Acid Base Balance

Question 1

What’s the normal blood pH range?

Answer 1

7.36 - 7.44

Question 2

What’s the main buffer system that aims to restore blood pH?

Answer 2

Bicarbonate buffer system:
H20 + Co2 <> H2CO3 <> H+ + HCO3-
Lungs regulate [CO2] and kidneys regulate [HCO3-]

Question 3

How do the lungs and kidneys maintain pH homeostasis?

Answer 3

Kidneys control ECF pH by excreting more/less H+ in urine and amount of HCO3- reabsorbed in PCT
Lungs increase/decrease ventilation rate to alter amount of Co2 puffed off

Question 4

By what mechanism does PCo2 and HCO3- affect blood pH, referring to Henderson-Hasselbalch equation?

Answer 4

Ratio of [Co2]:[HCO3-] determines pH, 20:1 maintains pH at 7.4
If concentration of either is changed then equation will become imbalanced, changing the pH

Question 5

Describe respiratory acidosis - how might the kidneys compensate?

Answer 5

Raised pCO2 and reduced pH

Kidney compensation: reabsorbing more HCO3- and excreting more H+ in urine, so [HCO3-] will increase if compensated

Question 6

Define acid

Answer 6

H+ proton donator

Question 7

Define base

Answer 7

H+ proton acceptor

Question 8

What’s the difference between a strong and weak acid?

Answer 8

Strong acids (HCL) completely dissociate in water, releasing lots of H+

Weak acids (H2CO3) incompletely dissociate in water, reaching equilibrium with its conjugate base, forming a buffer pair that responds to changes in H+ by reversibly binding H+

Question 9

What’s a 1 unit pH change equivalent to, with regards to [H+] and why?

Answer 9

1 unit pH change = 10-fold [H+] change

Because negative log base 10 to H+ gives pH scale 1-14

Question 10

How is homeostasis of most ions controlled, compared to H+?

Answer 10

Balance of intake, production and excretion (mainly by kidney)
Acid-base regulation controls H+ concentrations

Question 11

What are 2 reasons why H+ concentration needs to be tightly regulated?

Answer 11

Small changes alter protein/enzyme activity

Alters binding of other ions (low H+ increases Ca2+ binding to Albumin)

Question 12

What are the 2 sources of H+ in the body?

Answer 12

Volatile acids (easily vapourised and excreted by the lungs)

Non-volatile acids (eg organic acids excreted by the kidneys)

Question 13

What are the 3 main mechanisms to minimise changes in pH?

Answer 13

Buffer systems - rapid chemical reactions that minimise sudden change in pH

Lungs - rapidly adjust excretion of CO2

Kidneys - slowly adjust excretion of H+ into urine and alter HCO3- reabsorption

Question 14

What’s a buffer?

Answer 14

Any substance that can reversibly bind H+

Buffer + H+ <> HBuffer

Question 15

What are the 3 buffer systems in the body?

Answer 15

Bicarbonate buffer system (extracellular)

Phosphate buffer system (intracellular and urine)

Protein buffer system (mainly intracellular)

Question 16

Why is urine acidic?

Answer 16

pH 6

Kidneys must excrete H+ from non-volatile acid production to maintain the acid-base balance

Question 17

Where is the majority of filtered HCO3- reabsorbed and how?

Answer 17

PCT
In tubular lumen HCO3- + H+ (that’s transported via Na+/H+ exchanger) to form H2CO3
H2CO3 dissociates and CO2 diffuses across luminal membrane
When inside tubular cell H2O + CO2 -> H2CO3 via carbonic anhydrase which then dissociates to H+ + HCO3-
HCO3- moves across basolateral membrane into kidney interstitial fluid with Na+

Question 18

HCO3- can’t be directly transported from the lumen - what does it need?

Answer 18

Carbonic anhydrase and sectreted H+

Question 19

Where in the kidney is H+ secreted and how?

Answer 19

Late distal and cortical collecting tubules

In type A intercalating cells: H+/K+ ATPase pump

Question 20

What two things can increase the activity of the H+/K+ ATPase pump?

Answer 20

Aldosterone
Hypokalaemia
Increase H+ secretion and K+ reabsorption

Question 21

What are the 2 main urinary buffers and what are they essential for?

Answer 21

Ammonia and phosphate

Comfort and allow sufficient H+ to be excreted in urine

Question 22

How is H+ excreted by the urinary phosphate buffer?

Answer 22

H+ secreted into tubule lumen by Na+/H+ exchanger

H+ + NaHPO4- = NaH2PO4 (excreted in combination)

Question 23

Outline the urinary ammonium buffer

Answer 23

NH4+ synthesised from glutamine in PCT
NH3 + H+ -> NH4+
Ammonia secreted in collecting duct where it picks up excess H+ and is excreted at ammonium NH4+

Question 24

Why are renal responses to acid base imbalance slower than the lungs? (think about ammonia)

Answer 24

Decrease in pH stimulates renal glutamine metabolism = increased H+ secretion
Slower as requires protein synthesis

Question 25

What’s the difference between respiratory and metabolic acidosis?

Answer 25

Metabolic: primary problem affects [HCO3-]

Respiratory: primary problem affects CO2 excretion

Question 26

Outline compensation

Answer 26

Ratio of HCO3-:CO2 that gives pH
Therefore abnormality affecting one parameter can be compensated for by the other
Tries to minimise changes in pH and restore to normal

In compensated disorders BOTH HCO3- and CO2 lie outside their normal ranges (in same direction ie both raised/lowered)

Question 27

Outline respiratory acidosis (what it is, causes and compensation)

Answer 27

Low pH due to increased CO2 (retention)
Causes: any disorder affecting respiration/lungs
Compensation: kidney increases HCO3- production/reabsorption

Question 28

Outline respiratory alkalosis

Answer 28

Raised pH due to decreased CO2
Causes: hyperventilation, anxiety, altitude
Compensation: kidneys decrease HCO3- production

Question 29

Outline metabolic acidosis

Answer 29

Low pH due to decreased HCO3-
Causes: excessive HCO3- loss or addition of acid - failure of H+ secretion, severe diarrhoea losing HCO3-
Compensation: lungs increase ventilation to decrease CO2

Question 30

Outline metabolic alkalosis

Answer 30

Raised pH due to increased HCO3-
Causes: addition of alkali or excessive H+ loss - dehydration, prolonged vomiting
Compensation: lungs decrease ventilation to increase CO2