Acid/Base balance in the kidney

Question 1

What is alkalaemia?

Answer 1

• Reduced H+ in blood
• Occurs when body can no longer compensate for alkalosis

Question 2

What does alkalaemia cause?

Answer 2

• Lowers free calcium by causing Ca2+ ions to come out of solution
• Increases neuronal excitability
• Fires action potential at slightest signal

Question 3

Why do sensory changes occur with alkalaemia?

Answer 3

• Free calcium decreased
• Increases neuronal excitability
• Fire action potentials at slightest signal

Question 4

How does alkalaemia lower free Ca2+?

Answer 4

• Lots of albumin in blood
• Has carboxyl (COOH) groups
• When [H+] is low, H+ ions dissociate from COOH
• Forms COO-
• Ca2+ free in plasma binds to albumin

Question 5

How do patients with alkalaemia present?

Answer 5

• Sensory changes such as numbness, tingling, or muscle twitches
• If severe, tetany (sustained muscle contractions) that paralyse respiratory muscles

Question 6

What is acidaemia?

Answer 6

• Too many H+ ions in the blood
• Occurs when compensatory mechanisms become overwhelmed in acidosis

Question 7

How does acidaemia increase free calcium in the body?

Answer 7

• In acidaemia [H+] increases
• COO- of albumin binds with H+ to form COOH
• There are less negative charges for Ca2+ ions to bind to
• [Ca2+] in blood increases

Question 8

What are the consequences of increasing free calcium in the body as in acidaemia?

Answer 8

• Increases plasma K+ ion concentration
• Affects excitability
• Especially of cardiac muscles
• Causes arrhythmia
• Increased H+ denatures proteins

Question 9

What are sources of H+ in the body?

Answer 9

• Diet
• Metabolism

Question 10

How does the body buffer plasma pH?

Answer 10

• HCO3- in extracellular fluid
• Proteins, haemoglobin, phosphates in cells
• Phosphates and ammonia in urine
• Ventilation
• Renal regulation of H+ and HCO3-

Question 11

How do kidneys alter pH?

Answer 11

• Renal regulation is slower than buffers and ventilation
• Takes 1-2 days
• Excrete or reabsorb H+
• Can change the rate at which HCO3- is reabsorbed or excreted

Question 12

Outline the mechanisms by which the kidney can regulate pH in acidosis (PCT)

Answer 12

• HPO4^2- + H+ ==> H2PO4- (excreted in urine)
• Glutamine breaks down to ammonium ions and 2 alpha ketoglutarate
• Ammonium ions converted to ammonia and H+ (in cells of PCT)
• Ammonia diffuses into tubule
• H+ is exchanged for Na+
• Ammonia and H+ combine again to form ammonium ions
• Ammonium ions are then excreted in urine
• 2 alpha ketoglutarate is broken down into 2HCO3-
• 2HCO3- reabsorbed

Question 13

Outline the mechanisms by which the kidney can regulate pH in acidosis (DCT)

Answer 13

• alpha cells
• H2O + CO2 (from respiration) ==> H2CO3 ==> H+ + HCO3-
• H+ is secreted into lumen by H+ ATPase or H+-K+ ATPase
• K+ is reabsorbed into blood
• HCO3- reabsorbed into blood in exchange for Cl-
• HCO3- combines with H+ in the blood to form H2CO3

Question 14

Outline the mechanisms by which the kidney can regulate pH in alkalosis (DCT)

Answer 14

• beta cells
• H2O + CO2 ==> HCO3- + H+
• H+ is moved into blood by H+ ATPase or H+-K+ ATPase (exchanged for K+)
• K+ is lost from body
• HCO3- is secreted into lumen of tubule (exchanged for Cl-)

Question 15

How is potassium balance linked to acid/base balance?

Answer 15

• In acidosis cells take up H+ and release K+
• In alkalosis cells take up K+ and release H+
• Alkalosis can cause hypokalaemia
• Acidosis can cause hyperkalaemia

Question 16

When does respiratory acidosis occur?

Answer 16

• Alveolar hypoventilation results in CO2 retention
• Elevated pCO2
• Leads to increased H+ and HCO3-
• Any compensation must come from renal mechanisms that secrete H+ and reabsorb HCO3-

Question 17

What causes respiratory alkalosis?

Answer 17

• Much less common than acidotic conditions
• Often as a result of hyperventilation
• Alveolar ventilation increases without metabolic CO2 production increasing
• H+ and HCO3- have to compensate and make more CO2

Question 18

How do the kidneys compensate respiratory alkalosis?

Answer 18

• Any compensation must come from renal mechanisms
• HCO3- not reabsorbed in proximal tubule
• H+ reabsorbed at DCT (with K+)
• HCO3- secreted at DCT

Question 19

What causes metabolic acidosis?

Answer 19

• When dietary and metabolic input of H+ exceeds H+ excretion
• Lactic acidosis
• Ketoacidosis
• Can occur if body loses HCO3- (diarrhoea)
• Increased CO2 causes increased H+ and decreased HCO3-

Question 20

How does the body compensate for metabolic acidosis?

Answer 20

• Respiratory compensation instant
• Increased ventilation
• pCO2 decreases due to hyperventilation
• Renal compensation includes secretion of H+ and reabsorption of HCO3-

Question 21

What is the anion gap?

Answer 21

• Difference between measured cations and anions
• ([Na+] + [K+]) - ([Cl-] + [HCO3-])
• Gap is increased if HCO3- is replaced by other anions
• Normally 10-18 mmol/L

Question 22

What happens to the anion gap in acidosis?

Answer 22

• Increases
• Unless cause is renal (anion gap will not be changed)

Question 23

What causes metabolic alkalosis?

Answer 23

• Excessive vomiting of stomach acid contents
• Excessive ingestion of bicarbonate-containing antacid
• Leads to decreased H+ and increased HCO3-

Question 24

How does the body compensate for metabolic alkalosis?

Answer 24

• Hypoventilation is rapid respiratory compensation
• Creates more H+ and HCO3-
• Restores pH but produces more HCO3-
• HCO3- is not reabsorbed in proximal tubule
• HCO3- is secreted at DCT and H+ is reabsorbed