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Biomedical subjects

Y C Fung

Publications and source records attributed to Y C Fung.

At least 109 records · Page 6Linked to original sources
PubMed1978

Effect of velocity of distribution on red cell distribution in capillary blood vessels.

Through the use of simulated model experiments, data on blood cell distribution into a bifurcating capillary blood vessel are obtained. The results show that the movement of red blood cells at a bifurcation point is influenced by the difference in velocities of flow in the daughter branches. If the velocity of flow in one branch is slower than that in the other, the hematocrit decreases in the slower branch and increases in the faster branch. For velocity ratios sufficiently smaller than a certain critical value, the hematocrit ratio can be expressed by a linear relationship, (H1/H2) - 1 = a[v1/v2) - 1], in which v1, v2 and H1 H2 denote the particle velocities and tube hematocritis in the branches 1 and 2, respectively, and a is a dimensionless contant dependent upon a number of factors, the most important of which are the ratio of cell diameter to tube diameter, the shape and rigidity of the pellets, and the hematocrit in the feeding tube. For velocity ratios beyond a critical value, nearly all the cells flow into the faster branch. The smaller the feeding-tube hematocrit is, the smaller is the critical velocity ratio at which this phenomenon occurs.

Blood Flow Velocity↗
PubMed1978

Elasticity of excised dog lung parenchyma.

An experimental procedure was developed to measure the stress-strain relationship on rectangular slabs (5.0 X 5.0 X 0.5 cm) of excised dog's lung. The slabs were subjected to biaxial loading and the resulting triaxial deformations were measured. Deformations were measured in the central portion of the specimen by video dimension analysers in order to minimize boundary effects. Specimen thickness was measured with a magnetic reluctance proximeter system. The data were sampled and stored on-line by a PDP-8E computer. An electromechanical servo system was used to control the lateral force. Tests were performed at several pH values and at 20 and 37 degrees C. The tissue exhibited a highly nonlinear stress-strain relationship, compliant at low stress levels and stiff when the stress was high. Hysteresis was observed to be about 28% and was unaffected by a 250-fold change in strain rate. Biaxial loading revealed a new characteristic: there is a change in elastic behavior when the tissue undergoes a compressive strain. When the tissue was in tension increasing the lateral load decreased the compliance, but the opposite was true when compressive strain was present.

Animals↗
PubMed1977

Mechanical properties of taenia coli smooth muscle in spontaneous contraction.

The following mechanical tests have been performed on spontaneously contracting taenia coli: isometric step stretch, isotonic step loading, and length-tension. Passive stress-strain tests have also been done. Information on spontaneous contraction was more readily obtained in isometric tests. In length-tension tests, each isometric contraction was analyzed on a phase plane which is a representation of muscle stress, sigma(t), versus the rate of change of stress, sigma(t). The curve in the phase plane provided parameters for describing spontaneous contraction as a function of muscle length, L. The maximum active stress in a cycle of spontaneous contraction, S(L)max, and the maximum rate of increasing active stress, [S(L)]max, were shown to depend on L in the same way. Both parameters attain a maximum value (upper bound) at the same muscle length, Lmax. When L less than Lmax, the minimum stress in a spontaneous contraction, sigma(l)min, is negligible. If L greater than Lmax, sigma(L)min increases nonlinearly and is equal to the passive stress, tau(L), in an epinephrine-relaxed muscle. The results show that the ratio of [S(L)]max to S(L)max, eta(L), is constant for a physiological range of muscle length. This suggests that eta(L) may be used as an index of contractility for taenia coli.

Animals↗
PubMed1977

Inversion of Fahraeus effect and effect of mainstream flow on capillary hematocrit.

The well known Fahraeus effect (1929) states that if whole blood is allowed to flow from a large reservoir into a small circular cylindrical tube, the hematocrit in the tube is smaller than that in the reservoir, and the smaller the tube, the smaller will be the tube hematocrit. This is interpreted as a feature of particulate flow. We find that this relationship is not monotonic in a model experiment in which gelatin particles (circular disks) are suspended in a silicone fluid to simulate blood. When the diameter of the underformed cell is equal to or greater than the tube diameter, the volume fraction of the cells in the tubes increases to a value equal to or greater than that in the reservoir. Thus the Fahraeus effect has a point of inversion. Additional experiments show that the hematocrit in the tube could be greatly influenced by the flow condition outside the entrance of the tube. If the tube is perpendicular to the main direction of flow in the reservoir (as is the case of Barbee and Cokelet's experiment, or in most arteriole-capillary junctions), the velocity gradient and the velocity of flow in the reservoir just outside the entrance to the tube affects the hematocrit in the tube.

Blood Flow Velocity↗
PubMed1976

A noncontact method for three-dimensional analysis of vascular elasticity in vivo and in vitro.

