Acid-Base Balance PDF

Acid-Base Regulation  Acid-Base Balance Definition: Acid-base balance refers to the mechanisms the body uses to keep its fluids close to neutral pH (that is, neither basic nor acidic) so that the body can function normally.  Acid-Base Regulation Regulation of hydrogen ion (H+...

Acid-Base Regulation  Acid-Base Balance Definition: Acid-base balance refers to the mechanisms the body uses to keep its fluids close to neutral pH (that is, neither basic nor acidic) so that the body can function normally.  Acid-Base Regulation Regulation of hydrogen ion (H+ balance) is similar in some ways to regulation of other ions in the body. There must be a balance between the intake or production of H+ and net removal of H+ from the body to achieve homeostasis. H+ removal from the body is regulated by the kidney.  Acid-Base Regulation Multiple acid-base buffering mechanisms involving the blood, cells, and lungs also are essential in maintaining normal H+ concentrations in both the extracellular and intracellular fluid.  Alkalosis & Acidosis The term alkalosis refers to excess removal of H+ from the body fluids, in contrast to the excess addition of H+, which is referred to as acidosis.  Acids A hydrogen ion is a single free proton released from a hydrogen atom. Molecules containing hydrogen atoms can release hydrogen ions in solutions are referred to as acids. An example is hydrochloric acid (HCl), which ionizes in water to form hydrogen ions (H +) and chloride ions (Cl−). Likewise, carbonic acid (H2CO3) ionizes in water to form H+ and bicarbonate ions (HCO3 − ).  Bases The proteins in the body also function as bases because some of the amino acids that make up proteins have net negative charges that readily accept H+. The protein haemoglobin in the red blood cells and proteins in the other cells of the body are among the most important of the body’s bases.  Bases A base is an ion or a molecule that can accept an H+. For example, HCO3− is a base because it can combine with H+ to form H2CO3. Likewise, HPO4_ is a base because it can accept an H+ to form H2PO4.  Strong and Weak Acids A strong acid is one that rapidly dissociates and releases especially large amounts of H+ in solution like HCl. Weak acids are less likely to dissociate their ions and, therefore, release H+ with less vigor like H2CO3.  Strong and Weak Bases A strong base is one that reacts rapidly and strongly with H+ and, therefore, quickly removes H+ from a solution. A typical example is OH −, which reacts with H+ to form water (H2O). A typical weak base is HCO − because it 3 binds with H much more weakly than + does OH −. Most acids and bases in the extracellular fluid that are involved in normal acid-base regulation are weak acids and bases are(H2CO3) and HCO3 − base.  Normal H+ Concentration and pH of Body Fluids Blood H+ concentration is normally maintained within tight limits around a normal value of about 0.00004 mEq/L (40 nEq/L). Normal variations are only about 3 to 5 nEq/L. The H+ concentration can vary from as low as 10 nEq/L to as high as 160 nEq/L without causing death.  Normal H+ Concentration and pH of Body Fluids H+ concentration normally is low, and because these small numbers are cumbersome, it is customary to express H + concentration on a logarithm scale, using pH units. pH is related to the actual H+ concentration by the following formula (H+ concentration [H+] is expressed in equivalents per liter).  Normal H+ Concentration and pH of Body Fluids For example, normal [H +] is 40 nEq/L (0.00000004 Eq/L). Therefore, the normal pH is  Normal H+ Concentration and pH of Body Fluids From this formula, one can see that pH is inversely related to the H+ concentration. Therefore, a low pH corresponds to a high H+ concentration and a high pH corresponds to a low H+ concentration.  Normal H+ Concentration and pH of Body Fluids The normal pH of arterial blood is 7.4. The pH of venous blood and interstitial fluids is about 7.35. The normal pH of arterial blood is 7.4, a person is considered to have acidosis when the pH falls below this value and alkalosis when the pH rises above 7.4. The lower limit of pH at which a person can live more than a few hours is about 6.8, and the upper limit is about 8.0.  Normal H+ Concentration and pH of Body Fluids Intracellular pH usually is slightly lower than plasma pH because the metabolism of the cells produces acid, especially H2CO3. Depending on the type of cells, the pH of intracellular fluid has been estimated to range between 6.0 and 7.4. Hypoxia of the tissues and poor blood flow to the tissues can cause acid accumulation and decreased intracellular pH.  Normal H+ Concentration and pH of Body Fluids The pH of urine can range from 4.5 to 8.0, depending on the acid-base status of the extracellular fluid. The kidneys play a major role in correcting abnormalities of extracellular fluid H+ concentration by excreting acids or bases at variable rates. pH and H+ Concentration + of Body Fluids  DEFENDING AGAINST CHANGES IN H+ CONCENTRATION Three primary systems regulate the H+ concentration in the body fluids to prevent acidosis or alkalosis: 1. The chemical acid-base buffer systems of the body fluids, which immediately combine with an acid or a base to prevent excessive changes in H+ concentration. 