What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Physiological Changes: Types and Examples Across Systems

Name: Physiological Changes: Types and Examples Across Systems
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

A physiological change is a measurable, predictable shift in an organ system's function that occurs within the animal's range of normal operation. Physiological changes are reversible, they are proportional to the stimulus, and they serve homeostasis rather than threaten it.

Understanding that definition matters in practice because the same variable can move in two directions. A horse's heart rate climbs to 200 beats per minute during gallop without any disease. A resting cat's heart rate of 220 beats per minute is a medical emergency. The number is identical. The context, the direction of the trigger, and the recovery pattern decide whether a change is adaptation or pathology. Clinicians who cannot separate the two over-diagnose normal animals and miss early disease in abnormal ones.

This article organizes physiological changes by body system, gives concrete reference values where they are established, and pairs each normal adaptive change with its pathological counterpart.

Defining physiological change precisely

Three features separate a physiological change from a pathological one.

Reversibility. A change that returns to baseline when the stimulus ends is physiological. Plasma volume expands during training and contracts again during detraining. Cardiac hypertrophy from exercise regresses when training stops, while hypertrophy from hypertension or valvular disease does not [1].

Proportionality. The magnitude of the change tracks the magnitude of the stimulus. Mild cold exposure produces mild thermogenesis. The response saturates and stops before tissue is damaged.

Homeostatic direction. Physiological changes defend a regulated variable. Pathological changes push the same variable outside its regulated range. Exercise raises core temperature to about 38.5 to 39.0 °C in a trained dog and then plateaus through sweating and panting. Heat stroke occurs when that plateau fails and core temperature keeps climbing.

Adaptation, acclimatization and acclimation are not synonyms

These terms confuse students every year, and they are not interchangeable.

Adaptation is a genetic, population-level change across generations.
Acclimatization is a reversible, phenotypic change that develops in a natural environment with all its complexity (heat plus humidity plus photoperiod plus behavior) [2]. Two weeks of isothermic conditioning that holds core temperature at or above 38.5 °C produces measurable remodeling of the sweat proteome in humans, which is acclimatization [2].
Acclimation is the same process produced in a controlled, single-variable setting such as a laboratory heat chamber.

A useful memory hook: acclimatization happens outdoors, acclimation happens in a box.

The summary table

System	Normal adaptive change	Trigger	Pathological counterpart
Cardiovascular	Heart rate rises and stroke volume increases in proportion to workload	Exercise, fear, pregnancy	Persistent resting tachycardia, arrhythmia, or exercise intolerance at low workload
Respiratory	Tidal volume and respiratory rate rise, airways dilate	Exercise, heat, acidosis	Bronchoconstriction during or after exertion [3]
Renal	Glomerular filtration rate rises, urine concentrates or dilutes as needed	Protein load, dehydration, water load	Persistent azotemia, isosthenuria, proteinuria despite normal hydration
Endocrine	Cortisol follows a circadian curve and spikes with acute stress	Waking, exercise, handling	Flat or inverted cortisol curve, sustained hypercortisolemia [4]
Musculoskeletal	Muscle mass, capillary density and oxidative capacity increase	Repeated mechanical loading	Atrophy, fibrosis, or bone loss without unloading [5]
Nervous	Baroreflex gain and vestibular calibration tune to the prevailing gravity and activity level	Posture change, microgravity, training	Orthostatic intolerance, ataxia, seizure activity [5]

Cardiovascular changes

Resting values and exercise response

Resting heart rate varies enormously across domestic species. Dogs typically sit between 60 and 160 beats per minute depending on size, with small breeds at the high end. Cats run 140 to 220 beats per minute. Horses rest at 28 to 44 beats per minute. Cattle sit around 48 to 84 beats per minute.

During exercise, heart rate climbs toward a species-specific maximum. A galloping horse can reach roughly 220 to 240 beats per minute. A racing Greyhound can exceed 300 beats per minute. These are physiological changes driven by sympathetic tone and, in the horse, by the splenic contraction that injects a large volume of stored red cells into circulation.

The splenic reservoir and packed cell volume

Packed cell volume (PCV) is the percentage of blood volume occupied by red cells after centrifugation. Resting PCV is roughly 37 to 55 percent in dogs, 30 to 45 percent in cats, and 32 to 53 percent in horses. A horse in full gallop can transiently push its PCV above 60 percent through splenic contraction alone, with no disease present [1]. The same PCV in a resting horse suggests dehydration or polycythemia.

