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Highly Athletic Terrestrial Mammals: Horses and Dogs

David C. Poole, Howard H. Erickson

10.1002/cphy.c091001

Source: Volume 1, Issue 1, January 2011

Published online: January 2011

Full Article on Wiley Online Library



Abstract

Evolutionary forces drive beneficial adaptations in response to a complex array of environmental conditions. In contrast, over several millennia, humans have been so enamored by the running/athletic prowess of horses and dogs that they have sculpted their anatomy and physiology based solely upon running speed. Thus, through hundreds of generations, those structural and functional traits crucial for running fast have been optimized. Central among these traits is the capacity to uptake, transport and utilize oxygen at spectacular rates. Moreover, the coupling of the key systems—pulmonary‐cardiovascular‐muscular is so exquisitely tuned in horses and dogs that oxygen uptake response kinetics evidence little inertia as the animal transitions from rest to exercise. These fast oxygen uptake kinetics minimize Intramyocyte perturbations that can limit exercise tolerance. For the physiologist, study of horses and dogs allows investigation not only of a broader range of oxidative function than available in humans, but explores the very limits of mammalian biological adaptability. Specifically, the unparalleled equine cardiovascular and muscular systems can transport and utilize more oxygen than the lungs can supply. Two consequences of this situation, particularly in the horse, are profound exercise‐induced arterial hypoxemia and hypercapnia as well as structural failure of the delicate blood‐gas barrier causing pulmonary hemorrhage and, in the extreme, overt epistaxis. This chapter compares and contrasts horses and dogs with humans with respect to the structural and functional features that enable these extraordinary mammals to support their prodigious oxidative and therefore athletic capabilities. © 2011 American Physiological Society. Compr Physiol 1:1‐37, 2011.

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Figure 1. Figure 1.

Estimated maximum speeds for a selection of terrestrial mammals and the ostrich. Note that the horse is the only animal depicted here for whom the speed recorded is measured carrying a rider and also that the fastest horses clocked are Quarter Horses (∼55 mph, 88 km/h; ref. 341) rather than Thoroughbreds (as shown).
Figure 2. Figure 2.

Chaldean horse pedigree chart (circa 4000 BCE) thought to present evidence for selective breeding of horses at least 6 millennia ago.

Reproduced with permission from Lyons and Petrucelli 246 and the World Health Organization, Geneva
Figure 3. Figure 3.

(A) Body mass‐specific maximal O2 uptake (O2) plotted as a logarithmic function of body mass for a selection of mammals with body masses differing over 5 orders of magnitude (solid line from Linstedt et al. ref. 239). Notice the extraordinary values plotted for the horse (non‐elite, 222,239,318,322,326; elite, 465) and dog (Foxhound, 286; elite dog (Greyhound), 406). Human values from Astrand and Rodahl 12. (B) Comparison of absolute O2 in an elite human athlete 12 and the Thoroughbred race horse 465.
Figure 4. Figure 4.

Illustration of the pathway for O2 from atmosphere to its site of utilization within the muscle mitochondria. The enmeshed cogs illustrate the tightly coordinated function of the respiratory (lungs), cardiovascular (heart, blood and vessels), and muscle systems requisite for effective transport of O2 and support of locomotory muscle energetics. o2, mitochondrial O2 delivery; O2, oxygen uptake; CO2, carbon dioxide output; Creat‐PO4, creatine phosphate; Pyr, pyruvate; Lac, lactate; co2, mitochondrial CO2 production; o2, mitochondrial O2 consumption. Upper panel redrawn from Wasserman et al. 433. Values given are for elite Thoroughbred horse.
Figure 5. Figure 5.

