Advanced Physiology (part 3, Acid-base balance) فیزیولوژی تکمیلی Advanced Physiology (part 3, Acid-base balance) By: A. Riasi (PhD in Animal Nutrition & Physiology) Isfahan University of Technology (IUT)

Introduction Three aspects of the ECF and ICF that are crucial to the whole animal and to its individual cells: The osmotic balance Total fluid volume of the body Acid –base status The osmotic balance is determined primarily by the total concentration of major solutes. Total fluid volume of the body directly affects circulatory pressure Acid-base balance of body fluid may alters by hydrogen ions (measured as pH), This is crucial to the entire organism because protein structure and function is typically highly dependent on the concentration of hydrogen ions around them.

Introduction The term of acid-base balance refers to the precise regulation of free hydrogen ion (H+) and hydronium ion (OH-) concentration in the body fluids. Acids are a special group of hydrogen-containing substances. A strong acid has a greater tendency to dissociated in solution than does a weak acid. Acids are a special group of hydrogen-containing substances that dissociate when in solution to liberate free H+ and anions. Many other substances (for example carbohydrates) also contain hydrogen, but they are not classified as acids. Because the hydrogen is tightly bound within their molecular structure and is never liberated as free H+.

Acid-base balance in body fluids َAdapted from Animal Physiology by Sherwood et al. 2005

Acid-base balance in body fluids The normal pH of arterial and venous blood Acidosis exists whenever the blood pH falls below 7.35 Alkalosis occurs when the blood pH is above 7.45 Death can occur if arterial pH falls outside of the range of 6.8-8.0 The pH of venous blood is slightly lower than that of arterial blood (7.35 vs 7.45) because of H+ generated by the formation of H2CO3 from CO2 picked up at the tissue capillaries. Alkalosis occurs when the blood pH is above 7.45. Note that the reference point for determining the body’s acid-base status is not neutral pH (6.81 at 37◦C), but the normal plasma pH of 7.4. Thus a plasma pH of 7.2 is considered acidotic even though in chemistry a pH of 7.2 is considered basic.

Acid-base balance in body fluids َAdapted from Animal Physiology by Sherwood et al. 2005

Acid-base balance in body fluids Fluctuations in hydrogen ion concentration have profound effect on body chemistry. Even small changes in [H+] have dramatic effect on proteins. The most prominent whole body consequences of fluctuations in [H+] are changes in excitability of nerves and muscle cells. Even slight deviations in [H+] alter the shape and activity of protein molecules, for example, respiratory pigments do not deliver oxygen properly. Also because enzymes are proteins, a shift in the body’s acid-base balance disturbs the normal pattern of metabolic activity catalyzed by these enzymes. Some cellular chemical reactions are accelerated; others are depressed. Increased [H+] (acidosis) depress the central nervous system. In contrast, the major effect of alkalosis is overexcitability of the nervous system, first the peripheral and later the central nervous system.

Acid-base balance in body fluids Hydrogen ions are continually being added to body fluids Metabolic activities are main source for H+ in the body fluids Normally, H+ is continually being added to the body fluids by: Carbonic acid formation Inorganic acid produced during the breakdown of nutrients Organic acid resulting from intermediary metabolism Hydrogen ions are continually being added to body fluids as a result of metabolic activities. The input of hydrogen ions must be balanced an equal output. Cellular oxidation of nutrients yields energy, with CO2 and H2O. CO2 and H2O spontaneously react to form H2CO3. H2CO3 mostly dissociated to liberate free H+ and HCO3-.

Acid-base balance in body fluids CO2 + H2O H2CO3 H+ + HCO3- In some cell types such as erythrocytes, the reactions rapidly catalyzed by the enzyme carbonic anhydrase (CA). Although in an indirect way the change in OH- intern causes a change in H+ formation from water. These reactions are rapidly reversible and proceeding in either direction depending on the concentrations of the substances involved as dictated by the low mass action. Within vertebrate systematic capillaries, the CO2 level in the blood increases as metabolically produced CO2 enter from the tissues. This drive the reaction to the acid side. In the lungs the reaction is reversed: CO2 diffuses from the blood to the environment. The resultant reduction in CO2 in the blood drives the reaction toward the CO2 side. CO2 + OH- HCO3- CA H2O H+ + OH-

Acid-base balance in body fluids Sulfuric acid and phosphoric acid are produced in the body. Fatty acids and lactic acid that are produced during intermediary metabolism partially dissociate to yield free H+. In certain disease additional acids may be produced. Dietary protein and other ingested nutrient molecules contain sulfur and phosphorus. When these molecules are breakdown sulfuric and phosphoric acids are produced as by-products. In diabetes mellitus large quantities of keto acids may be produced as a result of abnormal fat metabolism.

