Physiology - Acid-Base Balance I: Renal Mechanism Involved in Regulation and [H+] in the Plasma

Question 1

An acidosis is defined as a blood pH of what?

Answer 1

<7.35

Question 2

An alkalosis is defined as a blood pH of what?

Answer 2

>7.45

Question 3

Why is venous blood more acidic than arterial blood?

Answer 3

Presence of more CO₂

(produces carbonic acid when mixed with water)

Question 4

How does an acidosis affect the CNS?

Answer 4

CNS depression

(alkalosis causes over-excitability of PNS and later CNS)

Question 5

Changes in hydrogen ion concentration in body fluids, impacts the concentration of which other ion?

Answer 5

Potassium

Question 6

In acidosis, the renal tubular cells _______ excretion of hydrogen ions and _________ excretion of potassium ions

Answer 6

In acidosis, the renal tubular cells increase excretion of hydrogen ions and decrease excretion of potassium ions

Question 7

In which 3 ways are hydrogen ions added to the body?

Answer 7

1. Carbonic acid formation
2. Inorganic acids produced due to nutrient breakdown
3. Organic acids resulting from metabolism

Question 8

What can “resist” initial increases in either acid or alkali?

Answer 8

Buffer solutions

Question 9

In the CO₂-HCO₃ buffer, which enzyme catalyses the formation of carbonic acid?

Answer 9

Carbonic anhydrase

Question 10

Which two organ systems are involved in maintaining the buffer system?

Answer 10

Bicarbonate concentration is controlled by the kidneys

PCO₂ is controlled by the lungs

Question 11

In which two ways can the kidneys alter bicarbonate levels?

Answer 11

1. Variable reabsorption of filtered bicarbonate
2. Kidneys can add new bicarbonate into the blood

Question 12

Where is bicarbonate mostly reabsorbed?

Answer 12

Proximal tubule

Question 13

How are bicarbonate ions reabsorbed in the proximal tubule?

Answer 13

1. Carbon dioxide diffuses across the basolateral (and apical) membranes
2. Along with water, carbonic acid is produced inside the tubular cell
3. Dissociation occurs to form bicarbonate and H⁺
4. The H⁺ are removed into the tubular fluid via sodium hydrogen transporter
5. Carbonic acid is produced in the tubular fluid (from this H+ and filtered bicarbonate)
6. Carbonic acid dissociates and CO₂ and H₂O can be reabsorbed into the tubular cell (these are used to produce carbonic acid inside the cell as per step 2)
7. By the sodium/bicarbonate co-transporter, bicarbonate is pumped across the basolateral membrane and reabsorbed

Question 14

How can the kindeys for “new” bicarbonate when buffer stores are depleted from high acid load, or when tubular bicarbonate is low in concentration?

Answer 14

Phosphate is used instead as it is also a plentiful filtered compound

1. CO₂ diffuses into the tubular cell via the basolateral membrane
2. CO₂ combines with water producing carbonic acid which dissociates forming bicarbonate and H⁺
3. H⁺ is pumped into the filtrate where it combines with phosphate (HPO₄^2-) to produce the more acidic (H₂PO₄^-) which is then excreted in urine
4. This way acid is lost and the bicarbonate produced in the tubular cell is reabsorbed via the sodium/bicarbonate cotransporter

Question 15

In which instance may bicarbonate levels in the tubular fluid be low?

Answer 15

Acidosis

(buffering has “wasted” bicarbonate)

Question 16

What happens in severe acidosis?

Answer 16

1. Glutamine produced by liver
2. Glutaminase produces ammonia from glutamine
3. Ammonia enters tubular fluid
4. Ammonium produced by combining with H⁺
5. Excretion of ammonium in urine

Question 17

Besides phosphate, which other compound can be used as a tubular buffer to aid in the excretion of excess acid and allow for new bicarbonate to be formed by removing such acid?

Answer 17

Ammonia

Question 18

What 3 things does H⁺ secretion by the tubule do?

Answer 18

1. Drives bicarbonate reabsorption
2. Forms acid phosphate
3. Forms ammonium ion