No / Tunica adventitia, the outer layer of blood vessels, is an important structural feature, predominantly consisting of collagen fibrils. This study uses pseudostatic atomic force microscopy (AFM) nanoindentation at physiological conditions to show that the distribution of indentation modulus and viscous creep for the tunica adventitia of porcine aorta and pulmonary artery are distinct. Dynamic nanoindentation demonstrates that the viscous dissipation of the tunica adventitia of the aorta is greater than the pulmonary artery. We suggest that this mechanical property of the aortic adventitia is functionally advantageous due to the higher blood pressure within this vessel during the cardiac cycle. The effects on pulsatile deformation and dissipative energy losses are discussed.