Notes on External/Internal Environments, Homeostasis, and Osmoregulation
External Environmental Context
Animal function is studied at the level of whole organism mechanics and how systems evolve across vertebrate lineages.
Digestive context: autotrophs/autotrophy (organisms that create energy) vs heterotrophs (organisms that consume energy, including herbivores and carnivores).
Herbivores eat plant material (consume autotrophs).
Carnivores eat herbivores or other animals; includes secondary and tertiary consumers.
Digestive differences between carnivores and herbivores will be discussed in the digestion section.
External environments are divided into two broad categories: terrestrial and aquatic.
Terrestrial environments feature stressors such as gas composition, water availability, temperature, and salinity (for aquatic systems).
Aquatic environments subdivide into fresh water and salt water.
Terrestrial stresses (xeric vs humid):
Xeric environments (dry) require adaptations to low water availability.
Humid/tropical environments have high water content and higher water availability.
Aquatic environments conceptualized as:
Saltwater (marine) environments, which impose high extracellular salt loads and water tends to leave the animal.
Freshwater environments, which have low ionic strength and can produce water influx into the animal.
Internal vs external environmental contrast:
External environment exposes organisms to different conditions; internal environment is maintained within a range by regulatory processes.
Organisms are either regulators (maintain internal conditions) or conformers (allow internal conditions to track the external environment).
External Environment: Detailed Stressors and Gas Exchange
External environment components (gas composition):
Oxygen concentration is about 21% at sea level; CO₂ is much less but becomes important depending on organism and environment.
At sea level, barometric pressure is approximately P_{total} \approx 760~\text{mmHg} \approx 101~\text{kPa}.
Dalton’s Law and partial pressures:
Total pressure is the sum of partial pressures: P{total} = \sumi P_i.
Partial pressure for a gas is the total pressure times its fractional concentration: Pi = P{total} \times f_i.
At sea level, approximate partial pressures:
Nitrogen: P{N2} = 760 \text{ mmHg} \times 0.78 \approx 600~\text{mmHg}.
Oxygen: P{O2} = 760 \text{ mmHg} \times 0.21 \approx 160~\text{mmHg}.
CO₂ is a minor component in the external air.
Altitude effects on gas availability:
As you rise, the total barometric pressure drops, but the percentage of O₂ remains about 21%; thus the partial pressure of O₂ decreases: the same fractional composition but a smaller total pressure, leading to lower P{O2}.
Example implication: at high elevation, a human’s available O₂ (partial pressure) can drop from ~160~\text{mmHg} to much lower levels, compromising brain and heart function during activity.
Adaptations to low O₂ in high altitude species:
Bar-headed geese exemplify the adaptation to fly over high elevations with low ambient O₂; they possess specialized lungs and other adaptations that enhance oxygen uptake during flight.
Water metabolism and terrestrial water loss:
Terrestrial animals face constant water loss to the environment; water content is a critical issue.
Nose breathing and other respiratory tract modifications can help recapture water vapor during exhalation; mouth breathing tends to lose more water.
Aquatic gas exchange specifics:
Water is a solvent; dissolved solutes include salts and ions; salinity varies along a spectrum from dilute to hypersaline (hyperhaline) and oligohaline conditions in different aquatic niches.
Gas solubility in water depends on temperature and salinity: CO₂ solubility is higher than O₂, and solubility generally decreases with higher temperature and higher salinity.
In aquatic systems, oxygen availability is determined by both the dissolved oxygen concentration and its solubility in water; CO₂ does not drive ventilation in aquatic animals as strongly as O₂ does.
Cold water holds more O₂ than warm water; tropical/high-salinity waters hold less O₂, which can affect respiration and activity; some aquatic animals breathe air to cope with low dissolved O₂.
Depth-related pressure changes and gas effects:
Pressure increases by approximately 1 atmosphere (≈ 760 mmHg) for every 10 meters of depth.
Partial pressures of dissolved gases scale with pressure; e.g., at depth, higher partial pressures of gases can lead to physiological effects (e.g., nitrogen narcosis in divers).
If a diver ascends too quickly, dissolved gases come out of solution and can form bubbles in the cardiovascular system, disrupting function (decompression sickness).
Specific aquatic environmental patterns:
Large bodies of water tend to have relatively stable temperatures, while smaller bodies experience larger fluctuations.
Polar aquatics can exhibit freeze-point depression due to salinity; higher salinity can lower the freezing point of water.
Buoyancy and pressure interactions contribute to aquatic biomechanics and physiology.
Brackish water and osmoregulation in amphibious or euryhaline species:
Brackish water (mixture of salt and freshwater) allows some species to move between salinities with adjustive physiology; bull sharks are cited as capable of moving into brackish environments.
Osmoregulation in these contexts involves osmoregulation vs. osmoconformation strategies.
