Respiratory systems

Thermoregulation

• Small‐bodied animals exploit behavioral thermoregulation more than large ones (seek microhabitats, shade, burrows, changing orientation to sun, etc.) due to their high surface-area-to-volume ratio, which leads to rapid heat exchange with the environment.

• Physiological Adaptations:

  • Heat Loss:

  • High surface-area structures (e.g., fennec fox ears, elephant ears) enhance heat loss through vasodilation, increasing blood flow to the surface. This is only effective while T{body} > T{env} (body temperature is higher than environmental temperature).

  • Sweating (e.g., humans) or panting (e.g., dogs) for evaporative cooling, which removes latent heat from the body.

  • Heat Gain/Retention:

  • Shivering: involuntary muscle contractions generate heat.

  • Vasoconstriction: reduces blood flow to the periphery, minimizing heat loss.

  • Piloerection/Feather fluffing: traps a layer of air for insulation.

  • Countercurrent heat exchange in limbs: arterial blood warms venous blood returning to the core, minimizing heat loss to extremities.

Dormancy Strategies

• Hibernation (deep, long-term torpor): An extreme state of metabolic depression.

  • Core body temperature (T_{body}) falls significantly, often close to ambient temperature; the animal becomes a thermal conformer.

  • Metabolic rate, heart rate, and respiration rate decrease drastically (e.g., heart rate may drop from 300 bpm to 5 bpm in ground squirrels).

  • Common in small mammals (e.g., groundhogs, ground squirrels, bats) as a strategy to conserve energy during periods of cold and food scarcity.

  • Animals exhibit periodic arousals from hibernation, though the exact reason is debated (e.g., immune function, sleep, processing waste).

• Brumation: A lighter, less profound state of dormancy, primarily found in ectotherms (e.g., reptiles and amphibians), similar to mammalian hibernation.

  • Core T still falls, but animals are more easily aroused than true hibernators.

  • Metabolically less drastic than hibernation.

• Torpor: A short-term, daily metabolic depression.

  • Often a daily response to food scarcity or cold (e.g., bats, hummingbirds).

  • Allows animals to conserve energy overnight or during a cold snap, with rapid arousal typically occurring within hours.

Feedback Regulation

• Homeostasis involves maintaining physiological variables within a narrow range around a set point.

• Negative feedback: The most common regulatory mechanism in biological systems. A deviation from a set-point triggers responses that reverse or counteract the change, stabilizing the system.

  • Components: Stimulus (change in variable) → Receptor (detects change) → Control Center (integrates information, compares to set point) → Effector (produces response) → Response (reduces stimulus).

  • Examples: Thermoregulation (if T{body} rises, sweating occurs to cool it down; if T{body} falls, shivering occurs to warm it up), blood glucose regulation (insulin/glucagon), blood pressure regulation.

• Positive feedback: Amplifies an initial change, moving the system further from the set-point, often leading to a rapid completion of a process.

  • Continues to amplify until an external event or endpoint stops the loop.

  • Examples: Childbirth (cervix stretch → oxytocin release → stronger uterine contractions → birth terminates the loop), blood clotting (platelet aggregation amplifies clotting factors until clot forms), action potential generation in neurons.

Why Oxygen Is Vital

• Oxygen is the final electron acceptor in the electron transport chain (ETC) located in the inner mitochondrial membrane.

• In the ETC, electrons from NADH and FADH2, derived from glucose breakdown, are passed down a series of protein complexes.

• This transfer of electrons drives the pumping of protons (H^+) from the mitochondrial matrix to the intermembrane space, creating a proton gradient (electrochemical gradient).

• Protons then flow back into the matrix through ATP synthase, a molecular motor, which harnesses this energy to synthesize the vast majority of cellular ATP (adenosine triphosphate) through oxidative phosphorylation.

• Without oxygen, the ETC cannot function, leading to a rapid cessation of aerobic ATP production and a switch to less efficient anaerobic pathways, which can only sustain the organism for a very short period.

Gas Exchange Principles

• Bulk flow (Convection): The mass movement of a fluid (air or blood) down a total pressure gradient. It is responsible for ventilation (movement of air into and out of lungs) and circulation (movement of blood through vessels).

  • Facilitates the rapid transport of gases over long distances in the body.

• Diffusion: The passive movement of individual gas molecules from an area of higher partial pressure to an area of lower partial pressure across a thin membrane.

  • Occurs over short distances, primarily at the respiratory surfaces (e.g., alveoli, gill lamellae) and between capillaries and tissues.

