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- umbilical cord: The structure that connects a developing mammalian fetus to the placenta, and which contains the veins and arteries that convey materials to and from the placenta.
- unsaturated: Of or referring to a fatty acid that has one or more double bonds between its carbon atoms. Compare saturated.
- upper respiratory system: A collective term for the nose, mouth, and pharynx. Compare lower respiratory system.
- urine: The filtrate from the kidneys that animals expel from their bodies.
- uterus (pl. uteri): The organ in female mammals in which the zygote implants and develops.