Effect of O2 uptake (O2) kinetics on the magnitude of the O2 deficit (shaded area) as a function of relative O2. The time constant, τ, denotes the time taken to reach 63% of the final response (steady‐state O2). Values for τ given, 10, 45, and 90 s, correspond to the Thoroughbred horse/elite human cyclist/marathon runner 185,217,222, healthy young human and heart failure patient, respectively 301. Notice that the longer τ values (i.e., slower O2 kinetics) mandate incurrence of a proportionally larger O2 deficit. Redrawn from Poole 318.
Figure 6. Figure 6.

O2 uptake (O2) response kinetics at the onset of moderate (top panel, i.e., sublactate threshold, 7 m/s) and heavy (bottom panel, i.e., supralactate threshold, 10 m/s) intensity running in the Thoroughbred horse (redrawn from Langsetmo et al. 222). I, II, and III correspond to the respective phases of the O2 kinetics (omitted from bottom panel for clarity). τ is the time constant of the phase II response. In the bottom panel, TD1 and TD2 denote the time delays prior to onset of phase II (primary fast exponential response) and slow component O2 response, respectively. Note the slowing of the fast exponential O2 (phase II) kinetics from moderate to heavy exercise and the emergence of the O2 slow component for heavy‐ but not moderate‐intensity running.
Figure 7. Figure 7.

Time constant, τ, of the O2 uptake (O2) kinetics plotted as a function of maximal O2 uptake (O2) for human heart transplant patients and their healthy controls 301, trained humans 339, elite humans 185, and the Thoroughbred horse (nonelite, 222).
Figure 8. Figure 8.

Nitric oxide synthase inhibition by L‐NAME (NG‐l‐nitro‐arginine methyl ester) significantly speeds O2 uptake (O2) kinetics at the onset of moderate‐intensity running (7 m/s) in the Thoroughbred horse.

Redrawn from Kindig et al. 201
Figure 9. Figure 9.

(A) O2 uptake (O2) response to an incremental exercise test where treadmill speed was increased 1 m/s each minute (from a 3 m/s baseline) until volitional fatigue in a nonelite Thoroughbred horse 222. Note highly linear (solid line) increase of O2 as a function of speed despite trot–canter–gallop transitions. Note only fleeting attainment of O2max at highest speed. (B) Determination of lactate, Tlac and gas exchange, Tlac(est) thresholds in the Thoroughbred horse during same test as in (A). Tlac(est) was discriminated by the nonlinearity of CO2 with respect to O2 which detects the additional CO2 produced consequent to HCO3− buffering of H+ emanating from the exercising muscles 222,266.
Figure 10. Figure 10.

The unique anatomy of the horse illustrating the following key features in the O2 transport pathway: (1) Unsupported nasal passages. (2) Laryngeal abductor muscles. (3) Long trachea. (4) Large lung capacity (but modest for cardiovascular system). (5) Dorsocaudal lobes where EIPH is most pronounced. (6) Large heart. (7) Large muscular spleen. (8) Great muscle mass with enormous absolute mitochondrial mass. See text for more details.
Figure 11. Figure 11.

Required alveolar ventilation (A, BTPS) plotted as a function of CO2. Note the phenomenally high A's and therefore C's (20%‐30% higher than A; ref. 304) that would be required if the horse chose to regulate its arterial Pco2 (Paco2) at 40 mmHg or hyperventilated (Paco2 30 mmHg) as do humans.
Figure 12. Figure 12.

Breathing patterns at rest and up to near‐maximal exercise in Thoroughbred horses and humans. Human data are from Clark et al. 47, horse data are from Hornicke et al. 173 and Pelletier and Leith 304. Note that, at the higher exercise intensities, horses achieve high mass‐specific C's (dotted isopleths) by means of increased respiratory frequency in contrast to humans in whom mass‐specific Vt increases preferentially.

From Pelletier and Leith 304 with permission
Figure 13. Figure 13.

