The exchange of water between plasma and interstitial fluid (ISF) along the length of a capillary is attributed to a balancing of the Starling forces, site-specific differences in hydrostatic and osmotic pressures that theoretically determine directional fluid movement. The osmotic forces for water movement are attributed to the osmotic effects of proteins, colloid osmotic pressure (COP). Several physiological inconsistencies question the role of proteins and COP in fluid flux. A reconsideration of Hulett's insights concerning the osmosis of water provides substantial evidence that the effect of COP does not cause osmosis, and therefore another force is needed to explain plasma–ISF exchange. Review of whole-body tissue and blood ion concentrations and/or ion differences across isolated tissue or secretory epithelia from a variety of species indicates that the diffusion of bicarbonate and strong ions within plasma is the dominant osmotic effect returning ISF to the capillary. Conceptually, as these ions diffuse along physiological gradients, they alter the chemical potential of water through which they are diffusing (solute–solvent drag), creating an osmotic effect on plasma water, and explain plasma–ISF exchange. Considering venous–arterial differences, diffusing and strong ions give rise to a net osmotic force (~35 Torr) in venous end capillary plasma water that is coupled to ISF through pores in the endothelium. More importantly, diffusing and strong ions provide an incremental osmotic force (~150 Torr) that is essentially matched to any change in metabolic rate (e.g. muscular work) when CO2 output and water production are increased. The proposed diffusing ion osmotic force does not negate the necessity for colloidal proteins in volume regulation. Proteins can have an essential effect on fluid exchange in plasma when blood flow is intermittent or changes in protein concentration in the ISF such that proteins exert a force against a distensible boundary (i.e. the endothelium and basement membrane) as they are reflected by it or diffuse through the membrane due to changes in permeability.

The fundamental tenets of exercise physiology are to describe energy transformations during physical work and make predictions about physical performance under different conditions. Historically, the most popular method to observe such responses during exercise has been the constant load or fixed-intensity protocol, based largely on the assumption that there is a certain threshold response of the organism under a given condition. However, constant load exercise does not fully allow for randomness or variability, as the biological system is overridden by a predetermined externally imposed load that cannot be altered. Conversely, in self-regulated (paced) exercise, there is almost an immediate reduction in power output and muscle recruitment upon commencing exercise. This observation suggests the existence of neural inhibitory command processes. This difference in regulation demonstrates the inherent importance of variability in the biological system; for in tightly controlled energy expenditure, as is the case during constant load exercise, sensory cues cannot be fully integrated to provide a more appropriate response to the given task. The collective evidence from conventional constant load versus self-regulated exercise studies suggests that energy transformations are indeed different, so that the inherent biological variability accounts for the different results achieved by the two experimental paradigms.

The impact of body density (BD) on an animal's capacity to sustain a workload is important in the accuracy of evaluating swimming exercise in rats and the associated training protocols. The aim of this study was to quantify the influence of BD the maximal lactate steady-state (MLSS) workload in swimming rats. The BD of ten 90-day-old rats and sixteen 120-day-old rats was determined by underwater weighing, and their aerobic capacity was determined by the MLSS test. The MLSS blood concentration values were 4.11 ± 0.96 mmol l− 1 in the 90-day-old rats and 4.81 ± 1.49 mmol l− 1 in the 120-day-old rats. There was no significant (P>0.05) difference between these values. The older rats were more dense (P < 0.001) and showed a significantly increased (P < 0.001) absolute effort and relative-to-body-weight effort to keep themselves on the water surface when compared with the younger rats. BD can significantly affect an animal's capacity to sustain work within this age range, and individual fluctuation in effort should be determined to avoid mistakes when interpreting the results. This is particularly important in longitudinal studies in which the intervention or ageing process can modify the animal's body composition. Our results quantify the effects of BD on the performance of rats in the MLSS test.

Recent research has shown that more successful performers interpret pre-competitive symptoms of anxiety as more facilitative than less successful performers. The aim of the study was to examine the effect of both trait anxiety and pre-competitive arousal intensity and direction on intermediate riding performance in the disciplines of dressage, showjumping and eventing, including whether differences exist between male and female riders. Sixty-two German equestrian riders competing in the equestrian disciplines of dressage (N = 21), showjumping (N = 25) and eventing (N = 16) were asked to complete a revised version of the ‘Wettkampf–Angst-Inventar-State’, which included directional scales for all items and the ‘Wettkampf–Angst-Inventar-Trait’ prior to competing. Final placings were used as performance indicators. Most important findings revealed that the correlation between direction of somatic state arousal and competitive placing was nearing significance (rs = − 0.23, P =0.07) and loss of focus was positively correlated to competitive placing (rs = 0.26, P < 0.05). Conclusions may be drawn that in equestrian sports positive interpretation of physical symptoms of arousal as well as the ability to remain focused on the task at hand may lead to more successful ridden performance.

