Lecture 14: Placental Physiology Flashcards
Functions of Placenta
- Diffusion of oxygen and carbon dioxide
- Diffusion of foodstuffs
- Excretion of waste products
Early Placenta:
- Thick
- Permeability low
- Small surface area
- Total diffusion conductance is miniscule
Late placenta:
- Thin
- Permeability high
- Large surface area
- Large increase in placental diffusion
Diffusion of Oxygen and Carbon Dioxide
- PO2 of mother: • 50mmHg
* PO2 of fetus: • 30mmHg
Reasons why adequate oxygenation can occur with such a low pressure gradient (mean diffusion gradient pressure = 20 mm Hg)
- Fetal hemoglobin has a higher affinity for oxygen.
- Fetal blood hemoglobin concentration is about 50% greater than maternal.
- Bohr effect
Bohr effect:
- Hemoglobin can carry more oxygen at a low PCO2
- Fetal blood coming into placenta carries more CO2.
- Excess CO2 diffuses into maternal blood.
- Fetal blood becomes more alkaline
- Maternal blood becomes more acidic.
- Changes cause:
- Increase in capacity of fetal blood to combine with oxygen
- Decrease in capacity of maternal blood to combine with oxygen
Double Bohr effect:
• Refers to the double shift in the maternal blood and in the fetal blood.
Bohr effect:
Bohr effect: shift of oxygen- hemoglobin dissociation curve to right and downward forcees oxygen away from the hemoglobin and delivers increased amounts of oxygen to the tissues.
Carbon dioxide diffusion:
• PCO2 of fetal blood = 2-3X higher than maternal blood.
Diffusion of foodstuffs:
- Facilitated diffusion of glucose via trophoblast cells
* Slower diffusion of fatty acids into fetal blood
Excretion of waste products:
Urea, uric acid, and creatinine diffuse from fetus to maternal blood.
Human chorionic gonadotropin:
• Secreted by the syncytial trophoblast cells into maternal fluids
• Measurable secretion 8-9 days after ovulation.
• Maximal secretion during 10th – 12th week of pregnancy.
• Lower levels secreted 16th through 20th weeks
• Functions:
Prevents involution of corpus luteum
Causes CL to increase secretion of progesterone and estrogens
Causes increased growth in CL
Exerts interstitial cell-stimulating effect on testes of male fetus:
Results in production of testosterone until birth
Estrogen:
- Secreted by syncytiotrophoblast cells of placenta
- Toward end of pregnancy, secretion level is 30X mother’s normal level
- Placental estrogens are formed almost entirely from androgenic steroid compounds:
- Formed in mother’s and fetal adrenal glands
- Converted by trophoblast cells into estradiol, estrone, and estriol
Estrogens:
• Functions:
- Uterine enlargement
- Breast enlargement
- Growth of breast ductal structure
- Enlargement of maternal external genitalia
- Relaxation of pelvic ligaments
- May also affect aspects of fetal development
Progesterone
- Secreted in small quantities by corpus luteum early
- Secreted in large quantities by placenta
- Functions:
- Causes decidual cells to develop in the endometrium
- Decreases contractility of pregnant uterus
- Increases secretions of fallopian tubes and uterus
- May work with estrogen to prepare breasts for lactation
