Acid Base Balance

Question 1

Arterial blood ranges from

Answer 1

7.36 to 7.44, Intracellular pH approx. 7.2

Question 2

carbonic anhydrase equation

Answer 2

CO2 + H2O Chemoreceptors & Lungs: Regulation of CO2 by ventilation
H2CO3 HCO3- + HKidneys:
HCO3- filtering or generation
H+ secretion

Question 3

Carbonic Anhydrase (CA)

Answer 3

Zinc Metalloenzymes
Carbonic anhydrase was first discovered in 1933 in bovine (cow) red blood cells
Expressed in a range of tissues including lung, kidney, eyes, intestine and red blood cells
Cytosolic and membrane located isoforms of CA
Also expressed in plants and coral

Question 4

HCO3- regulated by

Answer 4

kidneys

Question 5

PCO2 regulated

Answer 5

by lungs

Question 6

Transport of CO2 from tissues

Answer 6

CO2 generated by tissues

1. 10 % CO2 dissolves in plasma and red blood cells
2. 25 – 30 % CO2 reacts with Hb to form carbaminohemoglobin (HbCO2)
3. 60 – 65 % CO2 is converted by Carbonic anhydrase (CA) into HCO3- and H+
4. Chloride Shift – electroneutral exchange of HCO3- for Cl- ions
   A resting person generates 200 ml CO2 per minute

Question 7

Transport of CO2 to lungs

Answer 7

Blood CO2>Alveolar CO2 = net diffusion of CO2 into alveolus

1. Loss of CO2 leads to HCO3- to combine with H+ to generate CO2
2. Loss of CO2 leads to dissociation of Hb to release CO2
3. Expired CO2

Question 8

Transport of Hydrogen Ions between tissues and lungs
Blood flows through tissues – oxyhemoglobin loses oxygen and large quantity of CO2 enters the blood to form HCO3- and H+. What happens to the H+ ions?

Answer 8

1. Deoxyhemoglobin is generated at the tissues
2. Deoxyhemoglobin has a greater affinity for H+ than HbO2 – so binds most of the H+ generated by the CA reaction.

Question 9

Transport of Hydrogen Ions between tissues and lungs

Answer 9

Blood flows through lungs and the reaction is reversed

1. Deoxyhemoglobin binds to oxygen to form oxyhemoglobin HbO2
2. H+ reacts with HCO3 to generate CO2 and H2O
3. CO2 is expired

Question 10

Chemoreceptors

Answer 10

are responsible controlling alveolar ventilation

Question 11

Peripheral chemoreceptors

Answer 11

are found in the neck in the bifurcation of the common carotid artery (called carotid bodies) and in the thorax at the arch of the aorta (called aortic bodies)

Question 12

Central chemoreceptors are found in the

Answer 12

medulla

Question 13

Changes in pCO2 & Chemoreceptors

Answer 13

Peripheral chemoreceptors respond to an increase in arterial H+ concentration due to increase in pCO2

At the same time, increase in brain pCO2 that leads to an increase in brain extracellular H+ concentration that stimulates central chemoreceptors

Both central and peripheral chemoreceptors stimulate medullary inspiratory neurons to increase ventilation

Question 14

Renal control

Answer 14

The kidneys have an important role in the regulation of bicarbonate levels
Reabsorption of filtered bicarbonate
Production of new bicarbonate

Question 15

In the Cortical nephron it contains

Answer 15

bowmans capsule
glomerulus
loop of henie

Question 16

juxtamedullary nephron

Answer 16

diastol convoluted tubule
proximal convoluted tubule
cortex

Question 17

Bicarbonate regulation in kidneys

Answer 17

HCO3- filtered + HCO3- secreted – HCO3- reabsorbed = renal HCO3- excretion

Bicarbonate is completely filtered at the renal corpuscles and significant amounts are reabsorbed in the proximal collecting duct, loop of henie and cortical collecting duct
Collecting duct can also secrete HCO3-

Question 18

Bicarbonate reabsorption

Answer 18

proximal convoluted tubule reabsorbs 80 percent
distal convoluted tubule reabsorbs 15 percent
the collecting duct reabsorbs 5 percent

Question 19

Renal Carbonic Anhydrase

Answer 19

Carbonic anhydrase is a family of enzymes with different isoforms

Some are intracellular enzymes e.g. red blood cell and kidney epithelial cells lining tubules

Some enzymes are extracellular e.g. found in the brush boarder surface of epithelial cells in kidney

Question 20

Filtered bicarbonate
ions are

Answer 20

“reabsorbed” by facilitated diffusion

Question 21

Bicarbonate handling

Answer 21

Proximal tubule

(CA) Carbonic Anhydrase

Membrane bound CAIV

Cytosolic CAII

Net reabsorption of bicarbonate

Question 22

What happens when all filtered bicarbonate has been reabsorbed?

Answer 22

Additional “new”
bicarbonate ions
are synthesized
from catalysis of
the amino acid
glutamine.

Question 23

Collecting duct-Acidosis:

Answer 23

pH too low
Kidney excretes H+ and reabsorbs HCO3-
causes hypoventilation
ph over 7.4
pCO2 less than 40mm Hg
renal compression causes HCOL3- to be less than 24mEg/L

Question 24

Collecting duct-Alkalosis

Answer 24

pH too high
Kidney excretes K+ and HCO3- ; reabsorbs H+
Hyperventilation
ph less than 7.4
renal compensationn causes HCO3- TO BE MORE THAN 24mEg/L

Question 25

Aterial blood sample normal

Answer 25

Ph 7.4
pCO2 40 mm Hg

Question 26

Renal Response to Acidosis

Answer 26

Sufficient H+ is secreted to reabsorb the filtered HCO3-
More H+ is secreted and adds new HCO3- in the plasma due to non-bicarbonate urinary buffers
Tubular glutamine metabolism and ammonium secretion are increased and adds new bicarbonate to the plasma

Net result: More new HCO3- added to blood and plasma bicarbonate is increased = compensates for acidosis

Urine is highly acidic (potentially pH 4.4)

Question 27

Renal Response to Alkalosis

Answer 27

Amount of H+ secretion is not enough to reabsorb all the filtered bicarbonate so this is excreted in urine
Little or no excretion of H+ on nonbicarbonate buffers
Decreased Tubular glutamine metabolism and ammonium excretion
Type B intercalated cells secrete bicarbonate ions
Net result: Plasma bicarbonate is decreased and compensates for alkalosis
Urine is alkaline (pH >7.4)