Obstetric Physiology Dr Maddi Anupindi
Outline the cardiovascular, respiratory, haematological and genitourinary changes that occur during pregnancy
Cardiovascular: these include effects on cardiac output and mechanical effects
Respiratory: these include changes to the airways, changes to ventilation, changes to lung volumes and changes in respiratory mechanics
Haematological: include changes to coagulation and changes to full blood count
Genitourinary: mechanical changes and changes to blood flow
List the functions of the placenta
The placenta is a fetomaternal organ which consists of the fetal part that develops from the chorionic sac and the maternal part which develops from the decidua basalis of the endometrium. It connects the developing fetus to the uterine wall. Placental development begins at 6 weeks and is completed by the 12th week. The placenta has four main functions; transport, immunological, metabolic and endocrinological.
Outline the determinants of placental blood flow
Placental blood flow consists of a maternal and fetal part.
Maternal or utero-placental circulation:
The maternal blood supply to the uterus is via the uterine and ovarian arteries which then form the arcuate arteries from which radial arteries divide and penetrate the myometrium. The radial arteries then divide into spiral arteries which supply the intervillous space.
The basic structural unit of the placenta is the choronic villus which are vascular projections of fetal tissue surrounded by chorion. The chorion is composed of an outer syncytiotrophoblast in direct contact with maternal blood within the intervillous space, and the inner cytotrophoblast. The villous capillaries are branches of the chorionic arteries which are branches of the two umbilical arteries which bring blood from the fetus to the placenta. Oxygenated blood then leaves the placenta via the umbilical vein.
Determinants of utero-placental circulation:
Determinants of fetal-placental circulation:
List the mechanisms of transport across the placenta
Mechanism of transport across placenta:
What is the Double Bohr and Double Haldane effect?
Oxygen transfer from the maternal to the fetal circulation is via simple diffusion down a partial pressure gradient of 30mmHg. This oxygen transfer is facilitated by fetal haemoglobin which has a higher affinity for oxygen compared to adult haemoglobin and has a left shifted oxygen haemoglobin dissociation curve. Furthermore, the haemoglobin concentration of fetal blood is approximately 50% greater than that of maternal blood, thus increasing oxygen transport. The Bohr effect states that increases in the partial pressure of carbon dioxide of blood or a decrease in pH will lead to a rightward shift of the oxygen-haemoglobin dissociation curve thus decreasing the affinity of haemoglobin for oxygen. The Double Bohr effect describes this happening in opposite directions in the maternal and fetal circulations. Within the feto-placental circulation, blood is releasing co2 down its concentration gradient from fetal to maternal blood. The fetal blood is thus becoming more alkaline leading to a leftward shift in the o2-hb dissociation curve and increasing the fetal haemoglobin’s affinity for oxygen. Conversely, within the utero-placental circulation the maternal blood is gaining co2 leading to a right shift of the o2-hb curve and encouraging oxygen release from maternal haemoglobin.
The Haldane effect describes the effect of oxygen on co2 carriage with deoxygenated haemoglobin having a greater affinity for carbon dioxide. The Double Haldane effect refers to the fact this happens in opposite directions within the fetal and maternal interface. Within the utero-placental circulation oxygen is lost thus increasing haemoglobin’s affinity for carbon dioxide. Conversely within the feto-placental circulation, oxygen is gained thus decreasing haemoglobin’s affinity for c02.
List the differences in the respiratory system between newborns and adults
The differences can be divided into anatomical, lung mechanics, lung volumes and respiratory control.
Anatomical: can be sub-divided into upper and lower airways.
Lung volumes and gas exchange:
Maddi is an ICU registrar at St George Hospital who has a keen interest in teaching and education. She wishes that her recollection of medical knowledge was as strong as her recollection of song lyrics or quotes from episodes of ‘The West Wing’. When she’s not thinking of the next unquestionably funny dad joke for this podcast, she enjoys cryptic crosswords, musicals and anything cheese related. She hopes that this podcast helps in some small way to lessen the torment of the ICU primary exam. "
Dr Swapnil Pawar January 31, 2020
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