Bull sharks are noted as osmoregulators that can tolerate changing salinity by adjusting their internal osmotic state.
Burrow-dwelling terrestrial animals and elevated CO₂/low O₂ microenvironments:
Groundhog, naked mole rat, and other burrow-dwelling mammals can experience higher CO₂ and lower O₂ in confined spaces, prompting considerations of ventilation and respiratory adaptations.
Internal Environment: Compartments and Ionic Gradients
Internal environment subdivisions:
Extracellular environment: plasma and interstitial fluid (the fluid between cells).
Intracellular environment: fluid inside the cells.
Distinct ionic compositions:
Intracellular: high potassium (K⁺) and amino acids; relatively negative interior due to anions.
Extracellular: high sodium (Na⁺) and chloride (Cl⁻).
Driving forces across membranes:
Sodium tends to enter cells due to electrochemical gradients; potassium tends to leave cells.
These ion fluxes power other cellular processes, including action potentials and muscle activity.
Osmoregulation and compartments:
Osmotic balance involves maintaining appropriate ion concentrations across the extracellular and intracellular compartments.
Conformers vs regulators revisited:
Osmoconformers (osmoconformers) allow internal osmolality to track the external environment; often seen in certain invertebrates and some osmotic niches.
Osmoregulators actively regulate their internal osmolarity across a wide range of external conditions; many vertebrates, including most fish, are osmoregulators.
Body temperature regulation also follows a similar conformer vs regulator framework, though thermal strategies can be decoupled from osmotic strategies (e.g., crocodilians rely on behavioral thermoregulation).
Broad significance:
These strategies determine how organisms function across environments and underpin their capacity to occupy diverse ecological niches.
Homeostasis, Regulation, Adaptation, and Acclimation
Homeostasis: the capacity to maintain a relatively constant internal state around a set point.
Regulating organisms maintain variables within a narrow range; conformers allow fluctuations with the environment to save energy.
Components of regulatory feedback systems:
Sensor: detects deviations from the set point.
Integrator: processes information and determines the corrective response.
Effector (motor output): executes the corrective action to restore the set point.
Feedback loop types:
Negative feedback loops counteract deviations and restore the set point (e.g., room temperature control, blood glucose regulation).
Positive feedback loops amplify deviations, often leading away from the set point (e.g., childbirth, certain pathological states like fever amplification, insulin resistance in diabetes).
Examples of homeostatic regulation:
Blood glucose: deviation triggers insulin response to reduce glucose levels (negative feedback).
Temperature regulation: negative feedback via heating/cooling responses.
Positive feedback example: childbirth involves a stimulus that intensifies the response until delivery.
Osmoregulation and intracellular/extracellular balance:
Homeostasis encompasses maintaining ion and water balance across compartments to support cellular function.
Adaptation vs acclimation:
Adaptation: genetic changes across generations that improve fit to an environment; example: indigenous high-elevation populations with greater lung and cardiovascular capacity.
Acclimation: an individual’s reversible physiological adjustments to a new environment over time; example: acclimating to higher altitude before a journey; return to baseline after leaving the environment.
Important distinction: adaptation is long-term and heritable; acclimation is short-term and reversible.
Additional Considerations and Examples
Summary of key organisms/examples mentioned:
Bar-headed goose: high-altitude flight adaptation for low O₂ environments.
Naked mole rat: burrow-dwelling mammal with high CO₂/low O₂ microenvironments.
Bull sharks: euryhaline osmoregulators capable of tolerating brackish water by adjusting osmotic state.
Crocodilians: thermoregulate behaviorally; limited ability to regulate body temperature beyond environmental context.
Marlin and sailfish: represent saltwater inhabitants with distinctive osmoregulatory challenges.
Practical implications:
Understanding gas exchange across environmental contexts informs why organisms respond to altitude, depth, salinity, and temperature in particular ways.
The energy costs of regulation vs conforming impact ecological strategies and species distributions.
Extreme environments (high altitude, deep sea, deserts, brackish zones) reveal the boundaries and flexibility of homeostasis, regulation, and adaptation.
Key Formulas and Units to Remember
Dalton’s Law of Partial Pressures:
P{total} = \sumi Pi,\qquad Pi = P{total} \times fi.
Sea-level speakers (typical values):
P_{total} \approx 760\ \text{mmHg} \approx 101\ \text{kPa}.
P{O2} \approx 760\ \text{mmHg} \times 0.21 \approx 160\ \text{mmHg}.
P{N2} \approx 760\ \text{mmHg} \times 0.78 \approx 600\ \text{mmHg}.
Depth and pressure relationship:
Approximately, for every 10 m of depth, pressure increases by ~1 atm (≈ 760 mmHg).
