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Lecture 37

BIOL 1119 Lecture 37: Class 37

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Northeastern University
BIOL 1119
Christopher Richardson

Class 37 Acid-Base Balance - Normal pH of ECF: 7.35 to 7.45 - Important part of homeostasis o Cell metabolism depends on enzymes, and enzymes are sensitive to pH - Challenges to acid-base balance o Metabolism products non-volatile acids: lactic acids, phosphoric acids, fatty acids, and ketones o Volatile acid is carbonic acid o pH of a solution is determined solely by concentration of hydrogen ions (protons) - acids: release H+ in solution o strong acids ionize freely, release H+ greatly lower pH, cannot hold onto proton, 100% dissociate (HCl) o weak acids ionize only slightly, hold onto most protons, weak effect on pH (H2CO3) - bases: accepts H+ from solution o strong bases absorb H+ and greatly raise pH (OH-) o weak bases bind less of the available H and has weak effect on pH (HCO3-) - buffers: reversibly bind protons o resist changes in pH ▪ substance that can reversibly bind protons ▪ resist changes in pH by removing or adding protons to the system o physiological buffer ▪ system that control output of acids, bases or CO2 • urinary system buffers greatest quantity, takes hours to days o greatest capacity to buffer o kidneys normally excrete excess H form all acids except carbonic acid • respiratory system buffers limited quantity but within minutes o limited capacity in total o like first responder o chemical buffer system ▪ a substance that binds H+ ad removes from solution as H concentration increases or release H+ and ass into solution as H+ concentration decreases ▪ in general buffer does not eliminate H+ from body or add H to the body: keeps H+ temporarily bound or released until balance restored ▪ consists of a weak acid and weak base ▪ restore normal pH in fractions of seconds ▪ three systems: bicarbonate, phosphate and protein • bicarbonate system: main principle system of blood o solution of carbonic acid and bicarbonate ions ▪ CO 2 H O 2 H C2 3 HCO -3+ H+ o Reversible reaction important in ECF (tissue fluid and blood) ▪ CO 2 H O 2 H 2O 3 -> HCO -3+ H+ • Lowers pH by releasing H and add it to solution o Functions w/ urinary and respiratory systems • Phosphate buffer system 2 o H2PO 4 (phosphoric acid – weak acid) HPO - 4 (phosphate – weak base) + H+ ▪ As in bicarbonate system, reactions that proceed to the right, release H+ and lower pH ▪ Reactions that proceed to the left bind H+ and lower pH o Important in the ICD and renal tubules ▪ Where phosphates are more concentrated ▪ Less important in blood and tissue fluids • Protein buffer system o Protein buffering accounts for most of chemical buffering I body fluids o More concentrated than bicarbonate or phosphate systems especially in ICF o Most body fluid proteins are inside cells or in ICF o Proteins mostly function as buffers in ICF not in blood - Acid-Base and Potassium Imbalances o Acidosis: H+ diffuses into cells and K diffuses out ▪ Slow exchange, 1:1 ▪ H+ buffered by protein in ICF, so net loss of positive ▪ Bc of loss of K+: RMP is more neg than usual: increase polarity across membrane: shuts down volt gated Na channels and Na does not enter easily ▪ Causes membrane hyperpolarization, nerve and muscle cells are hard to stimulate; CNS depression may lead to death ▪ With time, forcing out K from these cells can create slow onset hyperkalemia for other cells: inhibits repolarization for those cells ▪ Slow entry of K on the other cells keeps membrane volt elevated: inhibits repolarization of skel muscle: heart tissue less excitable: elevated volt plateau is maintained for longer period (HR slow and irregular) o Alkalosis: H+ diffuses out of cells and K diffuses in ▪ Inhibits membrane repolarization ▪ w/ time taking K into some cells creates hypokalemia for other cells - disorder of Acid-Base Balances o respiratory acidosis (emphysema): rate of alveolar ventilation falls below CO 2roduction ▪ more CO i2 blood – more H+ in blood: lower ECF pH develops o respiratory alkalosis (hyperventilation): C2 eliminated faster than it is produced ▪ less CO 2n blood, less H+: higher ECF pH develops o metabolic acidosis: increase production of organic acids (lactic acid, ketones seen in starvation and diabetes) ▪ nonvolatile acids increase (not CO b2sed or acid independent of CO 2 ▪ lose of base (chronic diarrhea): loss of bicarbonate ion (HCO -) 3 o metabolic alkalosis (rare): loss of acid (chronic vomiting) ▪ metabolic acidosis or alkalosis is due to non-respiratory gain or loss of protons, H+ is gained or removed w/o initial changes in CO 2evels: before respiratory response - respiratory control of pH o neutralizes 2-3x as much acid as chemical buffers o Resp control of pH is altered by ventilation rate: CO is2constantly produced by aerobic mechanism: eliminated by lungs at equivalent rate o Coordinates with bicarbonate system 1. Ventilation increases: if pH is falling and rising CO i2 blood: CO 2expired) + H O 2- H CO 2 3
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