• Fick’s Law of Diffusion: Describes the rate of gas diffusion.

  • ext{Rate} ext{ of Diffusion} ext{ } oldsymbol{ ext{ proportional to }} ext{ } rac{(oldsymbol{ ext{P}} ext{1}-oldsymbol{ ext{P}} ext{2})oldsymbol{ ext{A}}}{oldsymbol{ ext{d}}}

  • (oldsymbol{ ext{P}} ext{1}-oldsymbol{ ext{P}} ext{2}) (Partial Pressure Gradient): A large difference in partial pressure across the membrane (oldsymbol{ ext{P}} ext{1} and oldsymbol{ ext{P}} ext{2} are partial pressures on either side) maximizes the diffusion rate.

  • oldsymbol{ ext{A}} (Surface Area): A large surface area for gas exchange (e.g., millions of alveoli in lungs, extensive gill filaments) maximizes the diffusion rate.

  • oldsymbol{ ext{d}} (Distance/Thickness): A small diffusion distance or membrane thickness (e.g., single-cell thick alveolar and capillary walls) minimizes the path length for gas, maximizing the diffusion rate.

• Boyle’s Law: States that for a fixed amount of an ideal gas at constant temperature, pressure and volume are inversely proportional.

  • P{oldsymbol{ ext{ proportional to }}} rac{1}{V} or P1V1 = P2V2 = ext{constant}. This principle explains how changes in thoracic cavity volume lead to pressure changes that drive air movement into and out of the lungs.

Mammalian Lungs

• Utilize tidal ventilation, where air flows in and out through the same pathway, leading to mixing of fresh and residual air in the lungs, reducing gas exchange efficiency compared to unidirectional flow.

• Breathing is primarily achieved via the diaphragm and intercostal muscles.

  • Inhalation (active): Diaphragm contracts and flattens (moves ↓), external intercostal muscles contract, lifting the rib cage up and out. This increases thoracic volume (V↑), which decreases intrapulmonary pressure (P↓) below atmospheric pressure, causing air to rush in.

  • Exhalation (passive at rest): Diaphragm relaxes and moves upward (↑), external intercostal muscles relax, allowing the rib cage to move down and in. This decreases thoracic volume (V↓), increasing intrapulmonary pressure (P↑) above atmospheric, forcing air out.

• Airway Branching: Air enters via the trachea → large bronchi → smaller bronchioles → terminates in tiny alveolar sacs (clusters of alveoli).

• Alveolus: The primary site of gas exchange.

  • Composed of a single layer of squamous epithelial cells (Type I pneumocytes), providing a very thin diffusion barrier.

  • Lined with a thin layer of water film and surfactant (produced by Type II pneumocytes) which reduces surface tension, preventing alveolar collapse during exhalation.

  • Each alveolus is densely enveloped by a network of capillaries (high perfusion) to match ventilation, ensuring efficient gas transfer.

• Partial Pressures: Typical atmospheric O2 is oldsymbol{ ext{ approximately }}21oldsymbol{ ext{ percent }} of total atmospheric pressure (approx. 760 ext{ mmHg} at sea level), meaning P{O2}oldsymbol{ ext{ and equals }}oldsymbol{ ext{ approximately }}160 ext{ mmHg} at sea level. In the alveoli, P{O2} is lower (around 100 ext{ mmHg}) due to mixing with residual air and continuous O2 absorption.

Fish Gills

• Highly specialized for aquatic gas exchange, consisting of many thin filaments arranged with numerous lamellae, providing a very large surface area.

• Employ countercurrent exchange for highly efficient oxygen uptake (>80oldsymbol{ ext{ percent }} efficiency) from water.

  • Blood flows through the capillaries in the lamellae in the opposite direction to the flow of water over the gill surface.

  • This maintains a continuous and steep partial pressure gradient for O2 (oldsymbol{ ext{ delta P }}_{O2}) along the entire length of the exchange surface, maximizing O2 diffusion into the blood.

  • In contrast, concurrent flow (same direction) would quickly reach equilibrium, significantly reducing efficiency.

• Ventilation methods:

  • Buccal/opercular pump: Most fish use synchronized movements of the mouth (buccal cavity) and operculum (gill cover) to actively pump water over the gills, even when stationary.

  • Ram ventilation: Highly active swimmers (e.g., sharks, tuna) keep their mouths open while swimming, forcing water continuously over their gills without needing a pump. If they stop swimming, they may suffocate.

