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Biomedical subjects

R S Seymour

Publications and source records attributed to R S Seymour.

16 recordsLinked to original sources
PubMed1996

Blood pressure and flow rate in the giraffe jugular vein.

Experimental measurements in the jugular veins of upright giraffes have shown that the internal pressure is somewhat above atmospheric and increases with height above the heart. A simple model of steady viscous flow in an inverted U-tube shows that these observations are inconsistent with a model in which the blood vessels in the head and neck are effectively rigid and the system resembles a siphon. Instead, the observations indicate that the veins are collapsed and have a high resistance to flow. However, laboratory experiments with collapsible drain tubing in place of the down arm of the U-tube show internal pressure to be exactly atmospheric and uniform with height. A model of viscous flow in a collapsible tube with non-uniform properties is used to suggest that the observed pressure distribution may be a consequence of the intrinsic cross-sectional area and/or compliance of the veins increasing with distance towards the heart, or the external, tissue pressure falling. Finally, the effect of fluid inertia on steady flow in vertical collapsible tubes with uniform intrinsic properties is analysed, and it is shown that a phenomenon of flow limitation is theoretically possible, in which the supercritical flow in the collapsed vein cannot return to the presumably subcritical flow in the open vena cava, even with the help of an 'elastic jump', if the flow rate is too large. The computed critical flow-rate, of about 80 ml s-1, is about twice the flow-rate estimated to be present in the normal giraffe jugular vein. If there were circumstances in which flow limitation occurred in the jugular veins, it would mean that the cerebral blood flow would be limited by downstream conditions, not directly by local requirements.

Animals↗
PubMed1994

Embryonic and larval respiration in the arboreal foam nests of the African frog Chiromantis xerampelina.

In Zimbabwe, female Chiromantis xerampelina construct spherical foam nests that are suspended above temporary water. The nests average 624 ml in volume and contain 854 eggs. The 1.7 mm ova have exceptionally thin jelly capsules and are dispersed in the foamy core of the nest, which is surrounded by a layer of eggless foam. At 25 degrees C, each embryo requires 3.5 days to reach hatching at developmental stage 22, during which it consumes 30 microliters of oxygen. After hatching, each larva remains in the nest for 2 more days and consumes a further 123 microliters of oxygen. The fresh foam contains 77% air, which is sufficient to supply all of the oxygen requirements of the embryos until well after they hatch. Therefore, the size of the egg mass is not limited by oxygen availability as it is in many other anurans. Oxygen also diffuses into the nest from the atmosphere, but the rate is severely restricted by the wet foam, despite the presence of bubbles. Drying of the outer layer of foam greatly increases its oxygen conductance, but the larvae remain in the inner core of wet foam, where they compete for oxygen at the periphery. With further drying of the nest, the wet foam diminishes in volume and concentrates the larvae at a time when their oxygen demands are approaching the maximum. Oxygen pressures within the wet foam drop below 10 kPa and oxygen uptake by the larvae becomes progressively limited, possibly stimulating their emergence from the nest. The delay between hatching and escape from the nest permits the larvae to grow and mature to a stage at which all of the clutch can emerge simultaneously.

Animals↗
PubMed1993

The heart works against gravity.

The circulatory systems of vertebrate animals are closed, and blood leaves and returns to the heart at the same level. It is often concluded, therefore, that the heart works only against the viscous resistance of the system, not against gravity, even in vascular loops above the heart in which the siphon principle operates. However, we argue that the siphon principle does not assist blood flow in superior vascular loops if any of the descending vasculature is collapsible. If central arterial blood pressure is insufficient to support a blood column between the heart and the head, blood flow ceases because of vascular collapse. Furthermore, the siphon principle does not assist the heart even when a continuous stream of blood is flowing in a superior loop. The potential energy gained by blood as it is pumped to the head is lost to friction in partially collapsed descending vessels and thus is not regained. Application of the Poiseuille equation to flow in collapsible vessels is limited; resistance depends on flow rate in partially collapsed vessels with no transmural pressure difference, but flow rate is independent of resistance. Thus the pressure developed by the heart to establish a given flow rate is independent of the resistance occurring in the partially collapsed vessels. The pressure depends only on the height of the blood column and the resistance in the noncollapsed parts of the system. Simple laboratory models, involving water flow in collapsible tubing, dispel the idea that the siphon principle facilitates blood flow and suggest that previously published results may have been affected by experimental artifact.

