oxygenated haemoglobin
oxygenated hemoglobin

Oxygenated blood is red due to the presence of oxygenated hemoglobin.

In short, deoxygenated hemoglobin is paramagnetic while oxygenated hemoglobin is diamagnetic.

In humans, oxygenated arterial blood contained oxygenated hemoglobin, which because of its iron matrix was diamagnetic and had, therefore, a small magnetic susceptibility effect.

This increase in blood flow produces an increase in the ratio of oxygenated hemoglobin relative to deoxygenated hemoglobin in that specific area.

More blood flows in to transport more glucose, also bringing in more oxygen in the form of oxygenated hemoglobin molecules in red blood cells.

For mothers to deliver oxygen to a fetus, it is necessary for the fetal hemoglobin to extract oxygen from the maternal oxygenated hemoglobin across the placenta.

Increased neural activity causes an increased demand for oxygen, and the vascular system actually overcompensates for this, increasing the amount of oxygenated hemoglobin relative to deoxygenated hemoglobin.

As an example, Fig. 2 shows the optical absorption spectra of oxygenated hemoglobin (HbO) and deoxygenated hemoglobin (Hb) in the visible and near infrared region.

In NIR, as the ratio of oxygenated hemoglobin (HbO) to deoxygenated hemoglobin (Hb) increases, the blood becomes less and less translucent and scatters more of the red light, instead of absorbing it.