I have bloody saliva sometimes but my heart and lungs are fine? what could cause it?

Gum disease: It's hard to say without a physical exam, but one of the more common causes of blood in the saliva is gum disease (especially if the blood is seen after brushing or flossing). Other sources of blood can be the sinuses, If you have a little blood in the sinuses sometimes it can drip down the back of your throat and end up in your saliva. If you haven't seen your dentist lately, I would get checked

Mouth or sinuses: Bloody saliva could be a sign of either gum disease where your gums are the source of the bleeding or could also be coming from your sinuses/nose via post nasal drip. Have your doctor check both to rule these out as causes.