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There are many different physiological outcomes produced by moderate physical exertion. These outcomes are due to the body systems attempting to maintain homeostasis during a temporary disruption. The effects of exercise influence many body systems. The main physiological mechanisms affected by moderate activity are cardiovascular and respiratory responses along with acid-base balance. There are natural variations of these effects between different species so a comparison in model organisms of man, a lower vertebrate and an invertebrate would be valuable. Therefore the similarities and differences in physiological responses to moderate physical exercise in humans (Homo sapiens), fish like salmon or rainbow trout (Oncorhynchus mykiss) and the crab such as the European Shore crab (Carcinus maenas) will be discussed in detail.
The threshold for moderate exercise (percentage of VO2) is difficult to define (Campbell-O'Sullivan et al., 2002). However, an assumption will be that moderate activity is mostly aerobic respiration and little if any anaerobic respiration in humans and little if any lactate is produced by contracting muscles in humans. Due to fish and crab having more simple cardiovascular and ventilation systems, the anaerobic threshold is much lower than that of the human. According to Hamilton and Houlihan (1992) a 1.4 fold increase in the concentration of lactate was recorded in a European Shore Crab under moderate exercise of 5.8mmin-1. In all three types of organisms considered prolonged moderate activity may cause increased anaerobic reactions to a greater extent than aerobic reactions of respiring skeletal muscles. Certainly in fish, an insufficient myocardial supply of oxygen causes most exercise to be anaerobic and prolonged swimming causes a decrease in the delivery of oxygen and anaerobic respiration to take over by white muscle (Farrell, 2002; Wang et al., 1994). Physiological responses are also less efficient with increasing age therefore an assumption that this discussion applies to an average fit and healthy young adult is considered (Ishida et al., 2000). A further assumption is that moderate exercise in each organism is under their standard environmental conditions as when their body surface is hotter or cooler the cardiovascular system responds to a higher or lower activity correspondingly in humans (Rowell et al., 1969). Both fish and crabs are aquatic ectotherms so they have a better tolerance to heat than humans, however they would also be affected by changes in temperature during moderate exertion (Schmidt-Nielson, 1997).
The cardiovascular system of Homo sapiens is majorly affected by moderate physical activity (Sherwood, 2004). The hormone epinephrine (adrenaline) is released during exercise by the expansion of sympathetic activity to the sinoatrial node which in turn causes increased energy demands and the heart rate to rise (Doohan, 2000). This is also the case in fish such as trout or salmon - adrenergic receptors are present in the circulatory system across the gills; the blood vessels vasodilate in the present of epinephrine whilst the blood vessels of the systemic circulation leading from the peripheral tissues to the heart vasoconstrict (Randall and Stevens, 1967). This is likely to be the case for Carcinus maenas too. The heart rate of lower vertebrates such as fish is generally lower than 120bpm which applies to this model of discussion (Oncorhynchus mykiss) whereas in humans the heart rate can reach much higher levels (Farrell, 1991; Farrell, 1996). The effect of epinephrine in fish is an increase in heart rate and blood pressure of the dorsal aorta; this maximizes the amount of oxygenated blood transmitted via the systemic circulation to the peripheral tissues (Axelsson & Farrell, 1993; Randall and Stevens, 1967). Baroreceptors are present in the blood vessels of humans to detect the rise in heart rate and send signals to the central nervous system (CNS) in order to dilate constricted arterioles during muscle contraction (Sherwood, 2004). As a result, more oxygen is delivered and utilized by respiring muscles, increasing the rate of perfusion (Sherwood, 2004; Mortola and Frappell, 2000).
