Blood-plasma Equilibrium with tissues

Somewhat surprisingly, microdialysis has also revealed that the time to maximum concentration (T ax) within the CNS is close to the Tj ax value in blood or plasma, irrespective of lipophilicity. For example, the CNS Tj ax for atenolol (log D7 4 = - 1.8) occurs at 2 min in the rat after intravenous administration [8]. In addition the rate of elimination (half-life) of atenolol and other polar agents from the CNS is similar to that in plasma or blood. The implication of these data is that poorly permeable drugs do not take longer to reach equilibrium with CNS tissue than more lipophilic agents... 

This section focuses on red cells since most of the particulate volume in human blood is occupied by red cells. Red cells ( erythrocytes ) are biconcave disks under equilibrium conditions in blood plasma as shown in Fig. 61. The diameter of the disk is between 6 and 8 pm. Red cells lack a nucleus and are largely filled with the protein hemoglobin, which binds O2 molecules and several other biologically important molecules. The red cells transport O2 molecules from the lung to tissues throughout the body. They also transport waste products, such as CO2, away from the tissues. 

When a person is exposed to a volatile organic solvent through inhalation, the solvent vapor diffuses very rapidly torough the alveolar membranes, fire connective tissues and the capillary endothelium and into fire red blood cells or plasma. With respiratory gases the whole process takes less than 0.3 seconds. This results in almost instantaneous equilibration between the concentration in alveolar air and in blood and, flierefore, the ratio of the solvent concentration in pulmonary blood to that in alveolar air should be approximately equal to the partition coefficient. As the exposure continues, the solvent concentration in the arterial blood exceeds that in the mixed venous blood. The partial pressures in alveolar air, arterial blood, venous blood and body tissues reach equilibrium at steady state. When the exposure stops, any unmetabolized solvent vapors are removed from the systemic circulation through pulmonary clearance. During that period the concentration in fire arterial blood is lower than in the mixed venous blood and the solvent concentration in alveolar air will depend on the pulmonary ventilation, the blood flow, the solubifity in blood and the concentration in the... 

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