Bicarbonate ions diffusion

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 bicarbonate ions diffuse into the plasma Most carbon dioxide in the plasma is transported in the bicarbonate ion form, moving from tissue cells to the lungs. 

In the moderately alkaline regime (I), bicarbonate ion diffuses co-currently with CO2, and carbonate serves as a counterion. At high pH (i.e., extremely low CO2 partial pressures in the steady state), carbonate ion becomes the OO2 carrier, hydroxyl becomes the counter-ion, and this defines regime II. Because... 

Na taken up from the tubule is removed from the epithelial cell into the extraeellular fluid by active transport charge neutrality is maintained because the export of Na is accompanied by export of HC03". The net effect is that for every H formed and secreted into the tubule, a bicarbonate ion diffuses into the extracellular fluid in cmnbination with Na absorbed from the tubule. In effect, bicarbonate is reabsorbed from the glomerular filtrate. 

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). 

We should consider the fate of the bicarbonate ion generated by the dissociation of the H2C03. The bicarbonate diffuses out of the red cell and into the plasma where it too... 

Carbon dioxide is continually produced by cellular aerobic metabolism of glucose and fatty acids. Carbon dioxide diffuses down its concentration gradient from the cell to the blood, which carries it to the lungs. It can interact with water to form carbonic acid (H2C03), a process catalyzed by carbonic anhydrase, an enzyme present in erythrocytes. Carbonic acid can then dissociate to liberate bicarbonate ion (HCOJ) and hydrogen ion (H+) as follows ... 

This process reverses in the lung, where hydrogen ion and bicarbonate ion combine to form carbonic acid, which then breaks down to form water and carbon dioxide, the latter diffusing into the alveolar space down its concentration gradient for removal by ventilation. 

As the concentration of HCO3 (i.e., of metabolic CO2) in red blood cells increases, an imbalance occurs between the bicarbonate ion concentrations in the red blood cell and plasma. This osmotic imbalance causes a marked efflux of HC03 to plasma and consequent influx of Cl from plasma in order to maintain the balance of electrostatic charges. The latter osmotic influx, known as the chloride shift, is accompanied by migration of water to red blood cells. Thus, transport of metabolic CO2 in the blood occurs primarily in the form of plasma bicarbonate formed after CO2 diffuses into red blood cells. 

Hydrogen ions are transported by an indirect mechanism in the upper small intestine. As sodium is absorbed, hydrogen ions are secreted into the gut. Hydrogen ions then combine with bicarbonate ions to form carbonic acid, which then dissociates into carbon dioxide and water. Carbon dioxide readily diffuses into the blood for expiration through the lung. The water remains in the chyme. 

When the venous blood returns to the lungs, the above processes are reversed. The bicarbonate ions now diffuse into the erythrocyte, where they react with hemoglobin to form carbonic acid ... 

The overall process by which parietal cells acidify the stomach lumen is Illustrated in Figure 7-28. In a reaction catalyzed by carbonic anhydrase the excess cytosolic OH combines with CO2 that diffuses in from the blood, forming HCO3 . Catalyzed by the basolateral anion antiporter, this bicarbonate ion is exported across the basolateral membrane (and ultimately into the blood) in exchange for a Cl ion. The Cl ions then exit through Cl channels in the apical membrane, entering the stomach lumen. To preserve electroneutrality, each Cl ion that moves into the stomach lumen across the apical membrane is accompanied by a ion that moves outward through a separate channel. In this way, the excess ions pumped Inward by the H /K ... 

The bicarbonate ion (HCO3A is the second-largest anionic contributor to maintaining acid-base balance, and its secretion from the pancreas helps to neutralize the contents of the small intestine. Respiration controlling the carbon dioxide concentration of the blood (PaCOj) and renal excretion of bicarbonate are the two main homeostatic influences on plasma bicarbonate. Within the renal tubular lumen, carbonic anhydrase converts carbonic acid into carbon dioxide, which diffuses into the epithelial cells and forms carbonic acid, which later dissociates to bicarbonate. 

Carbon dioxide (CO ) is produced by the body s metabohsm at approximately the same rate as consumption, about 3 mL/kg/minute at rest, increasing dramaticahy with heavy exercise. CO diffuses readily from the ceUs into the bloodstream, where it is carried partly as bicarbonate ion (HCO3 ), partly in chemical combination with hemoglobin and plasma proteins, and partly in soln-tion at a partial pressnre of 6 kPa (46 mm Hg) in mixed venons blood. CO is transported to the lung, where it is normaUy exhaled at the same rate at which it is produced, leaving a partial pressure of 5.2 kPa (40 mm Hg) in the alveoh and in arterial blood. An increase in Pco results in a respiratory acidosis and may be due to decreased ventilation or the inhalation of CO, whereas an increase in ventilation results in decreased Pco and a respiratory alkalosis. Since CO is freely diffusible, changes in blood Pco and pH soon are reflected by intraceUular changes in Pco and pH. 

This sequence of reactions illustrates the buffering of the blood by the carbonic acid-bicarbonate conjugate acid-base system. The protons generated at the tissue level by the ionization of carbonic acid are removed at the lungs when bicarbonate ion reenters the red blood cell and reacts with them to produce carbonic acid. H2CO3 then rapidly dissociates, under the influence of carbonic anhydrase, and the resultant CO2 diffuses into the alveolar space because its concentration in the plasma is higher than in the alveoli. 

Bicarbonate ions (HCOs") diffuse from the plasma into the red blood cells. 

The bicarbonate ions are replaced in the plasma by chloride ions that diffuse out of the blood cells. This step, called the chloride shift, maintains charge balance and osmotic pressure relationships between the plasma and red blood cells. 

The carbonic acid ionizes to give hydrogen ions and bicarbonate ions. The hydrogen ions diffuse into the developing urine. 

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