Placenta Flashcards
How much blood is contained within the intervillous space of a mature placenta?
150 mL which is replenished 3-4 x each minute.
What makes up the fetal portion of the placenta?
Villi of the chorion frondosum
What makes up the maternal portion of the placenta?
Decidua basalis
How many umbilical arteries does a normal umbilical cord contains, and what type of blood does it carry to the placenta?
There are TWO umbilical arteries and they carry DEOXYGENATED blood from the fetus to the placenta.
Across what type of chorionic villi does exchange between maternal and fetal blood occur?
Across BRANCH villi
Describe the order of blood flow of fetoplacental circulation
Poorly deoxygenated blood leaves the fetus –> umbilical arteries –> chorionic arteries –> arteriocapillary venous system within chorionic villi –> gas exchange occurs, oxygenated floor flows –> chorionic veins –> umbilical vein –> fetus
Describe the order of blood flow of the maternal-placental circulation
Oxygen rich blood leaves mother through spiral arteries in decidua basalis –> intervillous space –> exchange occurs around branch villi -→ main villli → umbilical vein → fetal circulation
List the functions of the placenta (5): TEEMP:
Transport of gases, nutrients, drugs and infectious agents
Excretion of waste products
Endocrine synthesis and secretion e.g. hCG
Metabolism e.g. synthesis of glycogen
Protection by maternal antibodies
Development of the placenta
- The maternal portion of the placenta develops from: decidua basalis and forms the basal plate
- The fetal portion of the placenta develops from: villi of the chorion frondosum and forms the chorionic plate
- Invasion of the synctiotrophoblast into decidua around day 6 triggers decidualisation along with progesterone secreted from corpus luteum
- Causes remodelling of spiral arteries to allow high flow, low impedance circulation to placenta
- Synciotrophoblast develops lacunae, which are filled with blood from the spiral arteries as they are invaded
- Cytotrophoblast grow onto the lacunae and form the chorionic villi, with branching villi forming the interface for gas and nutrient exchange within placenta, and anchoring villi grow towards the basal plate and attach to the decidual tissue
- Capillaries from the umbilical artery/vein grow into the villi
- The lacunae form in the intervillous space which is filled with maternal blood
- This provides high surface area interface for transfer of gas and nutrients
- Early placental circulation is formed by end of second week
Placental hormones and how they affect availability of glucose to baby
Later in pregnancy the placental syncytiotrophoblast release placental lactogen and placental growth hormone
These hormones affect fetal growth by increasing maternal insulin resistance and therefore available glucose.
GH increases placental growth and nutrient transporter activity Imbalance or dysfunction of this leads to abnormal growth in diabetes/obesity
Metabolic adaptations in pregnancy
First half of pregnancy:
- maternal food intake increases 10–15%,
- intestinal calcium absorption doubles,
- 60% rise in first-phase insulin secretion which facilitates maternal fat storage.
Second half:
- Maternal food intake and fat mass escalate
- Maternal metabolism becomes insulin resistant. This facilitates maternal utilization of free fatty acids as an energy source, sparing glucose, amino acids, essential fatty acids, and ketones for placental-fetal transport and fetal growth.
Facilitated by hormones produced by the placenta:
- Increase in oestrogen and progesterone
- Raised PRL
- Raised human placental lactogen (similar to growth hormone and prolactin)
- Placental growth hormone
Anatomy of the placenta
How is the fetus adapted to the relative hypoxic intrauterine state?
- Increased heart rate to expedite O2 delivery to tissues
- Higher O2 afffinity of fetal Hb
- Higher Hb concentration relative to adults
- The intervillous space creates a pool of blood to allow ongoing oxygen exchange even when the spiral arteries are compressed during a contraction
What affects the amount of oxygen diffusion from the intervillous space to the chorionic villi?
- Placental surface area (size, abrution, failure of spiral arteries to dilate)
- Uterine contractions will reduce blood flow via spiral arteries
- Fetal acidosis will affect fetal HB oxygen affinity
- Maternal hypoxia
- Maternal hypovolemia/hypotension
When does placental development begin?
At blastocyst implantation around day 7-8 of embryogenesis, where syncitiotrophoblast begin to invade into decidua basalis
What are the features of decidualisation and what factors drive the process?
Causes:
- Rising progesterone
- Blastocyst invasion
Features:
- Increased vascularisation and swelling of maternal vessels
- Increased oedema
- Increased glycogen and lipid stores in endometrial cells
- Synciototrophoblast driven erosion of endometrium at implantation site
When do cotyledons form and how?
Around 4 months - decidua septa divide placenta in 15-20 discrete cotyledons
What layers does gas exchange occur between fetal side of the placenta and the junctional zone?
Syncitiotrophoblast and endothelial cells of fetal circulation
What hormones are made by the placenta?
9 hormones!!
- HCG (secreted by syncitiotrophoblast; has 2 subunits - a and B - alpha unit is identical to LH/FSH/TSH; peaks at 9-12 wks)
- hPL (is anti-insulin - i.e increases insulin resistance)
- oestrogen
- progesterone
- corticosteroid
- CRH
- Relaxin (causes relaxation of ligaments and cervical stroma in preparation for childbirth)
- ACTH
- TSH
What are the roles of the placenta?
Gas exchange
- Branching villi provide high surface area for gaseous exchange
- Higher affinity of fetal Hb and delivery of CO2 from fetal circulation encourages passive transfusion of oxygen from maternal to fetal RBCs by the Bohr effect
- CO2 moves from fetal to maternal blood by passive transfer, driven by haldane effect - the increasing presence of deoxyhaemoglobin in mothers blood and increasing oxygenation of fetal Hb encourages CO2 transfer down diffusion gradient
Nutrient exchange
- Transfer of glucose, essential amino acids, fatty acids, electrolytes and vitamins for fetal growth and development, via active transport and passive transfusion
Removal of waste products
- Diffusion of urea and CO2 from fetal to maternal circulation
Immune defence
- Physical barrier and filtration of microbes
- Allows transfer of maternal IgG to allow passive immunity for fetus
Metabolism
- Secretes HPL and GH to reduce insulin sensitivity and increase serum glucose and delivery to fetus
- Converts maternal metabolism from predominantly glucose to predominantly fatty acid metabolism to maximise glucose available for fetal requirements
Endocrine function
- Secretes multiple hormones to support pregnancy and prepare for chilbirth, e.g. progesterone, estrogen, HCG, relaxin, HPL, GH etc
Protection from toxins/drugs
- Filters toxins and drugs to prevent transfer, or minimise transfer to fetal circulation
- Some drugs transfer (e.g. steroids and antiarythmics) is beneficial for treatment of fetus
