When word gets around that Henrietta needs blood, her friends and family all rush to help her and offer to give their blood for the blood transfusions that she needs. They do this because she had given so much to help them at various points in their lives, and therefore they feel that this small thing, donating their blood to help her, is the least that they can do to help her out. Note how this is described in Chapter 11:
They'd all slept on Henrietta's floor and eaten her spaghetti when they first came to Baltimore from the country, and anytime money ran low. She'd ridden from the streetcar to and from Sparrows Point to make sure they didn't get lot during their first weeks in the city.
Henrietta therefore is a character who has done so much for them that they automatically do what they can to help her out now that she is in need and in pain. In other words, the love that she showed them is returned when she is in need and they can do something to help her. When the opportunity arises, they glady donate blood to help her.