Biochem: Fluid and acid base balance

Question 1

Described what happens to the amount of extra and intracellular fluid in pregnancy.

Answer 1

Extracellular fluid increases significantly

Intracellular increases minimally

Question 2

What is the typical rise in plasma volume and interstitial fluid?

Answer 2

Plasma volume: 3 litres to 4.5 litres
Interstitial fluid: 11 to 15 litres

Accounts for 6-8kg of weight gain in pregnancy.

Question 3

How does this impact levels of plasma albumin and what effects does this have?

Answer 3

Fall in concentration of plasma albumin, decreases colloid osmotic pressure, accumulation of interstitial fluid (oedema)

Question 4

What 3 extracellular mechanisms can control pH

Answer 4

Buffer system in blood - responds in seconds
Change in ventilation - minutes
Renal modification to adjust H+ excretion

Question 5

What are the two most important buffers in the extracellular fluid

Answer 5

Bicarbonate

Haemoglobin

Question 6

Write the chemical equation for H2CO3 as a buffer.

Answer 6

C02 + H20 = H2C03 = H+ + HCO3-

Important as impacted by ventilation and exertion of CO2 pushes equation to the left.

Question 7

What is the Haldane effect

Answer 7

Reduced haemoglobin is a weaker acid, as Hb released O2 more H+ are buffered by reduced Hb, more CO2 will dissolve into plasma to maintain the H2CO3 conc. Aids Co2 transport.

Question 8

What is the bohr effect?

Answer 8

Increases in CO2 in RBCs lead to O2 release from haemoglobin.

Question 9

What 3 ways is carbon dioxide carried

Answer 9

• Simple solution in equilibrium with H2CO3
• Carbamino compounds with plasma proteins
• Red bloods cells

Question 10

Within the placenta what is the ‘double bowr effect’?

Answer 10

High levels of CO2 from fetal side of membrane, maternal Hb releases O2. HbF has higher affinity for O2

Question 11

What are the 3 broad categories of metabolic acidosis

Answer 11

Increased production of acid
Increased ingestion of acid
Failure to excrete acid

Question 12

2 broad acategories of metabolic alkalosis

Answer 12

Loss of acid (vomit)

Ingestion of alkali

Question 13

What physiological change to acid-base balance occurs in pregnancy?

Why does this occur?

How does this change in labour?

Answer 13

Slight resp alkalosis.

Compensatory drop in HCO3-
Low pCO2 needed to maintain diffusion gradient across placenta

Labour, more exaggerated maternal hyperventilation no time for renal compensation

Question 14

What is the potential effect of extreme hyperventilation on the foetus pH

Answer 14

Resp and metabolic acidosis

Alkolosis pushs oxyhaemoglobin to left, great affinity for O2 in mother, less available for fetes. More CO2 in solution (resp acidosis)