Solubility and gas exchange considerations:
CO₂ solubility in water > O₂ solubility under similar conditions; solubility decreases with rising temperature and increasing salinity.
Connections to Foundational Principles and Real-World Relevance
The distinction between external and internal environments reflects foundational systems biology: how organisms interface with, sense, and regulate their surroundings.
The regulator vs conformer framework links to bioenergetics and ecological strategy: regulators incur higher energy costs but gain functional versatility; conformers save energy but may be limited by external conditions.
Adaptation and acclimation differentiate evolutionary timescales from individual plasticity, with clear implications for species distributions, medical considerations (e.g., high-altitude health), and ecological resilience.
The interplay of gas exchange, solubility, and environmental factors underpins physiology of respiration across aquatic and terrestrial life, informing fields from veterinary science to human medicine and diving physiology.
External Environmental Context
Animal function is studied at the level of whole organism mechanics, exploring how physiological systems operate and how these systems have evolved across diverse vertebrate lineages to cope with environmental challenges.
Digestive context: Organisms are categorized by their energy acquisition.
Autotrophs/Autotrophy: Organisms that produce their own energy, primarily through photosynthesis (e.g., plants, algae) or chemosynthesis. They form the base of most food webs.
Heterotrophs: Organisms that consume other organisms for energy.
Herbivores: Consume primary producers (autotrophs). Their digestive systems are often adapted to break down tough plant material, typically having longer digestive tracts and specialized microbial symbionts (e.g., Ruminants, horses).
Carnivores: Consume other animals (herbivores or other carnivores). Their digestive systems are generally shorter and designed for rapid digestion of protein and fat (e.g., cats, wolves).
Omnivores: Consume both plant and animal material, exhibiting characteristics of both digestive strategies.
Digestive differences between carnivores and herbivores will be discussed in detail in the specific digestion section, focusing on gut morphology, enzyme production, and microbial fermentation.
External environments are broadly categorized into terrestrial and aquatic, each presenting unique physiological stressors.
Terrestrial environments: Present challenges such as significant fluctuations in gas composition (especially oxygen), highly variable water availability (from deserts to rainforests), wide temperature ranges, and less direct salinity stress compared to aquatic systems.
Aquatic environments: Include fresh water and salt water, offering different osmotic and gaseous challenges.
Terrestrial stresses regarding water availability (xeric vs. humid):
Xeric environments (dry): Characterized by low water availability and high evaporative loss, requiring significant adaptations for water conservation, such as specialized kidneys, behavioral avoidance of heat, and impermeable integument (e.g., desert animals).
Humid/tropical environments: Feature high water content and high water availability, but can pose challenges related to high humidity impacting evaporative cooling and facilitating pathogen spread.
Aquatic environments are conceptualized by their salinity:
Saltwater (marine) environments: Imposes high extracellular salt loads on most organisms. Water tends to leave the animal's body via osmosis due to the hypertonic external environment, requiring active osmoregulation to retain water and excrete excess salts (e.g., marine fish).
Freshwater environments: Characterized by low ionic strength, making the external environment hypotonic. This causes water to constantly influx into the animal and salts to leach out, necessitating active osmoregulation to excrete excess water and retain salts (e.g., freshwater fish).
Internal vs. external environmental contrast: A fundamental concept in comparative physiology.
The external environment exposes organisms to a wide range of conditions, which can fluctuate wildly.
The internal environment, despite external variations, is maintained within a relatively narrow, stable range through various regulatory physiological processes.
Organisms are classified based on their interaction with the external environment:
Regulators: Actively maintain their internal conditions (e.g., temperature, osmolality, pH) within a narrow optimal range, regardless of external fluctuations. This often requires significant energy expenditure (e.g., endothermic mammals, most aquatic vertebrates).
Conformers: Allow their internal conditions to closely track or match the external environmental conditions. This strategy saves energy but restricts the organism to specific environmental niches where external conditions are tolerable (e.g., many marine invertebrates, some fish in stable environments).
External Environment: Detailed Stressors and Gas Exchange
External environment components (gas composition):
Oxygen concentration: Approximately 20.95% (often rounded to 21%) at sea level, which is a major driver for aerobic life. This percentage applies to the fractional composition of dry air.
Carbon dioxide (CO₂): Much lower concentration in ambient air (around 0.04%). While minor in external air, it becomes physiologically important due to its role in pH regulation and its relatively high solubility in water.
At sea level, standard barometric pressure is approximately P_{total} \approx 760\ \text{mmHg} \approx 101.325\ \text{kPa}. This absolute pressure affects the partial pressure of all component gases.
Dalton’s Law and partial pressures: Crucial for understanding gas exchange in respiration.
Dalton's Law of Partial Pressures: States that the total pressure exerted by a mixture of non-reacting gases is equal to the sum of the partial pressures of the individual gases: P_{total} = \sum{i} P{i}.