Acid-base balance in body fluids Three line of defense against changes in [H+]: Chemical buffer systems Respiratory mechanisms of pH control Excretory mechanisms of pH control

pH Regulation: Buffers There are four buffer system in the vertebrate body: CO2-HCO3- buffer system The peptide and protein buffer system The hemoglobin buffer system The phosphate buffer system Note: A buffer system cannot buffer itself.

pH Regulation: Buffers The Co2-HCO3- buffer system in the ECF. An important example a buffer system in the body is carbon dioxide-bicarbonate. It is very effective ECF buffer system, for two reasons: 1- HCO3- is abundant in the ECF. 2- More importantly, key components of this buffer pair are closely regulated. The kidney regulate HCO3-, and the respiratory system regulates CO2, which generates H2CO3. Because of this relationship, not only do both the kidneys and respiratory organs normally participate in pH control but also, renal or respiratory dysfunction can induce acid-base imbalances by altering the [HCO3-]/[CO2] ratio. َAdapted from Animal Physiology by Sherwood et al. 2005

pH Regulation: Buffers The most plentiful buffers on the ICF are the cell proteins. The most important buffering amino acid is histidine. Hemoglobin (Hb) in erythrocytes buffers the H+ generated. The phosphate buffer system consists of an acid phosphate slat (NaH2PO4) and a basic phosphate salt (Na2HPO4). The phosphate system serve as an excellent urinary buffer. Proteins are excellent buffers because their contain both acidic and basic groups that can bind or release H+. The most important buffering amino acid is histidine. Because it is the only one with a pK close to 7. In addition, muscles of some birds and mammals have large concentrations of dipeptides containing histidine. The phosphate buffer system consists of an acid phosphate slat (NaH2PO4) that can donate a free H+ when [H+] falls and a basic phosphate salt (Na2HPO4) that can accept a free H+ when the [H+] rise. Even though the phosphate pair is a good buffer, its concentration in the ECF is rather low, so it is not very important as an ECF buffer. Because phosphates are more abundant within the cells, this system contributes significantly to intracellular buffering.

pH Regulation: Respiration The respiratory system plays an important role in acid-base balance through its ability to alter ventilation When arterial [H+] increases, the respiratory center in the brain stem is reflexly stimulated to increase ventilation. Conversely when the arterial [H+] falls, ventilation is reduced.

pH Regulation: Excretion The excretory organs are the third line of defense against changes in [H+] in body fluids. In mammals, the kidneys are the most potent acid-base regulatory mechanism. The kidneys can remove of H+ from any source The kidneys can variably conserve or eliminate HCO3- The excretory systems contribute powerfully to control of acid-base balance by controlling both hydrogen ion and bicarbonate concentrations in the ECF. The excretory organs require hours to days to compensate for changes in body fluid pH, compared to the immediate responses of the buffer systems and the few minutes of delay before respiratory system responds. During renal compensation for acidosis for each H+ excreted in the urine a new HCO3- is added to the plasma to buffer, by means of CO2-HCO3- system.

pH Regulation: Excretion The kidneys control the pH of the body fluids by adjusting three interrelated factors: H+ excretion H2CO3- excretion Ammonia secretion

pH Regulation: Excretion Hydrogen ion excretion by the kidneys The kidneys eliminate H+ derived from sulfuric, phosphoric, lactic, and other acids. Although lungs can adjust H+ by eliminating CO2, However the task of eliminating H+ derived from sulfuric, phosphoric, lactic, and other acids rest with the kidneys. Almost all the excreted H+ enters the urine by means of secretion and for this reason urine is usually acidic, having an average pH of 6.0. No only do the kidneys continuously eliminate the normal amount of H+ that is constantly being produced from non-CO2 sources, but they can also alter their rate of H+ secretion to compensate for changes in [H+] arising from abnormalities in the concentration of CO2.

pH Regulation: Excretion The magnitude of H+ secretion depends on a direct effect of the plasma’s acid-base status on the kidneys’ tubular cells. No neural or hormonal control is involved. When the [H+] of the plasma passing through the peritubular capillaries is elevated above normal, the tubular cells respond by secreting greater than usual amounts of H+ from the plasma into the tubular fluid to be excreted in the urine. Actually the kidneys cannot raise plasma [H+] by reabsorbing more of the filtered H+. َAdapted from Animal Physiology by Sherwood et al. 2005

pH Regulation: Excretion The H+ secretary process begins in the tubular cells with CO2 that has come from three sources: The CO2 diffused from plasma The CO2 diffused from the tubular fluid CO2 that has been metabolically produced within the tubular cells. Under the influence of carbonic anhydrase, CO2 and H2O increase H+ and HCO3-. An energy dependent carrier in the luminal membrane then transports H+ out of the cell into the tubular lumen. In part of the nephron, the carrier is a Na+-H+ exchanger. Thus H+ secretion and Na+ reabsorption are partially linked.

pH Regulation: Excretion The kidneys adjust H+ excretion to compensate for changes in both carbonic and noncarbonic acids. The kidneys regulate plasma [HCO3-] by two mechanisms: Variable reabsorption of the filtered HCO3- back to the plasma. Variable addition of new HCO3- to the plasma.

pH Regulation: Excretion Hydrogen ion secretion coupled with bicarbonate reabsorption (mammalian kidney). Because the disappearance of a filtered HCO3- from the tubular fluid is coupled with the appearance of another HCO3- in the plasma, HCO3- is considered to have been reabsorbed. َAdapted from Animal Physiology by Sherwood et al. 2005

pH Regulation: Excretion Hydrogen ion secretion and excretion coupled with the addition of new HCO3- to the plasma. (mammalian kidney). Secreted H+ does not combine with filtered HPO42- and is not subsequently excreted until all the filtered HCO3- has been reabsorbed. Once all the filtered HCO3- has combined with secreted H+, further secreted H+ is excreted in the urine, primarily in associated with urinary buffers such as basic phosphate. Excretion of H+ is coupled with the appearance of new HCO3- in the plasma. The new HCO3- represents a net gain rather than being merely a replacement for filtered HCO3-. َAdapted from Animal Physiology by Sherwood et al. 2005