Stroke volume increases in moderate exertion of humans due to a greater venous return to the heart and more intense contractions of the myocardium (Sherwood, 2004). With exertion an increased heart rate and stroke volume causes the cardiac output of the heart to increase specifically to the skeletal muscle, heart and skin diverting a higher amount of blood to these tissues (Doohan, 2000; Sherwood, 2004). This is a result of a decrease in oxygen, myogenic activity, histamine release, heat and sympathetic stimulation along with an increase in carbon dioxide and nitric oxide from exercise (Ishida et al., 2000). In trout, cardiac output increases during moderate swimming whilst PO2 increases in the venous blood and maintains oxygen supplies similar to at rest (Farrell and Clutterham, 2003). The amount of oxygen in venous blood decreases in exercise (Farrell, 2002). Adenosine levels are increased in cardiac myocytes of humans during metabolic activity, causing an increased blood flow and hence vasodilation of coronary vessels of humans, posing an increased oxygen requirement (Sherwood, 2004). There is a decrease in blood flow to the digestive system, liver, kidneys and bone due to the sympathetic nervous system activating vasoconstriction of arteriole vessels (Sherwood, 2004). Total peripheral resistance decreases during exercise due to fact that resistance in vasoconstricting vessels to tissues decreases to a greater extent than to tissues with vasodilating vessels, for example in the kidneys (Sherwood, 2004). This decrease in total peripheral resistance is to a lower extent than the increase in cardiac output therefore there is an increase in mean arterial blood pressure (Sherwood, 2004). Carcinus maenas also have low pressures, a high cardiac output along with a low peripheral resistance during moderate exertion (Taylor, 1982). This is case for fish too; exertion cardiac output increases dramatically and the resistance in coronary vessels decreases (Gamperl et al., 1995). However, according to Taylor (1982) the cardiac output is higher in Carcinus maenas than fish; 755mlkg-1min-1 during moderate exercise in the crab compared to 40mlkg-1min-1 in trout (Table 1). The heart rate during moderate exercise is higher for Carcinus maenas compared to the other two model organisms which could be due to the crab having an open circulation system so more oxygen needed to be pumped around their bodies. They are also smaller creatures too so there can be more oxygen provided per kilogram of weight from a pump providing an appropriate amount of pressure. Raising adenosine levels has the same effect on fish - vasoconstriction of the efferent filamentary artery, increased branchial vascular resistance and hence decreasing gill blood flow (Evans et al., 2005). The increased energy requirements during exercise by contracting muscles causes a greater utilization of oxygen in both humans and fish and the metabolizing muscle promotes the production of carbon dioxide with an adequate alveolar ventilation level in humans (Sherwood, 2004). In fish oxygenated blood is received by peripheral tissues from the gills and deoxygenated blood enters and leaves the pumping heart (Farrell, 2002). An increased activity causes an increase in oxygen consumption from cardiac muscle and hence extraction by skeletal muscles also rises, lowering oxygen levels in the blood so increasing blood flow (Farrell, 2002). In humans the arterial partial pressure is greater than or equal to normal levels whilst the partial pressure of carbon dioxide is less than or equal to the human homoeostatic threshold level (Sherwood, 2004). Fish and crabs have a less efficient oxygen supply due to the denser, more viscous medium which requires more energy to push water over their gills (Evans et al., 2005). The uptake of oxygen is increased during swimming as more water is moved over the gills. There is less dependence on the cardiovascular systems in fish and more on the respiratory system compared to humans. Cardiovascular systems in fish provide an increased flow of blood rather than a direct increase in oxygen supply and oxygen impairs maximum cardiac performance in fish hearts (Kikuchi et al., 1985). Myocardial oxygen supply limits maximum cardiac performance in fish and the amount of oxygen in venous blood plays a central role in the maximum cardiac activity (Axelsson and Farrell, 1993). Oxygen levels in the venous blood have little effect on cardiac performance during moderate exercise in humans as almost the entire heart is supplied with a coronary circulation (Doohan, 2000).
Table 1: A comparison of cardiovascular and ventilatory responses at rest and during moderate exercise (Eddy, 2010; Farrell, 1996; Hamilton and Houlihan, 1992; Jones and Bushnell, 1994; Kikuchi et al., 1985; Sherwood, 2004; Stevens and Randall, 2006).