Partial pressure (Pi) for a gas $i$ is calculated by multiplying the total pressure by its fractional concentration in the gas mixture: Pi = P_{total} \times f_i. This determines the driving force for gas diffusion into or out of body fluids.
At sea level, approximate partial pressures (using a total pressure of 760 mmHg):
Nitrogen (f_{N2} \approx 0.78): P_{N2} = 760\ \text{mmHg} \times 0.78 \approx 592.8\ \text{mmHg}. (Note: 600 mmHg is an approximation).
Oxygen (f_{O2} \approx 0.21): P_{O2} = 760\ \text{mmHg} \times 0.21 \approx 159.6\ \text{mmHg}. (Note: 160 mmHg is an approximation).
CO₂ is a minor component in the external air, so its partial pressure is very low, but it is highly soluble in water and thus critical for internal pH and respiration.
Altitude effects on gas availability: As elevation increases, the number of gas molecules per unit volume decreases.
As you rise in altitude, the total barometric pressure (P_{total}) drops significantly. For instance, at Mount Everest's summit, P_{total} is less than 30% of sea level pressure.
Crucially, the percentage of O₂ in the air remains constant (about 21%). However, because P_{total} is lower, the partial pressure of O₂ (P_{O_2}) decreases proportionally.
Lower P_{O_2} means a reduced driving force for oxygen to diffuse from the lungs into the blood, making it harder for organisms to get enough oxygen.
Example implication: At high elevation, a human’s available O₂ (partial pressure) can drop from ~160\ \text{mmHg} at sea level to below 50\ \text{mmHg} at extreme altitudes. This severe hypoxia rapidly compromises brain and heart function, making sustained activity or even survival without acclimatization or adaptation extremely difficult.
Adaptations to low O₂ in high-altitude species: Evolution has equipped many species with remarkable solutions.
Bar-headed geese: Exemplify extreme adaptation, capable of flying over the Himalayas at altitudes where many other birds cannot survive. Their adaptations include highly efficient lungs with cross-current gas exchange, larger lung volume, increased capillary density in tissues, hemoglobin with a higher affinity for oxygen, and more efficient mitochondria.
Other adaptations include increased red blood cell count, increased ventilatory rate, and enhanced cardiovascular performance at lower oxygen partial pressures.
Water metabolism and terrestrial water loss: A constant challenge for terrestrial animals.
Terrestrial animals face continuous water loss primarily through evaporation from respiratory surfaces (lungs or skin), urine, and feces. Maintaining hydration (water content) is a critical homeostatic issue.
Nose breathing and other respiratory tract modifications: Play a key role in recapturing water vapor during exhalation. The nasal passages cool exhaled air, causing water vapor to condense and be reabsorbed before leaving the body. Animals in xeric environments often have elongated and convoluted nasal passages to maximize this effect. Mouth breathing bypasses this mechanism and tends to lose significantly more water.
Other strategies include nocturnality, specialized kidneys (e.g., loops of Henle), impermeable skin, and metabolic water production.
Aquatic gas exchange specifics: Governed by unique physical properties.
Water is a solvent; dissolved solutes include salts, ions, and gases. Salinity varies widely, from oligohaline (very low salt) to brackish (mixed salt/fresh) to hypersaline (very high salt) conditions.
Gas solubility in water: A critical factor. CO₂ solubility is significantly higher than O₂ solubility under the same conditions (e.g., at 0°C, CO₂ is about 30 times more soluble than O₂).
Solubility generally decreases with higher temperature and higher salinity. This means warm, salty water holds less dissolved O₂ than cold, fresh water.
In aquatic systems, oxygen availability is determined by both the dissolved oxygen concentration and its solubility. Low dissolved O₂ (hypoxia) is a major stressor.
Unlike terrestrial animals where CO₂ buildup is a primary driver of ventilation, in aquatic animals, O₂ levels are often the primary driver for ventilation due to its lower solubility and frequent scarcity.
Implications: Tropical oceans and high-salinity waters naturally hold less O₂, which can severely limit the metabolic activity and distribution of aquatic organisms. Some aquatic animals (e.g., certain fish, marine mammals) have evolved to breathe atmospheric air to cope with low dissolved O₂ environments or to support high metabolic rates.
Depth-related pressure changes and gas effects: Pressures increase dramatically with depth.
Pressure increases by approximately 1 atmosphere (≈ 760 mmHg or 101 kPa) for every 10 meters (33 feet) of depth in water.
Partial pressures of dissolved gases in the blood and tissues scale directly with ambient pressure. At depth, higher partial pressures of gases (especially nitrogen) can lead to physiological effects.
Nitrogen narcosis: At increased P_{N_2}, nitrogen can have anesthetic effects on the central nervous system, impairing judgment and motor control (often called