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module 12 - acid-base balance.docx

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Western University
Physiology 2130
Ingrid L.Stefanovic

Module 12 – Acid-Base Balance Intro  Relationship btwn H+ and acidity Why is it Important?  Many chemical rxns that take place in the body and most of the machinery inside all the cells are very sensitive to the presence of H+ bc H+ can alter the shape of proteins that act as enzymes that speed up chemical rxs o Therefore, as a result any change in the concentration of H+ will affect the activity of almost every cell o It is essential that H+ concentration be very carefully regulated Hydrogen Atom and Hydrogen Ion  Atom = single proton (+) and single electron (-)  Ion = single proton (+) What are Acids and Bases?  Acids = any molecule that will release H+ when put in a solution o Ie. HCl will dissociate (break apart) to free H+ and Cl- o The presence of free H+ is what makes a solution acidic  The more H+  more acidic the solution and vice versa  Strong acid will dissociated rapidly and release large amounts of H+ in solution (HCl is a strong acid)  Base = any molecule that will accept a H+ o Ie. HCO3- will bind with H+ to form carbonic acid H2CO3 o Bases lower the concentration of free H+ in solution by combining with H+ o Less free H+  decrease the acidity of the solution or it becomes more basic or alkaline The pH Scale  A way of quantifying the concentration of H+ in any solution  Ph = negative logarithm (to the base 10) of the H+ concentration o Because it is a negative logarithm, the more free H+, the more acidic, the lower the pH and vice versa  A solution with a pH below 7 = acidic (more free H+)  A solution with a pH above 7 = alkaline/basic (less free H+)  The normal pH of body fluids varies slightly btwn 7.35-7.45 and has an average of 7.4 (slightly alkaline) o Arterial blood pH = 7.45 o Venous blood pH = 7.35  Acidosis = body fluids when the pH is below 7.4  Alkalosis = body fluids is above 7.4  When the pH of body fluids is below 6.8 or above 7.8 for long periods of time, death will occur The Source of Acid in the Body  When cells in the body make energy (ATP) they will produce CO2 as a byproduct  This CO2 can with the help of the enzyme carbonic anhydrase, combine with water in RBCs to produce carbonic acid H2CO3  The carbonic acid will dissociate into free H+ and bicarbonate ions HCO3-  In the lungs, the rxn will then revers – carbonic acid will reform, convert to CO2 and H2O and the CO2 will be removed and exhaled  As a result, there is generally no NET increase in free H+ in the plasma bc the carbonic acid reforms into CO2, which is then removed at the lungs  Carbonic acid is known as a volatile acid (interaction of CO2 + H20=H2CO3)  The metabolic breakdown of various proteins will produce a number of acids incl sulphuric, phosphoric, lactic and other acids  Also the stomach is a large source of HCl- acid and therefore = nonvolatile acids o These acids are a significant source of free H+ and are constantly being produced throughout the body o As a result, they must be dealt with in order to maintain a constant pH Regulation of H+ Concentration  Bc of the various sources of acids and the negative effects that these acids can have on cellular rxns, the body must have a way to deal with potential increases in free H+  The body regulates free H+ concentrations by 3 mechanisms: o Buffers in body fluids (incl blood) bind free H+. This system reacts almost immediate to sudden, brief changes in free H+ and consequently in the body’s first line of defense.  Buffers do not directly remove H+ from the body nor do they alter the pH instead they ‘bind up’ free H+, stabilize the pH until balance can be reestablished by the next 2 systems o Respiratory system can regulate H+ conc from volatile acids within seconds-minutes  Volatile acids incl carbonic acid formed when CO2 combines with water o Kidneys which respond more slowly (hours-days) have an extremely powerful control over H+ concentrations, particularly nonvolatile acids Regulation of H+ Conc – Buffers  Buffer = any molecule that can reversibly bind (or release) free H+  Very rapid way of dealing with sudden changes in the pH  Bc buffers bind free H+ and thereby reduce the amount of free H+ in solution, they help to stabilize the pH  General rxn btwn a buffer and free H+: o X (buffer) + H+  XH o With less H+, the pH of the solution is stabilized o Buffers do not prevent the pH from changing, they only help to minimize any pH change until the free H+ can be removed from the body by either the lungs or kidneys  Some buffers incl bicarbonate ions and hemoglobin (Hb)  Free H+ can bind with buffers in both the intracellular and extracellular fluid  Intracellular buffers incl phosphates, intracellular proteins (Hb inside RBCs)  Extracellular buffer incl bicarbonate ion HCO3- (most powerful)  Hb inside RBCs can reversibly bind with free H+ to help stabilize the acidity inside the RBCs  Hb can also bind CO2 to reduce the potential acidity should the CO2 combine with H20 to form carbonic acid H2CO3 (volatile acid) Regulation of H+ Conc – Respiratory System  Main function of respiratory system = remove CO2, keep gas levels relatively constant and help keep the pH of the blood relatively constant  Very rapid acting system that helps maintain pH levels of the blood  Can detect both CO2 levels (PCO2) and drops in pH (increases in H+)  Regulation of CO2 levels in the blood (and thus the pH) involves detection of this gas by central and peripheral chemoreceptors  When CO2 levels increase, both these receptors detect the change and cause an increase in ventilation o The increased ventilation causes more CO2 to be
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