Medial elastin in the thoracic and abdominal aorta of sheep and lambs.

Aortas were removed from six mature lambs and four sheep and pressure fixed at 100 and 120 mmHg pressure (1 mmHg = 133.322 Pa), respectively, in 10% buffered formalin. The numbers of elastin layers were counted from cross sections at different distances down the aorta, from the distal arch to the iliac bifurcation, and showed a linear decrease in the thoracic aorta. In the abdominal aorta there was no difference in values from the diaphragm to the aortoiliac bifurcation in the lamb, but a slight decrease in the sheep. If y = mx + b, where y is the number of medial lamellar units (MLU), b the intercept, x the distance from the last brachiocephalic branch in centimetres, and m the slope, we obtained the following equations (with standard deviations): lamb thoracic aorta; y = -6.29 (+/- 0.71)x + 106 (+/- 12); sheep thoracic aorta; y = -3.46 (+/- 0.40)x + 140 (+/- 21.4); lamb abdominal aorta; y = 0.51 (+/- 1.48)x + 37 (+/- 7.7); sheep abdominal aorta; y = -0.85 (+/- 0.28)x + 66 (+/- 5.9). Tension per lamellar unit was calculated and plotted versus distance yielding a linear increase in the entire lamb aorta but an abrupt increase between thoracic and abdominal aortas in the sheep. This causes the pulse wave to move uniformly with increasing speed along the length of the lamb aorta but would cause an abrupt change in the wave at or about the diaphragm in the sheep. Distortions in the pulse wave could produce local stress concentrations in the abdominal aorta which might render it more susceptible to atherosclerosis than the thoracic aorta.