Fig. 1.

A summary of hemoglobin-based delivery of oxygen molecules to peripheral tissues in a normal patient's capillary and in a COVID-19 patient's capillary. Hb = hemoglobin; O₂ = oxygen; SaO₂ = arterial oxygen saturation; SvO₂ = venous oxygen saturation. The oxygen tension of tissues will dedictate whether one of several oxygen molecules will be released from oxyhemoglobin (HbO₂) and diffuse from the capillary (pink area) into the tissue. The fraction of HbO₂ relative to total Hb in arterial blood is the SaO₂, which is normally > 95%. Normal mixed SvO₂ is about 65–75%. A decrease in SvO₂ usually indicates low tissue oxygen tension, resulting in increased extraction of oxygen molecules from Hb.