Mass in kilograms
Stroke volume (ml/beat)
Heart rate (beats/min)
Cardiac output (Q) (ml/kg/min)
Ventilation (V) (ml/kg/min)
Ventilation systems of fish and humans may involve different structures - gills in fish and lungs in humans, but they both seem to function equally well at rest (Schmidt-Nielson, 1990). Crabs have chitinous gills so there is limited gaseous exchange and their haemolymph carries relatively low levels of oxygen, however the haemocoel is large and there is a high affinity of oxygen in the haemocyanin to compensate (Taylor, 1982). Under a higher oxygen demand like in moderate exercise each organism responds differently apart from the general trend that ventilation is increased during activity in order to obtain more oxygen. Ventilation is fifteen times greater than the normal resting rate of humans compared to a fivefold increase for fish (Table 1). The efficiency of oxygen lowers in response to exercise since the rise in heart rate and cardiac output increases perfusion and reduces the time for the diffusion of oxygen across their respiratory surface (Schmidt-Nielson, 1997; Sherwood, 2004). Therefore to compensate, they must have an appropriate respiratory surface area and a highly efficient means of gaining enough oxygen for the organism's level of active lifestyle. According to Fick's Law in order to maximize diffusion rate a large surface area, small distance between the source and destination as well as a high concentration gradient (Eddy, 2010). The rate of diffusion is lower in fish and crab compared to humans due to osmoregulation (Sherwood, 2004). Also, since humans are terrestrial animals they can obtain oxygen much more readily than aquatic organisms such as fish and crab, due to the higher oxygen content in the air and the development of lungs (Farrell, 2002). However, oxygen is extracted much more readily by fish and crab due to their countercurrent flow system (Farrell, 2002). Crabs use a countercurrent flow system in their gills, but the diffusion barrier is much greater than that found in fish and water flow is less effective (Schmidt-Nielson, 1997). Therefore fish can extract a greater amount of oxygen from the surrounding water compared to either humans or the crab (Schmidt-Nielson, 1997). Fish have to pump water over their gills in order to obtain oxygen which requires more energy due to the higher density and viscosity of water compared to air (Schmidt-Nielson, 1990). Although lungs are less efficient in this respect, they utilized a higher amount of oxygen in ventilation from a medium with a greater percentage of oxygen (Sherwood, 2004). Crabs possess scaphognathites which draw water over the gills and the beat of this action is increased with increasing heart rate in moderate physical exertion (Taylor, 1982). Fish force water over their gills whilst swimming so they are more efficient at oxygen extraction than lungs due to their countercurrent flow system (Evans et al., 2005). Water flows past lamellae which are two cells thick in one direction whilst the blood flows in the opposite direction, optimizing gaseous exchange (Evans et al., 2005). Since lungs are better at increasing the ventilation rate during exercise than fish and crab, they have a more efficient method of oxygen uptake during exercise when a higher rate of perfusion is required (Hamilton and Houlihan, 1992). In particular alveolar walls of humans are only one cell thick (Sherwood, 2004). Water breathers such as fish and crab have higher pulmonary ventilation: VO2 ratio due to the low oxygen content in water (Mortola and Frappell, 2000).
Acid-base regulation occurs in the lungs of humans where the reversible reaction: CO2 + H2O = H+ + HCO3- takes place (Sherwood, 2004). The concentration of CO2 is the major determinate for raising ventilation levels in humans (Doohan, 2000). An increased oxygen demand causes increased respiration and oxygen extraction. After moderate exercise in humans, respiratory acidosis occurs due to decreased HCO3-, increased H+ ions and the concentration of CO2 as well as a rise in lactate at a lower extent (Sherwood, 2004). The increased amount of CO2 and H+ ions in the metabolizing muscles are a key physiological effect of moderate exercise (Stringer et al., 1992). Peripheral chemoreceptors are part of the sympathetic nervous system in human lungs and are stimulated by CO2 levels in the blood to control pulmonary ventilation by increasing ventilation (Mortola and Frappell, 2000). The levels of lactate produced during exercise changes the pH of the blood by increasing the acidity (Stringer et al., 1992). This acidity is lowered by the release of carbonic acid in order to regulate the acid-base balance (Sherwood, 2004). Haemoglobin is the respiratory pigment in humans that buffers the increased H+ ions from exercise by the metabolic effects of CO2 production being delivered from the tissues to the lungs via the following reaction: H+ + Hb â†’ HHb (Sherwood, 2004). Filtration and selective reabsorption of ions in the loop of Henle aids acid-base balance after exercise (Sherwood, 2004).