This is one of the cleanest teaching examples in veterinary physiology: the same number means two different things depending on when the sample was drawn.

Exercise-induced cardiac hypertrophy

Sustained aerobic training enlarges cardiac myocytes and increases chamber dimensions. The molecular pathways involved include IGF1/PI3K/AKT signaling, hepatocyte growth factor, and the calcineurin and MAPK/ERK cascades, with downstream transcriptional factors such as GATA4, MEF2 and NFAT [1]. The result is improved contractile function and greater oxidative capacity, not a weakened heart.

The dividing line from pathology is functional. Physiological hypertrophy improves cardiac output. Pathological hypertrophy from hypertension or valvular disease stiffens the wall and reduces filling.

Comparative notes across mammals

Horses show an extreme exercise-induced PCV rise because of their large splenic reservoir. Dogs show a pronounced stroke volume increase with a relatively smaller heart rate ceiling. Cats rely heavily on heart rate because their stroke volume is small and relatively fixed. These differences mean a feline stress tachycardia of 240 beats per minute is normal in the exam room, while the same rate in a calm dog warrants an electrocardiogram.

Respiratory changes

Ventilation during exercise

Minute ventilation equals tidal volume times respiratory rate. Both components rise during exercise. In dogs, respiratory rate can climb from 20 to 40 breaths per minute at rest to well over 100 during heavy work, and tidal volume roughly doubles.

Respiratory rate is also a core thermoregulatory variable. Dogs and cats pant rather than sweat, and cats add grooming as a saliva-based evaporative strategy. A panting dog with a respiratory rate of 300 breaths per minute after a walk on a hot day may be entirely normal.

Exercise-induced bronchoconstriction

Exercise-induced bronchoconstriction (EIB) is a transient narrowing of the airways during or after exertion. It affects a substantial proportion of asthmatics and roughly 5 to 20 percent of the general human population [3].

Detection matters because standard spirometry, which depends on forced expiration, measures large airway function and misses small airway changes. Impulse oscillometry measures peripheral airway resistance without forced maneuvers. In a study of healthy young adult males, peripheral airway resistance (R5-R20) increased significantly at 5 minutes post-exercise, while FEV1 fell significantly at 5 minutes but had recovered by later time points [3].

The veterinary parallel

Equine asthma and canine chronic bronchitis both produce exaggerated bronchoconstriction to inhaled triggers. A horse with inflammatory airway disease may show normal respiratory rate at rest and elevated resistance during exercise. The physiological change (airway diameter tuning to demand) becomes pathological when the constriction persists and impairs gas exchange.

Renal changes

Glomerular filtration rate

Glomerular filtration rate (GFR) is the volume of plasma filtered by the glomeruli per unit time. It is the single best index of renal function because it changes before azotemia appears.

GFR is not a fixed number. It rises after a protein-rich meal (renal reserve) and falls during dehydration. A clinically useful rule of thumb is that GFR falls by roughly 75 percent before creatinine rises above the reference interval in dogs and cats, which is why a normal creatinine does not guarantee normal kidney function.

Species differences matter. Cats have a lower GFR per unit body mass than dogs, and both decline with age. A 15-year-old cat with a modestly elevated SDMA (symmetric dimethylarginine) is showing an age-related physiological change, not necessarily disease.

Urine concentration

Urine specific gravity (USG) is the practical bedside proxy for renal concentrating ability. Normal dogs can concentrate to 1.030 to 1.045 or higher. Cats are more efficient and often reach 1.050 to 1.080. Horses concentrate modestly, typically 1.020 to 1.050.

A dehydrated working dog with USG of 1.050 has produced a physiological change. The same dog with USG stuck at 1.010 despite dehydration has lost its concentrating ability, which is a pathological change.

Fluid balance under microgravity and unloading

The microgravity literature provides clean insight into how fluid distribution drives renal physiology. In spaceflight, plasma volume contracts, causing relative hemoconcentration, natriuresis, and impaired baroreflex gain, which converge to produce post-flight orthostatic intolerance [5]. This demonstrates that the kidney is not a passive filter. It responds to the same stretch, pressure, and volume signals that govern the cardiovascular system.

Endocrine changes

Cortisol reference intervals and the circadian rhythm

Cortisol is the primary glucocorticoid in dogs and cats, organized by the hypothalamic-pituitary-adrenal (HPA) axis. The HPA axis is a circadian-regulated, multi-oscillator system in which the paraventricular nucleus, pituitary, and adrenal gland each show intrinsic rhythmicity coordinated by the suprachiasmatic nucleus [4]. Glucocorticoids act both as stress effectors and as systemic zeitgebers that synchronize peripheral clocks [4].