Breath‐by‐breath ventilation (C), breathing frequency (f), and tidal volume (Vt) for a Thoroughbred horse at rest, during, and following (trotting recovery) a maximal incremental treadmill exercise test. Arrows denote the beginning of trotting recovery period. Note that C does not decrease upon cessation of galloping but is retained at or above maximal exercise levels for ∼15 to 30 s by reciprocal changes in f (decrease) and Vt (increase). Indeed C's were maintained at more than 1200 liters/min for over 3 min following the transition to the trotting recovery.

With kind permission from Padilla et al. 294
Figure 14. Figure 14.

Ventilatory equivalents for O2 uptake (C/O2) and CO2 output (C/CO2) (top) and arterial partial pressure of O2 (Pao2) and CO2 (Paco2) for the same Thoroughbred horse performing rest–incremental test to gallop–trot transition shown in Figure 13. Arterial blood gases are temperature corrected 94. Arrows denote the gallop–trot transition. Note the pronounced hypoventilation evidenced by the inexorable decrease of C/O2 and particularly C/CO2 that elevates Paco2 and contributes to the exercise‐induced arterial hypoxemia (EIAH) neither of which is characteristic of, or prevalent in, healthy humans. See text for additional information.

With kind permission from Padilla et al. 294
Figure 15. Figure 15.

Schematics representing (left) Partial pressure of O2 (Po2) in the red blood cell (RBC) as a function of pulmonary capillary transit time at rest and during maximal exercise. Note the reduction in alveolar Po2 during exercise (due, in part, to alveolar hypoventilation, see Figures 13 and 14) and the extreme decrease in RBC Po2 as it enters (0 s) and leaves (<0.25 s later) the capillary during maximal exercise compared with rest. (right) O2 dissociation curves in the Thoroughbred horse at rest (open symbols) and during maximal exercise (closed symbols). Pao2 denotes the arteriolar partial pressure of O2. The arrows denote the arterial points under each condition. Note that the exercise‐induced hemoconcentration elevates the O2‐carrying capacity almost twofold above rest, but during maximal exercise, the alveolar hypoventilation and short RBC transit times in the pulmonary capillary (diffusion limitation) conspire to reduce the arterial O2 content far below the potential limit. See text for further details.

Redrawn from Poole 318
Figure 16. Figure 16.

Rupture of the fragile blood‐gas barrier. Left: Red blood cell (RBC) escaping from a fracture in the alveolar epithelium into the alveolar space.

Reproduced with kind permission from Fu et al. 101. Right: Exercise‐induced pulmonary hemorrhage (EIPH) in the alveolus of the equine lung. Reproduced with kind permission from Erickson et al. 79. Abbreviations: R, RBC; P, proteinaceous material. Scale bar = 5 μm
Figure 17. Figure 17.

It is not simply heart mass (and therefore pumping capacity and muscle O2 delivery) that facilitates superb O2 transport potential and athletic ability in the Thoroughbred horse but the ratio of heart mass to body mass. Dr. Le Gear, one of the world's largest horses (body mass 3940 lb or 1791 kg), must surely have possessed an enormous heart (top panel). Photograph courtesy of Mr. Weldon Dudley. However, among species differing in body mass (center panel) from the 250 g rat to the 4,000 to 12,000 kg (8,800‐26,400 lb) elephant, the horse's heart, and in particular the Thoroughbred's heart, is outstandingly large and averages more than 1% body mass, reaching an estimated 2% in the extreme 318,321,322. Note also that the mixed‐breed dog's heart averages close to 1% body mass (and up to 1.7% for Greyhound, see The Athletic Dog section). Both are proportionally larger than that of the human, rat, or elephant. Lower panel: Comparison of heart size in a champion Thoroughbred (Key to the Mint, 7.2 kg, 15.8 lb, left‐hand side) with that of an average stallion of similar body mass (5.5 kg, 12 lb, right‐hand side).

Photograph courtesy of Dr. Thomas Swerzek
Figure 18. Figure 18.