Extracellular acidosis and increased plasma lactate are produced during continuous swimming in fish which impairs maximum cardiac function (Wang et al., 1994). During swimming at a moderate speed, oxygen content in the venous blood increases, protons are released during the oxygenation of haemoglobin and release of protons compared to consumption by HCO3- dehydration (Brauner et al., 2000). Increase in ventilation and cardiac output and an increased rate of perfusion across the gills as well as red muscle. Rainbow trout are characteristic of a low haemoglobin buffer value and a huge Haldene effect (Brauner et al., 2000). The difference between the amounts of arterial-venous oxygen in the blood is increased with intensity of sustained activity (Brauner et al., 2000). There is a reduced oxygen binding capacity during exercise. This encourages oxygen unloading at the tissues and the pH of the blood increases in fish during moderate exercise due to the rise in water passing over the gills which also forces out CO2 (Kikuchi et al., 1985; McKenzie et al., 2004). Moderate exercise promotes hyperventilation in fish due to the increased ventilatory movements and there becomes a greater concentration of lactate in the respiring muscles (Kikuchi et al., 1985).
The balance of ion concentrations in fish is done by specialized mitochondrial rich chloride cells of the brachia of gills (Perry, 1997). Since most species of trout and salmon are anadromous they generally inhabit salt water except during migration to freshwater for reproduction. Salmon, trout along with the European Shore crab are all euryhaline so can tolerate a variety of salinities (Evans et al., 2005; Hamilton and Houlihan, 1992). Organisms that live in seawater like the majority of fish and some crabs such as Carcinus maenas are hypotonic to their environments (Eddy, 2010). In hypotonic fish, Na+ and Cl- are taken into the cells as they move down their diffusion gradients whilst water leaves osmotically (Evans et al., 2005). The gills are site of water loss and active excretion of Na+ and Cl- so they possess well developed kidneys and have multiple scales on their skin in order to avoid water loss and excrete excess salts in their urine (Evans et al., 2005). Acidosis conditions as a result of moderate exercise in these fish causes chloride cells to reduce in size to decrease the surface area exposed to the surrounding salt water reducing the rate of diffusion of Cl- and HCO3- ion exchange into the cells (Perry, 1997; Mortola and Frappell, 2000). Different species of crab habituate marine and freshwater environments as well as terrestrial, the majority being freshwater species (Taylor, 1982). The pH of extracellular and intracellular white muscle of fish decreases after exercise (Wang et al., 1994). The concentration of haemoglobin and haematocrit in red blood cells of fish increases after exercise in order to increase the amount of oxygen transfer to the peripheral tissues just as there is the same type of increase in humans to maximize the oxygen provided to tissues (Wang et al., 1994). Oxygen is unloaded from haemoglobin rich red blood cells in metabolizing muscles whilst excess carbon dioxide is removed in both humans and fish (Sherwood, 2004). In crabs oxygen is unloaded during exercise from haemocyanin respiratory pigments (Schmidt-Nielson, 1997). Fish gills have many control mechanisms of homoeostasis of the internal (e.g. acidosis) and external (e.g. salinity) environments (Evans et al., 2005).
The similarities of physiological responses of Carcinus maenas, Oncorhynchus mykiss and Homo sapiens to moderate exercise include an increase in oxygen demand by respiring tissues and the myocardium hence an increasing heart rate, ventilation rate and H+, K+, Na+ ion concentrations. There was also a slight rise in lactate recorded for all of the model organisms; however more lactate was produced by Carcinus maenas and Oncorhynchus mykiss due to their lower anaerobic threshold and their denser and lower percentage of oxygen surrounding respiratory medium. Humans can uptake a greater amount of oxygen since their surrounding environmental medium (air) is more oxygen rich than the medium of aquatic organisms; crabs and fish require more energy to actively force water over their gills. There appears to be different types of circulatory blood vessel receptors; Baroreceptors and chemoreceptors in humans, adrenergic receptors in crab and fish which both types have the same general trend: vasoconstriction of the blood vessels leading from the tissues to the heart in the presence of epinephrine. In Carcinus maenas and Homo sapiens the heart is supplied with oxygenated blood whereas fish both receive oxygenated blood directly from the heart whereas the heart of fish receives deoxygenated blood from the peripheral tissues. This difference in physiology means that there is a higher ventilation and heart rate for Carcinus maenas over Oncorhynchus mykiss.