Biomedical subjects
Publications and source records attributed to G Karreman.
It is often necessary to determine the selectivity and cooperativity constants of ions and other substances which are selectively and cooperatively adsorbed at surface sites of biological cells. A computer program based on the Levenberg-Marquardt method is given to determine the selectivity and cooperativity constants from the non-linear equation for the resting potential.
van der Pol developed a mathematical model for self-sustained radio oscillations described by his non-linear differential equation D2X + epsilon(X2-1)DX + X = 0 in which X is a function of time T and D/DT the differential operator to T. For epsilon = 0, this is the differential equation for the harmonic oscillator which has sinusoidal solutions. For epsilon not equal to 0 the equation is non-linear. If epsilon > 1 van der Pol coined the name relaxation oscillations for its solutions. These are non-linear and quite different from simple sinusoidal oscillations. They are mathematical models for many physical and biological phenomena. van der Pol suggested that his equation is also a model for the heartbeat. However, biomedical oscillations, including the heartbeat, have a threshold which the mathematical model described by van der Pol's equation does not possess. It has, in addition to an unstable origin, only a stable limit cycle of Poincaré. In this paper, van der Pol's equation is extended in such a way that it has in addition to a stable origin and a stable limit cycle, an unstable limit cycle. Because it possesses such an unstable limit cycle, the extension obtained is a mathematical model for a threshold oscillation. It is also shown that an asymmetric analogy of the extended equation is a mathematical model for an isometric contraction of the mammalian cardiac muscle.
In the absence of peripheral chemoreceptors, the effects of graded hypoxemia on the carotid sinus control of central and regional hemodynamics were studied in anesthetized mongrel dogs. Baroreceptor stimulation was effected by carotid sinus isolation and perfusion under controlled pressure. Blood flows were measured in the aorta and the celiac, mesenteric, left renal, and right iliac arteries. Carotid sinus reflex set-point pressures were well maintained until hypoxemia was severe. Carotid sinus reflex set-point gain was maximal during mild hypoxemia. Reflex operating point regional flows were unaffected by hypoxemia. A factorial analysis of overall reflex increases in mean aortic pressure, flow, and power during reduced baroreceptor stimulation showed potentiation by increasing hypoxemia. Corresponding effects of baroreceptor stimulation and hypoxemia on aortic resistance and heart rate were additive. Celiac, renal, and iliac blood flows increased during both hypoxemia and reduced baroreceptor stimulation. Only in the celiac bed were blood flow changes independent of concomitant changes in cardiac output. Thus, at maximum sympathetic stimulation (low carotid sinus pressure) during hypoxemia, the cardiovascular system maintained both central and regional blood flows at high systemic blood pressures independent of the peripheral chemoreceptors.
Explore the source record for details and available documents.
The equilibrium parameters of potassium-sodium distribution in human lymphocytes, determined experimentally in the preceding study (Negendank and Shaller, '79), were incorporated into a stochastic treatment of the cooperative adsorption model in order to predict the kinetics of "active" potassium-sodium exchange. The rate of uptake of potassium, in potassium-depleted, sodium-loaded cells, is complex and deviates markedly from simple exponential functions. The sigmoid form of the exchange data closely followed the predicted curve. This result enhances one's confidence in the usefulness and applicability of the cooperative adsorption model, and adds further support to the association-induction hypothesis as a coherent theory of cell physiology.
Comparison was made between intentional and postural tremor amplitudes with a 2-to 50-Hz bandwidth in four men rapidly compressed from surface to 13.1, 25.2, and 37.3 ATA in 4 h while breathing helium-oxygen mixtures (Predictive Studies IV, University of Pennsylvania). Excursions with compression rates of 20 and 40 ft/min were then made to 49.4 ATA on exposure days 2 and 3, returning to saturation at 37.3 ATA for 6 additional days. During compression from 25.2 to 37.3 ATA, there were increases in tremor amplitudes for both intentional and postural tremor in the 3-to 7-Hz and 8- to 12-Hz ranges. Both types of tremor showed increases in amplitudes at 49.4 ATA on exposure days 2 and 3. Amplitude changes in postural tremor, which occurred only on the fastest compression to 49.4 ATA, were less evident than those in intentional tremor during the remaining dive profile, and adaptation to pressure exposure could be defined by day 8. Intentional tremors did not show adaptation at pressure.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Properties of the ion exchange mechanisms in the arterial wall were investigated by comparing water and electrolyte contents, and by measuring the steady-state entry of (22)Na, (42)K, and (26)Cl under similar in vitro conditions. Overnight incubation of freshly dissected slices at 2 degrees C resulted in an accumulation of sodium, chloride, and water and a loss of potassium. Subsequent incubation at 37 degrees C in a physiological solution resulted in a reversal of these processes. Loss of water, sodium, and chloride at 37 degrees C could also take place into a potassium-free solution. Under all conditions studied the quantity of fast exchanging electrolyte (half time less than 3 min) exceeded that contained in the inulin and sucrose spaces. The excess could not be attributed to connective tissue adsorption. A kinetic model was applied to the flux data which incorporated two simultaneous processes: bulk diffusion and a reversible reaction. The assumption that the cell membrane behaved as a discrete barrier for the exchange of all cell electrolyte was relaxed in this approach. A theory based upon the physicochemical properties of proteins, ions, and water in biological systems provided a physical basis for the kinetic model, and for interpreting the ion exchange properties of the vascular wall.