Relationship between cardiac output () and either heart mass (left panel) or O2 uptake (O2) (right panel) during maximal exercise. The solid symbols at the left are from the data of Evans and Rose 91,92, whereas the hollow symbols are extrapolated from that relationship using either the measured or estimated (Secretariat) heart weights published for each named horse 151. For the to O2 relationship at the right, an arterial‐venous O2 difference () of 22.8 ml/100 ml blood is assumed to estimate maximal O2 uptake (O2) values for Secretariat, Sham, and Mill Reef. Note Secretariat's (∼540 liters/min) and O2 (>120 liters/min or ∼240 ml/kg/min at 500 kg of body mass).

Redrawn from Poole and Erickson 321,322 and Poole 318
Figure 19. Figure 19.

Distribution of cardiac output () at rest and during maximal exercise in the Thoroughbred horse. It is likely that skeletal muscle blood flow may reach 90% of . Values from Erickson 74, Armstrong et al. 5, and Erickson and Poole 80.
Figure 20. Figure 20.

Determination of maximal O2 uptake (O2) by conductive (o2) and diffusive (Do2) movement of O2 by the cardiovascular and muscle microcirculatory systems (“Wagner” diagram; 362,430). The curved line denotes mass balance according to the Fick principle and the straight line from the origin represents Fick's law of diffusion. Do2 is the effective diffusing capacity and K is a constant that relates venous Po2 to mean capillary Po2. Pvo2, Cao2, and Co2 are the partial pressure of venous O2 and the concentrations of O2 in arterial and venous blood, respectively. O2 occurs at the confluence of the two relationships. The top panel is a general schematic, whereas the bottom panel presents values for an elite Thoroughbred at maximal exercise. Understanding the conductive and diffusive determinants of O2 is essential for interpreting the structural and functional mechanisms that increase O2 with exercise training (see the Adaptations to Training section). See the text for additional details.
Figure 21. Figure 21.

Top panel: Relationship between maximum O2 uptake (O2) and mass‐specific mitochondrial volume for a variety of species varying in body mass over several orders of magnitude. Note that humans fall furthest from this relationship, likely, in part, because of their two‐legged anatomy, bipedal gait, and muscle activation patterns as well as their locomotory patterns facilitated by that bipedalism.

Based, in part, upon the data summarized in Wagner et al. 430. Point for the elite horse is taken from the data of Young et al. 465 and that for the pronghorn antelope from Lindstedt et al. 239; for both, the mitochondrial volume is estimated from O2. Bottom panel: Select data from top panel expressed in absolute terms with the addition of Secretariat (data from Figure 18). This relationship supports the notion that the Thoroughbred horse achieves its extraordinary O2max by means of its very large muscle mass, and therefore total mitochondrial volume, and the ability to supply that mitochondrial volume with O2. In contrast, the mass‐specific muscle blood flow (m) and capillarity as well as the mitochondrial volume density and function appear quite ordinary. See text for additional information
Figure 22. Figure 22.

Exercise training improves maximal O2 uptake (O2) by elevating both conductive (curved line, due to increased stroke volume and small elevation in Cao2 see Table 3) and diffusive (straight line, due, in part, to increased capillarity and events within those capillaries) O2 transport. It is interesting that even a relatively modest decrease (5%‐10%) in venous O2 partial pressure (Po2) 215,362 after training (Table 3) requires a far greater percentage elevation (∼30%) in muscle O2 diffusing capacity (Do2m). In addition, from this figure, it is apparent that if training solely acted to increase convective O2 delivery in the absence of increased Do2m, venous Po2 would have to rise (see posttraining solid star). In contrast, if Do2m increased in the absence of an elevated cardiac output and muscle O2 delivery, venous Po2 would fall but the increase in O2 would be minimal (open star). Figure is redrawn from Poole 318. See the caption to Figure 20 and the text for additional information.
Figure 23. Figure 23.