Practical resting cortisol reference intervals in dogs vary by laboratory but commonly fall around 1 to 5 µg/dL (roughly 28 to 138 nmol/L), with lower values in the evening. Cats run similar ranges with slightly higher baselines.

A physiological cortisol spike follows acute stress, exercise, and waking. A pathological pattern is a flattened or inverted curve, such as that seen in Cushing's disease, or sustained hypercortisolemia from chronic inflammation [4]. Endocrine shifts in microgravity include insulin resistance and cortisol dysrhythmia alongside hypercalciuria [5].

Thyroxine reference intervals

Total thyroxine (T4) in dogs is typically reported as 1.0 to 4.0 µg/dL (13 to 51 nmol/L). Cats are higher, commonly 1.0 to 5.0 µg/dL, with wide age-dependent variation. Free T4 is a small fraction of total and is more specific for disease detection.

Physiological changes that move T4 include time of day, pregnancy, and non-thyroidal illness, which can lower T4 without true hypothyroidism. This is why a single low T4 in a sick dog is not diagnostic. It is a physiological shift until proven otherwise.

Insulin and dietary adaptation

Sustained changes in dietary composition produce coordinated homeostatic responses. In humans switching to plant-based diets, increased intestinal iron absorption, upregulated endogenous creatine and carnosine synthesis, improved nitrogen economy, and microbiota remodeling maintain physiological function [6]. Adaptive capacity is not unlimited though. Pregnancy, chronic inflammation, malabsorptive disease, and prolonged insufficiency can exceed it [6].

Musculoskeletal changes

Loading and unloading

Bone and muscle respond to mechanical load through mechanotransduction. Microgravity removes that load, producing measurable deconditioning. Skeletal muscle atrophy begins within days, and strength loss precedes structural loss because neural changes happen before fiber size shrinks [5]. Bone mineral density falls 1 to 2 percent per month at weight-bearing sites, and recovery after return to Earth is incomplete [5].

The same principle applies on Earth. A dog in a hindlimb splint loses muscle mass in that limb within two weeks. Swimming and underwater treadmill work maintain some load while reducing joint impact.

Exercise-induced thermal change

Muscle contraction generates heat, and infrared thermography can detect it non-invasively. In a study of three Warmblood horses over 10 treadmill sessions, mean local surface temperature rose from 33.75 to 35.36 °C over the deltoid region and from 32.37 to 33.33 °C over the gluteal region after exercise [7]. These are physiological changes from increased muscular activity and metabolic heat production [7].

The pathological counterpart would be a sustained thermal asymmetry between limbs without exercise, which suggests inflammation.

Endurance adaptation

Six weeks of fartlek-style cycle training in untrained humans raised VO2max by 5 mL/kg/min and maximal power output by 19 W [8]. The same principle applies to canine athletes, though temperature-specific adaptations vary. A meta-analysis comparing exercise in hot versus thermoneutral conditions found that heat exercise produced greater reductions in resting and exercise core temperature and greater increases in sweat rate [9].

Nervous system changes

Baroreflex and vestibular adaptation

The baroreflex maintains blood pressure during posture change by adjusting heart rate and vascular tone. In microgravity, plasma volume contraction and impaired baroreflex gain produce orthostatic intolerance on return to Earth [5]. This is a physiological change driven by unloading, not a primary neurological disease.

Vestibular calibration follows the same logic. A dog wearing a head tilt from vestibular disease will, over days to weeks, develop central compensation. The compensation is a physiological change built on neuroplasticity.

Circadian regulation of the stress response

The circadian clock and the integrated stress response (ISR) interact at the cellular level. The ISR converges on phosphorylation of serine 51 of eIF2α, and ISR signaling is temporally regulated by the circadian clock, controlling time-of-day-dependent protein synthesis [4]. Physiological ISR activity supports circadian robustness and resetting, while excessive or chronic ISR activity contributes to pathology [4].

Caffeine provides a clean model. In a study of C57BL/6 male mice, caffeine improved balance and coordination after forced treadmill fatigue and altered expression of clock genes including Clock, Bmal1, Per2, Cry1 and Cry2 after 16 days [10]. Age mattered: young (1 month 2 weeks) and mature (10 months 2 weeks) mice showed different responses [10].

