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Question 1 Report
At a neonatal unit, staff study Fig. 1 to show how an unborn human obtains materials for growth. The diagram shows chorionic villi pressed against maternal blood spaces in the uterus. Table 1 records concentrations measured in blood entering and leaving the placenta on the fetal side. The mother and fetus have separate circulatory systems, even though useful substances move across a very short distance.
(a) State the role of the umbilical cord. [2]
(b) Use Table 1 to identify one substance taken into fetal blood at the placenta and one substance removed from fetal blood. [2]
(c) Explain how three features of the placenta make exchange between maternal blood and fetal blood efficient. [4]
(d) Explain why it is important that maternal and fetal blood do not normally mix directly. [2]
(e) Give one substance, other than carbon dioxide, that can pass from fetal blood to the mother’s blood. [1]
Answer Details
(a) The umbilical cord connects the fetus to the placenta [1] and carries fetal blood to and from the placenta. [1]
(b) Oxygen is taken into fetal blood at the placenta. [1] Carbon dioxide, or urea, is removed from fetal blood. [1]
(c) Exchange is efficient because:
- Many chorionic villi provide a large surface area. [1]
- The villi have thin walls, producing a short diffusion distance. [1]
- A rich maternal blood supply maintains concentration gradients. [1]
- A dense fetal capillary network also maintains concentration gradients. [1]
- The membranes are permeable to suitable small molecules. [1]
Any four points are required. Diffusion is fastest when surface area is large, the path is short, and a steep concentration gradient is maintained.
(d) Maternal and fetal blood may have incompatible blood groups. [1] Direct mixing could trigger an immune response or agglutination, harming blood cells. [1]
(e) Urea can pass from fetal blood to the mother's blood. [1]
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