Mary L. Ellsworth

Assessment and impact of heterogeneities of convective oxygen transport parameters in capillaries of striated muscle: Experimental and theoretical

Convective oxygen transport parameters were determined in arteriolar (n = 5) and venular (n = 5) capillary networks in the hamster cheek pouch retractor muscle. Simultaneously determined values of red blood cell velocity, lineal density, red blood cell frequency, hemoglobin oxygen saturation (SO2), oxygen flow (QO2), longitudinal SO2 gradient, and diameter were obtained in a total of 73 capillaries, 39 at the arteriolar ends of the network (arteriolar capillaries) and 34 at the venular ends (venular capillaries). We found that the hemodynamic variables were not different at the two ends. However, not unexpectedly, SO2 and QO2 were significantly higher at the upstream end of arteriolar capillaries (60.8 +/- 9.8 (SD)% and 0.150 +/- 0.081 pl/sec, respectively) compared with the downstream end of venular capillaries (39.9 +/- 13.6% and 0.108 +/- 0.095 pl/sec, respectively). Heterogeneities in red blood cell velocity, lineal density, SO2, and QO2, assessed by their coefficients of variation, were significantly greater in venular capillaries. To evaluate the impact of these heterogeneities on oxygen exchange, we incorporated these unique experimental data into a mathematical model of oxygen transport which accounts for variability in red blood cell frequency, lineal density, inlet SO2, capillary diameter, and, to some degree, capillary flow path lengths. An unexpected result of the simulation is that only the incorporation of variability in capillary flow path lengths had any marked effect on the heterogeneity in end-capillary SO2 in resting muscle due to extensive diffusional shunting of oxygen among adjacent capillaries. We subsequently evaluated, through model simulations, the effect of these heterogeneities under conditions of increased flow and high oxygen consumption. Under these conditions, the model predicts that heterogeneities in the hemodynamic parameters will have a marked effect on oxygen transport in this muscle.

Estimation of red cell flow in microvessels: Consequences of the Baker-Wayland spatial averaging model

The dual sensor cross-correlation method of H. Wayland and P.C. Johnson [1967), J. Appl. Physiol. 22, 333-337) has become a standard technique for determining the velocity of red blood cells (RBCs) in glass tubes and blood vessels. M. Baker and H. Wayland [1974), Microvasc. Res. 7, 131-143) found that under a variety of conditions the ratio of dual sensor velocity at the centerline of a glass tube to the blood velocity averaged over the lumen was close to 1.6. They provided an explanation of this factor based on spatial averaging of RBC velocity vertically through the tube as well as laterally across the face of the sensor. Their spatial averaging model could also account for the apparent blunting of RBC velocity profiles determined with the dual sensor technique. We used Baker and Waylands spatial averaging model to calculate how the above velocity ratio depends on sensor size. A nonlinear relation between the velocity ratio and sensor size was found such that the velocity ratio varied from 1.6 to 1.33 as the ratio of sensor width to vessel or tube diameter was varied from 0 to 1. These results also hold for vessels or tubes of elliptic cross section. Some investigators have found that the velocity of red cells near the walls of blood vessels can be a substantial fraction of centerline velocity which suggests that RBC velocity distributions can be blunter than a Poiseuille distribution. We repeated the above calculation for blunted parabolic profiles and we found that the velocity ratio ranged from 1 for plug flow to 1.6 for Poiseuille flow. These calculations show that reliable estimates of RBC flow from dual sensor centerline velocity measurements require one to take into account the relative size of the sensor and blood vessel diameter as well as the bluntness of the RBC velocity distribution.

