A planet in the Rigel system whose physicians tested a drug that speeds blood replacement, used on Spock so he could donate enough blood to save Sarek.
Rigel V is an inhabited world of the Rigel system, mentioned in the Original Series episode Journey to Babel.
When Ambassador Sarek needs heart surgery, the Enterprise does not carry enough of his rare blood type. Spock recalls that an experimental drug has been tried successfully on Rigel V to raise the rate of blood production, and that Rigelian physiology is close to Vulcan.
Doctor McCoy objects that the drug puts a severe strain on the body and has never been used on a Vulcan, but it is finally given to Spock. With the stimulant he supplies enough blood for the operation, and both father and son recover.