K7 - Acid Base Balance I: Renal Mechanisms involved in Regulation of [H+] Plasma

Question 1

What is the pH of arterial blood?

Answer 1

7.45

Question 2

What is the pH of venous blood?

Answer 2

7.35

Question 3

What is the average pH of blood?

Answer 3

7.4

Question 4

What do small changes in pH reflect?

Answer 4

large changes in [H+]

Question 5

An increase in [H+] reduces what?

Answer 5

pH

Question 6

what is acidosis?

Answer 6

increase in H ion secretion which is coupled with a decrease in K ion resulting in K ion retention which has a knock on effect of the membrane potential

Question 7

what can acidosis lead to?

Answer 7

depression of the CNS – result in a state of coma

Question 8

what can alkalosis lead to?

Answer 8

over excitability of the peripheral NS and later the CNS

Question 9

What does [H+] exert a marked influence on?

Answer 9

enzyme activity

Question 10

What does changes in [H+] influence?

Answer 10

K+ levels in the body

Question 11

H+ is continually added from what 3 sources?

Answer 11

1. Carbonic acid formation
2. Inorganic acids produced during breakdown of nutrients
3. Organic acids resulting from metabolism

Question 12

What disease is a result from the build up of H+ ions in body fluids

Answer 12

Diabetes mellitus

Question 13

what does a buffer system consist of ?

Answer 13

a pair of substances –
one can yield free H+ as the [H+] decreases
the other can bind free H+ when [H+] increases

Question 14

What is the most important physiological buffer system

Answer 14

CO2- HCO3 buffer

Question 15

What is the formula for carbonic acid?

Answer 15

H2CO3

Question 16

What is the formula for bicarbonate?

Answer 16

HCO3-

Question 17

what is the equation for the CO2- HCO3 buffer?

Answer 17

CO2 + H2O >< H2CO3 >< H+ + HCO3-

Question 18

How is H2CO3 formed?

Answer 18

from CO2 and water, catalysed by carbonic anhydrase (CA)

Question 19

What is the CO2- HCO3 buffer in Henderson-Hasselbalch notation?

Answer 19

pH = 6.1 + log x [HCO3-]/PCO2 x 0.03

where 0.03 is solubility coefficient

Question 20

Why is the buffer system so important?

Answer 20

[HCO3-] is controlled by the kidneys and PCO2 is controlled by the lungs.

Interplay of these two systems ensures precise control

Question 21

what sets normal plasma pH at 7.4?

Answer 21

pH = 6.1 + log [24 mmol/l]/ 40mmHg x 0.03

Question 22

what is the role of the kidney in control of [HCO3]p?

Answer 22

1. Variable reabsorption of filtered HCO3-

2. Kidneys can add “new” HCO3- to the blood

Question 23

what are the two roles of the kidney n control of [HCO3]p dependent on?

Answer 23

upon H+ secretion into the tubule

Question 24

How much HCO3- is filtered per day? (working)

Answer 24

Rate of HCO3- filtration

= GFR x [HCO3-]plasma
= 125 ml/min x 24 mmol/l
= 180 l/day x 24 mmol/l
= 4320 mmol/day

Question 25

how is bicarbonate re absorbed in the proximal tubule?

Answer 25

no membrane transporter

1. infiltrate bicarbonate reacts with H+ secreted from epithelial cells forming carbonic acid
2. this is broken down to CO2 and H2O
3. H2O taken into epithelial cells and combined with CO2 which has been absorbed from interstitial fluid making carbonic acid
4. this is broken down to bicarbonate and H+
5. bicarbonate leaves the cells in symport with Na+ into interstitial fluid
6. H+ enter the filtrate in antiport with sodium and used to from more carbonic acid etc

Question 26

What does secreted H+ combine with when [HCO3-]tubular fluid is low?

Answer 26

phosphate

Question 27

what is the action, location and importance carbonic anhydrase?

Answer 27

action - catalyses the formation of carbonic acid from carbon dioxide and water

location - luminal membrane of epithelial cells lining proximal tubule

Question 28

What is “titratable acid”?

Answer 28

The amount of H+ excreted as H2PO4-

Question 29

How can “titratable acid” be measured?

Answer 29

Measure amount of strong base (NaOH) added to titrate the urine pH back to 7.4

Question 30

What is the maximum amount of titratable acid that can be excreted?

Answer 30

40mmol/day

Question 31

How does ammonia act as a tubular buffer?

Answer 31

1. Tubular cells produce and from NH3 from glutamine
2. this is a gas it diffuses through apical membrane
3. binds with the H ion to form ammonium ion which is excreted through same process as bicarbonate

Acid excreted as NH4+ & 2. “new” HCO3- generated

Question 32

What does measurement of titratable acid not measure?

Answer 32

NH4+

Question 33

What three things does H+ secretion by the tubule do?

Answer 33

1. Drives bicarbonate ion reabsorption
2. Forms acid phosphate which removes H and adds HCO3 into blood
3. Forms ammonium and new bicarbonate ions are added to the blood

Question 34

what is the role of tirateable acid in acid excretion?

Answer 34

leads to addition of new bicarbonate ions to the body

Question 35

What is the total H+ secretion into the tubule?

Answer 35

HCO3- reabsorption = 4300 mmol/day
TA excretion = 20 mmol/day
NH4+ excretion = 40 mmol/day

TOTAL H+ SECRETION = 4360 mmol/day

Question 36

What is the total H+ excretion into the tubule?

Answer 36

TA excretion = 20 mmol/day
NH4+ excretion = 40 mmol/day

TOTAL H+ EXCRETION = 60 mmol/day

Question 37

What is the vast majority of H+ secretion used for?

Answer 37

HCO3- reabsorption to prevent generation of acidosis

Question 38

What does excretion of TA and NH4+ do?

Answer 38

Simultaneously rids the body of acid load and regenerates buffer stores