Hasselbalch was born in Åstrup, near
Hjørring, Denmark on 1 November 1874. His parents were landowner and lawyer Hans Peter Jansen Hasselbalch (1843–1916) and Hedevig Alberta Rebekka Spärck (1845–76). In 1879, his father remarried to Wilhelmine Marie Koppel (1858–1950). Hasselbalch received his medical degree in 1898 and his
doctorate in 1899 for his thesis on the respiratory metabolism in the
chicken embryo. ==Career==