Urinary System 5 PDF

S2 L5 Urinary System 5 Explained by : Fadhloallah najih Edited by : Mohammad Talib Abbood Objectives ❑Regulation of Acid-Base Balance ❑Defense Against Change in [H+] 1. Acid-Base Buffer System 2. Respiratory Center 3. Kidneys ❑Buffer S...

S2 L5 Urinary System 5 Explained by : Fadhloallah najih Edited by : Mohammad Talib Abbood Objectives ❑Regulation of Acid-Base Balance ❑Defense Against Change in [H+] 1. Acid-Base Buffer System 2. Respiratory Center 3. Kidneys ❑Buffer Systems of the Body 1. Proteins Buffer System 2. Phosphate Buffer System 3. Bicarbonate Buffer System ❑Respiratory Regulation of Acid-Base Balance ❑Renal Regulation of Acid-Base Balance Regulation of Acid-Base Balance ❑ Metabolism depends on the functioning of enzymes, and enzymes are very sensitive to pH. ❑ Slight deviations from the normal pH can shut down metabolic pathways as well as alter the structure and function of other macromolecules. ❑ Consequently, acid base balance is one of the most important aspects of homeostasis. Regulation of Acid-Base Balance Regulation acid-base balance means regulation of [H+] in the body fluid. ❑ Only slightly changes in [H+] from the normal value can cause marked alteration in the rates of chemical reactions in the cell. ❑ For this reason the regulation of [H+] is one of the most important aspects of homeostasis. ❑ The symbol pH is used for expressing the [H+]. pH = log 1/[H+] = - log [H+] Note: from the formula (1) that a low pH corresponds to a high [H+], which is called acidosis, and conversely, a high pH corresponds to a low [H+] which is called alkalosis. Defense against change in + [H ] There are three primary systems that regulate the H+ concentration in the body fluids to prevent acidosis or alkalosis: 1. Acid-base buffer system 2. Respiratory center 3. Kidneys 1. Acid-base buffer system: Which are present in all body fluids that immediately combine with any acid or alkali and thereby prevent excessive change in [H+] this system can act within a fraction of a second to prevent excessive changes in [H+]. Defense against change in + [H ] 2. Respiratory center: Upon the changes in [H+] the respiratory center is immediately stimulated to alter the rate of breathing. As a result, the rate of CO2 removal from the body fluids is automatically changed and this causes the [H+] to return toward normal. This mechanism takes 1 to 15 minutes to readjust the [H+] after sudden changes have occurred. 3. Kidneys: When the [H+] changes from normal the kidneys excrete either an acid or alkaline urine thereby also helping readjust the [H +] of the body fluids back to normal. The kidneys provide the most powerful of all the acid-base regulatory systems but requires many minutes to several days to readjust the [H+]. A. The Buffer systems of the body fluids The three major buffer systems of the body fluids are: 1. The protein buffer system 2. The phosphate buffer system 3. The bicarbonate buffer system The kidneys help control acid-base balance by excreting hydrogen ions and generating bicarbonate that helps maintain blood plasma pH within a normal range. A. The Buffer systems of the body fluids 1- Protein buffer system: One of the most important protein buffers is hemoglobin in red blood cells. Hemoglobin is the principal protein inside of red blood cells and accounts for one-third of the mass of the cell. During the conversion of CO 2 into bicarbonate, hydrogen ions liberated in the reaction are buffered by hemoglobin, which is reduced by the dissociation of oxygen. H+ + Hb → HHb This buffering helps maintain normal pH. The process is reversed in the pulmonary capillaries to re-form CO2, which then can diffuse into the air sacs to be exhaled into the atmosphere. A. The Buffer systems of the body fluids 2- Phosphate buffer system: Phosphates are found in the blood in two forms: ❖ Sodium