A buffer of carbonic acid (H2CO3) and bicarbonate (HCO3-) is present in the human blood stream, in order to maintain a pH between 7.35 and 7.45. Blood accounts for ~8% of body mass. Adult humans have ~60 millilitres of blood per kilogram of body weight. This amounts to ~4-5 litres (roughly a gallon) of blood in an adult.
See also: human anatomy