topic 9: acid base balance - ex phys

normal levels

arterial blood pH: 6.7-7.5 (neutral)

• acid 0 (drop, acidosis) — increase in H+ and accumulation of acids

• alkaline: 14 (rise, alkalosis) — decrease in H+ and accumulation of bases

muscle pH: 7.10

H+ sources

1) production of CO2 in skeletal muscle (aerobic metabolism)

2) production of lactic acid/lactate in skeletal muscle

3) ATP breakdown (ATP hydrolysis)

CO2 production

CO2 + H2O →← H2CO3 →← H+ + HCO3-

• aerobic metabolism of glucose makes ATP and produces CO2 in skeletal muscle

• CO2 reacts with water → bicarbonate → H+ and bicarbonate

lactic acid

during high intensity exercise, anaerobic glycolysis rapidly forms ATP

• lactic acid →← H+ + lactate

• glycolysis forms pyruvate then lactic acid and instantly converted into lactate

• can be cleared from blood quickly so theres not as much H+ released

• lactate is a fuel source: not the main reason why H+ accumulates

atp breakdown

ATP + H2O →← ADP + HPO4- + H+

• the number one source of H+ accumulation during hard exercise

• reaction is required for muscle contraction — for actin-myosin to form crossbridge

• harder exercise = harder contraction = more ATP = more H+

  • more motor units recruited

acid base regulation

importance:

heavy exercise can result in large production of H+

• sports lasting ≥45 sec can produce significant amounts (ex: 400 m run, repeated sprints)

increased H+ within skeletal muscle can impair performance

impair performance

increased H+ within skeletal muscle can ______ ________

1) inhibit enzymes in aerobic and anaerobic ATP production — ex: decreased PFK activity → slows glycolysis → less ATP → fatigue and less sustainable

2) hinder muscle contractile process by competing with Ca2+ for binding sites on troponin

• H instead of ca2+ → troponin → doesn’t move tropomyosin from actin binding sites → reduced contraction

3) H+ impede myosin and actin crossbridge formation

buffers

molecules in blood and muscle that sequester (take up/bind/hold on) H+ to keep pH from dropping during exercise

• heavy exercise → H+ increases = take extra H+

• pH high → H+ decreases = release H+

• purpose is to transport acids from muscles to the lungs (exhaled) or kidneys (excreted)

  • H+ to CO2 and exhaled

  • excess acid excreted in urine

• bicarbonate is the #1 mechanism that directly sequesters H+ — 64%

  • H+ + HCO3- →← H2CO3 (carbonic acid) →← CO2 + H2O to the lungs

  • others: Hb, protein, phosphates

low pH

respiratory influence on ___ __ (short term influence)

• HCO3- + H+ → H2CO3 → CO2 + H2O

• convert FROM H+ to raise to normal levels

• ex: intense exercise

• arterial PCO2 increases → drops pH (high H+) → stims carotid bodies → stims RCC → respiratory muscles increase VE → decease PCO2

acidosis → stims brain and arterial receptors → increases respiration rate → decrease blood CO2 (alveoli) → decrease blood carbonic acid → increase pH with less H+

high pH

respiratory influence on ___ __ (short term influence)

• CO2 + H2O → H2CO3 → HCO3- + H+

• convert TO H+ to lower to normal levels

• ex: dehydration, vomiting, rapid breathing

alkalosis → stims brain and arterial receptors → decrease respiration rate → increase blood CO2 (alveoli) → increase blood carbonic acid → decrease pH with more H+

kidney influence

reabsorb bicarbonate into the blood to make sure it is not excreted so it can buffer hydrogen atoms (long term influence)

blood pH decreases → reduced rate of bicarbonate excretion

• reabsorb HCO3- back in blood

• secrete H+ in urine, and generate new HCO3-

• also form H2CO3 to be broken down into CO2 to be exhaled

blood pH increases → increased rate of bicarbonate excretion

• less HCO3- in blood and more H+ stays in blood

• maintaining H+

other buffers

Hb inside RBCs: H+ binds to Hb and reduces affinity for O2

• Hb has preference for H+ → results in no oxygen on Hb (deoxyhemoglobin)

proteins: amino acids contain charge group with a negative charge that attracts H+

• ex: phosphocreatine

phosphates: accepts H+ and can weaken an acid

• ex: sodium phosphate

during exercise

low-mod intensity keeps pH stable, but high intensity is where H+ exceeds buffering

• HCO3-: constant at lower intensities than drops sharply at 50% VO2max

  • from sequestering faster

• lactate: little change at low intensities than rises sharply at 50% VO2ax

  • anaerobic glycolysis from high ATP demand

• muscle pH: gradual decrease then steeper drop at high intensity

  • ATP hydrolysis drops pH, local acidity more severe

• arterial blood pH: slight decrease than larger drop at high intensity

  • from buffering with HCO3-, Hb, proteins

  • lesser decrease than muscle (H+ in muscle then enters blood)