Analysis of vascular pattern and dimensions in arteriolar networks of the retractor muscle in young hamsters

A quantitative analysis of the distribution of microvascular blood flow and oxygen delivery requires a detailed description of the vascular network geometry. The distributions of lengths and diameters were determined in terminal arteriolar networks of the cheek pouch retractor muscle of young (34 +/- 2 days) hamsters. We compared the Strahler centripetal vessel ordering scheme, which assigns lowest order to the capillaries and proceeds upstream toward the larger vessels, with the centrifugal ordering scheme, which begins with the input arteriole and proceeds downstream toward the capillaries. The terminal networks of the retractor muscle typically contain 2 to 4 Strahler orders and 2 to 6 centrifugal orders. The coefficients of variation of diameter and length are smaller for Strahler ordering than for centrifugal ordering. In addition, for Strahler ordering, we found that the sequence of number of vessels obeyed Hortons law. We have compared three different methods of calculating the bifurcation, diameter, and length ratios. As an alternative method for analyzing network topology, we also studied the distribution of the number of segments on each pathway from the inlet of a network to a capillary. The information obtained from this analysis is useful for the mathematical modeling of flow in the microvascular network.

Rate of oxygen loss from arterioles is an order of magnitude higher than expected

The experimental data on oxygen flux from arterioles in the hamster cheek pouch retractor muscle [L. Kuo and R. N. Pittman, Am. J. Physiol. 254 (Heart Circ. Physiol. 23): H331-H339, 1988] were analyzed under the assumption that the permeability to oxygen is the same in both perfused and unperfused tissue; permeability is defined as the product of the diffusion and solubility coefficients. However, our analysis indicated that the observed oxygen flux was inconsistent with this assumption and that permeability to oxygen of a blood-perfused tissue may be an order of magnitude higher than previously assumed.

Measurements of oxygen flux from arterioles imply high permeability of perfused tissue to oxygen.

A mathematical model developed for the analysis of oxygen flux from an arteriole surrounded by perfused tissue was used to analyze experimental data in the resting hamster cheek pouch retractor muscle. The flux predicted by the model, with the commonly accepted values of tissue permeability to oxygen (the Krogh diffusion coefficient), was an order of magnitude smaller than the average value of experimentally observed oxygen flux. The values of permeability required by the model to equate the predicted and observed oxygen flux are one to two orders of magnitude higher than the accepted values. Also, the values of the oxygen tension gradient in the arteriolar wall predicted by Fick’s law are an order of magnitude greater than the measured values reported in the literature. Since the accepted values of permeability are based on experiments with unperfused tissue and the values predicted by the model are for blood-perfused tissue, we conjecture that tissue permeability is a function of the perfusion conditions. Hence, there is a need for re-examination of the distribution of resistance to oxygen transport along the pathway between red blood cells and mitochondria. Theoretical estimates based on accepted values of tissue permeability to oxygen show resistances to oxygen transport inside and outside the capillaries to be of similar magnitude. However, if the tissue component of the resistance is significantly reduced because of greater permeability, the intracapillary resistance becomes dominant and is responsible for most of the drop in the oxygen tension between the red blood cells and the tissue.

B-Lymphocyte Cells in Lymphatic Tissue of the Duck, Anas platyrhynchos

SUMMARY Fluorescent-antibody techniques were used to identify Blymphocytes in the duck. These cells were restricted primarily to the germinal centers of a number of lymphatic tissues. In some instances, these germinal centers were associated with stromal or vessel-like structures.

The Pineal Body of the Dog

A technique is described for finding the pineal body of the dog. The posterior half of the skull is cut a little behind the parietofrontal suture, through the occipital condyles. The cerebral hemispheres and cerebellum are carefully sliced, disclosing the pineal at the frontal edge of the colliculi. Two types of cells are present, those with completely round nuclei, and others with vesicular and variably shaped nuclei. In the histological pattern, ependymal cells were observed on the edges, pinealocytes and glial cells within the gland.

Videodensitometric Determination of Oxyhemoglobin Saturation (SO2) in Muscle Capillaries

The transport of oxygen to striated muscle cells is comprised of two principal components: one convective and the other diffusive. To evaluate the convective component it is necessary to quantify the amount of oxygen delivered to the site of exchange. Therefore, one must determine the number of red cells in the delivery vessel, the velocity of the red cells and their level of oxygenation. Oxygen is carried in the blood in two forms. About 98% of the oxygen is bound to red cell hemoglobin at normal hematocrit with the remaining 2% dissolved in the plasma. Therefore, a relatively accurate estimate of the level of oxygenation can he obtained from measurements of oxygen saturation.