BPK 305 - Lecture 31

why is H+ regulation essential?

the activities of almost all enzymes and proteins regulated by H+

acidosis

pH below 7.35

alkalosis

pH above 7.45

what is the blood buffered by?

blood proteins and haemoglobin

what do buffers do?

reversibly bind H+ and stabilize pH

buffer power

# moles strong base/1 pH unit change

why is pH lower in venous blood?

higher CO2

Henderson-Hasselbalch equation

pH = 6.1 + log [HCO3-]/0.03xPaCO2

plasma pKa

6.1

what is blood pH proportional to?

[HCO3-] / PaCO2

what are pH disturbances due to?

- a change in [HCO3-]

- or a change in PaCO2

changes in HCO3-

metabolic

changes in PaCO2

respiratory

how is HCO3- controlled?

kidneys

PaCO2 controlled

lungs

how are acid-base disturbances compensated?

1. Chemical buffering

-fast but limited capacity

2. Change in ventilation

-slower but less limited

3. Change in kidney secretion

-slowest but greatest capacity

how are metabolic disturbances compensated?

respiratory changes

how are respiratory disturbances compensated?

metabolic changes

Davenport diagram

Helps graphically estimate simultaneous HCO3- and non HCO3- buffer equilibrium

what is the slope of the Bn buffers?

buffering power

non HCO3- buffer line

- Hb and blood protein buffers

- change in HCO3- required for equilibrium

why does respiratory acidosis occur?

when lung gas exchange is

impaired

- barbiturate drugs or

diseases like pneumonia or emphysem

what happens during respiratory acidosis?

- PaCO2 increases

- decreases pH

- increases [HCO3-] along the buffer line

metabolic compensation of respiratory acidosis

- increased renal H+ secretion

- drives renal reabsorption of

HCO3-

- PcCO2 stays elevated

renal H+ secretion and HCO3- generation

• The kidneys reabsorb almost all filtered HCO3-

• Basolateral membrane is permeable to CO2 which converts to HCO3- + H+ with the help of carbonic anhydrase (CA)

• H+ moves across the apical membrane into tubular fluid by different mechanisms,

• Increased H+ excretion drives HCO3- uptake into the blood

what is respiratory alkalosis due to?

Develops due to hyperventilation

- altitude or anxiety

what does respiratory alkalosis do?

- PaCO2 decreases

- increases pH

- decreases HCO3- along buffer line

metabolic compensation for respiratory alkalosis

- decreased renal H+ excretion

- decreases plasma [HCO3-]

- PCO2 low

what causes metabolic acidosis?

ketoacidosis, diarrhea

what does metabolic acidosis do?

- decreases [HCO3-]

- decreases pH

respiratory compensation for metabolic acidosis

- increased [H+] stimulates peripheral and central

chemoreceptors

- resulting hyperventilation reduces PCO2

- HCO3- reduces along buffer line

metabolic alkalosis causes

vomiting

metabolic alkalosis affects

- increased pH

- increases [HCO3-] along isopleth

respiratory compensation for metabolic alkalosis

- decreased H+ reduces chemoreceptor

stimulation inducing hypoventilation

- causes plasma CO2 retention

- causes pH to fall to normal along buffer line

acid base nomogram

• shaded area: normal compensatory limits

• clinically useful

• only require measurement of blood pH and

PCO2 for diagnosis