ACID-BASE

Question 1

Define the pH

Answer 1

The pH is log10 [H+].

Question 2

What is the pH of blood?

Answer 2

7.36–7.44.

Question 3

Where does the acid load (H) in the body come

from?

Answer 3

Most of the H in the body comes from CO2 generated from
metabolism. This enters solution, forming carbonic acid
through a reaction mediated by the enzyme carbonic anhydrase.
co2+ h20 –h2co3–h + hco2
Acid is also generated by
Metabolism of the sulphur-containing amino acids
cysteine and methionine
Anaerobic metabolism, generating lactic acid
Generation of the ketone bodies acetone, acetoacetate and
-hydroxybutyrate

Question 4

What are the main buffer systems in the intravascular,

interstitial and intracellular compartments?

Answer 4

In the plasma the main systems are
 The bicarbonate system
 The phosphate system
 Plasma proteins
 Globin component of haemoglobin
Interstitial: the bicarbonate system
Intracellular: cytoplasmic proteins

Question 5

What does the Henderson–Hasselbalch equation

describe, and how is it derived?

Answer 5

This equation, which may be applied to any buffer system,
defines the relationship between dissociated and undissociated
2 +
4 24 (HPO + H H PO )   
CO H O H CO H HCO 2 2 23 3
\+ − + +  
SURGICAL CRITICAL CARE VIVAS
AACID-BASE
 7
AACID-BASE
acids and bases. It is used mainly to describe the equilibrium
of the bicarbonate system.

Question 6

Which organ systems are involved in regulating

acid-base balance?

Answer 6

The main organ systems involved in regulating acid-base
balance are

Respiratory system: this controls the pCO2 through
alterations in alveolar ventilation. Carbon dioxide
indirectly stimulates central chemoceptors (found in the
ventro-lateral surface of the medulla oblongata) through
H released when it crosses the blood-brain barrier
(BBB) and dissolves in the cerebrospinal f luid (CSF)

Kidney: this controls the [HCO3
], and is important for
long term control and compensation of acid-base
disturbances

Blood: through buffering by plasma proteins and
haemoglobin

Bone: H may exchange with cations from bone mineral.
There is also carbonate in bone that can be used to
support plasma HCO3
levels

 Liver: this may generate HCO3
 and NH4
 (ammonia)
by glutamine metabolism. In the kidney tubules, ammonia
excretion generates more bicarbonate

Question 7

How does the kidney absorb bicarbonate?

Answer 7

There are three main methods by which the kidneys increase
the plasma bicarbonate
Replacement of filtered bicarbonate with bicarbonate
that is generated in the tubular cells
Replacement of filtered phosphate with bicarbonate that
is generated in the tubular cells
By generation of ‘new’ bicarbonate from glutamine that is
absorbed by the tubular cell

Question 8

Define the base deficit

Answer 8

The base deficit is the amount of acid or alkali required to
restore 1 l of blood to a normal pH at a pCO2 of 5.3 kPa and
at 37°C. It is an indicator of the metabolic component to an
acid-base disturbance.The normal range is 2 to 2 mmol/l.