To help resolve the mechanistic bases for haematological adaptations (~28% increase in red blood cell volume) of equids to high altitude (3800 m, barometric pressure Pb, 487 mm Hg) and exercise, plasma erythropoietin concentration ([EPO]) was measured at rest and following exercise in six, moderately fit equids (four Arabians, one Quarter Horse and one Shetland Pony; four females and two males; age 9.0 ± 4.5 years (mean ± SD)). [EPO] was measured on 2 days at 225 m (i.e. ~sea level; Pb, 743 mm Hg), over the course of a 10-day altitude exposure, and then again for 2 days after return to sea level. A standard track exercise test (submaximal, speed set-to-heart rate of 110 (trot), 150 (canter), 180 (gallop) bpm) was performed 2 days pre-high-altitude exposure and on three separate days at high altitude. In addition, a maximal incremental exercise test was performed on a high-speed motor-driven treadmill at sea level and 2 days following return to sea level from high altitude. Resting [EPO] increased from 28 ± 29 at sea level to 144 ± 46 mU ml− 1 (P < 0.05) on the first day at high altitude. By day 2 at high altitude, [EPO] had returned to baseline (31 ± 24 mU ml− 1, P>0.05 vs. pre-high altitude) and did not change over the remaining 8 days at high altitude nor over the 2 days after return to sea level. [EPO] was not significantly altered by acute exercise at sea level or at 3800 m. These results indicate that [EPO] increases rapidly (though transiently) in response to hypobaric hypoxia but not to acute exercise, and that exercise does not appear to potentiate the altitude response. Thus, if any [EPO]-derived haematological adaptations to high altitude are present, these appear to result from a transient ~4-fold elevation of [EPO] rather than any sustained increase in this signalling mechanism, at least in the equid.

Nations intending to send teams to compete in the equestrian sports at the 2008 Olympic Games in August in Hong Kong were concerned that safe competition would not be possible in this environment. Our objective was to demonstrate that horses could safely compete in the Hong Kong climate, where identifying and treating dehydration and electrolyte losses would be especially important to horse welfare. We hypothesized that evaluating serial changes in body weight, urine specific gravity (USG), urine Na concentration (UNa) and urinary clearance of Na (CLNa) would allow us to identify dehydration and Na deficits. Seventeen horses were housed in air-conditioned stables maintained at 25°C. Vital signs and body weight were recorded twice daily. Twenty-four-hour water consumption, and number and duration of exercise periods, were recorded daily. Haematological and biochemical variables, USG, UNa and CLNa, were measured intermittently in subsets of horses. Environmental conditions were monitored daily. Fifteen horses completed the competition; vital signs and all mean values for haematological and biochemical variables (except a high mean total protein on day 1 after arrival) remained within reference ranges. Daily water intake was (mean ± SE) 38 ± 1.5 l (range 18–74 l). Mean change in body weight during the entire study was − 3 ± 2.4 kg (range − 17 to 14 kg). Horses spent at most 3.5 h outside and at least 20.5 h in the stables daily. The mean value for USG was 1.035 ± 0.0008 and 57% of determinations were >1.035. The mean UNa was 61 ± 8 mmol l− 1 and 57% of values were ≤ 20 mmol l− 1. The mean CLNa was 0.14 ± 0.0.04%, with 76% of determinations being ≤ 0.1%. An 88% completion rate demonstrated that safe competition was possible. Access to climate-controlled stables probably helped to minimize daily fluid and electrolyte losses, and facilitated recovery following exercise. A majority of USG values >1.035, UNa < 20 mmol l− 1 in 29% of determinations, and CLNa < 0.1% in 57% of determinations suggested that some horses may have had insufficient Na intake while in Hong Kong. Monitoring USG, UNa and CLNa may aid in identifying and treating fluid and Na deficits in competition horses.

Foot impact force during running has been suggested as one of the important causes of haemolysis in runners. However, intravascular haemolysis has also been reported among athletes involved in sports in which foot impact does not occur. The purpose of our study was to analyse intravascular haemolysis in athletes during running and compare it with cycling. Twenty male long- and middle-distance runners volunteered to participate in this study. They were divided into two groups (group 1: running and group 2: cycling), with 10 participants in each group. Each athlete completed 1 h of running or cycling at their calculated target heart rate (60–70%). Venous blood samples were collected before and immediately after the running and cycling protocols. Unconjugated bilirubin (UBR; P < 0.01) and lactate dehydrogenase (LDH; P <0.05) levels were increased significantly after running, indicating the occurrence of haemolysis in this group of athletes. A significant variation was observed in the mean values of haematological variables between post-run (UBR: 1.40 ± 0.29, LDH: 225 ± 69.13, haemoglobin (Hb): 11 ± 1.09) and pre-run (UBR: 0.88 ± 0.31, LDH: 183 ± 40.42, Hb: 12.10 ± 1.19) during the running protocol. No significant differences in haematological variables were found between athletes who did cycling and non-exercising group participants. Our results indicate that intravascular haemolysis occurred significantly during running when compared with cycling. Hence, we conclude that the mechanical trauma due to footstrike is the major cause of intravascular haemolysis during running.