Foetal Physiology Flashcards
How does meternal-foetal exchange occur?
Happens at the placenta
Chorionic villi increase the surface area of foetal capillaries.
Maternal side has umbilical arteries and veins.
Uterine arteries and veins - maternal blood lakes in the intervillous spaces.
How is gas exchange at the placenta effective?
Small diffusion barrier that decreases as pregnancy proceeds.
Gradient of partial pressures required - to make this work, maternal pO2 increases marginally and, foetus is in Hypoxic conditions to maintain gradient.
How do you increase foetal oxygen content?
Hb variant - HbF
Foetal haematocrit increased over adult
Increased maternal production of 2,3 BPG - secondary to physiological respiratory alkalosis of pregnancy.
Double bohr effect
What other factors promote oxygen exchange to foetus at placenta?
2,3 BPG increased
Foetal Hb
Double Bohr effect
What is the difference betwen HbF and HbA?
2 gamma subunits instead of beta
Greater affinity for oxygen because it doesn’t bind 2,3-BPG
HbF is thepredominant form of Hb from 12 weeks.
What is the double Bohr effect?
- Speeds up the process of oxygen transfer
- As CO2 passes into intervillous blood, pH decreases
- Bohr effect
- Decreasing affinity of Hb for O2
- At the same time as CO2 is lost, pH rises
- Bohn effect
- Increasing affinity of Hb for O2
The mother physiologically adapts to pregnancy by progesterone-driven hyperventilation which lowers the pCO2 in maternal blood and creates a concentration gradient.
What is the double haldane effect?
- As Hb gives up oxygen, it can accept increasing amounts of CO2.
- Foetus gives up carbon dioxide as oxygen is acceptoed
- No alterations in local pCO2
Describe the foetal circulation and shunts.
- Recieves oxygenatedblood from mother via placenta in umbilical vein
- Lungs are non-functional so they are bypassed
- Returns to the placenta via umbilical arteries.
- The three shunts are:
- Foramen Ovale - RA-LA
- Ducts arteriosus - Pulmonary Trunk-Aorta
- Ductus Venosus - Placenta-IVC
Why do we need the ductus venosus?
- DV connects umbilical vein carrying oxygenated blood to the IVC
- Blood enters right atrium
- By ensuing shunting of blood around the liver, saturation is mostly maintained -drops from 70% to 65%
Why do we need the foramen ovale?
- Right atrial pressure is greater than that in left atrium.
- Forces leaves of FO apart and blood flows into LA
- Free border of septum secundum forms a ‘crest’ -Crista dividens
- Creates a stream of blood flow
- Majority flows to LA
- Minor proportion flows to RV - mixing with blood from SVC
What organs get the most blood?
Brain and heart - needed to keep alive.
Why do we need the ductus arteriosus?
Shunts blood from RV and PT to aorta.
Joins aorta distal to supply to the head and heart to minimise the drop in oxygen stats
How does the foetus respond to hypoxia?
- Has HbF and increased [Hb]
- Redistribution of flow to protect supply to heart and brain (reducing supply to GI tract, kidneys, limbs)
- Foetal heart rate SLOWS in response to hypoxia to reduce oxygen demand
- Foetal chemoreceptors detecting decreased pO2 or increased pCO2.
- Vagal stimulation leadign to bradycardia
- cf adult where vagal stimulation leads to tachycardia
- Chronic hypoxaemi
- Growth restirction
- Behavioural changes - impact on development
What hormones are necessary for foetal growth?
- Insulin
- IGFI - nutrient dependant, dominates in T2 and T3
- IGFII - nutrient independant, dominant in first trimester
- Leptin - Placental production
- EGF
- TGFa
What is the dominant cell growth mechanism during the different phases of pregnancy?
- 0-20 weeks - Hyperplasia
- 20-28 weeks - Hypertrophy and hyperplasia
- 28 weeks- term - Hypertrophy
How is amniotic fluid produced and recycled?
Amniotic fluid is mostly foetal urine after 9 weeks. In the 3rd trimester up to 800ml a day can be porduced.
- Amniotic fluid is swallowed
- It goes through foetal lungs and GI tract where it absorbs water and electrolytes
- Debris acculumates in the gut
- Then the remainer leaves as foetal urine
