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MEDSCI 205 > Fetal physiology > Flashcards

Fetal physiology Flashcards

1
Q

Placenta purpose

A

Gas (O2/CO2) and nutrient transport, waste removal

Endocrine organ - maintenance of pregnancy and fetal development

Barrier to prevent immunologic attack by mum

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2
Q

Haemochorial placenta

A

Fetal and maternal blood do not mix directly

Fetal placental tissue is suspended in maternal blood

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3
Q

Basic structure of mature placenta

A

Fetal side: Chorionic plate and villi
Maternal side: Decidua basalis, intervillus space
Exchange area: Intervillus space

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4
Q

Intervillous space - maternal exchange area

A

Holds about 500-600mls

Placental perfusion ~500-800ml/min

Blood completely replaced ~2-3 min

Spiral arteries eject blood at ~70mmHg into intervillous space
Intervillous pressure low ~10mmHg
Gradient promotes efficient diffusion

Fetal placental villi are suspended in the intervillous space so are
bathed in maternal blood

Remember: Maternal and fetal circulations and thus blood do not
touch directly.

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5
Q

Villi - fetal part of the placenta

A

Site of gas/nutrient/waste exchange unit between mum & fetus

Gases diffuse across the villi

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6
Q

Intervillous pool

A

Location: Within the placenta, between chorionic villi.
Function: Facilitates the exchange of nutrients, gases, and waste between maternal and fetal blood.
Blood Supply: Maternal blood enters through spiral arteries.

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7
Q

Diffusion of Gases in Intervillous Pool

A

Oxygen Diffusion: From maternal blood in the intervillous pool to fetal blood in the chorionic villi.

Carbon Dioxide Diffusion: From fetal blood in the chorionic villi to maternal blood in the intervillous pool.

Exchange Mechanism: Passive diffusion driven by concentration gradients.

Importance: Ensures the fetus receives oxygen and expels carbon dioxide effectively, critical for fetal health.

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8
Q

Amniotic fluid functions

A
  1. Hydraulic brace – protective buffer (like CSF in your brain) – protects the cord from compression.
  2. Permits fetal body and breathing movements (fetal behaviour).
  3. Fluid reservoir – blood volume, electrolyte balance, fetal and
    maternal fluid balance.
  4. Nutrient reservoir – swallowing
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9
Q

In and Out of amniotic fluid

A

In: Amniotic fluid is made of fetal urine/lung liquid – but
mainly fetal urine

Out: swallowed, absorbed by placenta, umbilical cord,
skin (until mid-gestation when the skin starts to
keratinise)

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10
Q

Why does the fetus make body and breathing movement

A

Exercising muscles ready for birth
Movements also co-ordinate brain-body
connections
Growing lungs:
Lungs fluid filled with lung liquid
Respiratory muscle contractions moves lung fluid
Lung fluid movements stretches lungs develops
alveoli
If this is prevented, lungs don’t grow very well (lung
hypoplasia)

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11
Q

Why does the mature fetus not continue to do everything at once like the immature fetus?

A

From 32 weeks, the fetus significantly increases growth rate
(and thus energy demand)
Mum cannot supply significantly more food or oxygen (energy)
Solution: practice different movements at different times regulated by sleep states, so there is energy to grow and
exercise

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12
Q

Mature sleep states and behaviours

A

Mature fetuses compartmentalise behaviour relative to sleep state for energy management – to grow and be physically active at the same time.

These general behaviours go with these sleep states
Rapid eye movement (REM) sleep
^Fetal breathing movements (FBMS)
^Swallowing
^ Licking,
^ Eye movements
X Few body movements (atonia or sleep paralysis)

Non-REM (NREM) sleep
^ Body movements
X Fetal breathing movements (FBMS)
X Swallowing
X Licking,
X Eye movements

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13
Q

Fetus survival at low PaO2

A

The fetus has a low PaO2 but a high saturation (SaO 2) > 70% (left shifted oxygen dissociation curve)

Has more O2 than needed – usually a surplus

Needs a lot of O2 due to high metabolic demand (growth and organ
function). Nearly 2x the adult requirement

The fetal PaO2 is low only because oxygen diffuses down a gradient from mother to fetus.

The fetus cannot be higher than the lowest PO2 in mum (her venous PO2).

The fetus compensates for the low PaO2 to ensure good saturation.

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14
Q

How does the fetus compensate for a low PaO2

A

Different type of haemoglobin – alpha and gamma, can
hold 2x the amount of O2 than an adult.

