Acid-Base Balance

Question 1

Why is the acid-base balance important

Answer 1

The activity of enzymes is dependent on H+ concentration and only function within a narrow range

Question 2

What is the equation for pH

Answer 2

pH = -log[H+]

Question 3

What is the normal pH range

Answer 3

7.36-7.44

Question 4

What does the metabolism of carbs and fats produce

Answer 4

Large amounts of CO2 and volatile acids

Question 5

What kind of acids does the metabolism of proteins produce

Answer 5

Non-volatile acids (AAs, uric acid etc.) that cannot be excreted from the lungs

Question 6

Why do non-volatile acids need to be excreted

Answer 6

The kidneys excrete non volatile acids to maintain acid-base balance

Question 7

Why don’t volatile acids need to be excreted by the kidneys

Answer 7

They can be excreted by the lungs as they can be turned into gases

Question 8

What is the capacity of buffer systems determined by

Answer 8

• dissociation constant (pK), the relationship between pK and pH is determined using the Henderson-Hasselbach equation
• quantity of buffer present

Question 9

Why is the bicarbonate buffer unique

Answer 9

It remains in equilibrium with atmospheric air (it acts on CO2)

Question 10

What is the concentration of bicarbonate ions controlled by

Answer 10

Kidneys

Question 11

What buffer system is used in erythrocytes

Answer 11

Haemoglobin

H+ + Hb HHb

Question 12

What buffer system is used in the ICF and ECF

Answer 12

Phoshphate

H+ + HPO4^2- H2PO4-

Question 13

What buffer system is used in the kidney

Answer 13

Ammonia

H+ + NH3- NH4+

Question 14

What organs and parts of the blood contribute to the acid-base balance

Answer 14

Lungs
Kidneys
Plasma/erythrocytes

Question 15

Describe how CO2 forms H+ and how these are buffered

Answer 15

CO2 from tissues is transported in the plasma as bicarbonate and haemoglobin in erythrocytes buffers the H+ derived from carbonic acid.

Question 16

Where in the nephron is filtered bicarbonate reabsorbed

Answer 16

In the PCT and DCT, where there is a net secretion of hydrogen ions

Question 17

What is the site of bicarbonate reabsorption and hydrogen ion excretion in the nephron

Answer 17

The carbonic anhydrase in the the PCT and DCT

Question 18

Describe the mechanism of Bicarbonate reabsorption in the PCT

Answer 18

• HCO3- and H+ = H2CO3
• Carbonic anhydrase in the brush border catalyses the dehydration of H2CO3 to produce CO2 and H2O
• Within cell H+ and HCO3- produced by reaction catalysed with carbonic anhydrase
• H+ secreted and HCO3- diffuses into blood
• Majority of HCO3- exits via symporter with Na+ and some exits in exchange with Cl-

Question 19

Describe the mechanism of Bicarbonate generation and hydrogen ion excretion in the DCT

Answer 19

• majority of bicarbonate is reabsorbed in the PCT, DCT gets small amount remaining
• In cell H+ and HCO3- produced by H2O and CO2 reaction catalysed by carbonic anhydrase
• H+ are secreted into the tubular fluid via H+/ATPase and K+/H+/ATPase
• The HCO3- exits cell across basolateral membrane in exchange for Cl-

Question 20

What is acidosis

Answer 20

An abnormal increase in the H+ concentration of the blood that lowers the arterial pH below 7.35

Question 21

What is alkalosis

Answer 21

An abnormally high alkalinity of the blood and body fluids, pH above 7.45

Question 22

What mechanisms are in place to minimise changes in pH of body fluid

Answer 22

• extracellular and intracellular buffering
• adjustments of blood PaCO2 (ventilation)
• adjustments in renal [H+/HCO3-] secretion

Question 23

What might be the primary regulatory problems that lead to a disturbance in acid-base balance

Answer 23

Problems affecting regulation of PaCO2 or bicarbonate concentration

Question 24

What is respiratory acidosis

Answer 24

Reduction in ventilation due to the action of drugs or lung disease resulting in reduced pH and raised PaCO2

Question 25

How does the body carry out renal compensation for respiratory acidosis

Answer 25

Increased acid excretion and enhanced reabsorption of HCO3- to buffer the H+

Question 26

How does metabolic acidosis occur

Answer 26

The addition of non-volatile acid to the body e.g. diabetic ketoacidosis or in kidney failure

Question 27

What is the respiratory response to metabolic acidosis

Answer 27

Decrease in pH stimulates respiratory centres and increases ventilation rate as a reduction in PaCO2 minimises fall in plasma pH

Question 28

What is the renal response to metabolic acidosis

Answer 28

Increase in acid secretion and enhanced reabsorption of HCO3-

Question 29

What is and what causes metabolic alkalosis

Answer 29

Caused by the addition of non-volatile alkalis (ingestion of antacids) to the body or loss of non-volatile acid (vomiting/gastric HCl)
Shows high pH and high plasma HCO3-

Question 30

What is the respiratory response to metabolic alkalosis

Answer 30

Increase in pH inhibits respiratory centres and decreases ventilation rate, leading to an increase of PaCO2

Question 31

What is the renal response to metabolic alkalosis

Answer 31

Increase in HCO3- excretion (reduced reabsorption)

Question 32

What is and what causes respiratory alkalosis

Answer 32

Caused by increased ventilation (stimulation of respiratory centres e.g. drugs) or hyperventilation through anxiety
Shows high pH and reduced PaCO2

Question 33

What is the renal response to respiratory alkalosis

Answer 33

Reduced acid excretion and reduced reabsorption of HCO3-