Physiological changes at birth

Question 1

fetal circulation intro

Answer 1

Fetal circulation is adapted to best use a poor oxygen (O2) supply. Non-functional organs (which are bypassed) include the lungs, gut, and liver.
Even if blood is flowing to/from fetal heart, remember though that the exchange (of e.g. O2) occurs at the placenta..!
- Placenta is an exchange organ  high blood flow.
- Non-functional organs, i.e. are not ‘in use’  so, are restrictions on blood flow:
3 fetal (vascular) shunts differentiate the fetal from adult circulation.

Question 2

fetal adaptations: 3 shunts

Answer 2

Foramen ovale and the ductus arteriosus enable circulation to mostly bypass the lungs:
- Foramen ovale is a gap formed by the incomplete fusion of the (atrial) septum.
- Ductus arteriosus is a vessel connecting the pulmonary trunk(/artery) to the aorta.
Ductus venosus enables the circulation to mostly bypass liver:
- Is a continuation of the umbilical vein; connects to the inferior vena cava.

Question 3

fetal circulation overview

Answer 3

Note the purple colour, i.e., mixing oxygenated (from umbilical vein) and deoxygenated blood in the fetal vessels.
Little blood flow to lungs results in left atrium containing little blood; has very low blood pressure  ensures blood flows from right, to left atrium via the foramen ovale…

Question 4

steps of fetal circulation

Answer 4

1. Blood (mostly) bypasses lungs by flowing from the pulmonary trunk through the ductus arteriosus to aorta.
2. Blood also bypasses lungs by flowing from right to left atrium through the foramen ovale.
   - The opening/gap is covered by a flap of tissue that acts as a 1-way valve.
3. Blood bypasses the liver sinusoids by flowing through the ductus venosus.
4. Oxygenated blood is returned to fetus from placenta by the umbilical vein.
5. Deoxygenated blood is carried from fetus to placenta through the umbilical arteries.

Question 5

placenta exchange by diffusuion: partial pressure gradients

Answer 5

Blood is quite deoxygenated and hypercapnic: i.e. has abnormally high level of CO2 in arterial blood (PaCO2 >45 mm Hg; 6 kPa).
Fetal blood is not in equilibrium with maternal blood – why..?
(1) Not all maternal blood is in contact with chorionic villi.
(2) Placenta itself uses 20% of the maternal O2 supplied.

Question 6

fetal vs adult circulation

Answer 6

Fetus: the two sides of the heart work in parallel; largely bypass organs with little/no function (via 3 shunts).
Adult: serial arrangement

Question 7

fetal circulation: route taken by blood

Answer 7

draw flow chart

Question 8

Partuition triggers

Answer 8

Parturition: events leading to, and including delivery of fetus.
Parturition is a multifactorial process: both maternal and fetal nervous and endocrine systems are involved (usually, weeks 38-40).
The signals that bring pregnancy to an end (i.e. the triggers for the onset of parturition) in women are not well understood:
- Fetal cortisol, rapid rise in oestrogens (from placenta), prostaglandins (PGF2α) and oxytocin all likely play a role in initiation of parturition

Question 9

Parturition- at the end of gestation

Answer 9

Corticotropin-releasing hormone (CRH)  adrenocorticotropic hormone (corticotropin; ACTH)  cortisol.
Also: stretching of myometrium; thins and ↑ in excitability…
- Delivery is produced by strong rhythmic contractions of the myometrium (and by mums pushing!).
↓ in progesterone  less myometrial relaxation/quiescence; now ↑ in sensitivity to other agents which ↑ myometrial excitability.
↑ in oestrogens  ↑ myometrial excitability, so ↑ contractile activity; ↑ sensitivity to other agents (e.g. oxytocin).

Question 10

bringing forth young

Answer 10

draw flowchart with positive feedback