Gamma Hb resistance to 2,3 DPG – the
organophosphate that encourages Hb to release O2

Acidity at tissues and within placenta encouraging
release of O2 from haemoglobin (Bohr effect)

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15
Q

Fetal haemoglobin

A

Hb structure allows Hb to hold 2x the amount of O2 vs.
adult

Adults = 4 moles of O2/1mole of Hb.
Fetus = 8 moles of O2/1mole of Hb

Fetal Hb has different protein structures
Adults: 2 alpha chains & 2 beta chains.
Fetus: 2 alpha chains & 2 gamma chains
Gamma chain differs in amino acid sequences; specifically has
serine not histidine at position 143.

Different amino acid sequence effects 2,3DPG binding

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16
Q

2,3 - DPG

A

2,3 DPG: Organophosphate from erythrocytes

2,3 DPG acts to reduce Hb O2 affinity

It encourages Hb release of oxygen

Adults: 2,3 DPG binds to the beta Hb protein mainly due to positively charged amino acid histidine 143

Fetus: in the gamma chain, histidine is replaced by serine which
has a neutral charge

Neutral charge reduces 2,3 DPG binding. It encourages Hb holding of O2

↑2,3 DPG binding = O2 release
↓2,3 DPG binding = O2 attraction

17
Q

Bohr effect

A

Tissue respiration during metabolism = PCO2 ^ acidity

Bohr effect = in the presence of acid, Hb release O2

PaO2 gas diffusion gradient blood vessel (higher) to tissue (lower)

18
Q

Haldane effect

A

Empty Hb preferentially takes up CO2.

PaCO2 gas diffusion gradient tissue (higher) to blood vessel (lower)

19
Q

Bohr & Haldane in the placenta

A

Fetal CO2 diffusion into the in the pool acidifies maternal blood (Bohr)

Acidification causes maternal O2 release (Bohr) – diffusion to fetus

++ maternal increase in 2,3DPG throughout pregnancy to help O2 release

Empty maternal Hb takes up CO2 (Haldane), CO2 transported to maternal lungs

++ Fetuses ^ metabolism with advancing age

Mum compensates with changes in
breathing decreasing her CO2 (compensated
respiratory alkalosis) to increase gas
gradient

20
Q

Adult oxygen dissociation curve

A

Describes PaO2 - SaO2 relationship and how haemoglobin acquires and releases O2 molecules

21
Q

Fetal circulation and shunts

A

Umbilical cord
Ductus venosus - liver
IVC
Foramen ovale = atrium
Ductus arteriosus = lung bypass

22
Q

Stream one - oxygenated blood from placenta

A

Placenta
Umbilical vein
Ductus venosus
IVC, no mixing
Right atrium
Foramen ovale
Left atrium
Left ventricle
Ascending Aorta

23
Q

Umbilical cord has

A

2x arteries - carry deoxygenated blood to the placenta for oxygenation

1 x vein: carries oxygenated blood from the placenta to the fetus

Whartons jelly

24
Q

Whartons jelly

A

Surrounds blood vessels and protects them from being
squeezed

Gelatinous substance similar to that found in eyeballs, contained
in a connective sheath

Rich source of stem cells
Umbilical cord

25
Q

Ductus venosus (from liver to IVC)

A

The DV = duct between UV
and the IVC

Arterial blood is shunted
through the DV into the IVC

The DV duct is very narrow

DV is narrow = greater
pressure ejection (faster) of
arterial blood into IVC

26
Q

DV to IVC

A

The IVC now has 2 streams of differently oxygenated blood

Oxygenated blood is travelling faster than the deoxygenated blood

The speed prevents mixing of the 2 streams of blood

Can do this effectively for the short distance from DV to right atrium

27
Q

Foramen ovale

A

FO is a flap between left and right atria

FO flap kept open by higher pressures in right atrium due to high-speed arterial blood flow. DV ejection blood flow

This streams highest oxygenated blood into ascending aorta

Circulation that feeds organs with highest metabolic demand - brain and heart

28
Q

Mixing in the right atrium

A

Stream one adds oxygenated blood to stream two in the right atrium

This increases the oxygen saturation of stream two

29
Q

Stream two - returning deoxygenated blood

A

IVC and SVC
IVC, no mixing
Right atrium – pick up some O2 in the RA from stream 1
Right ventricle
Pulmonary artery
Most blood bypasses lung through ductus arteriosus
Descending aorta – pick up more O2 blood from the aortic arch spillover - aortic isthmus

30
Q

Ductus arteriosus - lung bypass

A

Blood does not go to lung to be oxygenated

Lungs need some blood for growth, around 8% of cardiac output

Remaining blood is shunted past lungs and into the descending aorta via the ductus arteriosus.

