12 – Renal Regulation of Acid-Base Balance

Question 1

Why does H+ concentration need to be tightly regulated?

Answer 1

-regulate normal enzymatic activities
*normal in body fluid, pH=7.4

Question 2

Definition of acid:

Answer 2

-molecules that can release H+ in solution
Ex. carbonic acid (H2CO3)

Question 3

Definition of base:

Answer 3

-molecules that can accept H+
Ex. bicarbonate (HCO3-)

Question 4

Acidosis:

Answer 4

-condition where H+ concentration is higher than normal
-kidneys secret more H+ and absorb more HCO3- and produce HCO3-

Question 5

Alkalosis:

Answer 5

-condition when H+ concentration is lower than normal
-kidneys secret less H+ and reabsorb less HCO3

Question 6

What acids and bases are found in EC fluid?

Answer 6

-WEAK acids and bases

Question 7

pH range our body can still function within:

Answer 7

-6.8-8.0

Question 8

Why is pH in arterial blood 7.4, but in venous blood it is 7.35?

Answer 8

-CO2 is dissolved in venous blood and produces carbonic acid=lowers pH

Question 9

What is the pH of intracellular pH?

Answer 9

-lower
>6-7.4

Question 10

What is the pH of urine?

Answer 10

-4.5-8.0

Question 11

What are the 3 defense mechanisms against changes in H+ concentration?

Answer 11

1. Acid-base buffer systems of body fluids
2. Respiratory system
3. Kidneys

Question 12

Acid-base buffer systems of body fluids: (defense against changes in H+)

Answer 12

-react within seconds
-keep H+ tied up until removed

Question 13

Respiratory system: (defense against changes in H+)

Answer 13

-works within minutes
-removes acids in form of carbonic acid (to get H2O and CO2)

Question 14

Kidneys: (defense against changes in H+)

Answer 14

-work within hours and days
-excrete H+ and/or HCO3- to adjust acid-base

Question 15

Acid-base buffer system: mechanism

Answer 15

-converts strong acids to weak acids and strong bases to weak bases
>composed of a weak acid and a salt

Question 16

What does the salt do in the acid-base buffer system?

Answer 16

-provides basis for the weak acid
-when stronger acid is added to system, H+ from strong acids binds to base of weaker acid=form a weaker acid

Question 17

Example of acid-base buffer system: HCl

Answer 17

-ionized to H+ and Cl- in presence of buffer system
-H+ is removed by buffer and gives ‘H buffer’

Question 18

Example of acid-base buffer system: NaOH

Answer 18

-ionized as Na+ and OH-
-OH- reacts with weak acid=H20
-Na+ reacts to form salt

Question 19

What are the common buffer systems in the body (3)?

Answer 19

-bicarbonate buffer system
-phosphate buffer system
-proteins

Question 20

Bicarbonate buffer system:

Answer 20

-carbonic acid (H2CO3) and bicarbonate salt (NaHCO3)
>carbonic acid ionizes weakly so H+ is mostly sequestered
>ionized form is the DOMINANT form bicarbonate salt

Question 21

How does bicarbonate buffer a strong acid?

Answer 21

-converts strong acid to weak acids
>additional H+ combines with HCO3- in system=H2CO3 to CO2 + H2O
*need to make a lot of weak acid to ‘equal’ the strong acid

Question 22

How does bicarbonate buffer a strong base?

Answer 22

-H2CO3 + NaOH to NaHCO3 + H2O

Question 23

Phosphate buffer system:

Answer 23

-H2PO4- (acid) and HPO4 2- (base)
-less important than bicarbonate system in ECF
*important for buffering intracellular fluid

Question 24

Proteins (common buffer system):

Answer 24

-important cellular buffering molecules
-extracellular pH changes affect intracellular pH
>moving H+ and HCO3- across membrane is slow (hours)
*important role in fast buffering of INTRACELLULAR pH changes

Question 25

Respiratory regulation of acid-base balance is based on:

Answer 25

-CO2 exchange in lungs
>increased ventilation=decreases EC CO2 and H+ (pH goes up)
>decreased ventilation=increase EC CO2 and H+ (pH goes down)

Question 26

Doubling ventilation and pH:

Answer 26

-7.4 to 7.63
*increases pH

Question 27

Reducing ventilation by ¼ and pH:

Answer 27

*reduces pH to 6.95

Question 28

Respiratory regulation of acid-base balance:

Answer 28

-increase in H+ can increase ventilation
-decreased H+ can suppress ventilation, but it is LIMITED due to reduction of PO2 and stimulation of ventilation

Question 29

Respiratory regulation of acid-base balance: ‘effectiveness’

Answer 29

-cannot completely compensate for changes in pH
>effective for 50-75% compensation

Question 30

Why is renal regulation important?

Answer 30

-our body produces 80mEq of non-volatile acids per day
>lungs CANNOT remove them

Question 31

What is the main principal of renal acid-base adjustment?

Answer 31

-HCO3- is filtered and if less reabsorbed=pH goes down
>almost all HCO3- is reabsorbed and kidneys need to secrete H+ for reabsorbing HCO3-
-if lots of H+ secreted into tubules and more H+ excreted=pH goes up

Question 32

Bicarbonate reabsorption:

Answer 32

-all segments of nephron except thin descending and ascending limps
>80% in proximal tubules
>10% in thick part of loop of Henle
>remaining in distal tubules and collecting ducts
*mechanisms in early and late segments are different

Question 33

Secretion of H+ and reabsorption of HCO3- in proximal tubule, thick segment of loop of Henle and early distal tubule:

Answer 33

-reabsorbing HCO3- is a priority so kidneys secret H+
>H+ comes from carbonic acid (carbonic anhydrase)
>carbonic acid is ionized and H+ is secreted
-bicarbonate diffuses into peritubular capillaries (Na/HCO3 or Cl-/HCO3 exchanger)
-H+ combines with filtered bicarbonate (get CO2 + H2O)
*pH can only be reduced to 6.7

Question 34

Secretion of H+ and reabsorption of HCO3- in late distal and collecting tubules (intercalated cells):

Answer 34

-95% of bicarbonate has been reabsorbed
-H+ secreted directly by primary active mechanism (H+ pumps)
>free H+ =pH can decrease to 4.5 (w/o damaging epithelium as much)

Question 35

What are the 2 main differences in the late distal and collecting tubules compared to earlier segments with regards to secretion of H+ and reabsorption of HCO3-?

Answer 35

1. Primary active mechanism of H+ secretion is located here
2. Acidic tubular fluid can form here (previously free H+ was removed by bicarbonate)

Question 36

What is done to excrete the non-volatile acids that are produced?

Answer 36

-phosphate buffer system
-ammonia buffer system

Question 37

Why is phosphate an important buffer?

Answer 37

-phosphate in tubular fluid is less reabsorbed than water=it’s concentration in tubule increases

Question 38

Phosphate buffer system:

Answer 38

-neutralize free H+ ions =form salt and is excreted
-additional bicarbonate is produced and is important for maintaining extracellular bicarbonate stores

Question 39

Ammonia buffer system: (proximal tubular cells)

Answer 39

-due to metabolism of glutamine in epithelial cells forming ammonium and bicarbonate is released to tubule
>glutamine diffuses from tubular lumen
>bicarbonate goes to interstitial fluid
>ammonium exchanges with Na to go into the tubular lumen

Question 40

Ammonia buffering system (collecting tubules):

Answer 40

-secreted H+(via ATPase) binds to ammonia in the tubule and forms ammonium
>luminal membrane of cells is NOT permeable to ammonium, and it is TRAPPED in the tubule and excreted
*ammonia can freely diffuse from cells to the tubular lumen

Question 41

Under normal conditions how much acid is excreted and HCO3 produced by the ammonia buffering system?

Answer 41

-50% of acid excreted
-50% of produced HCO3-
*quantitatively important