Renal Handling of Acid Base Balance

Question 1

What are the 7 general guidelines of Acid base

Answer 1

1. Input= output
2. Acidity is measured in terms of pH- pH of 7= 10^-7
3. Body fluids are bufferes. Buffers prevent large changes in pH when H+ is added (or removed).
4. THe most important buffer is CO2- bicarb system
5. CO2 is not an acid but it reacts with H20 and makes H- since H20 is ubiquitious in the body then CO2 acts as an acid
6. gain or loss of fixed acids will always gain/lose a bicarb

7 Plasma is defined by the ratio of HCO3/CO2 by Henderson Hasselbach where pH= 6.1 + Log (HCO3)/.03 x Plasma CO2

Question 2

How is acid produced in the body

Answer 2

Food 20 mEq/day

Body’s metabolism 20mEq/day

Feces- loss of bas of 20meq/day which is like gaining acid

Kidneys have to excrete 60mEq/day

Question 3

Is filter H+ enough to excrete extra acid

Answer 3

the filtered H+ load per day is very small.
filtered H+ load is 0.0072 mEq/day!! This is more than 8000 times less than the ~60 mEq acidthat enters the body per day.

The point is that H+ filtration alone can not possibly account for the
amount of acid excreted in the urine per day. The only way ~60 mEq/day acid can get into thenephron (in order to be excreted) is by secretion.

Question 4

Discuss bicarb filtering

Plasma amoutn

Answer 4

Bicarb is freely filtered.

Plasma levels is 24mM

Question 5

Where is bicarb reabsorbed

Answer 5

PT- 80%- transcellular

Thick Ascending Loop- 15%

Cortical CCD- 5%

Question 6

Explain the process of bicarb reabsroption in the proxima tubule

Answer 6

1. BL Na/K pump making an Na graident
2. Na/H antiporter on apical- secretion of H+ into lumen

3 Secreted H+ associates with bicarb making carbonic acid

4. Carbonic acid disscoaites to H20 and CO2 which can easily enter the cell and reassociate to carbonic acid.
5. In the cell the carbonic acid dissociate and through the Na+/HCO3 symporter, bicarb enters the intersittum. The h+ can go back to the apical membrane Na/H+ anitporter and be secreted again

Question 7

What’s special about the bicarb that ends up in the interstitum in bicarb reabsorption?

how about the H+?

Answer 7

It is interesting to note that the HCO3- that ends up in the interstitial space was neveractually in the tubular lumen. Nevertheless, a HCO3- in the tubular lumen disappeared and another HCO3
-, which formed in the cell, ended up in the blood. The net result is reabsorption of a HCO3- molecule

** also note that the H+ is just recycled back through the H+/Na apical antiporter***

Question 8

Explain the steps of bicarb reabsorption in the thicking ascening loop

Answer 8

Bicarb is reabsorbed in Type A intercalated cells– very similar to reabsorption in proximal tubule

1. Apical H+ secretion through the K/H ATPase antiporter or H+ ATPase (Key difference with proximal- it uses Na/H anitporter)
2. H+ associates with the filtered HCO3
3. The carbonic acid dissociates to water and CO2 and enters the cells
4. H20 and CO2 reassociate to H2CO3 and then disscoaites to H+ and HCO3-
5. The HCO- is reabsorbed through the HCO3/Cl- antiporter (key difference with proximal- proximal uses Na/HCO3 symporter)

Question 9

What does the kdiney do with excess bicarb?

Answer 9

1. Absorbs less of the filtered so it can be excreted in urine
2. Secreted bicarb at the Cortical CD into the tubular fluid for excretion with urine

Question 10

The CCD both secretes and absorbs bicarb.. how?

Answer 10

Intercalted cells- secrete HCO3

A-principle cells reabsorb HCO3

Question 11

Explain the steps of CCD secretion of bicar

Answer 11

It happens on the TYPE B intercalated cells. Type B is just a “flipped around TYpe A”

1. Basolateral H+ through the K/H antiporter and the H-ATPase
2. H+ associates with bicarb making carbonic acid
3. CArbonic acid disscoiates to water and bicarb which enters the cells
4. Apical HCO/CL anitporter secreted HCO3

Question 12

What key part happens when handling excess acid?

Answer 12

NEW bicarb is formed with each H+ excretion

Question 13

What is the most important titrable acid in the body

Answer 13

PHOSPHATE

Question 14

What are the two main ways to excrete excess H+

Answer 14

Phosphate (2/3 of excess H+ binds to phosphate)

Ammonium

Question 15

Explain the steps of Ammonium binding to H+ for excretion

Answer 15

Onlky when the body needs to get rid of H+

1. NH3 deliberatly produced from glutamine int he proximal tubule (this step is on demand as needed)
2. THe NH3 diffuses the apical membrane and binds with secreted H+
3. This results in one new HCO3- to enter the blood for each H+ excreted.

Question 16

What main thing are we controlling in acid/base?

What 3 factors must we consider to see the net bicarb amounts

Answer 16

HCO3-

1. Amount of bicarb in urine
2. Amount o titrable acid in urine (amounf of new bicarb from H2PO4 excretion)
3. Amount of ammounium in urine (this is equal to new bicarb from NH4+ excretoin)

Net Gain/loss of bicarb= Total new bicarb (phosphate/ammonium)- Bicarb lost in urine

Question 17

What signals H+ secretion to increase or decrease

Answer 17

Plasma CO2 and arterial PH- process is not well understood.

Question 18

What is acidosis?

explain the two kinds

Answer 18

Acidosis- plsma pH less than 7.35

Respiratory acidosis- high plasma CO2

Metaboli acidosis- low plasma HCO3-

Remember that pH is A RATIO of bicarb and Plasma CO2

Question 19

Define alklosis what are the two kinds

Answer 19

Alkalosis is clinically defined as a situation in which plasma pH is greater than 7.45 (the normal range of plasma pH is 7.35 to 7.45).

Alkalosis can also be either of respiratory origin
(low PCO2) or metabolic origin (high HCO3
-).

During alkalosis the bicarb/CO2 ratio is above 20

And the renal response is to reduce the amount of bicarb reabsorbed

Question 20

What is anion gap

Answer 20

defined (in its simplest form) as the difference between the
concentration of the major plasma cation (Na+) and the major plasma anions (Cl- and HCO3 -).

This can be expressed as:
Anion gap = [Na+]PLASMA – ([Cl-]PLASMA
+ [HCO3 -]PLASMA)

important to remember that there is never a measureable gap in plasma or elsewhere