De-oxygenated blood returning to the heart from IVC and SVC picks
up oxygen
In the right atrium
From aortic isthmus spill-over

31
Q

Causes of hypoxia

A

Cord occlusion, cord knots, prolapse
Placenta: impaired formation, abruption, haemorrhage

Delivery in the wrong position – e.g. feet first

Premature closure of the ductus arteriosus

Maternal respiratory illness/anaemia

32
Q

How do adults and fetuses
detect hypoxia?

A

Peripheral chemoreceptors – carotid and
aortic arch (fetus = carotid dominance)

Central (brain) chemoreceptors -
brainstem, midbrain

Collectively they detect changes in O2,
CO2 - pH

Initiate quick chemoreflex responses

Added to by slower acting factors (eg
endocrine, endothelial factors) to change
respiration and cardiovascular function

33
Q

Fetal response to hypoxia

A

Fetal heart rate: Reduce heart rate to reduce metabolic demand -
protect the heart

Blood flow: Send more blood to key organs (e.g. brain, heart) to
support organs with higher metabolic demand

Reduce blood flow to liver: Increase blood flow from placenta (stream one) to key organs and stream more blood through foramen
ovale

Haemoconcentrate: reduce fluid in blood vessels – send it to extracellular space – this aggregates red blood cells and increases O2 delivery per unit of blood

Behaviour: Reduce energy/O2 demands such as body and breathing
movements

34
Q

Fetal heart: Parasympathetic chemoreflex

A

Role for parasympathetic in
bradycardia can be demonstrated by
1. Cut the vagus nerves
2. Giving a parasympathetic
muscarinic receptor blocker like atropine.

Chemoreflex control can be
demonstrated by
1. Cutting carotid sinus nerve

Purpose of the fall in FHR?
1. Reduces cardiac work, protects the heart from injury during low O2

Prolonged bradycardia may also be caused by the heart becoming gradually hypoxic itself

35
Q

Peripheral vascular resistance: Sympathetic chemoreflex

A

Role for sympathetic in
vasoconstriction can be
demonstrated by
1. Giving an alpha adrenergic
antagonist.

Chemoreflex control can be
demonstrated by
1. cutting carotid sinus nerve

Why does peripheral blood flow fall?
1. Sends blood flow to vital organs with highest metabolic demand

  1. Supports blood pressure when heart rate falls
36
Q

Peripheral vasoconstriction

A

Blood pressure = determined by CO
(stroke volume and heart rate) +
peripheral resistance.

Fetal CO is mainly controlled by heart rate as immature heart structure limits stroke volume.

The fall in heart rate can cause a big change in BP.

Peripheral vasoconstriction supports BP during hypoxia.

Blood pressure is vital for blood flow and O2 delivery and CO2 removal

37
Q

Blood flow to the brain : not a carotid chemoreflex
Endothelial activity

A

The change in blood flow to
the brain is not a chemoreflex

Cutting the carotid sinus nerve
does not change brain blood
flow.

It is not due to the increase in blood pressure

It is actively mediated by
vasodilatation due mainly to
endothelial vasodilators like
nitric oxide

This is demonstrated by using nitric oxide donor inhibitors

38
Q

Haemoconcentration and shunting

A

Fetus will concentrate red blood cells.

Does this by removing fluid out of the blood vessels to the extracellular space.

This helps concentrate red blood cells, reduces diffusion area at the
tissue site

Ductus venosus – less blood to liver.

Greater streaming into IVC – due to increased flow

Less mixing in RA - due to increased flow

More well oxygenated blood (stream 1) crosses the foramen ovale

39
Q

Energy conserving behaviour

A

Switch to non-REM brain activity or reduced brain activity

NREM lower metabolic state – conserves energy

Isoelectric (near complete suppression) if severe.
Activity and growth

Stop making breathing movements, licking, swallowing

Not chemoreflex as carotid denervation does not change them. Apnea is due to
brainstem centres.

Stop making body movements

Not chemoreflex. In fact the fetus makes some body movements to begin with to
try and move away from cord obstructions.

If hypoxia is prolonged and the fetus has adapted, some movements will return to help lungs and body prepare for birth but at the expense of somatic (body) growth

MEDSCI 205 (7 decks)

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