Schematic of velocity vs. time‐to‐exhaustion for high‐intensity exercise. The curve is traditionally constructed by having the individual run to fatigue at four different constant velocities, selected to induce exhaustion in 2 to 20 min, on four or more occasions (points 1‐4, separated by at least 1 day). The asymptote parameter is critical velocity, CV, and the curvature constant W′. W′ denotes a finite amount of work (intramuscular energy store, principally composed of creatine phosphate + anaerobic glycolysis) that can be performed above CV and, as shown by the hatched rectangles that denote energy, W′ is the same for all velocities above CV. Exhaustion occurs when W′ is expended. Tlac denotes the lactate threshold as determined by a separate ramp exercise test. Each exercise intensity domain evinces a discrete oxygen uptake and blood lactate response. Note that CV demarcates the heavy and severe exercise intensity domains and also that even a modest elevation in CV, which results from endurance training, will facilitate a disproportionate increase in time‐to‐fatigue at any velocity that was originally more than CV. See the text for more details.


Figure 1.

Estimated maximum speeds for a selection of terrestrial mammals and the ostrich. Note that the horse is the only animal depicted here for whom the speed recorded is measured carrying a rider and also that the fastest horses clocked are Quarter Horses (∼55 mph, 88 km/h; ref. 341) rather than Thoroughbreds (as shown).



Figure 2.

Chaldean horse pedigree chart (circa 4000 BCE) thought to present evidence for selective breeding of horses at least 6 millennia ago.

Reproduced with permission from Lyons and Petrucelli 246 and the World Health Organization, Geneva


Figure 3.

(A) Body mass‐specific maximal O2 uptake (O2) plotted as a logarithmic function of body mass for a selection of mammals with body masses differing over 5 orders of magnitude (solid line from Linstedt et al. ref. 239). Notice the extraordinary values plotted for the horse (non‐elite, 222,239,318,322,326; elite, 465) and dog (Foxhound, 286; elite dog (Greyhound), 406). Human values from Astrand and Rodahl 12. (B) Comparison of absolute O2 in an elite human athlete 12 and the Thoroughbred race horse 465.



Figure 4.

Illustration of the pathway for O2 from atmosphere to its site of utilization within the muscle mitochondria. The enmeshed cogs illustrate the tightly coordinated function of the respiratory (lungs), cardiovascular (heart, blood and vessels), and muscle systems requisite for effective transport of O2 and support of locomotory muscle energetics. o2, mitochondrial O2 delivery; O2, oxygen uptake; CO2, carbon dioxide output; Creat‐PO4, creatine phosphate; Pyr, pyruvate; Lac, lactate; co2, mitochondrial CO2 production; o2, mitochondrial O2 consumption. Upper panel redrawn from Wasserman et al. 433. Values given are for elite Thoroughbred horse.



Figure 5.

Effect of O2 uptake (O2) kinetics on the magnitude of the O2 deficit (shaded area) as a function of relative O2. The time constant, τ, denotes the time taken to reach 63% of the final response (steady‐state O2). Values for τ given, 10, 45, and 90 s, correspond to the Thoroughbred horse/elite human cyclist/marathon runner 185,217,222, healthy young human and heart failure patient, respectively 301. Notice that the longer τ values (i.e., slower O2 kinetics) mandate incurrence of a proportionally larger O2 deficit. Redrawn from Poole 318.



Figure 6.

O2 uptake (O2) response kinetics at the onset of moderate (top panel, i.e., sublactate threshold, 7 m/s) and heavy (bottom panel, i.e., supralactate threshold, 10 m/s) intensity running in the Thoroughbred horse (redrawn from Langsetmo et al. 222). I, II, and III correspond to the respective phases of the O2 kinetics (omitted from bottom panel for clarity). τ is the time constant of the phase II response. In the bottom panel, TD1 and TD2 denote the time delays prior to onset of phase II (primary fast exponential response) and slow component O2 response, respectively. Note the slowing of the fast exponential O2 (phase II) kinetics from moderate to heavy exercise and the emergence of the O2 slow component for heavy‐ but not moderate‐intensity running.



