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Blood is one of the most important biological fluids. It derives its importance from the ability of red blood cells (RBCs) to reversibly bind and carry oxygen. Most living cells rely on oxidative metabolism to perform their functions, while the circulatory and respiratory systems work together to provide the necessary supply of oxygen. Nearly all circulating oxygen is bound to the hemoglobin molecule, while the rest is dissolved in the cytosol of RBC and in blood plasma. Therefore, the delivery of oxygen to different organs is determined by the blood oxygen content and blood flow at the particular location. Optical investigations of blood properties have a long history. Slare was the first to report in 1700 that blood changes color when exposed to air, and Ångström was the first, in 1855, to apply a spectrophotometric technique to study spectral characteristics of blood. Oxyhemoglobin and hemoglobin absorption bands in the visible spectral range were investigated by Hoppe-Seyler in 1862.2 Soret described hemoglobin’s optical properties in the ultraviolet in 1878. In 1873, Vierordt invented a method to determine the hemoglobin content of blood samples. Since that time, numerous investigations have been devoted to the optical properties of hemoglobin derivatives and whole blood. For example, the differences in the absorption spectra of oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) in the visible and near-infrared spectral ranges stimulated the development of blood oximetry in the 1930s. In 1977, Jöbsis discovered that near-infrared light can transilluminate a cat’s head and that the optical signal can provide in vivo information on the biotissue HbO2/Hb ratio and on blood volume. This investigation led to the development of tissue oximetry and, consequently, to near-infrared spectroscopy (NIRS) of different biological tissues. Besides oximetry, knowledge of blood optical properties is required for many medical diagnostic and therapeutic applications. During the past 20 years, considerable efforts were directed toward developing NIRS imaging, optical biopsy, photodynamic therapy, and laser-induced interstitial thermo-therapy. The efficacy of these procedures greatly depends on the propagation and the fluence rate distribution of optical radiation within tissue. Since most human tissues contain blood, many theoretical and experimental studies were undertaken to predict and determine its optical properties.
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