Acid Base Balance

Question 1

Why does body pH need to be regulated within fine limits?

Answer 1

Small changes in pH have a large effect on body function. This can cause denaturing of enzymes, sometimes permanently, which disables their catalytic activity

Question 2

What is the normal pH range?

Answer 2

7.35 - 7.45

Question 3

What is the pH of alkalosis?

Answer 3

Above 7.45

Question 4

What is the pH of acidosis?

Answer 4

Below 7.35

Question 5

What are some examples of systems where pH sits outside the average?

Answer 5

Gastric secretions (0.7), cerebrospinal fluid (7.3), pancreatic secretions (8.1), final urine (5.4)

Question 6

What are some sources of acids and alkalis?

Answer 6

Metabolism - large amount of acid

Diet - alkali (fruit) and acid

Question 7

What are the systems that act to compensate for changes in the acid base balance?

Answer 7

Blood and tissue buffers (work in seconds)
Respiration (works in minutes)
Renal (takes hours/days)

Question 8

What is the common cause of death from kidney failure?

Answer 8

Acidosis

Question 9

What are some examples of pH buffers?

Answer 9

Haemoglobin, HCO3-, inorganic phosphate, weak acids/bases on proteins

Question 10

How do buffers work?

Answer 10

When an acidic substance enters the blood stream: bicarbonate ions neutralise the hydronium ions forming carbonic acid and water
When a basic substance enters the blood stream: carbonic acid reacts with the hydroxide ions to produce bicarbonate ions and water

Question 11

Discuss the Davenport diagram

Answer 11

It is a graphic display of the acid-base state
They illustrate the dynamic relationship between arterial blood pH, buffers, and pCO2
<7.4 and <24 [HCO3-] = metabolic acidosis
<7.4 and >24 [HCO3-] = respiratory acidosis
>7.4 and <24 [HCO3-] = metabolic alkalosis
>7.4 and >24 [HCO3-] = respiratory acidosis

Question 12

What are the three mechanisms that the kidney utilises to excrete hydrogen ions?

Answer 12

HCO3- handling (90% in proximal tubule, 10% in distal tubule)
Urine acidification
Ammonia synthesis
(UA and AS use bicarbonate generated in the cell)

Question 13

Discuss how HCO3- handling leads to the excretion of acid in the urine

Answer 13

Recycles bicarbonate by combining with H+ to form H2CO3
Carbonic anhydrase converts H2CO3 into CO2 and H20 and they can now pass into the proximal cell
Carbonic anhydrase inside the cell converts then back to H2CO3
H2CO3 is coverted into H+ and HCO3-
H+ moved back into proximal tubule through NHE3
HCO3- moved into the blood

Question 14

Discuss how urine acidification leads to the excretion of acid in the urine

Answer 14

An alkaline salt present in the tubular fluid will combine with the H+ to produce an acid phosphate (Na2HPO4 to NaH2PO4)
Urine pH can’t get too low so this system buffers the urine, picks up a H+ ion and secretes it
NaK ATPase creates a driving force for Na to enter through the NHE3 exchanger, simultaneously removing H+
Filtered Na2HPO4 loses a sodium ion to become NaHPO4- where it can then pick up a H+
CA converts CO2 + H20 into H2CO3 which can then be broken down into H+ + HCO3-
This H+ is used in the NHE3 exchanger and HCO3- is moved into the blood

Question 15

Describe how ammonia synthesis leads to the excretion of acid in the urine

Answer 15

Glutamine is broken now into alpha keto-glutarate, producing NH3 + H+ in the process
NH3 is secreted from the cell and combines with H+ to generate NH4+
H+ secreted in exchange for Na+ moving in and H+ present in the tubular fluid combine with NH3
Diffusion trapping
HCO3- absorbed

Question 16

Why can H+ and NH3 not combine within the cell and then be secreted?

Answer 16

NH3 is permeable whereas NH4+ isn’t. The cell therefore has to generate NH3 and secrete it where it can combine with H+ in the tubule