Phsyiology Of Pregnancy And The Fetus

Question 1

What are the main functions of the placenta?

Answer 1

Fetal gut supplying nutrients
Fetal lung exchanging O2 and CO2
Fetal kidney regulating fluid volumes and disposing of waste metabolites
Endocrine gland: synthesizes steroids and proteins that affect both maternal and fetal metabolism

Question 2

Describe maternal and fetal blood flow across the placenta

Answer 2

Transport from atmosphere to alveoli -> diffusion across alveolar membrane -> transport from lungs to placenta -> diffusion across the placenta -> transport from placenta to fetus -> diffusion into fetal tissues

Question 3

The placental interface is organized to facilitate what?

Answer 3

Exchange between maternal and fetal circulations
Large SA for exchange
Highly developed vascularity of both fetal and maternal components
Intimately juxtaposed but physically separate

Question 4

What are the 3 major features of the maternal-fetal circulation?

Answer 4

Chorionic villi, intervillous space, decidua basalis

Question 5

What represents the functional unit of the placenta?

Answer 5

Chorionic villi
Has extensive branching which increases SA for exchange
Spiral arteries from maternal side empty into intervillous space which is drained by maternal veins

Question 6

Describe maternal blood flow

Answer 6

Arterial blood discharged from 120 spiral A
Supply blood in spurts into intervillous
Filling of intervillous spaces dissipates the force and reduces blood velocity
Slowing of blood flow allows adequate time for exchange of nutrients
Blood drains through venous orifaces and enter placental vein - no capillaries are present

Question 7

Describe fetal blood flow

Answer 7

Originates from two umbilical A
Carry deoxygenated blood*
Umbilical A branch and penetrate the chorionic plate to form the chorionic villi capillary network
Obtain oxygen and nutrients and returns to the fetus from single umbilical vein
Terminal dilations in capillary network (slower blood flow and exchange of nutrients)

Question 8

Describe gas and solute exchange

Answer 8

Maternal blood entering intervillous space
Diffusion of O2 into the chorionic villi causes the PO2 of the blood in intervillous space to fall to 3-35mmHg (lower than what it was in the uterine A) and lower in the umbilical vein of the fetus
Differences in hemoglobin structure allows for sufficient Hb saturation
Fetus has higher oxygen affinity

Question 9

Describe CO2 transfer

Answer 9

Driven by a concentration gradient difference
Near term PCO2 in umbilical arteries is slightly higher than in intervillous space
Fetal blood has a slightly lower affinity for CO2 than maternal blood
All factors factor transfer of CO2 from fetus to mother

Question 10

What structures undergo passive exchange across the placenta?

Answer 10

Non-protein nitrogen wastes (urea/creatine) from fetus to mother
Lipid soluble hormone transfer between mother, placenta and fetus

Question 11

What substance undergoes facilitated diffusion from the mother to the fetus?

Answer 11

Glucose

Question 12

Which structures undergo primary and secondary active transport to the fetus?

Answer 12

Amino acids, vitamins, minerals

Support growth of fetus

Question 13

Which structures undergo receptor mediated endocytosis?

Answer 13

Large molecule exchange like LDL, hormones (insulin), Abs

Question 14

What is the endocrine function of the placenta

Answer 14

Plays a key role in the manufacture of steroid hormones, amines, polypeptides (hormones and neuropeptides), proteins/glycoproteins
Placenta can regulate in a paracrine fashion (release of local placental hormones and release of hormones into fetal or maternal circulations)

Question 15

What are the general functions of placental hormones during pregnancy?

Answer 15

Maintains pregnant state of the uterus
Stimulating lobuloalveolar growth and function of maternal breasts
Adapting aspects of maternal metabolism and physiology to support a growing fetus
Regulating aspects of fetal development
Regulating timing and progression of parturition

Question 16

Describe hCG

Answer 16

Produced by syncytiotrophoblasts
Structurally related to LH
-binds with high affinity to LH receptors
-glycosylation increases half life
-rapidly accumulates in maternal circulation
Serum levels double daily up to 10 weeks

Question 17

What is the primary function of hCG?

Answer 17

To stimulate LH receptors in the CL which prevents luteoloysis and maintains high levels of luteal derived progesterone
Thought to be responsible for nausea associated with morning sickness
Small amounts enter fetal circulation (stimulate fetal Leydig cells to produce testosterone)

Question 18

Describe human placental lactogen (hPL)

Answer 18

Aka human somatomammotropin (hCS)
Structurally related to growth hormone and prolactin
Produced by syncytiotrophoblast
Detected at day 10 in syncytiotrophoblast and in maternal serum at 3 weeks

Question 19

What are the functions of hPL?