Animals↗
PubMed1992

Polyunsaturated dietary lipids lower the selected body temperature of a lizard.

Cold acclimation lowers the selected body temperature (Tb) in many ectothermic vertebrates. This change in behavioural thermoregulation is accompanied by an increase in the proportion of polyunsaturated fatty acids in tissues and cellular membranes. We investigated how diets containing different fatty acids, known to significantly alter the fatty acid composition of animal tissues and membranes, affect the selected Tb of the lizard Tiliqua rugosa. Lizards on a diet containing many polyunsaturated fatty acids (10% sunflower oil) showed a 3-5 degrees C decrease in Tb, whereas Tb in animals on a diet containing mainly saturated fatty acids (10% sheep fat) did not change. Our study suggests that the composition of dietary lipids influences thermoregulation in ectothermic vertebrates and may thus play a role in the seasonal adjustment of their physiology.

Acclimatization↗
PubMed1988

Aeration of the shell membranes of avian eggs.

The inner and outer shell membranes (ISM, OSM) of fertile hen's eggs become aerated during the first ten days of incubation. The volume of gas in the compound membrane increases from 0 to 65% by day 11 when practically all of the space between the fibers is gas filled. The amount of gas depends on a balance between the capillary tension produced by fluid menisci in the interstices between fibers and the colloid osmotic tension of the albumen. Capillary tension, measured directly with a pressure plate apparatus, and colloid osmotic pressure, measured with a new technique, are in virtual equilibrium during aeration. These tensions increase from 0.15 bar at laying to 2.25 bar on day 11 because of evaporation from the membranes and active removal of water from the albumen by the embryo. Water leaving the membranes by evaporation is only partly replaced from the albumen. Thus the water contents of the membranes and albumen decrease (ISM: 79 to 56%, OSM: 71 to 52%, albumen: 86 to 58%). As the membranes dry, gas enters the larger pores first. Physiological estimates of effective radius show smaller (x = 1.3 micrograms), bimodally distributed pores in the ISM and larger (x = 3.4 micrograms), unimodally distributed pores in the OSM.

Animals↗
PubMed1987

Blood flow uphill and downhill: does a siphon facilitate circulation above the heart?

1. Despite the continuity of the circulatory system, blood flowing down the veins of the neck does not assist blood flowing up the arteries. Because of collapsible veins, gravitational pressure gradients are not matched in arterial and venous sides of circulatory loops above the heart as would be necessary for a siphon to operate. 2. All animals have arterial blood pressures capable of lifting the blood to the head without the assistance from a siphon. In cases of longer vertical distances, such as in giraffes and climbing snakes, the heart does more work against the effects of gravity on the arterial blood column. 3. The potential energy of the blood in the head is lost as frictional heat produced in the veins.

Animals↗
PubMed1983

Functional venous admixture in the lungs of the turtle, Chrysemys scripta.

Pulmonary functional venous admixture was determined during forced, unidirectional ventilation with pure O2 and air. At a normal lung volume of 140 ml X kg-1, anatomical shunts (physical bypass of gas exchange surface by the blood) averaged 10% of pulmonary blood flow but it increased to 28% as lung volume declined to 30 ml X kg-1. Diffusion limitation and possibly inhomogeneity in ventilation: perfusion ratio also contributed to a total functional venous admixture of about 25% at normal lung volume.

Animals↗
PubMed1982

Cardiovascular function during treadmill exercise in the turkey.

1. Heart rate, arterial pressure and blood lactate levels were measured in turkeys over a range of running speeds on a treadmill. 2. Heart rates showed a hyperbolic increase with speed to a level approximately twice that recorded at rest. Arterial pressures were found to be lower than those previously reported for the turkey. During severe exercise, the mean value increased 37% above the resting level. 3. Blood lactate levels were greater at increased work loads, but after an initial rise, the levels remained steady during locomotion. 4. At rest, heart rate increased about 46% following parasympathetic blockade and decreased 19% with sympathetic blockade. During locomotion, parasympathetic tone is minimal. 5. In turkeys free of parasympathetic and beta-adrenergic influences, heart rate still increased at the highest running speed. Neither lactate nor ionotrophic effects appear to cause this increase.

Animals↗
PubMed1978

Regulation of arterial blood pressure in Australian tiger snakes.

