Bicarbonate carbonic acid dissociation

Carbonic acid dissociates to produce bicarbonate and hydrogen ions... 

Carbon dioxide reacts with water to form carbonic acid, H CX) i. ITiis reaction is accelerated by carbonic anhydnise, an enzyme abundant in red blood cells that will be considered extensively in Chapter 9. Carbonic acid is a strong acid with a pK, of 3.5. Thus, once formed, carbonic acid dissociates to form bicarbonate ion, HCO.Ct cind H, resulting in a drop in pH. This drop in pH stabilizes the T state by the mechanism discussed previously. 

Because bicarbonate is a small ion, it is freely filtered at the glomerulus. The bicarbonate load delivered to the nephron is approximately 4,500 mEq/day. To maintain acid-base balance, this entire filtered load must be reabsorbed. Bicarbonate reabsorption occurs primarily in the proximal tubule (Fig. 51-1). In the mbular lumen, filtered bicarbonate combines with hydrogen ion secreted by the apical Na+-H+-exchanger to form carbonic acid. The carbonic acid is rapidly broken down to CO2 and water by carbonic anhydrase located on the luminal surface of the brush border membrane. The CO2 then diffuses into the proximal tubular cell, where it reforms carbonic acid in the presence of intracellular carbonic anhydrase. The carbonic acid dissociates to form hydrogen ion, that can again be secreted into the tubular lumen, and bicarbonate that exits the cell across the basolateral membrane and enters the peritubular capillary. 

The carbonic acid dissociates, releasing hydrogen ions, which are buffered by nonbicarbonate buffers (i.e., proteins, phosphate, and hemoglobin) and bicarbonate. Thus on the basis of physicochemical factors, increases in PaC02 raise the serum bicarbonate concentration. In general, in acute respiratory acidosis, the bicarbonate concentration increases by 1 mEq/L above 24 for each 10-mm Hg increase in PaC02 above 40 (see Table 51 ). 

Carbonic acid dissociates to give hydrogen ions and bicarbonate ions. 

These reactions describe the dissolution of atmospheric carbon dioxide (CO2) in water (reaction 1) and the subsequent reaction forming carbonic acid (2). Once in solution, carbonic acid dissociates forming bicarbonate (HCO3-) and a hydrogen ion (H+ reaction 3). Subsequently, this bicarbonate ion may dissociate to form a carbonate ion and a further hydrogen ion (4). Concurrently, calcium hydroxide (Ca(OH)2) dissolves in water forming calcium and hydroxyl ions (OH- 5). 

The enzyme carbonic anhydrase promotes the hydration of COg. Many of the protons formed upon ionization of carbonic acid are picked up by Hb as Og dissociates. The bicarbonate ions are transported with the blood back to the lungs. When Hb becomes oxygenated again in the lungs, H is released and reacts with HCO3 to re-form HgCOj, from which COg is liberated. The COg is then exhaled as a gas. 

Hemoglobin carbamates account for about 15% of the CO2 in venous blood. Much of the remaining COj is carried as bicarbonate, which is formed in erythrocytes by the hydration of COj to carbonic acid (H2CO3), a process catalyzed by carbonic anhydrase. At the pH of venous blood, HjCOj dissociates into bicarbonate and a proton. 

Figure 6-9. The Bohr effect. Carbon dioxide generated in peripheral tissues combines with water to form carbonic acid, which dissociates into protons and bicarbonate ions. Deoxyhemoglobin acts as a buffer by binding protons and delivering them to the lungs. In the lungs, the uptake of oxygen by hemoglobin releases protons that combine with bicarbonate ion, forming carbonic acid, which when dehydrated by carbonic anhydrase becomes carbon dioxide, which then is exhaled.

Acid-base reactions of buffers act either to add or to remove hydrogen ions to or from the solution so as to maintain a nearly constant equilibrium concentration of H+. For example, carbon dioxide acts as a buffer when it dissolves in water to form carbonic acid, which dissociates to carbonate and bicarbonate ions ... 

The carbon dioxide produced during cellular metabolism diffuses out of the cells and into the plasma. It then continues to diffuse down its concentration gradient into the red blood cells. Within these cells, the enzyme carbonic anhydrase (CA) facilitates combination of carbon dioxide and water to form carbonic acid (H2C03). The carbonic acid then dissociates into hydrogen ion (H+) and bicarbonate ion (HC03). 

This entire reaction is reversed when the blood reaches the lungs. Because carbon dioxide is eliminated by ventilation, the reaction is pulled to the left. Bicarbonate ions diffuse back into the red blood cells. The hemoglobin releases the hydrogen ions and is now available to load up with oxygen. The bicarbonate ions combine with the hydrogen ions to form carbonic acid, which then dissociates into carbon dioxide and water. The carbon dioxide diffuses down its concentration gradient from the blood into the alveoli and is exhaled. A summary of the three mechanisms by which carbon dioxide is transported in the blood is illustrated in Figure 17.8. 

