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L Hoofd

Publications and source records attributed to L Hoofd.

At least 37 records · Page 2Linked to original sources
PubMed1995

Calculation of oxygen pressures in tissue with anisotropic capillary orientation. I. Two-dimensional analytical solution for arbitrary capillary characteristics.

In tissue with a distinct orientation of the oxygen supplying structures, the capillaries, a mathematical description of oxygen transport is feasible in terms of two-dimensional diffusion in a plane perpendicular to the capillaries. Muscle tissue is an example of a highly anisotropic tissue. With some additional simplifying assumptions, a solution can be constructed in terms of capillary sources for arbitrary capillary characteristics, in particular, capillary locations. The solution includes facilitated diffusion by myoglobin in the tissue. For homogeneous tissue, the solution becomes explicit allowing direct calculation of tissue oxygen pressure at any location in a field of simple geometry (circular, rectangular). Also, the size of the area into which each capillary distributes its oxygen, the oxygen supply area, is readily calculated.

Animals↗
PubMed1995

Calculation of oxygen pressures in tissue with anisotropic capillary orientation. II. Coupling of two-dimensional planes.

The mathematical descriptions of oxygen transport in terms of two-dimensional diffusion in planes perpendicular to the capillaries can be coupled for an ensemble of planes to yield a three-dimensional description. In tissue with a distinct orientation of the oxygen supplying structures, the capillaries, this leads to a description of oxygen pressure in a whole tissue volume. Muscle tissue is an example of such tissue. The method allows calculation of oxygen pressure (pO2) at any location in the volume. Tissue volume oxygen status can be assessed by constructing a pO2 histogram, gathering pO2's at a large number of locations in the tissue.

Animals↗
PubMed1994

Local plasma convection can be important for oxygen release in tissue capillaries.

Aroesty and Gross investigated the effect of mixing in gaps between two successive RBCs on the local mass transfer at the capillary wall. They found that the effect was insignificant. In this presentation it is shown that their conclusion results from their choice of boundary conditions. When boundary concentrations are chosen which are more similar to those in diffusional models the flux at the capillary wall increases significantly when mixing increases. The calculations presented indicate that mixing of plasma may enhance oxygen transport, although it is impossible to assess the physiological importance. To be able to investigate that, a model has to be developed that includes oxygen transport in both the RBCs and tissue, the oxygen release in the RBCs, and the oxygen consumption in the tissue.

Animals↗
PubMed1993

Capillarisation and fibre types in hypertrophied m. plantaris in rats of various ages.

Influences of age, overload obtained through denervation of synergists, and training on the capillarisation of the m. plantaris were compared in 5-, 13- and 25-month-old rats in relation to different fibre types. Overload resulted in about 30% hypertrophy in each age group. Age effects were significant only in the deep (more oxidative) region of the muscle. From 5 to 13 months, the percentage of FOG fibres increased at the expense of FG fibres, while the fibre cross-sectional areas (FCSA) of each fibre type increased. From 13 to 25 months, the FCSA of FG fibres decreased, as did the local capillary-to-fibre ratio (LCFR) of each fibre type, indicating capillary loss and a declined capillary density for each fibre type (CFD). Overload effects were identical for both the superficial (more glycolytic) and the deep region for each age group. With overload, FCSA and LCFR of each fibre type increased, while CFD decreased, indicating that capillary proliferation occurred with overload, even at old age, although lagging behind increases in FCSAs. Training showed minor effects.

Aging↗
PubMed1991

Computed myocardial PO2 histograms: effects of various geometrical and functional conditions.

A model of myocardial oxygenation was developed that allows calculation of Po2 histograms under varying conditions. The model consists of parallel tissue cylinders with varying radii, simulating the heterogeneity of capillary spacing, in agreement with our previous experimental results. The facilitated diffusion of O2 by myoglobin, an additional resistance to diffusion at the capillary level, and the Michaelis-Menten type of O2 consumption were also incorporated. The shape of the histograms depends on input data. When no additional barrier to O2 transport is included, the histograms resemble those obtained with Po2 surface electrodes, and they are strongly dependent on heterogeneity in capillary spacing and capillary blood flow. On the other hand, an inclusion of an additional capillary barrier combined with the Michaelis-Menten type of O2 consumption can generate Po2 histograms similar to those derived from myoglobin cryospectroscopy. In this case, the Po2 histograms are relatively independent of heterogeneity of capillary spacing and blood flow. The facilitation of O2 diffusion by myoglobin has only a modest effect on the form of the histograms in all situations considered.

Animals↗
PubMed1989

Microelectrode studies of facilitated O2 transport across hemoglobin and myoglobin layers.

Experimentally measured PO2 profiles across layers of hemoglobin and myoglobin solutions were compared with profiles predicted from facilitated transport theory assuming chemical equilibrium. Measurements across myoglobin layers were in excellent agreement with theory, but measurements across hemoglobin layers departed from theory at low PO2. This departure was greatest for salt-free hemoglobin solution, which may be caused by an electrical potential formed by a pH gradient in the layer as oxyhemoglobin is deoxygenated.

Animals↗
PubMed1987

Morphometric analysis of sparse capillary networks.

Two methods were used to assess the heterogeneity of capillary supply to muscles of widely differing metabolic capacity and fibre size. Using the method of capillary domains (DOM; Hoofd et al., 1985) and the closest-individual method (CI; Kayar et al., 1981) radii of Kroghian cylinders (R) can be calculated, and the heterogeneity of their lognormal distribution represented by the logarithmic standard deviation (Log SD). Both methods yield similar values for mean R in a tissue. DOM is more direct and quicker than CI, and may be particularly useful in the analysis of capillary oxygen supply during functional hypertrophy and in muscle regeneration where a broad distribution of fibre areas may be found. Despite a 500-fold range of capillary density, to a minimum of 20 capillaries mm-2, heterogeneity of capillary supply was similar in all muscles, indicating a functionally homologous spatial distribution. The relationship between number of fibres overlapped by a capillary domain, and domain area has zero correlation in most tissues but shows a negative trend in fish fast muscle, reflecting hyperplastic and hypertrophic growth. Capillary/fibre ratio is inappropriate for sparse networks whereas the cumulative fraction of domains vs fibre area shows a strong correlation, suggesting that maximal oxygen supply to muscle fibres is not restricted to contiguous capillaries, but also involves those remote from the fibre surface.

Animals↗