1. The fetus depends on the placenta for the exchange of gas, nutrients, and metabolic byproducts. 2. The efficiency of gas exchange at the placenta depends on the species-variable arrangement of fetal and maternal blood vessels. 3. The fetal circulation mixes oxygenated and deoxygenated blood at several points, so the fetus exists in a state of hypoxemia. 4. Fetal oxygen transport is assisted by fetal hemoglobin, which has a high affinity for oxygen. 5. The lung develops in three stages, and pulmonary surfactant must be present at birth. 6. At or shortly after birth, umbilical vessels rupture, pulmonary vascular resistance decreases, and the foramen ovale and ductus arteriosus close. TABLE 51-1 Placentation in Domestic Mammals
Fetal and Neonatal Oxygen Transport
The Fetus Depends on the Placenta for the Exchange of Gas, Nutrients, and Metabolic Byproducts
Species
Classification
Gross
Histological
Horse
Diffuse
Epitheliochorial
Pig
Diffuse
Epitheliochorial
Cow
Cotyledonary
Epitheliochorial
Sheep
Cotyledonary
Epitheliochorial
Goat
Cotyledonary
Epitheliochorial
Dog
Zonary
Endotheliochorial
Cat
Zonary
Endotheliochorial
Rabbit
Discoid
Hemochorial
Guinea pig
Discoid
Hemochorial
The Fetal Circulation Mixes Oxygenated and Deoxygenated Blood at Several Points, So the Fetus Exists in a State of Hypoxemia
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