Figure 7.

Time constant, τ, of the O2 uptake (O2) kinetics plotted as a function of maximal O2 uptake (O2) for human heart transplant patients and their healthy controls 301, trained humans 339, elite humans 185, and the Thoroughbred horse (nonelite, 222).



Figure 8.

Nitric oxide synthase inhibition by L‐NAME (NG‐l‐nitro‐arginine methyl ester) significantly speeds O2 uptake (O2) kinetics at the onset of moderate‐intensity running (7 m/s) in the Thoroughbred horse.

Redrawn from Kindig et al. 201


Figure 9.

(A) O2 uptake (O2) response to an incremental exercise test where treadmill speed was increased 1 m/s each minute (from a 3 m/s baseline) until volitional fatigue in a nonelite Thoroughbred horse 222. Note highly linear (solid line) increase of O2 as a function of speed despite trot–canter–gallop transitions. Note only fleeting attainment of O2max at highest speed. (B) Determination of lactate, Tlac and gas exchange, Tlac(est) thresholds in the Thoroughbred horse during same test as in (A). Tlac(est) was discriminated by the nonlinearity of CO2 with respect to O2 which detects the additional CO2 produced consequent to HCO3− buffering of H+ emanating from the exercising muscles 222,266.



Figure 10.

The unique anatomy of the horse illustrating the following key features in the O2 transport pathway: (1) Unsupported nasal passages. (2) Laryngeal abductor muscles. (3) Long trachea. (4) Large lung capacity (but modest for cardiovascular system). (5) Dorsocaudal lobes where EIPH is most pronounced. (6) Large heart. (7) Large muscular spleen. (8) Great muscle mass with enormous absolute mitochondrial mass. See text for more details.



Figure 11.

Required alveolar ventilation (A, BTPS) plotted as a function of CO2. Note the phenomenally high A's and therefore C's (20%‐30% higher than A; ref. 304) that would be required if the horse chose to regulate its arterial Pco2 (Paco2) at 40 mmHg or hyperventilated (Paco2 30 mmHg) as do humans.



Figure 12.

Breathing patterns at rest and up to near‐maximal exercise in Thoroughbred horses and humans. Human data are from Clark et al. 47, horse data are from Hornicke et al. 173 and Pelletier and Leith 304. Note that, at the higher exercise intensities, horses achieve high mass‐specific C's (dotted isopleths) by means of increased respiratory frequency in contrast to humans in whom mass‐specific Vt increases preferentially.

From Pelletier and Leith 304 with permission


Figure 13.

Breath‐by‐breath ventilation (C), breathing frequency (f), and tidal volume (Vt) for a Thoroughbred horse at rest, during, and following (trotting recovery) a maximal incremental treadmill exercise test. Arrows denote the beginning of trotting recovery period. Note that C does not decrease upon cessation of galloping but is retained at or above maximal exercise levels for ∼15 to 30 s by reciprocal changes in f (decrease) and Vt (increase). Indeed C's were maintained at more than 1200 liters/min for over 3 min following the transition to the trotting recovery.

With kind permission from Padilla et al. 294


Figure 14.

Ventilatory equivalents for O2 uptake (C/O2) and CO2 output (C/CO2) (top) and arterial partial pressure of O2 (Pao2) and CO2 (Paco2) for the same Thoroughbred horse performing rest–incremental test to gallop–trot transition shown in Figure 13. Arterial blood gases are temperature corrected 94. Arrows denote the gallop–trot transition. Note the pronounced hypoventilation evidenced by the inexorable decrease of C/O2 and particularly C/CO2 that elevates Paco2 and contributes to the exercise‐induced arterial hypoxemia (EIAH) neither of which is characteristic of, or prevalent in, healthy humans. See text for additional information.

With kind permission from Padilla et al. 294


Figure 15.

