entering the plasma from H 2 PO 4 – :
Removal of alkaline equivalents with phosphate buffer
The role of the phosphate buffer is especially high in the intracellular space and in the lumen of the renal tubules. Acid-base reaction urine depends only on the content of dihydrogen phosphate (H2 PO4 –), because Sodium bicarbonate is reabsorbed in the renal tubules.
Bicarbonate buffer system
This system is the most powerful, accounting for 65% of the total buffering power of the blood. It consists of bicarbonate ion (HCO 3 –) and carbonic acid (H 2 CO 3). Normally, the ratio of HCO 3 - to H 2 CO 3 is 20 : 1.
When H + ions (i.e. acid) enter the blood, sodium bicarbonate ions interact with it and carbonic acid is formed:
When the bicarbonate system operates, the concentration of hydrogen ions decreases, because Carbonic acid is a very weak acid and does not dissociate easily. At the same time in the blood doesn't happen parallel significant increase in the concentration of HCO 3 – .
If substances with alkaline properties enter the blood, they react with carbonic acid and form bicarbonate ions:
The work of the bicarbonate buffer is inextricably linked with respiratory system(with ventilation). In the pulmonary arterioles with a decrease in plasma CO 2 concentration and due to the presence of an enzyme in erythrocytes carbonic anhydrase carbonic acid quickly breaks down to form CO 2, which is removed with exhaled air:
H 2 CO 3 → H 2 O + CO 2
In addition to erythrocytes, significant carbonic anhydrase activity was noted in the epithelium of the renal tubules, cells of the gastric mucosa, adrenal cortex and liver cells, in small quantities - in the central nervous system, pancreas and other organs.
Protein buffer system
Plasma proteins are primarily albumen, play the role of a buffer due to their amphoteric properties. Their contribution to blood plasma buffering is about 5%.
IN acidic environment the dissociation of COOH groups of amino acid radicals (in aspartic and glutamic acids) is suppressed, and NH 2 groups (in arginine and lysine) bind excess H +. In this case, the protein becomes positively charged.
IN alkaline environment, the dissociation of COOH‑groups increases, H+ ions entering the plasma bind excess OH‑ions and the pH is maintained. Proteins in this case act as acids and are charged negatively.
Change in the charge of protein buffer groups at different pH values
Hemoglobin buffer system
Has high power in the blood hemoglobin buffer, it accounts for up to 28% of the total buffer capacity of the blood. As sour part of the buffer is oxygenated hemoglobin H‑HbO2. It has pronounced acidic properties and gives off hydrogen ions 80 times more easily than reduced H‑Hb, which acts as a base. The hemoglobin buffer can be considered as part of the protein buffer, but its feature is work in close contact with the bicarbonate system.
A change in the acidity of hemoglobin occurs in tissues and in the lungs, and is caused by the binding of H + or O 2, respectively. The direct mechanism of action of the buffer is the addition or donation of H + ion histidine residue in the globin part of the molecule (Bohr effect).
In tissues, a more acidic pH is normally the result of the accumulation of mineral (carbonic, sulfuric, hydrochloric) and organic acids (lactic). When the pH is compensated with this buffer, H + ions attach to the incoming oxyhemoglobin (HbO 2) and convert it into H‑HbO 2. This instantly causes oxyhemoglobin to release oxygen (Bohr effect) and it turns into reduced H‑Hb.
HbO 2 + H + → → H-Hb + O 2
As a result, the amount of acids, primarily H 2 CO 3, decreases, HCO 3 ions are produced and the tissue space is alkalized.
In the lungs, after removal of CO 2 (carbonic acid), the blood becomes alkalized. In this case, the addition of O 2 to deoxyhemoglobin H-Hb forms an acid stronger than carbonic acid. It donates its H + ions to the medium, preventing an increase in pH:
H-Hb + O 2 → → HbO 2 + H +
The work of the hemoglobin buffer is considered inseparably from the bicarbonate buffer:
