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UT Arlington NURS 5315 - Acid Base Balance Transcript

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1 N5315 Advanced Pathophysiology Acid Base Balance Acid Base Balance Introduction Acid base balance is the regulation of hydrogen ion concentration in our bodies. The maintenance of an optimal pH is important for cellular function. A minor alteration of pH can affect cellular function dramatically. For instance, a drop of one pH unit decreases the function of the Na+-K+ pump by half. This can have a profound effect on cellular ion concentration and therefore impair homeostasis. Basic Principles The normal pH ranges from 7.35-7.45. Optimal pH is required to maintain optimal cellular function. An increase in the concentration of hydrogen ions decreases pH and the environment becomes more acidic. A decrease in the concentration of hydrogen ions increases pH and makes the environment more alkalotic. Acids enter the blood from multiple sources continuously and needs to be excreted, hence the need to have mechanisms to maintain the pH balance. Acids are produced in our bodies as a byproduct of cellular metabolism on a daily basis. Carbonic acid (H2CO3) is part of the byproduct of aerobic metabolism. The actual byproduct of aerobic metabolism is carbon dioxide (CO2) which is converted to carbonic acid by the enzyme carbonic anhydrase. Lactic acid is a byproduct of anaerobic metabolism of glucose. Sulfuric acid results from the oxidation of sulfur containing amino acids. Phosphoric acid results from the metabolism of phosphoproteins and ribonucleotides which are used as an energy source. Ketone bodies are an acid and result from the breakdown of fats. This formula represents acid base physiology: CO2 + H20 H2CO3 H+ + HCO3- The left side of the equation represents what occurs in the lungs. The right side of the equation is the process which occurs in the kidneys. A loss of hydrogen ions results in the equation shifting to the right. This means the lungs retain more CO2 in order to convert it into carbonic acid, which will then convert into hydrogen ions and bicarbonate thereby replacing the lost hydrogen ions. A hydrogen ion gain will result in the equation shifting to the left. In this instance the lungs are stimulated to increase ventilation in an effort to expel more CO2. The conversion of CO2 + H20 to H2CO3 and H2CO3 back to CO2 + H2O is completed by the enzyme carbonic anhydrase. Carbonic anhydrase either adds water to CO2 (in other words it hydrates CO2) and makes H2CO3, or it dissociates water from CO2 (in other words it takes water away or dehydrates it) and breaks down H2CO3 to CO2 and H2O. Control Mechanisms of acid base balance The control mechanisms of acid base balance include the chemical buffer systems, the kidneys and the lungs. Chemical buffer systems include bicarbonate, phosphate, plasma proteins and hemoglobin. The kidney’s role in the control of acid base balance is three-parts and includes the reabsorption of filtered bicarbonate, renal excretion of hydrogen, and the excretion of hydrogen as ammonium.2 Bicarbonate (HCO3-/CO2 Buffer) is the most important extracellular buffer. It is the first line of defense against alterations in the acid base balance. Phosphates can be either inorganic or organic. Inorgranic phosphate is an extracellular buffer. The chemical formula is as you see it on your screen (HPO4-2/H2PO4- Buffer). Organic Phosphates operate as intracellular buffers. They include ATP, ADP, AMP, glucose 1-phosphate, and 2,3 diphosphoglycerate (2,3 DPG). Plasma Proteins act as extracellular buffers. Albumin is the main plasma protein and it has a negative charge; therefore, it has a role in buffering H+, which has a positive charge. Albumin also has a role in binding calcium. Albumin binds approximately 40% of calcium. In acidotic states there is an excess of hydrogen ions which causes albumin to bind more hydrogen ions and consequently it binds less calcium ions. This results in the calcium ions being displaced and a higher level of free calcium. In alkalotic states there is an insufficient amount of hydrogen ions available to bind with albumin. As such albumin binds more calcium thus decreasing the amount of free calcium. Hemoglobin is the most important intracellular buffer and plays a pivotal role in acid base homeostasis. After releasing oxygen to the tissues CO2 diffuses into the RBC. Once inside the RBC, CO2 and H2O are combined by carbonic anhydrase and form carbonic acid (H2CO3). Carbonic acid is a weak acid and it immediately dissociates into H+ and HCO3-. H+ is then buffered by hemoglobin. When the RBC reaches the lung, the process reverses and CO2 and H2O are then formed. CO2 then diffuses out of the RBC and is expired by the lungs. Chemical buffers prevent sudden changes in the pH balance. If chemical buffers are unable to maintain the pH, the lungs will begin to compensate within a minute or two. If the lungs are ineffective, the kidneys will begin to compensate in approximately 24 hours. The lungs remove 30Liters of carbonic acid from venous blood on a daily basis. The three roles of the kidney in maintaining acid base balance The first role is the reabsorption of filtered HCO3-. Reabsorption of filtered HCO3- occurs to maintain the buffer. Nearly 100% of filtered bicarbonate is reabsorbed. The majority of filtered HCO3- is reabsorbed in the proximal tubule of the kidney. Much smaller amounts are reabsorbed in the loop of Henle, the distal tubule and the collecting ducts. Let us look at the steps of reabsorption. The picture is a representation of the process. The left side of the picture represents the lumen of the proximal tubule. The bluish purple square in the middle of the picture represents one of the cells which line the lumen of the proximal tubule and the right side of the picture represents the blood. The cells of the proximal tubule that line the lumen act as a bridge between the lumen and the blood. The proximal tubule contains a membrane lining which houses the Na+- H+ exchanger. You can see this on the left hand side of your screen. This exchanger is responsible for the movement of sodium and hydrogen in and out of the luminal cells. The exchanger moves Na+ into the luminal cells and moves H+ into the lumen of the proximal renal tubule. The H+ which was secreted into the lumen now binds with filtered HCO3- (that which was already in the lumen) and forms carbonic acid (H2CO3). The luminal carbonic acid (H2CO3) then is catalyzed by carbonic anhydrase and forms CO2 and H2O.


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