Schematics representing (left) Partial pressure of O2 (Po2) in the red blood cell (RBC) as a function of pulmonary capillary transit time at rest and during maximal exercise. Note the reduction in alveolar Po2 during exercise (due, in part, to alveolar hypoventilation, see Figures 13 and 14) and the extreme decrease in RBC Po2 as it enters (0 s) and leaves (<0.25 s later) the capillary during maximal exercise compared with rest. (right) O2 dissociation curves in the Thoroughbred horse at rest (open symbols) and during maximal exercise (closed symbols). Pao2 denotes the arteriolar partial pressure of O2. The arrows denote the arterial points under each condition. Note that the exercise‐induced hemoconcentration elevates the O2‐carrying capacity almost twofold above rest, but during maximal exercise, the alveolar hypoventilation and short RBC transit times in the pulmonary capillary (diffusion limitation) conspire to reduce the arterial O2 content far below the potential limit. See text for further details.

Redrawn from Poole 318


Figure 16.

Rupture of the fragile blood‐gas barrier. Left: Red blood cell (RBC) escaping from a fracture in the alveolar epithelium into the alveolar space.

Reproduced with kind permission from Fu et al. 101. Right: Exercise‐induced pulmonary hemorrhage (EIPH) in the alveolus of the equine lung. Reproduced with kind permission from Erickson et al. 79. Abbreviations: R, RBC; P, proteinaceous material. Scale bar = 5 μm


Figure 17.

It is not simply heart mass (and therefore pumping capacity and muscle O2 delivery) that facilitates superb O2 transport potential and athletic ability in the Thoroughbred horse but the ratio of heart mass to body mass. Dr. Le Gear, one of the world's largest horses (body mass 3940 lb or 1791 kg), must surely have possessed an enormous heart (top panel). Photograph courtesy of Mr. Weldon Dudley. However, among species differing in body mass (center panel) from the 250 g rat to the 4,000 to 12,000 kg (8,800‐26,400 lb) elephant, the horse's heart, and in particular the Thoroughbred's heart, is outstandingly large and averages more than 1% body mass, reaching an estimated 2% in the extreme 318,321,322. Note also that the mixed‐breed dog's heart averages close to 1% body mass (and up to 1.7% for Greyhound, see The Athletic Dog section). Both are proportionally larger than that of the human, rat, or elephant. Lower panel: Comparison of heart size in a champion Thoroughbred (Key to the Mint, 7.2 kg, 15.8 lb, left‐hand side) with that of an average stallion of similar body mass (5.5 kg, 12 lb, right‐hand side).

Photograph courtesy of Dr. Thomas Swerzek


Figure 18.

Relationship between cardiac output () and either heart mass (left panel) or O2 uptake (O2) (right panel) during maximal exercise. The solid symbols at the left are from the data of Evans and Rose 91,92, whereas the hollow symbols are extrapolated from that relationship using either the measured or estimated (Secretariat) heart weights published for each named horse 151. For the to O2 relationship at the right, an arterial‐venous O2 difference () of 22.8 ml/100 ml blood is assumed to estimate maximal O2 uptake (O2) values for Secretariat, Sham, and Mill Reef. Note Secretariat's (∼540 liters/min) and O2 (>120 liters/min or ∼240 ml/kg/min at 500 kg of body mass).

Redrawn from Poole and Erickson 321,322 and Poole 318


Figure 19.

Distribution of cardiac output () at rest and during maximal exercise in the Thoroughbred horse. It is likely that skeletal muscle blood flow may reach 90% of . Values from Erickson 74, Armstrong et al. 5, and Erickson and Poole 80.



Figure 20.