Bird Respiratory System

• Highly efficient system due to unidirectional airflow and fixed lung structure.

• Consists of relatively rigid lungs and a system of oldsymbol{ ext{ approximately }}9 compliant (flexible, non-gaseous exchange) air sacs (anterior and posterior).

• Two-breath cycle for a single parcel of air to pass through the system:

  1. Inhalation 1: Air enters trachea, bypasses the lungs, and flows primarily to the posterior air sacs.

  2. Exhalation 1: Posterior air sacs squeeze, forcing the air from the posterior sacs into and through the parabronchi (the actual gas exchange surfaces within the rigid lungs).

  3. Inhalation 2: Fresh air enters the posterior sacs (like Inhalation 1), while the spent air from the parabronchi moves into the anterior air sacs.

  4. Exhalation 2: Anterior air sacs squeeze, expelling the COoldsymbol{ ext{ 2 }}‐rich air out of the body.

• This continuous, unidirectional flow of fresh, oxygen-rich air over the parabronchi (unlike the tidal flow in mammals) allows for extremely efficient gas exchange, making it the highest terrestrial gas-exchange efficiency among vertebrates.

• There is minimal mixing of incoming fresh air with residual, 'stale' air.

Insect Tracheal System

• A unique respiratory system that delivers O*2 directly to almost every cell of the body without involving the circulatory system.

• Spiracles: External openings along the body segments that can be opened and closed to regulate gas exchange and minimize water loss.

• Air enters via spiracles into a network of highly branched, air-filled tubes called tracheae.

• Tracheae branch into finer and finer tubes called tracheoles.

• Tracheoles are minute, fluid-filled tubes that directly invade individual cells, allowing oxygen to diffuse directly to body cells and COoldsymbol{ ext{ 2 }} to diffuse away.

• Gas exchange at the cellular level relies on simple diffusion down partial pressure gradients. In larger insects or active flight, muscular contractions (active pumping) can assist in ventilating the larger tracheal tubes.

• This system is highly effective for insects but limits their maximum body size due to the reliance on diffusion alone for gas transport to distant cells.

Cutaneous Respiration

• Gas exchange that occurs directly across the skin or outer body surface.

• It is minor in most vertebrates (e.g., contributes less than 2oldsymbol{ ext{ percent }} of O*2 uptake in humans).

• Major in amphibians, especially those with moist, thin skin and a large surface area (e.g., some salamanders, like lungless species, rely almost 100oldsymbol{ ext{ percent }} on their skin for both O*2 uptake and COoldsymbol{ ext{ 2 }} release).

• Requirements for effective cutaneous respiration: a highly vascularized (rich blood supply) and moist skin surface, and often a relatively high surface-area-to-volume ratio.

• Limitations: Only effective in aquatic or highly humid environments, as the skin must remain moist for gases to dissolve and diffuse. It is less efficient than specialized respiratory organs, limiting the metabolic rate an animal can sustain.

Blood Gas Transport & pH Buffer

• Oxygen Transport:

  • O2 is primarily transported bound to hemoglobin in red blood cells (RBCs). Hemoglobin’s affinity for O2 changes based on local conditions.

  • In tissues, P{O2} is low (due to cellular respiration), causing O2 to unload from hemoglobin. Metabolically active tissues also have higher CO2 levels and a lower pH, further promoting O*2 release (Bohr effect).

• Carbon Dioxide Transport:

  • In tissues, COoldsymbol{ ext{ 2 }} generated by cellular respiration diffuses from cells into the capillaries.

  • Three main forms of COoldsymbol{ ext{ 2 }} transport in blood:

  1. Dissolved in plasma (oldsymbol{ ext{ around }}7oldsymbol{ ext{ percent }}).

  2. Bound to hemoglobin (forming carbaminohemoglobin) or other plasma proteins (oldsymbol{ ext{ around }}23oldsymbol{ ext{ percent }}). The binding of O*2 to hemoglobin decreases its affinity for COoldsymbol{ ext{ 2 }} and H^+ (Haldane effect), facilitating COoldsymbol{ ext{ 2 }} release at the lungs.

  3. As bicarbonate ions ( ext{HCO}*3^-) in the plasma (oldsymbol{ ext{ around }}70oldsymbol{ ext{ percent }}). This is the most significant form.

