systemic circulation

This term is a precise medical descriptor used primarily in anatomy and physiology to distinguish the body-wide blood flow from the pulmonary circulation, which only services the lungs. It describes a closed-loop process of nutrient delivery and waste removal across all peripheral tissues.

Because it refers to a specific, singular biological system within the human body, it is treated as an uncountable noun. It is almost never used in the plural form, as it describes a functional process rather than a countable object.