The formation and degradation of planktonic POC and PIC influence pH and C.A. as follows. The remineralization of POC produces CO2, which is rapidly hydrolyzed to carbonic acid, bicarbonate, and carbonate via the reactions given in Eqs. 5.53 through 5.57. Carbonic acid and bicarbonate are both weak acids, so their dissociation generates H. This acid enhances the dissolution of PIC through the following reaction ... 

Pharmacokinetics Sodium bicarbonate in water dissociates to provide sodium and bicarbonate ions. Sodium is the principal cation of extracellular fluid. Bicarbonate is a normal constituent of body fluids and normal plasma level ranges from 24 to 31 mEq/L. Plasma concentration is regulated by the kidney. Bicarbonate anion is considered labile because, at a proper concentration of hydrogen ion, it may be converted to carbonic acid, then to its volatile form, carbon dioxide, excreted by lungs. Normally, a ratio of 1 20 (carbonic acid bicarbonate) is present in extracellular fluid. In a healthy adult with normal kidney function, almost all the glomerular filtered bicarbonate ion is reabsorbed less than 1% is excreted in urine. 

Carbonic acid, H2CO3, is a weak acid that dissociates into a proton and the bicarbonate anion, HCOg" (Figure 1-2). 

After administration, sodium bicarbonate dissociates to sodium and bicarbonate ions. With increased hydrogen ion concentrations bicarbonate ions combine with hydrogen ions to form carbonic acid, which then dissociates to COj, which is excreted by the lungs. 

One of the most important buffer systems in the human body is that which keeps the pH of blood around 7.4. If the pH of blood fall below 6.8 or above 7.8, critical problems and even death can occur. There are three primary buffer systems at work in controlling the pH of blood carbonate, phosphate, and proteins. The primary buffer system in the blood involves carbonic acid, H COj and its conjugate base bicarbonate, HCO3. Carbonic acid is a weak acid that dissociates according to the following reaction ... 

Carbonic anhydrase influences the tubular reabsorption of sodium in proximal tubule where biocarbonate absorption occurs and in the distal tubule where sodium is exchanged for potassium or hydrogen ion and bicarbonate is formed as the accompanying anion. The hydration of carbon dioxide takes place under the influence of enzyme carbonic anhydrase which forms carbonic acid which dissociates and breaks into hydrogen and carbonate ions. 

Explain why the equality of the hydrogen ion and hydroxyl ion concentrations is violated when certain salts are dissolved in water. Compare the values of the dissociation constants of water, acetic acid, carbonic acid, the bicarbonate ion, and aluminium hydroxide. How can the hydrolysis process be explained from the viewpoint of the law of mass action In what cases is hydrolysis reversible and in what cases does it proceed virtually to the end ... 

Carbonic acid in the blood readily dissociates into hydrogen and bicarbonate ions. If the serum pH of... 

Carbon dioxide gas dissolves readily in water and is spontaneously hydrated to form carbonic acid, which rapidly dissociates to a proton and a bicarbonate ion ... 

Although C02 does not contain hydrogen ions it rapidly reacts with water to form carbonic acid (H2C03), which further dissociates into hydrogen and bicarbonate ions (HC03 ). This reaction is shown below ... 

Filtered bicarbonate combines with secreted hydrogen ions forming carbonic acid. Carbonic acid then dissociates to form C02 and water. This reaction is catalysed by carbonic anhydrase, which is present in the brush border of the renal tubular cells. This C02 readily crosses into the tubular cell down a concentration gradient. 

Inside the cell the C02 recombines with water, again under the influence of carbonic anhydrase, to form carbonic acid. The carbonic acid further dissociates to bicarbonate and hydrogen ions. The bicarbonate passes back into the blood stream whilst the H+ passes back into the tubular fluid in exchange for sodium. In this way, virtually all the filtered bicarbonate is reabsorbed in the healthy individual. 

Bicarbonate is much more suitable as a major environmental water pH buffer. Its parent acid is H2C03, which is diprotic (Fig. 1.11). The dissociation of carbonic acid is described as follows ... 

The concentrations of the various forms of C02 present in an aqueous phase are temperature dependent and extremely sensitive to pH (the concentrations also depend on the presence of other solutes, which presumably is a small effect for the cell wall water). For instance, the equilibrium concentration of C02 dissolved in water divided by that of C02 in an adjacent gas phase, Cc 7cccv decreases more than two-fold from 10°C to 40°C (Table 8-3 the decreasing solubility of C02 as the temperature increases is a characteristic of dissolved gases, which fit into the interstices of water, such space becoming less available as molecular motion increases with increasing temperature). This partition coefficient is not very pH dependent, but the equilibrium concentration of HCO3- in water relative to that of dissolved C02 is markedly affected by pH. In particular, C02 dissolved in an aqueous solution can interact with water to form carbonic acid, which then dissociates to form bicarbonate ... 

A shift in community composition may also be important to the biological pump if it is from a calcareous to a noncalcareous phytoplankton, as precipitation of CaCOs diminishes the ocean s ability to hold dissolved CO2. DIC (XCO2) is present in seawater as several species, dissolved CO2, carbonic acid, and the dissociated forms, bicarbonate ion, and carbonate ion ... 

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