1. Blood pressure was measured in the dorsal aorta of restrained, unanaesthetized tiger snakes (Notechis scutatus) at different body temperatures during graded, passive tilt. Aortic blood pressure in horizontal snakes showed no significant change over a range of body temperatures between 18 and 33 degrees C (mean of measurements on 16 snakes = 42.2 +/- I.98 mmHg), while heart rate increased logarithmically (Q10 approximately 2.5). Blood pressure was stable during heating and cooling between body temperatures of 15 and 30 degrees C, but the pressure was 10--50% higher during heating than during cooling. 2. Head-up tilt usually caused a brief fall in pressure at heart level followed by partial or complete recovery and tachycardia. At the cessation of tilt, there was a characteristic overshoot of the blood pressure followed by readjustment to control (pretilt) levels. Head-down tilt typically increased pressure which then either stabilized or returned toward pretilt levels. Heart rate changes during head-down tilt were not consistent in direction or magnitude. Stabilized pressures at mid-body usually increased following head-up tilt and decreased following head-down tilt, indicating physiological adjustment to posture change. Blood pressure control was evident at body temperatures ranging from 10 to 38 degrees C, but was most effective at the higher and behaviourally preferred temperatures. 3. Propranolol lowered heart rate but did not influence pressure in horizontal snakes. During head-up tilt propranolol eliminated or reduced tachycardia and sometimes reduced the efficacy of pressure compensation for tilt. Phentolamine increased heart rate, lowered blood pressure, and eliminated pressure regulation during tilt. The results suggest that sympathetically mediated reflexes assist central blood pressure regulation in the tiger snake, with vasomotor adjustments having greater importance than changes in heart rate.

Animals↗
PubMed1975

Gas transport and blood acid-base balance in diving sea snakes.

The values of hemoglobin concentration, Hb-O2 affinity and buffering capacity of the blood of six sea snake species considerably overlap values from terrestrial squamates. Decreased blood pH had little effect on the P50 but increased the n-value of Hb-O2 equilibrium curves. The O2 saturation of blood in the dorsal aorta varied between about 30 and 70% during voluntary diving in Acalyptophis peronii and Lapemis hardwickii. Voluntary dives ended when the lung PP02 was about 50 mm Hg and the arterial PO2 about 30 mm Hg indicating that roughly half of the O2 reserves had been used. In conjunction with relatively stable blood lactate concentration and pH, this indicates that voluntary dives occurred largely aerobically. In contrast, forced dives resulted in depletion of O2 reserves and large changes in blood acid-base balance. Long recovery periods following forced dives are inconsistent with field observations and thus suggest that extensive anaerobic metabolism does not normally occur in sea snakes. Bradycardia was not evident during forced dives. Large differences in PO2 between the lung and dorsal aorta indicated considerable right to left shunting either in the heart or in the lung. Venous blood represented over 50% of the systemic flow when there was considerable O2 in the lung. Therefore blood PO2 may remain relatively low despite elevated lung PO2 resulting from diving. In view of substantial capability for extra-pulmonary gas exchange, high shunting reduces the possibility of losing O2 through the skin and also may help prevent decompression sickness following deep dives.

Acid-Base Equilibrium↗
PubMedDate not supplied

Osmotic balance in the eggs of the turtle Chelodina rugosa during developmental arrest under water.

The tropical Australian turtle Chelodina rugosa normally lays its hard-shelled eggs in mud, under shallow freshwater, during the monsoon season. The eggs undergo developmental arrest until the water recedes and oxygen is able to diffuse into the embryo. This period of arrest can exceed 12 wk without embryonic mortality. To understand how the eggs avoid osmotic absorption of water leading to shell rupture and embryonic death, this study investigates the solute concentrations and volumes of the albumen and yolk compartments during submergence in distilled water. The albumen loses considerable sodium through the shell, particularly during the first week, and its osmotic concentration drops from 234 mmol/kg at laying to about 23 mmol/kg. Meanwhile, water from the albumen slowly moves through the vitelline membrane into the yolk compartment, which enlarges at a constant rate until it approaches the inside of the shell at about 22 wk. Osmotic uptake dilutes yolk solutes, decreasing the osmotic concentration from 281 mmol/kg at laying to 132 mmol/kg at 157 d. Loss of embryonic viability is associated with contact of the vitelline membrane with the inside of the shell. The principal adaptation of this species for protracted developmental arrest under water is a vitelline membrane of such low permeability to water that the expansion of the yolk compartment occurs about 10 times more slowly than in other chelonians.

Animals↗