Growth plates are the soft areas at the ends of the bones and are the parts of the bones that grow. Homones greatly affect growth. The article I linked to explains it better. Growth plates close when a pup becomes sexually mature, different for each dog but anywhere from 9-14 months. Closure may be determined by xray.
Canine Sports Productions: Early Spay-Neuter Considerations for the Canine Athlete
In Yorkies, there is usually not alot of blood, mostly nickel sized spots here and there. Heats generally last 3 weeks, bleeding lasts about one week or less, and starts. More info:
Yorkie Heat | Yorkshire Terrier Information Center
And absolutely request that pro-op bloodwork be done before the surgery.