Answer 19

Role in coordinating fuel economy of fetoplacental unit
-conversion of glucose to FAs and ketones
-can have antagonistic action to maternal insulin, contributing to diabetogenicity of pregnancy
Lipolytic actions help mother shift to free FA use for energy
Promote development of maternal mammary glands during pregnancy

Question 20

Describe why high levels of progesterone are required throughout pregnancy

Answer 20

Required for implantation and early maintenance of pregnancy
Derived from CL and provides window of receptivity (increased adhesion proteins in endometrium)
Stimulates endometrial gland secretions for early nutrient transfer
Reduces uterine motility
Inhibits propagation of uterine contractions
Induces mammary growth and differentiation

Question 21

Describe the role of estrogen

Answer 21

Induces endometrial growth, progesterone receptor expression, and LH surge just prior to ovulation
Increases uteroplacental blood flow
Increases LDL receptor expression in syncytiotrophoblasts
Induces prostaglandins and oxytocin receptors necessary for parturition
Increase the growth and development of mammary glands

Question 22

What is the maternal placental fetal unit?

Answer 22

During pregnancy maternal levels of progesterone and estrogens (estradiol, estrone and estriol) rise to levels substantially higher than during a normal menstrual cycle
The placenta is an imperfect endocrine organ and cannot produce these alone
Coordination between the maternal, placental, and fetal tissues are required (MPF unit, mother supplies cholesterol, fetal adrenal gland and liver supply enzymes the placenta lacks)

Question 23

Describe progesterone in the maternal placental fetal unit

Answer 23

Luteal phase shift occurs around week 8
Progesterone production is largely unregulated
Syncytiotrophoblasts import cholesterol from maternal blood and express CYP11A1 and 3-beta-HSD1
Released primarily into maternal compartment
Maternal serum levels rise throughout pregnancy

Question 24

Describe estrogen in the MPF unit

Answer 24

Placenta cannot produce cholesterol and lacks 17 alpha hydroxylase as well as 17,20 desmolase needed for estrone and estradiol
Lacks 16 alpha hydroxylase needed for estriol
MPF unit overcomes this
-mother supplies cholesterol
-production of DHEAS and 16 OH DHEAS produced by fetal adrenal gland (weak androgens)

Question 25

What are the 3 shunts the fetal circulatory system uses?

Answer 25

Bypass the liver and lungs (not fully functional until late pregnancy/just after birth)
Ductus venosus to bypass the fetal liver
Foramen ovale moves blood from the RA to the LA
Ductus arteriosus moves blood from pulmonary A to aorta

Question 26

Describe the ductus venosus

Answer 26

Liver bypass (bc liver is not functional yet) 
A shunt that takes blood from the placenta via the umbilical vein and the IFC to bypass the liver into the RA of the heart 
Some does go to the liver for oxygen/nutrients

Question 27

Describe the foramen ovale

Answer 27

Hole in septum dividing the atria posterior aspect of the RA
Right to left atrial shunt
Bypass around RV
PO2 ~27 mmHg IFC right through to LV to supply carotid and brain
Of blood entering RA 27% shunted through foramen ovale

Question 28

Describe the ductus arteriosus

Answer 28

Bypasses the pulmonary artery to aorta
Another right to left shunt
From the RV blood moves to the pulmonary artery where a small amount goes to the lungs
Most is shunted through the ductus arteriosus to the descending aorta
Around 50% fo the blood from descending aorta will enter umbilical arteries
The rest will travel to tissues and return via the IFC and mix with oxygenated blood from umbilical V

Question 29

Describe closure of the ductus arteriosus

Answer 29

As the lungs expand there is a major reduction in pulmonary pressure and an increase in aortic pressure
Leads to closure of the ductus arteriosus
Now well oxygenated aortic blood flows through ductus arteriosus
1-8 days constriction sufficient
1-4 months anatomically occluded

Question 30

What is a patent ductus arteriosus (PDA)?

Answer 30

Heart problem that occurs soon after birth
Can be heard as heart murmur
Oxygenated blood in aorta mixes with deoxygenated blood in pulmonary A
Puts strain on heart and increases pulmonary bp

Question 31

Describe closure of the foramen ovale

Answer 31

Reversal of pressure gradient across atria
Pushes foramen ovale’s valve shut
Due to increased venous return to LA and elevate LA pressure as well as decreased RA pressure
Eventually flap seals
Establishes left and right circulatory system

Question 32

What is a patent foramen ovale (PFO)?

Answer 32

In 20% of individuals by 2 years of age foramen ovale does to close permanently
Increase RA pressure can push flap open
Sustained pulmonary HTN or transient increases (e.g. bowel movement, coughing, sneezing)

Question 33

Describe closure of the ductus venosus

Answer 33

Within 1-3 hours muscle wall of ductus venosus contracts and closes due to decreased blood flow and pressure changes
Portal venous pressure rises forcing venous blood through liver sinuses
Permanent closure within 1-3 months
Rarely fails to close