dihydrogen phosphate (Na2H2PO4-), which is a weak acid ❖ Sodium monohydrogen phosphate (Na2HPO42-), which is a weak base. When Na2HPO42- (the weak base) comes into contact with a strong acid, such as HCl, the base picks up a second hydrogen ion to form the weak acid Na2H2PO4- and sodium chloride, NaCl. When Na2H2PO4- (the weak acid) comes into contact with a strong base, such as sodium hydroxide (NaOH), the weak acid reverts back to the weak base and produces water. A. The Buffer systems of the body fluids Acids and bases are still present, but they hold onto the ions. When [H+] increases: HCl + Na2HPO4 → NaH2PO4 + NaCl (strong acid) + (weak base) → (weak acid) + (salt) When [OH¯] is increased: NaOH + NaH2PO4 → Na2HPO4 + H2O (strong base) + (weak acid) → (weak base) + (water) A. The Buffer systems of the body fluids 3- Bicarbonate buffer system: It is the most important buffer system in ECF (ExtraCellular Fluid). As with the phosphate buffer, a weak acid or weak base captures the free ions, and a significant change in pH is prevented. Bicarbonate ions and carbonic acid are present in the blood in a 20:1 ratio if the blood pH is within the normal range. With 20 times more bicarbonate than carbonic acid, this capture system is most efficient at buffering changes that would make the blood more acidic. This is useful because most of the body’s metabolic wastes, such as lactic acid and ketone bodies, are acids. A. The Buffer systems of the body fluids The bicarbonate-carbonic acid buffer works in a fashion similar to phosphate buffers. The bicarbonate is regulated in the blood by sodium, as are the phosphate ions. When sodium bicarbonate (NaHCO 3), comes into contact with a strong acid, such as HCl, carbonic acid (H2CO3), which is a weak acid, and NaCl are formed. When carbonic acid comes into contact with a strong base, such as NaOH, bicarbonate and water are formed. A. The Buffer systems of the body fluids When [H+ ] is increased: HCl + NaHCO3 → NaCl + H2CO3 → H2O + CO2 … (CO2 to be expired by the lungs). (strong acid) + (sodium bicarbonate) → (salt) + (weak acid) While when [OH¯] is increased: NaOH + H2CO3 → H2O + HCO3- … (HCO3- to be excreted by kidneys). (strong base) + (weak acid) → (water) + (bicarbonate) B. Respiratory Regulation of Acid-Base Balance ❑ The second line of defence against acid-base disturbances is control of extracellular fluid CO2 concentration by the lungs. ❑ The respiratory system contributes to the balance of acids and bases in the body by regulating the blood levels of carbonic acid H2CO3. ❑ CO2 is formed continually in the body cells by metabolism, and then it diffuses to the interstitial fluids and blood flow which transports it to the lungs alveoli and then expired by pulmonary ventilation. ❑ CO2 in the blood readily reacts with water to form carbonic acid H 2CO3, and the levels of CO2 and carbonic acid in the blood are in equilibrium. B. Respiratory Regulation of Acid-Base Balance CO2 + H2O H2CO3 H+ + HCO3– When the CO2 level in the blood rises, the excess CO2 reacts with water H2O to form additional carbonic acid H2CO3, lowering blood pH. Increasing the rate and/or depth of respiration allows you to exhale more CO 2. The loss of CO2 from the body reduces blood levels of carbonic acid and thereby adjusts the pH upward, toward normal levels. B. Respiratory Regulation of Acid-Base Balance This process also works in the opposite direction. Excessive deep and rapid breathing (as in hyperventilation) rids the blood of CO2 and reduces the level of carbonic acid H2CO3, making the blood too alkaline. This brief alkalosis can be remedied by rebreathing air that has been exhaled into a paper bag. Rebreathing exhaled air will rapidly bring blood pH down toward normal. B. Respiratory Regulation of Acid-Base Balance B. Respiratory Regulation of Acid-Base Balance ❑ The body regulates the