2. The respiratory center, which regulates the removal of CO2 (and, therefore, H2CO3) from the extracellular fluid. 3. The kidneys, which can excrete either acid or alkaline urine, thereby readjusting the extracellular fluid H+ concentration toward normal during acidosis or alkalosis.  Buffering System When there is a change in H+ concentration, the buffering systems of the body fluids react within seconds to minimize these changes. Buffer systems do not eliminate H+ from or add H+ to the body but only keep them tied up until balance can be re-established.  The respiratory system The second line of defense, the respiratory system, acts within a few minutes to eliminate CO2 and, therefore,H2CO3 from the body.  Kidneys system These first two lines of defense keep the H+ concentration from changing too much until the more slowly responding third line of defense, the kidneys, can eliminate the excess acid or base from the body. Although the kidneys are relatively slow to respond compared with the other defenses, over a period of hours to several days, they are by far the most powerful of the acid-base regulatory systems.  BUFFERING OF H+ IN THE BODY FLUIDS A buffer is any substance that can reversibly bind H+. The general form of the buffering reaction is  BUFFERING OF H+ IN THE BODY FLUIDS + In this example, a free H + combines with the buffer to form a weak acid (H buffer) that can either remain as an unassociated molecule or dissociate back to the buffer and H+. When the H+ concentration increases, the reaction is forced to the right and more H+ binds to the buffer, as long as buffer is available. Conversely, when the H+ concentration decreases, the reaction shifts toward the left and H+ is released from the buffer. In this way, changes in H+ concentration are minimized.  BUFFERING OF H+ IN THE BODY FLUIDS + The importance of the body fluid buffers can be quickly realized if one considers the low concentration of H+ in the body fluids and the relatively large amounts of acids produced by the body each day. For example, about 80 milliequivalents of H + is either ingested or produced each day by metabolism, whereas the H+ concentration of the body fluids normally is only about 0.00004 mEq/L. Without buffering, the daily production and ingestion of acids would cause lethal changes in body fluid H+ concentration.  BICARBONATE BUFFER SYSTEM The bicarbonate buffer system consists of a water solution that contains two ingredients: 1. A weak acid, H2CO3. 2. A bicarbonate salt, such as sodium bicarbonate (NaHCO3).  BICARBONATE BUFFER SYSTEM H2CO3 is formed in the body by the reaction of CO2 with H2O.  BICARBONATE BUFFER SYSTEM This reaction is slow, and exceedingly small amounts of H2CO3 are formed unless the enzyme carbonic anhydrase is present. This enzyme is especially abundant in the walls of the lung alveoli, where CO2 is released; carbonic anhydrase is also present in the epithelial cells of the renal tubules, where CO2 reacts with H2O to form H2CO3.  BICARBONATE BUFFER SYSTEM H2CO3 ionizes weakly to form small amounts of H+ and HCO3−.  BICARBONATE BUFFER SYSTEM The second component of the system, bicarbonate salt, occurs predominantly as NaHCO3 in the extracellular fluid. NaHCO3 ionizes almost completely to form HCO3 − and Na +, as follows:  BICARBONATE BUFFER SYSTEM Because of the weak dissociation of H2C03, the H+ concentration is extremely small. When a strong acid such as HCl is added to the bicarbonate buffer solution, the increased H released from + the acid (HCl → H++ Cl-) is buffered by HCO-3.  BICARBONATE BUFFER SYSTEM As a result, more H2CO3 is formed, causing increased CO2 and H2O production. From these reactions, one can see that H+ from the strong acid HCl reacts with HCO3− to form the very weak acid H2CO3, which in turn forms CO2 and H2O. The excess CO2 greatly stimulates respiration, which eliminates the CO2 from the extracellular fluid.  BICARBONATE BUFFER SYSTEM The opposite reactions take place when a strong base, such as sodium hydroxide (NaOH), is added to the bicarbonate buffer solution.  BICARBONATE BUFFER SYSTEM In this case, the OH− from the NaOH combines with H2CO3 to form additional HCO3−. This, the weak base NaHCO3 replaces the strong base NaOH. At the same time, the concentration of H2CO3 decreases (because it reacts with NaOH), causing more CO2 to combine with H2O to replace the H2CO3. BICARBONATE BUFFER SYSTEM  BICARBONATE BUFFER SYSTEM The net result, therefore, is a tendency for the CO2 levels in the blood to decrease, but the decreased CO2 in the blood inhibits respiration and decreases the rate of CO2 expiration. The rise in blood HCO3− that occurs is compensated for by increased renal excretion of HCO3 −.  RESPIRATORY REGULATION OF ACID-BASE BALANCE The second line of defense against acid- base disturbances is control of extracellular fluid CO2 concentration by the lungs. An increase in ventilation eliminates CO2 from extracellular fluid, which, by mass action, reduces the H + concentration. Conversely, decreased ventilation increases CO2, thus also increasing H + concentration in the extracellular fluid.