5. Acid-base balance

Question 1

Why is pH so important in human physiology?

Answer 1

Due to the effect it can have on proteins and their structure
Especially important with enzymes as changes in pH can alter their active site

pH also controls the speed of the body’s chemical reactions and the speed of enzyme activity

Question 2

What is meant by and acid and what is meant by a base?

Answer 2

Acid is a substance that donates a H+ and a base is a substance that accepts a H+

Question 3

What happens when an acid is added to water?

Answer 3

The acid (HA) dissociates reversibly according to the reaction: HA --> H+ + A-
SO the acid yields a free H+ and its conjugate base

Question 4

What is the pH range for human blood?

Answer 4

Normal blood is between 7.35 at the venous end (because it has more CO2 so is more acidic) and 7.45 at the arterial end
Below 7.35 is acidosis and above 7.45 is alkalaemia

Question 5

What happens if the blood ph is outside of the normal range and when can this lead to morbidity?

Answer 5

If the blood pH is outside of the normal range then this is still compatible with life but can lead to disturbances of certain body functions as it disrupts many enzyme systems and the ETC in mitochondria

Below 6.8 and above 8.0 leads to death

Question 6

How are acids present in the body i.e. how are they taken in/formed?

Answer 6

Acids are present in the diet and are released during the breakdown of foods especially proteins
CO2 is metabolically produced and during respiraton and this forms carbonic acid
Acids are produced from other metabolic activities e.g. lactic acid in exercise

Question 7

Which systems act to regulate the concentration of H+ in the blood and how long do they take?

Answer 7

Chemical buffer systems in the blood - have an immediate action
Respiratory centre in the brainstem - acts within 1-3 minutes
Renal mechanisms - Requires hours to days to affect pH changes

Question 8

What is meant by a ‘buffer’?

Answer 8

A solution that can resist pH change upon addition of a small amount of acid or base
Acts quickly to temporarily bind or release H+
Consists of a weak acid and the salt of that acid

Question 9

What are the three chemical buffer systems in the body?

Answer 9

Bicarbonate (HCO3-) (this is the most important)
Proteins (Hb. and albumin)
Phosphate (PO4 3-)

Question 10

Give the equation for the bicarbonate buffer system

Answer 10

CO2 + H20 H2CO3 H+ + HCO3-

Question 11

What is the anion gap?

!!!! need to know this !!!!!

Answer 11

The difference between the measured cations and the measured anions (generally excludes potassium as this is negligible)

!!! need to know this !!!

Question 12

What is the normal value range for the anion gap?

Answer 12

8-16mEg/L

Question 13

What do abnormal values of the anion gap indicate?

Answer 13

Raised anion gap indicates metabolic acidosis

Low anion gap indicates metabolic alkalosis (very rare)

Question 14

Why might a pathology in the kidneys cause metabolic acidosis and a raised anion gap?

Answer 14

Kidneys cannot excrete acids
SO there is an increased release of bicarbonate to the kidneys buffer this so there is a drop in the bicarbonate anion
Raises the anion gap

Question 15

Why might a low anion gap occur?

Answer 15

Common in liver cirrhosis due to hypoalbuminia

Question 16

How does the respiratory centre regulate the pH?

Answer 16

Increases/decreases rate of breathing via mechanisms of the lungs altering the CO2 levels

Question 17

What is the role of the kidneys in regulating pH?

Answer 17

Third line of defence and is the ultimate acid base regulatory organ
This is the only organ that can prevent metabolic acidosis as it is the only organ that can rid the body of metabolic acids

Question 18

How does the kidney regulate the pH if there is metabolic acidosis?

Answer 18

Kidney must reabsorb all filtered bicarbonate in the nephron and also excrete excess H+

Kidney has intercalated cells (a and B) which have a reverse polarity SO they secrete acid on one end and reabsorb bicarbonate on the other, basal end (a and B have opposite end polarity)

Question 19

How does the kidney regulate the pH if there is metabolic alkalosis?

Answer 19

Tubular cells secrete bicarbonate and reclaim H+ to acidify the blood

NB. there is also trading with e.g. potassium to maintain the electric change

Question 20

What other mechanisms can the kidney use to regulate pH?

Answer 20

SO reabsorption of HCO- as mentioned and excretion of H+ as mentioned

Also excretion of NH4+

Question 21

How does the kidney control the blood pH?

Answer 21

1 - Reabsorption of bicarbonate: glomerulus filters out all components greater than a certain mass (including bicarbonate) and so the proximal convoluted tubule acts to reabsorb this
2 - Active excretion of H+ at the distal convoluted tubule - as required

Question 22

What is the enzyme involved in the reabsorption of HCO3- in the proximal convoluted tubule?

Answer 22

Carbonic anhydrase

Question 23

How is the H+ actively excreted from the distal convoluted tubule? What cells allow this?

Answer 23

Distal tubule contains intercalated cells with reverse polarity - alpha cells and beta cells that function according to what regulation is required

Question 24

How do the alpha-intercalated cells of the distal tubule work?

Answer 24

These absorb HCO3- and secrete H+

Question 25

How do the beta cells of the distal tubule work?

Answer 25

These absorb H+ and secrete HCO3-

Question 26

How does the kidney react when the body is in alkalosis?

Answer 26

Tubular cells function when the body is in alkalosis

These secrete HCO3- and reclaim hydronium (H3O+) to acidify the blood (K+ traded for H+)

Question 27

How does the body react when the body is in acidosis?

Answer 27

Excretion of H+ for K+ using the phosphate/ammonium buffer

Question 28

What is the Henderson Hasselbach equation used for?

Answer 28

Used to find the pH of a buffer solution or the ratio of a conjugate base to acid of the system

Question 29

Give three common causes for a state of metabolic acidosis

Answer 29

Diarrhoea
Keto-acidosis
Lactic-acidosis

Question 30

Give three common causes for a state of metabolic alkalosis

Answer 30

Vomiting
Hypokalaemia
Ingestion of HCO3-