Nervous system thermoregulation

Thermoregulation is a nervous system function that integrates afferent thermal input, hypothalamic set-point, and efferent motor and autonomic output. Exercise in hot conditions produces greater reductions in resting core temperature (Hedges' g = 0.51) and exercise core temperature (g = 0.55) than thermoneutral training [9]. These are physiological adaptations to heat acclimatization.

How physiological changes are tested in practice

Veterinarians don't grade adaptation by looking at animals. They use structured tests.

Resting baseline first. Heart rate, respiratory rate, temperature, and PCV all need a calm, acclimated baseline before any challenge.
Controlled provocation. Exercise testing, water deprivation trials (short, supervised), and ACTH stimulation tests all measure how far the system moves.
Recovery curves. How fast a variable returns to baseline is often more informative than the peak value.
Species-appropriate comparison. A horse's PCV above 60 percent during gallop needs no treatment. A dog's PCV above 60 percent at rest is a red flag.
Trend over time. Single measurements mislead. Serial data shows whether the change is adaptive or drifting into pathology.

Clinical Relevance, Limitations and Common Mistakes

Clinical relevance. Normal physiological changes set the baseline against which every diagnostic test is interpreted. A student or clinician who cannot distinguish exercise tachycardia from pathological tachycardia over-diagnoses. A practitioner who misses the pathological shift over-treats or delays treatment.

Limitations. Reference intervals are population statistics, not individual guarantees. They vary with age, sex, breed, pregnancy, altitude, and laboratory methodology. Pregnancy alone shifts nearly every cardiovascular, renal and endocrine variable. A single value outside the interval is not a diagnosis.

Common mistakes students make.

Confusing acclimatization with acclimation. The first happens in the real environment with all variables changing, the second in a controlled chamber.
Forgetting pregnancy as a normal physiological state. Heart rate rises, GFR rises, cortisol and thyroxine shift, and the animal is not sick.
Treating microgravity findings as science fiction. The same unloading biology explains how a splinted limb loses muscle [5].
Reading endocrine curves as flat numbers. Cortisol is a rhythm, not a single value [4].
Assuming the reference interval covers the individual. It covers the 95 percent central population, so 2.5 percent of normal animals sit outside each end by definition.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Quick Review

Physiological changes are reversible, proportional, and homeostatic. Pathological changes are not.
Acclimatization happens in nature, acclimation in a controlled setting.
A horse's PCV can exceed 60 percent during exercise and still be normal [1].
Cortisol follows a circadian curve maintained by the HPA axis as a multi-oscillator system [4].
Microgravity unloading causes muscle atrophy within days and 1 to 2 percent bone loss per month at weight-bearing sites [5].
Heat exercise changes core temperature and sweat rate more than thermoneutral exercise [9].
All reference intervals are species-, age-, and pregnancy-dependent.

Frequently Asked Questions

What is a physiological change?

A physiological change is a reversible, stimulus-proportional shift in organ function that serves homeostasis. It returns toward baseline when the stimulus ends.

Is acclimatization the same as acclimation?

No. Acclimatization develops in a natural environment with all its variables changing at once. Acclimation develops in a controlled setting where one variable is manipulated.

Why does a horse's packed cell volume rise during exercise?

The spleen stores a large reservoir of red blood cells and contracts during sympathetic activation, releasing them into circulation. This raises PCV transiently.

Why does cortisol vary during the day?

The HPA axis is a multi-oscillator system with intrinsic rhythmicity in the hypothalamus, pituitary, and adrenal gland. Glucocorticoids also act as systemic zeitgebers that synchronize peripheral clocks.

Can pregnancy cause changes that look like disease?

Yes. Pregnancy raises heart rate, increases GFR, and shifts endocrine values. These are normal physiological states, not pathologies.

Why do different species have different normal values?

Species differ in body size, metabolic rate, splenic reservoir size, and thermoregulatory strategy. A cat concentrates urine better than a horse because its renal anatomy and ecology demand it.

flowchart TD
    A[Stimulus arrives] --> B{Is the stimulussustained or brief}
    B -->|Brief| C[Transient shift]
    B -->|Sustained weeks| D[Acclimatization begins]
    C --> E[Returns to baseline]
    D --> F{Is the direction homeostatic}
    F -->|Yes| G[Physiological adaptation]
    F -->|No| H[Pathological shift]
    G --> I[Reversible on detraining]
    H --> J[Persists and worsens]