     • Inside red blood cells, carbonic anhydrase (CA), a highly efficient enzyme, rapidly catalyzes the reaction: ext{CO}2+ ext{H}2 ext{O}
       ightleftharpoons ext{H}2 ext{CO}3
       ightleftharpoons ext{H}^++ ext{HCO}*3^-

     • The resulting bicarbonate ion ( ext{HCO}*3^-) is then transported out of the RBC into the plasma in exchange for a chloride ion ( ext{Cl}^-, known as the Chloride Shift), maintaining electrical neutrality.

     • The H^+ ions produced are largely buffered by hemoglobin within the RBC, preventing a drastic drop in blood pH.

• At the Lungs: The partial pressure gradient reverses. High P{O2} causes O2 to bind to hemoglobin, releasing H^+ and COoldsymbol{ ext{ 2 }} (Haldane and Bohr effects reverse). The reactions reverse: ext{HCO}*3^- re-enters the RBC (in exchange for ext{Cl}^-) and converts back to COoldsymbol{ ext{ 2 }} and H2O, and the COoldsymbol{ ext{ 2 }} diffuses into the alveoli to be exhaled.

• Blood pH Buffer: The bicarbonate buffer system ( ext{HCO}3^-/ ext{H}2 ext{CO}*3) is critical for maintaining blood pH within a narrow physiological range (oldsymbol{ ext{ approximately }}7.35-7.45), directly linking respiration (COoldsymbol{ ext{ 2 }} removal) to acid-base homeostasis.

Key Equations / Values

• Fick’s Law of Diffusion: Rate_{ ext{diffusion}} = k rac{oldsymbol{ ext{ delta P }} ext{ } imes ext{ } A}{d}

  • Rate: speed of gas transfer.

  • k: diffusion constant (depends on gas solubility and molecular weight).

  • oldsymbol{ ext{ delta P }}: partial pressure difference of the gas across the exchange surface.

  • A: surface area available for diffusion.

  • d: diffusion distance (thickness of the barrier).

• Boyle’s Law: P V = ext{constant}, or P1V1 = P2V2.

  • P: pressure of a gas.

  • V: volume it occupies.

  • Describes the inverse relationship between pressure and volume in a closed system.

• Atmospheric Pressure (P*{atm}): Approximately 760 ext{ mmHg} at sea level.

• Atmospheric Oxygen Partial Pressure (P{O2}): Approximately 21oldsymbol{ ext{ percent }} of P{atm}, which is oldsymbol{ ext{ approximately }}160 ext{ mmHg} at sea level.

• Alveolar Oxygen Partial Pressure (P{A ext{O}2}): Approximately 100-104 ext{ mmHg} (lower than atmospheric because of mixing with residual air and continuous O2 absorption).

Core Takeaways

• Size and Thermoregulation/Gas Exchange: Small body size leads to a higher surface-area-to-volume ratio, favoring behavioral and surface-area (e.g., large ears, thin skin) strategies for heat and gas exchange. While large animals face challenges with heat dissipation, small animals struggle more with heat retention during cold periods.

• Dormancy Diversity: Dormancy depth varies from deep, prolonged hibernation (true metabolic arrest, mainly small mammals) to lighter brumation (ectotherms) and short, daily torpor (saves energy for quick recovery).

• Feedback Control: Negative feedback loops are crucial for maintaining stability and homeostasis by counteracting deviations. Positive feedback loops amplify changes, driving processes to completion, and require an external event to stop them.

• Efficient Gas Exchange Fundamentals: Requires a steep partial pressure gradient (oldsymbol{ ext{ delta P }}), a large surface area (A), and a thin diffusion distance (d) across the respiratory membrane. It also critically depends on matched perfusion (blood flow) to ventilation (airflow or water flow) at the exchange surface.

• Specialized Respiratory Adaptations: Evolution has led to diverse, highly efficient respiratory systems in different environments:

  • Countercurrent exchange in fish gills maximizes O*2 extraction from water.

  • Unidirectional airflow and air sacs in bird lungs allow for continuous fresh air over exchange surfaces, increasing efficiency.

  • Direct tracheal delivery in insects bypasses the circulatory system for gas transport, enabling efficient O*2 supply directly to cells.

  • Cutaneous respiration provides a supplementary (or primary in some amphibians) means of gas exchange across moist skin surfaces.

• Blood Gas Chemistry & pH: COoldsymbol{ ext{ 2 }} transport within the blood, notably its conversion to bicarbonate ( ext{HCO}*3^-) via carbonic anhydrase and the chloride shift, forms a key buffer system, intrinsically linking respiration to blood pH homeostasis.