Arteries

Arteries consist of layers of oriented elastin interspersed with randomly oriented collagen (in a matrix of water and polysaccharides). This complex structure gives arteries a strong J-curve.

Elastin is the main elastic protein of vertebrates. It is plasticised by water and has a coiled coil structure. Its elasticity has two components: one obeying ordinary rubber elasticity theory, and the second resulting from an elastic restoring force due to free energy changes associated with the hydration of non-polar groups at the centre of a deformed molecule. Collagen is a protein that forms characteristic fibres in most of the animal kingdom. It is widely found in the human body, including in bone, skin, tendon, ligament and cartilage, as well as in artery. A single collagen molecule shows a left-handed helix, and three of these coil together to form the collagen triple helix, tropocollagen. These pack to form fibrils, which are good stress-transmitters.

The elastin fibres are aligned around the circumference of the artery, and as the artery is strained, the randomly oriented collagen fibres begin to straighten and align. More become load-bearing and the Young's Modulus increases. Due to the initial random orientation of the collagen fibres, some fibres are stressed and yield even at low strains, giving toughness, but even at high strains not all fibres are load-bearing, giving a high failure strain. This effect is also observed in skin, which consists of a random feltwork of collagen and elastin fibres. In an artery, the elastin fibres deflect longitudinal crack growth and help to counteract the high hoop stresses (in the wall of a pressurised cylindrical tube, the hoop stress is twice the longitudinal stress). The overall properties of an artery are elastic, which allows energy to be returned to the blood.