Renal Physiology 4 (Renal)

Question 1

pH of normal arterial and venous blood?

Answer 1

Arterial - 7.4
Venous - 7.35 (more PCO2)

Question 2

Definitions for acidosis and alkalosis

Answer 2

Acidosis = ARTERIAL pH \< 7.35 
Alkalosis = ARTERIAL pH \>7.45

Question 3

What are the three broad impacts that pH has on the body

Answer 3

Metabolism - enzymes are pH dependent

Neuromuscular - pH impacts binding of plasma Ca2+ to albumin. ACIDOSIS decreases protein binding, increases free Ca2+, blocking voltage-gated Na+ channels, RAISING THE AP THRESHOLD.

Potassium - acidosis increases potassium in serum, alkalosis decreases potassium in serum

Question 4

List H+ inputs and outputs

Answer 4

Inputs: Acids produced from CHO/fat metabolism, CO2, lactic acid

Outputs: Ventilation (CO2 output), Renal H+ excretion

Question 5

Describe the three mechanisms to protect against pH change, their time taken to take effect and their capacity.

Answer 5

Chemical buffering (immediate but exhaustible) = solutions that resist pH change

Pulmonary regulation (minutes-hours, limited capacity) = changes in ventilation (removal of CO2) changes blood pH

Renal regulation (hours-days, “infinite” capacity) = Kidneys control HCO3- and H+ that is secreted.

Question 6

What is pKa and what does it mean?

Answer 6

pKa is the dissociation constant of the weak acid in the buffer.

pKa determines optimal pH for maximum buffeirng capacity.

E.g. blood has pH of 7.4, so we want buffer system with pKa of 7.4

However, we use bicarbonate buffer, which as pKa of 6.3. So we just use more of it.

Question 7

What are three chemical buffer systems in the body?

Answer 7

Bicarbonate (pKa 6.4)(Main ECF one)

Ammonia (pKa = 9.3)(ECF, renal tubular fluid buffering)

Proteins (pKa 7.4)(ICF, Hb in RBCs)

Question 8

Describe in detail the renal control of pH.

Answer 8

Kidneys constantly remove HCO3- from the blood. To maintain balance, they must reabsorb HCO3- back into blood.

However, HCO3- cannot be reabsorbed in this form, so quantitative H+ secretion determines HCO3- reabsorption

H+ secretion > HCO3- filtration = acid loss

H+ secretion < HCO3- filtration = base loss

Question 9

How does pH impact potassium levels?

Through what mechanisms?

Answer 9

Acidosis = hyperkalaemia

Alkalosis = hypokalaemia

Alkalosis results in K+ shift into ICF< as Na+/H+ exchange takes H+ out and K+ in

K+ is also passively secreted as a cation partner to excess HCO3-

Question 10

What blood do you take to see if someone is acidotic or alkalotic?

Answer 10

Arterial blood gas analysis.

Venous blood pH fluctutates so it isn’t helpful.

ΔPCO2 reflects respiratory component

Δ[HCO3-] reflects metabolic component

Question 11

What are the two broad causes of pH changes?

Answer 11

MEtabolic - due to production or loss of acids/bases. Reflected in plasma [HCO3]-

Respiratory - hyper/hypo-ventilation. Reflected in PCO2

Question 12

Describe metabolic and respiratory compensation for acidosis and alkalosis.

Answer 12

Metabolic compensation:

Acidosis: Complete HCO3- reabsorption, increase H+ excretion, increased HCO3- production.

Alkalosis: Decreased HCO3- reabsorption.

Respiratory compensation:

Acidosis: Increased ventilation, decreased PCO2

Alkalosis: Decreased ventilation, increased PCO2

Question 13

Describe values involved in PCO2 and HCO3- for pH disorders.

Answer 13

Acidosis:

Metabolic = <24mEq/L HCO3. Respiratory compensation = <40mmHg PCO2

Respiratory = >40mmHg PCO2. Renal compensation = >24mEq/L HCO3

Alkalosis

Metabolic = >24mEq/L HCO3. Respiratory compensation = <40mmHg PCO2

Respiratory = >40mmHg PCO2. Renal compensation <24mEq/L HCO3.

Question 14

What causes an increased anion gap?

Answer 14

Organic acidosis (starvation, ketoacidosis, lactic acidosis), poisoning, aspirin.