respiratory rate by the use of chemoreceptors, which primarily use CO2 as a signal. ❑ Peripheral blood sensors are found in the walls of the aorta and carotid arteries. These sensors signal the brain to provide immediate adjustments to the respiratory rate if CO2 levels rise or fall. ❑ Yet other sensors are found in the brain itself. ❑ Changes in the pH of CSF affect the respiratory center in the medulla oblongata, which can directly modulate breathing rate to bring the pH back into the normal range. C. Renal Regulation of Acid-Base Balance ❑ The kidneys control acid-base balance by excreting either acidic or basic urine (urine pH ranges from 4.5 to 8.0). ❑ The kidneys regulate extracellular fluid H+ concentration (maintain acid-base balance) through three fundamental mechanisms which are accomplished through the same basic mechanism: 1. Secretion of H+ ions into the urine 2. Reabsorption of filtered HCO3– from the urine back to the blood 3. Production of new HCO3– C. Renal Regulation of Acid-Base Balance ❑ In ↓ECF H+ conc. (alkalosis), the kidneys fail to reabsorb all the filtered bicarbonate, so ↑ bicarbonate excretion which normally buffers hydrogen in the ECF, this loss of bicarbonate is the same as adding an H+ to ECF → ↑H+ conc. back toward normal. ❑ In acidosis, the kidneys do not excrete bicarbonate into the urine but reabsorb all the filtered bicarbonate and produce new bicarbonate, which is added back to ECF→ ↓ ECF H+ conc. back toward normal. C. Renal Regulation of Acid-Base Balance ❑ For each bicarbonate reabsorbed, an H+ must be secreted, but different tubular segments accomplish this task differently. ❑ About 80 to 90 % of the bicarbonate reabsorption (and H+ secretion) occurs in the proximal tubule, (the descending and ascending thin limbs of the loop of Henle, no bicarbonate reabsorption and H+ secretion). ❑ In the thick ascending loop of Henle, another 10 % of the filtered bicarbonate is reabsorbed, and the remainder of the reabsorption takes place in the distal tubule and collecting duct. ❑ The renal tubule secrete H+ into the tubular fluid by sodium-hydrogen counter-transport & by the hydrogen-ATPase mechanism at the intercalated cells. C. Renal Regulation of Acid-Base Balance ❑ Filtered bicarbonate ions by the glomerulus do not reabsorbed directly through the luminal membranes of the renal tubular cells; Instead, HCO 3– is first combines with H+ (secreted by the tubular cells) to form H2CO3, which eventually becomes CO 2 and H2O. ❑ The CO2 can move easily across the tubular membrane & diffuses into the tubular cell, where it recombines with H2O, under the influence of carbonic anhydrase, to generate a new H2CO3 molecule. ❑ This H2CO3 in turn dissociates to form HCO3– and H+ ; the HCO3– then diffuses through the basolateral membrane into the interstitial fluid and is taken up into the peritubular capillary blood. C. Renal Regulation of Acid-Base Balance ❑ The transport of HCO3– across the basolateral membrane is facilitated by two mechanisms: 1. Na+-HCO3– co-transport 2. Cl–-HCO3– exchange. C. Renal Regulation of Acid-Base Balance Note: The partial pressure of carbon dioxide (PCO2) is the measure of carbon dioxide within arterial or venous blood. C. Renal Regulation of Acid-Base Balance Lungs, kidneys & erythrocytes contribute to the maintenance of the acid–base balance. C. Renal Regulation of Acid-Base Balance ❑ The lungs control the gas exchange with the atmospheric air. Carbon dioxide CO2 generated in tissues is transported in plasma as bicarbonate HCO3; ❑ The erythrocyte hemoglobin contributes to CO2 transport. Hemoglobin also buffers the hydrogen ion H+ derived from carbonic acid H2CO3. ❑ The kidneys reabsorb filtered bicarbonate in the proximal tubules and generate new bicarbonate in the distal tubules, where there is a net secretion of hydrogen ion. Hb, hemoglobin.  S2 L5 THANK YOU