Final answer:
Deoxygenated hemoglobin is more likely to be found at active sites in the brain, as these regions have a higher metabolic demand and rate of oxygen consumption.
Step-by-step explanation:
Deoxygenated hemoglobin is more likely to be found at active sites in the brain. When brain tissue is active, such as during a visual task, the metabolic demand for oxygen increases. This leads to an elevation in local blood flow to supply the needed oxygen.
The active nerve cells increase their consumption of oxygen, leading to a higher rate of oxygen dissociation from hemoglobin. This process is illustrated by the oxygen-hemoglobin saturation/dissociation curve, which shows how hemoglobin releases oxygen in response to the metabolic needs of tissues. Functional MRI (fMRI) scans can detect these changes in blood oxygen levels, capitalizing on the differing magnetic properties of oxygenated and deoxygenated hemoglobin to map brain activity.
Conversely, in non-active, or resting, tissues with a lower metabolic demand, such as adipose tissue, oxygen consumption is slower. This results in a lower rate of oxygen unloading from hemoglobin, which means deoxygenated hemoglobin is less likely to be present in high amounts. Interestingly, even deoxygenated venous blood retains some bound oxygen, serving as an oxygen reserve that can be utilized when a sudden demand arises within the tissue.