A new method is described to measure the deformation of the blood vessel wall simultaneously in longitudinal and circumferential directions. This information is of paramount importance for further characterization of the elastic properties of the arterial wall. The new method consists of a closed-circuit TV system in conjunction with a video dimension analyzer (VDA). The VDA utilizes the video signal from the TV camera and forms a DC voltage proportional to the distance between two selected points in the scene. The resulting analog voltage, calibrated in dimensional units, is recorded. Dimensional changes in two directions, due to intraluminal pressure oscillations, are tracked continuously. The measurement can be performed in vivo on exposed vessels as well as in vitro on excised specimens. Distortion caused by the end effects is completely eliminated. For further data analysis, the wall thickness of the artery is determined microscopically. Stress and strain relationship in longitudinal and circumferential directions is calculated for every intraluminal pressure change. This approach yields data on mechanical properties of the vessel wall in a far more physiological way than the hitherto used direct-contact techniques.

Animals↗
PubMed1975

A device for testing mechanical properties of biological materials--the "Biodyne".

An electromechanical servo-controlled device has been developed. This device can be used to test the mechanical behavior of a wide variety of biological soft tissues. Control and execution of material testing procedures such as stress-strain, vibration, relaxation, creep etc. can be performed by manual operation of the device or by interfacing it with a laboratory type minicomputer. Experiments on excitable tissues such as muscle can also be executed. The design details and system performance are discussed.

Biomechanical Phenomena↗
PubMed1975

Vascular endothelium-leukocyte interaction; sticking shear force in venules.

To determine the shear force acting on a white blood cell sticking to the endothelium of a blood vessel, the flow field about a single white blood cell in a venule was determined by hign-speed motion picture photomicrography. The force acting on the white blood cell was then calculated according to the principles of fluid mechanics. In this paper, the calculation was made using an experimentally determined dimensionless shear force coefficient obtained from a kinematically and dynamically similar model. The large physical model of the hemodynamic system could be easily instrumented, and the shear force acting on the model cell and the flow field around it were measured. The data were then used to calculate a shear force coefficient. On the basis of dynamic similarity, this shear force coefficient was applied to the white blood cell in the venule. The shear force coefficient was strongly influenced by the hematocrit, so in vivo hematocrits were measured from electron micrographs. It was found that in the venules of the rabbit omentum a white blood cell sticking to the endothelial wall was subjected to a shear force in the range of 4 times 10--5 dynes to 234 times 10--5 dynes; the exact value depended on the size and motion of the white blood cell, the size of the blood vessel, the velocity of the blood flow, and the local hematocrit, which varied between 20% and 40% in venules of about 40 mum in diameter. The contact area between the white blood cell and the endothelial cell was estimated, and the shear stress was found to range between 50 dynes/cm-2 and 1060 dynes/cm-2. The normal stress of interaction between the white blood cell and the endothelium had a maximum value that was of the same order of magnitude as the shear stress. The accumulated relative error of the experimental procedure was about 49%. The instantaneous shear force was a random function of time because of random fluctuations of the hematocrit.

Adhesiveness↗
PubMed1975

Stress, deformation, and atelectasis of the lung.

The lung parenchyma as a tissue has a rather unusual stress-strain relationship. A theoretical derivation of this relationship is presented which connects the surface tension and the tissue elastic stress in the alveolar septa with the alveolar geometry. The mathematical expression contains a few meaningful physical constants which can be determined by in vitro and in vivo experiments. With this stress-strain relationship, the general equations of lung mechanics are formulated, and solutions to some simpler problems are presented. First, the equilibrium of a lung subjected to a uniform inflation pressure (definition: alveolar air pressure - intrapleural pressure - pleural tension X mean curvature of pleura) is analyzed, and the stability of the equilibrium states with respect to small perturbations is examined. Second, an exact solution for a lung in a chest under the influence of gravity is presented; the solution is "exact," of course, for only a particular lung, but it can serve as a standard to check numerical procedures being developed in many laboratories. Finally, three types of possible atelectasis-planar, axial, and focal-are analyzed. The planar type can exist in a normally inflated lung, provided the layers of alveoli are forced to collapse toward a plane by some external agent. But axial atelectasis (alveoli collapse into a cylinder) can occur only if the dimension (at which the elastic tension in the alveolar septa vanishes). Similarly, focal atelectasis can occur only if the entire lung is smaller than the resting volume.

Elasticity↗
PubMed1975

Does the surface tension make the lung inherently unstable?

Many authors regard the human lung as a collection of 300 million bubbles independently connected by cylindrical tubes. Under surface tension such a model is inherently unstable in the sense that the small alveoli would empty into the large ones so that the lung would consist only of collapses and hyperinflated alveoli. It has been demonstrated that this basic model is wrong. My observation is based on the well-known fact that both sides of each interalveolar septum are exposed to ventilated air. When the topological relationship between the alveolar septa is properly taken into account, it can be shown that each interalveolar septum is a minimal surface and that there is no problem of inherent instability in the sense mentioned earlier. However, the lung structure is flimsy and can become unstable in the same sense that an airplane structure or an Atlas rocket can become unstable. The clarification of lung inflation and atelectasis can proceed in a rational manner when the confusion of an erroneous model is removed.

Elasticity↗