Acid/Base balance in the kidney Flashcards
What is alkalaemia?
- Reduced H+ in blood
- Occurs when body can no longer compensate for alkalosis
What does alkalaemia cause?
- Lowers free calcium by causing Ca2+ ions to come out of solution
- Increases neuronal excitability
- Fires action potential at slightest signal
Why do sensory changes occur with alkalaemia?
- Free calcium decreased
- Increases neuronal excitability
- Fire action potentials at slightest signal
How does alkalaemia lower free Ca2+?
- 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
How do patients with alkalaemia present?
- Sensory changes such as numbness, tingling, or muscle twitches
- If severe, tetany (sustained muscle contractions) that paralyse respiratory muscles
What is acidaemia?
- Too many H+ ions in the blood
- Occurs when compensatory mechanisms become overwhelmed in acidosis
How does acidaemia increase free calcium in the body?
- 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
What are the consequences of increasing free calcium in the body as in acidaemia?
- Increases plasma K+ ion concentration
- Affects excitability
- Especially of cardiac muscles
- Causes arrhythmia
- Increased H+ denatures proteins
What are sources of H+ in the body?
- Diet
- Metabolism
How does the body buffer plasma pH?
- HCO3- in extracellular fluid
- Proteins, haemoglobin, phosphates in cells
- Phosphates and ammonia in urine
- Ventilation
- Renal regulation of H+ and HCO3-
How do kidneys alter pH?
- 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
Outline the mechanisms by which the kidney can regulate pH in acidosis (PCT)
- 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
Outline the mechanisms by which the kidney can regulate pH in acidosis (DCT)
- 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
Outline the mechanisms by which the kidney can regulate pH in alkalosis (DCT)
- 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-)
How is potassium balance linked to acid/base balance?
- 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
When does respiratory acidosis occur?
- 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-
What causes respiratory alkalosis?
- 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
How do the kidneys compensate respiratory alkalosis?
- Any compensation must come from renal mechanisms
- HCO3- not reabsorbed in proximal tubule
- H+ reabsorbed at DCT (with K+)
- HCO3- secreted at DCT
What causes metabolic acidosis?
- 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-
How does the body compensate for metabolic acidosis?
- Respiratory compensation instant
- Increased ventilation
- pCO2 decreases due to hyperventilation
- Renal compensation includes secretion of H+ and reabsorption of HCO3-
What is the anion gap?
- 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
What happens to the anion gap in acidosis?
- Increases
- Unless cause is renal (anion gap will not be changed)
What causes metabolic alkalosis?
- Excessive vomiting of stomach acid contents
- Excessive ingestion of bicarbonate-containing antacid
- Leads to decreased H+ and increased HCO3-
How does the body compensate for metabolic alkalosis?
- 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
