Physiology of pregnancy

Question 1

State three demands of the foetus

When might these be increased

Answer 1

Nutrients (O2, AAs, glucose)
Amniotic fluid production
Removal of foetal waste products (nitic oxides, CO2)

Require increased nutrient content (GI) and oxygen content (pulmonary and CVD)

Question 2

Consider volume homeostasis

How does this change during pregnancy?
What is plasma colloid osmotic pressure? What happens to it?

Answer 2

Rapid increase in plasma volume by 40% (from 2.5L to 3.7L)

Plasma colloid osmotic (oncotic) pressure is a form of osmotic pressure induced by proteins (albumin), in a blood vessel’s plasma that displaces water molecules, thus creating a relative water molecule deficit with water molecules moving back into the circulatory system.

It falls in pregnancy causing a shift of fluid into extracellular space, increased hydration of CT, oedema of lower limbs, hands and face.

Question 3

Consider volume homeostasis

Outline the mechanism by which plasma colloid oncotic pressure falls in pregnancy

What is the function of this

Answer 3

Kidneys secrete RENIN which converts angiotensinogen (from liver) to angiotensin 1. This is stimulated by oestrogen

ACE from the lung converts angiotensin 1 into AT2

AT2 causes the secretion of aldosterone. This is stimulated by oestrogen.

Aldosterone causes increased Na+ and water reabsorption by the nephron.

Slight decrease in ANP, decrease thirst threshold (increased fluid intake), resetting osmostat, increased plasma volume

Question 4

Consider the physiological changes to red blood cells in pregnancy

How does the [RBC] change?
How is this facilitated?

Why?

Answer 4

RBC mass increases by 25%
- Iron is required for the increase in RBC mass: a fall in ferritin levels increases iron absorption from guy. Only need exogenous Fe supplementation in twin cases

Delivery of blood to the uterus increases 3.5 fold

Question 5

Consider the physiological changes to red blood cells in pregnancy

Discuss haemostasis
Effect?

Answer 5

Hypercoagulable state: increase plasma fibrinogen, platelets, factor 8 and vW factor

This has a marked effect at delivery as 500ml/min blood loss at placental seperation
- myometrial contraction uses up 10% of ALL fibrinogen

This is an evolutionary balance between thrombosis and haemorrhage

Question 6

Consider the physiological changes to white blood cells in pregnancy

What happens?

Answer 6

Concentration doesnt fall during pregnancy . Total WBC increases
- increase in neutrophils (due to reduction in apoptosis)

Marked increase around delivery

Question 7

Consider the physiological changes to the CVS in pregnancy

What are the implications of increased blood volume?

Answer 7

Effects
- CO (=SV x HR)

Heart enlarges by 12% due to increased venous return. This is the cause of innocent systolic murmurs commonly heard (90%). Reflect high blood flow across AV valves. Diastolic murmurs (20%) should be investigated to rule out any pathology

• peripheral resistance (progesterone causes peripheral vasodilation by 35%)
• BP (decreased resistance partly compensated by increase in CO results in small changes in BP. SBP and DBP slightly decrease until week 20, where they both increase)

Question 8

What are the physiological changes to the respiratory system in pregnancy

Answer 8

• Increased pulmonary blood flow matched by increased tidal flow, decreased maternal pCO2 and increased maternal pO2

Increased maternal pO2 aids passive diffusion at the placenta (greater concentration gradient)