Acid-base balance

Question 1

What is the difference between an acid and a base?

Answer 1

An acid is any chemical that can donate a H+ whilst a base is any that can accept one

Question 2

What is the difference between a strong and weak acid?

Answer 2

Strong acids - completely dissociate in water releasing large amounts of H+
Weak acids - don’t completely dissociate, instead reach an equilibrium with its conjugated base forming a buffer pair that responds to changes in H+ by reversibly binding Hydrogen ions

Question 3

What is acid-base regulation and why is it important?

Answer 3

The regulation of H+ concentrations (H+ has inverse relationship with pH so the more H+, the more acidic the pH)

Important due to H+ ability to alter protein function (e.g. enzymes) and ability to alter binding between other ions

Question 4

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

Answer 4

Volatile and non-volatile acids.

Volatile - main source, generated from aerobic metabolism and CO2 production in tissues
Easily vaporised so able to leave solution and enter the atmosphere - excreted by the lungs

Non-volatile - generated from other metabolic processes - excreted by the kidneys

Question 5

What are the 3 main mechanisms used to regulate pH in the body?

Answer 5

1. Buffer systems (rapid but unable to change overall H+)
2. Lungs (can rapidly adjust excretion of CO2)
3. Kidneys (slowly adjusts excretion of H+ into urine)

Question 6

What are the 3 main buffer systems in the body and where do they act?

Answer 6

1. Bicarbonate buffer system (Extracellular)
2. Phosphate buffer system (Intracellular and urine)
3. Protein buffer system (mainly intracellular)

Question 7

Explain the bicarbonate buffer system and it’s importance

Answer 7

Bicarbonate Buffer System highlights how the lungs and kidneys can compensate for each other:

1. Hydrogen + Bicarbonate can bind reversibly to form Carbonic Acid
2. Carbonic Acid can dissociate reversibly to form water + Carbon Dioxide (catalysed by Carbonic Anhydrase)

So when H+ is high, bicarbonate can be increased to form Carbonic Acid and respiration can be increased to then excrete the excess CO2 from dissociated Carbonic Acid

Question 8

What is the importance of the Henderson-Hasselbalch equation?

Answer 8

Allows us to calculate pH based on Bicarbonate and CO2 levels in the blood.
Normal ratio of Bicarbonate:CO2 should be approximately 20:1

Question 9

What are the roles of the lungs and kidneys in acid-base regulation?

Answer 9

The lungs are able to quickly respond to changes in pH by altering excretion of CO2 (increased ventilation would decrease CO2 levels whilst decreased ventilation would increase CO2 levels)

The kidneys respond more slowly to change in the pH but respond by altering production of Bicarbonate (increased production in response to low pH and vice versa) and secretion of H+ (reduced secretion in response to low pH and vice versa)

Question 10

Where in the kidneys is the majority of Bicarbonate reabsorbed?

Answer 10

Proximal Convoluted Tubule

Question 11

What is the process for Bicarbonate reabsorption in the kidneys?

Answer 11

1. H+ pumped out of cell by Na+/H+ exchange
2. H+ combines with HCO3- in the tubular lumen to form H2CO3 (Carbonic Acid)
3. Carbonic Acid dissociates into CO2 and H2O in the lumen
4. CO2 and H2O cross the luminal membrane and recombine in the cell to form Carbonic Acid (catalysed by Carbonic Anhydrase)
5. Carbonic Acid then dissociates into H+ and HCO3-
6. HCO3- crosses basolateral membrane into the renal interstitial fluid with Na+

Question 12

How is phosphate used as a urinary buffer?

Answer 12

Two forms of filtered phosphate that form a buffer pair:

Monoprotic phosphate + Hydrogen ion Diprotic phosphate

Hydrogen ions excreted in to the lumen in combination with NaHPO4- to form NaH2PO4 which is excreted in the urine

Process leads to production of HCO3-

Question 13

How is ammonia used as a urinary buffer?

Answer 13

Ammonia and Ammonium form a buffer pair:
Ammonia + Hydrogen ion Ammonium

Ammonia (NH3) secreted in collecting ducts and picks up excess H+ which is excreted in the urine as Ammonium (NH4+). Process leads to production of HCO3-

[Nb. this buffer can respond to acid-base status by detecting decrease in pH which stimulates renal glutamine metabolism leading to increased excretion of H+ - Glutamine metabolised to 2NH4+ and HCO3- in the PCT]

Question 14

What is acidosis?

Answer 14

Any process that results in the blood becoming more acidic than normal (lower pH) through addition of acid or loss of alkali (base)

Question 15

What is alkalosis?

Answer 15

Any process that results in the blood become more alkaline than normal (higher pH) through addition of alkali (base) or loss of acid

Question 16

What is the difference between a metabolic and a respiratory (alkalosis or acidosis)?

Answer 16

A metabolic disorder is when the primary problem is affecting Bicarbonate levels in the blood whilst a respiratory disorder would refer to a primary problem being the CO2 levels in the blood

Question 17

What is compensation?

Answer 17

An abnormality in the parameters of Bicarbonate or CO2 in the blood leading to deviation from the standard ratio (of approx. 20:1) can cause the non-affected component to also alter its parameters in order to maintain the normal ratio moving the pH levels closer to its optimum value

Question 18

What are the main causes of respiratory acidosis?

Answer 18

Inappropriate reduction in ventilation caused by disorders affecting the lungs (e.g. chest wall, nerves or muscles) or a problem with the CNS

Question 19

What are the main causes of respiratory alkalosis?

Answer 19

Inappropriate increase in ventilation caused by disorders such as anxiety, hyperventilation or altitude

Question 20

What are the main causes of metabolic acidosis?

Answer 20

Addition of acid: can be exogenous (e.g. methanol) or endogenous (e.g. lactic or keto acid build up)
Failure to excrete H+ or loss of Bicarbonate (e.g. through severe prolonged diarrhoea)

Question 21

What are the main causes of metabolic alkalosis?

Answer 21

Addition of an alkali 
Excessive loss of H+ (e.g. through severe prolonged vomiting) 
Excess aldosterone (e.g. due to dehydration which stimulates secretion of H+ in the distal tubule)

Question 22

What are the general principles for treating acidosis and alkalosis?

Answer 22

Important to identify the underlying cause of the problem and treat that.
In extreme situations, substances can be used to neutralise the acid or base:
Acidosis can be treated with sodium bicarbonate
Alkalosis can be treated with ammonium chloride

Question 23

What are the steps in interpreting results for treating acid-base disorders?

Answer 23

1. Look at pH (identify whether this is normal, acidosis or alkalosis)
2. Look at CO2 and Bicarbonate values (identify whether this is respiratory or metabolic problem)
3. Look for evidence of compensation

Question 24

What is the average pH of blood?

Answer 24

7.4 [Range 7.36-7.44]

Question 25

How is Hydrogen secreted into the urine in the kidneys?

Answer 25

Transported out of Type A intercalated cells into the tubular lumen in the late DCT and cortical collecting tubule via H+ ATPase and H+/K+ ATPase (Can be stimulated by Aldosterone or hypokalaemia)

Also via phosphate and ammonia buffers!

[Nb. processes of excreting H+ leads to production of HCO3- which is reabsorbed back into the blood]