Determination of maximal O2 uptake (O2) by conductive (o2) and diffusive (Do2) movement of O2 by the cardiovascular and muscle microcirculatory systems (“Wagner” diagram; 362,430). The curved line denotes mass balance according to the Fick principle and the straight line from the origin represents Fick's law of diffusion. Do2 is the effective diffusing capacity and K is a constant that relates venous Po2 to mean capillary Po2. Pvo2, Cao2, and Co2 are the partial pressure of venous O2 and the concentrations of O2 in arterial and venous blood, respectively. O2 occurs at the confluence of the two relationships. The top panel is a general schematic, whereas the bottom panel presents values for an elite Thoroughbred at maximal exercise. Understanding the conductive and diffusive determinants of O2 is essential for interpreting the structural and functional mechanisms that increase O2 with exercise training (see the Adaptations to Training section). See the text for additional details.



Figure 21.

Top panel: Relationship between maximum O2 uptake (O2) and mass‐specific mitochondrial volume for a variety of species varying in body mass over several orders of magnitude. Note that humans fall furthest from this relationship, likely, in part, because of their two‐legged anatomy, bipedal gait, and muscle activation patterns as well as their locomotory patterns facilitated by that bipedalism.

Based, in part, upon the data summarized in Wagner et al. 430. Point for the elite horse is taken from the data of Young et al. 465 and that for the pronghorn antelope from Lindstedt et al. 239; for both, the mitochondrial volume is estimated from O2. Bottom panel: Select data from top panel expressed in absolute terms with the addition of Secretariat (data from Figure 18). This relationship supports the notion that the Thoroughbred horse achieves its extraordinary O2max by means of its very large muscle mass, and therefore total mitochondrial volume, and the ability to supply that mitochondrial volume with O2. In contrast, the mass‐specific muscle blood flow (m) and capillarity as well as the mitochondrial volume density and function appear quite ordinary. See text for additional information


Figure 22.

Exercise training improves maximal O2 uptake (O2) by elevating both conductive (curved line, due to increased stroke volume and small elevation in Cao2 see Table 3) and diffusive (straight line, due, in part, to increased capillarity and events within those capillaries) O2 transport. It is interesting that even a relatively modest decrease (5%‐10%) in venous O2 partial pressure (Po2) 215,362 after training (Table 3) requires a far greater percentage elevation (∼30%) in muscle O2 diffusing capacity (Do2m). In addition, from this figure, it is apparent that if training solely acted to increase convective O2 delivery in the absence of increased Do2m, venous Po2 would have to rise (see posttraining solid star). In contrast, if Do2m increased in the absence of an elevated cardiac output and muscle O2 delivery, venous Po2 would fall but the increase in O2 would be minimal (open star). Figure is redrawn from Poole 318. See the caption to Figure 20 and the text for additional information.



Figure 23.

Schematic of velocity vs. time‐to‐exhaustion for high‐intensity exercise. The curve is traditionally constructed by having the individual run to fatigue at four different constant velocities, selected to induce exhaustion in 2 to 20 min, on four or more occasions (points 1‐4, separated by at least 1 day). The asymptote parameter is critical velocity, CV, and the curvature constant W′. W′ denotes a finite amount of work (intramuscular energy store, principally composed of creatine phosphate + anaerobic glycolysis) that can be performed above CV and, as shown by the hatched rectangles that denote energy, W′ is the same for all velocities above CV. Exhaustion occurs when W′ is expended. Tlac denotes the lactate threshold as determined by a separate ramp exercise test. Each exercise intensity domain evinces a discrete oxygen uptake and blood lactate response. Note that CV demarcates the heavy and severe exercise intensity domains and also that even a modest elevation in CV, which results from endurance training, will facilitate a disproportionate increase in time‐to‐fatigue at any velocity that was originally more than CV. See the text for more details.

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Article Title: Highly Athletic Terrestrial Mammals: Horses and Dogs
 

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David C. Poole, Howard H. Erickson. Highly Athletic Terrestrial Mammals: Horses and Dogs. Compr Physiol 2011, 1: 1-37. doi: 10.1002/cphy.c091001