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The adult human body holds close to 5 liters of blood, with normal variation by sex, body size, and health status.

Most healthy adults can donate whole blood once every eight weeks, which means one standard donation fits within a single month.

A standard whole blood donation is 450–500 milliliters, a volume chosen to stay within healthy limits for most adults.

Most adults start sliding into shock after losing 15–20% of their blood, and the odds of collapse rise fast as loss gets near one-third.

A whole-blood donation usually collects about 450–470 mL (just under a pint), which is a small share of the blood an adult body carries.

Bleeding that soaks more than 2 large pads an hour for 2 hours, or bleeding with fainting, severe pain, fever, or shoulder pain needs urgent care now.

Period bleeding can be too heavy if you soak a pad or tampon each hour for 2+ hours, pass large clots, or feel dizzy or faint.

Spotting is light bleeding that shows as a few drops or streaks and usually needs only wiping or a panty liner, not a pad.

Most daily blood loss is tiny, but repeated bleeding, heavy periods, or any bleeding with dizziness, fainting, or confusion needs urgent care.

Light pink or brown spotting for a few hours to 2 days can fit; soaking pads, clots, or sharp one-sided pain calls for urgent care.