I was with a friend of mine and we had unprotected oral sex. I was the one receiving. One or two of my gums were bleeding, very little blood, just a small red dot, more pinkish, on the napkin. He did not ejaculate, but there was a little bit of pre sum. At the end, he told me that he was on descovy and showed me his most recent lab report (less than a month earlier) with was less than 20/undetectable, and the RNA, PCR, ULTRA were also undetectable. Am I at risk? Like I said, the blood was very little and I read here that blood must be heavy to transmit. Thanks
Thank you for your inquiry. From what we gather from the question, you are asking about the risk of HIV acquisition from partaking in oral sex. From the information given, this scenario is considered to be low risk (evidence of transmission occurs through these activities when certain conditions are met), as this scenario does satisfy some of the transmission equation (see below). The transmission equation is satisfied because there may have been direct access to the bloodstream via the bleeding gums. In addition, ways to increase HIV transmission through oral sex as mentioned by CDC are "sores in the mouth or vagina or on the penis, bleeding gums, oral contact with menstrual blood, and the presence of other sexually transmitted diseases (STDs).  Also, there are ways to reduce the risk of transmission such as using a condom, dental dam, pre-exposure prophylaxis (PrEP), having the HIV partner regular take antiretroviral therapy (ART), and avoiding ejaculation into the mouth. 
Recommendation: We would recommend speaking to your physician about getting tested for HIV.
To answer your second question about the amount of blood necessary to transmit infection: there is no specific amount, and transmission is possible even with only small amounts of infected bodily fluids.
Thank you, and good luck!
AIDS Vancouver Helpline/Online, Sara
- CDC. Oral Sex and HIV risk. Available from: https://www.cdc.gov/hiv/risk/oralsex.html