15 Human physiology (Option D)
This option covers concepts in physiology that are relevant to human health, and that exemplify recurring themes in biology - structure and function, homeostasis, and so on. The SL program includes human nutrition, digestion, functions of the liver, and the heart. Hormones and metabolism and respiratory gases are additional topics in the HL program.
Only one option topic is required (15 hours at Standard Level; 25 hours at Higher Level)