Female mosquitoes are attracted to carbon dioxide. Humans and other animals exhale carbon dioxide with every breath.
After locating and landing on an attractive host, the mosquito probes the skin with her mouth parts (knows as a proboscis) to find a capillary in the skin. She pierces the skin with her serrated proboscis, not really biting, but sucking blood. She injects a pain killer, present in her saliva, which makes the piercing less noticeable. Her saliva also contains a blood thinner so that the blood will flow into her rather than clot. If her salivary glands are infected with a pathogen, it may be transmitted to the host when she takes the blood meal.

The volume of blood one female can ingest depends on her size. Generally, mosquitoes take in about five micro liters of blood. How much is that? when blood is donated, the typical amount taken is a pint.
It would take 94,600 of the five micro liter bites to get a pint of blood!