Velocity profile measurements have been made for steady and pulsatile flow within rigid tube models of a curved and a planar arterial bifurcation with the intent of providing some insight into the fluid mechanical forces existing at such branches. An example in the cardiovascular system of a curved bifurcation is the branching of the left common coronary artery into the left circumflex and left anterior descending coronary arteries where the vessels curve over the base of the heart and an example of a planar bifurcation is the branching of the abdominal aorta into the left and right iliac arteries. Point velocity measurements were carried out using a constant temperature hot-film anemometer system.

The uptake of 131I-albumin by the wall of the isolated pig thoracic aorta was investigated in vitro to achieve good control of the experimental conditions. The lumena of the arteries were perfused with a balanced salt solution (Hanks') containing the radioactively labelled albumin. Albumin flux was studied at three static intralumenal pressures, viz 6.7, 13.3 and 20 kPa (50, 100 and 150 mmHg) and uptake (normalised per unit dry tissue weight) was found to be approximately doubled on increasing the pressure from 6.7 to 20 kPa (50 to 150 mmHg). With a mean pressure of 6.7 kPa (50 mmHg) the uptake was studied in the presence of sinusoidal pressure oscillations of amplitude in the range 1.3 to 6.0 kPa (10 to 45 mmHg) at frequencies 1, 5 and 11 Hz. At each frequency the uptake was found to be dependent on the pressure amplitude. The dependence at 5 Hz was significantly stronger than at 1 Hz (P < 0.05) and although not significant there was a suggestion that the uptake was higher at 5 than 11 Hz. There was no significance and no suggestion of a difference at 1 and 11 Hz. The possible influence on uptake of the associated oscillatory wall shear component has been considered. The relevance of these findings to macromolecular transport and to the characteristic localisation of atheroma within the arterial tree is discussed.

The influence of abdominal aortic stenosis on the uptake of the protein-binding trypan blue dye and 131I human serum albumin (HSA) has been studied. The major change was a region of high uptake proximal to the stenosis, returning to normal by the level of the renal arteries. There was reduced uptake distal to the stenosis, apart from occasional small areas of high uptake probably due to turbulence. The increase in uptake immediately proximal to the stenosis was dependent on the severity and duration of the stenosis. Removal of the stenosis immediately before the injection of dye and 131I-HSA still resulted in elevated uptake in the proximal region. Some haemodynamic modifications resulting from a stenosis are described. It is suggested that the most satisfactory haemodynamic explanation of the observed uptake change is a proximal increase in oscillatory pressure/strain. The relevance of these findings to atherosclerotic development at and above arterial junctions is discussed.

Reflection from an arterial discontinuity, such as a stenosis, would be expected to produce partial standing waves of pressure and flow proximal to the stenosis. This phenomenon is demonstrated by determining the amplitude changes of the harmonic components of pressure and flow waves recorded at three sites at different distances from an experimental stenosis of the abdominal aorta in dogs. To minimise the effect of reflections from other arterial sites, such as the peripheral beds, the animals were vasodilated. Impedance and propagation velocity measurements were made to estimate the contribution of peripheral reflections in the harmonic components of the aortic pressure and flow pulses. In general, the peripheral contribution appeared to be small for harmonics greater than the first. The results indicate that, for the 'closed' type of reflection at the stenosis, a pressure antinode and a flow node occur immediately proximal to the stenosis. As the distance from the stenosis to the measurement site is increased, nodes and antinodes of pressure and flow occur at frequencies which correspond to integer multiples of lambda/4. Similar fluctuations take place in the impedance modulus proximal to the stenosis, such that close to the stenosis the modulus is a maximum, at the lambda/4 distance the modulus is a minimum and at the lambda/2 distance it is again a maximum. The extent of these impedance changes with distance indicates that the attenuation of the reflected, backwardgoing waves is greater than found for forwardgoing waves and that the diagnostic assessment of a vascular obstruction by means of a proximal measurement of pressure or flow may be subject to error if the measurement is not made close to the obstruction.

The uptake of trypan blue and [131I]human serum albumin (HSA) has been studied in the dog's abdominal aorta between 1 and 42 days after removal of an experimental stenosis (approximately 90%) applied 1 week previously. Previous work has shown that when the stenosis was present during circulation of these markers, their uptake was increased immediately proximal to the stenosis decreasing to normal by the renal artery level. Distal to the stenosis uptake was reduced apart from small areas of high uptake probably due to turbulent jet impacts. Within the stenosed section the uptake was normal. In this present study it was found that, after removal of the stenosis, proximal uptake initially remained elevated, returning to normal after approximately 15 days whilst the distal uptake returned to normal after approximately 10 days. In the previously stenosed section uptake was increased markedly following the release of the stenosis but returned to normal within approximately 20 days. The relationship of these findings to alterations in the local haemodynamic state and to possible changes in endothelial morphology are discussed.