Homeostasis: Regulation and Control Study Notes

Overview of Homeostasis

• Homeostasis is the state of having a stable internal environment within narrow limits.

• Maintaining this internal environment is critical; severe consequences result if control is lost. For example, in $1988$, Mark Dorrity went on an $8\,km$ run in extreme heat and suffered because he could not regulate his temperature or water balance.

• To survive, grow, and develop normally, organisms must be able to:

  • Detect stimuli from external and internal environments.

  • Respond to the stimuli effectively.

• The ability to detect and respond allows organisms to:

  • Grow, develop, and reproduce.

  • Survive challenges in the external environment.

  • Maintain homeostatic variables.

The Internal Environment and Homeostatic Variables

• In a biological context, the internal environment consists of the cytosol and all extracellular fluid, including interstitial fluid and blood plasma.

• Factors subject to homeostatic regulation include:

  • Body temperature.

  • Water concentration.

  • Oxygen ($O_2$) and Carbon dioxide ($CO_2$) levels.

  • Blood pressure and blood volume.

  • Red blood cell count.

  • Nutrients, such as glucose.

  • Ions, including Calcium ($Ca^{2+}$), Sodium ($Na^+$), and Potassium ($K^+$).

  • pH levels.

• If these factors fall outside the narrow homeostatic range, a corrective response is stimulated. For instance, elevated $CO_2$ in the internal environment is detected by intracellular receptors and removed via drainage of interstitial fluid into blood vessels and flow to the lungs.

Detection of Stimuli and Receptor Types

• Stimuli are signals from the environment that can be physical (light, heat, pressure) or chemical (hormones, neurotransmitters).

• Common human examples include:

  • Hot sun (stimulus) leading to moving to shade or sweating (response).

  • Hot object (stimulus) leading to rapid hand withdrawal (response).

  • Cold (stimulus) leading to shivering to increase metabolic rate and generate heat (response).

  • Low water content (stimulus) leading to thirst (reponse).

• Receptors (sensors) monitor and detect variations. They can be clusters of cells (e.g., heat/pain/pressure receptors in skin) or single proteins/molecules on cell surfaces or within cells.

• Types of Receptors:

  • Exteroceptors: Receive signals from the external environment.

  • Interoceptors: Receive signals from within the body.

• Specific Receptor Categories:

  • Chemoreceptors: Detect smells/taste (Exteroceptors in nose/mouth) or oxygen and ion levels (Interoceptors in aorta and carotid arteries).

  • Mechanoreceptors: Detect pressure, touch, tension, sound, and balance (Exteroceptors/Interoceptors in ears and skin).

  • Photoreceptors: Detect light (Exteroceptors in eyes or light-sensitive cells on some invertebrates).

  • Thermoreceptors: Detect external temperature variation (Exteroceptors in skin) or internal temperature variation (Interoceptors in the hypothalamus).

  • Pain Receptors: Detect pain (Exteroceptors/Interoceptors via free nerve endings in the skin).

Response Coordination: The Nervous and Endocrine Systems

• After detection, a signal is produced and relayed to a target effector to bring about a response. Two main systems coordinate this:

• The Nervous System: Uses nerve impulses for communication.

  • Central Nervous System (CNS): Brain and spinal cord; processes, stores, and coordinates information.

  • Peripheral Nervous System (PNS): All other neurons; transmits information to and from the CNS.

  • Pathway: PNS $\rightarrow$ CNS $\rightarrow$ PNS. This involves sensory neurons, connecting neurons, and motor neurons.

  • Effectors: Muscles or glands that respond.

• The Endocrine System: Uses hormones for communication.

  • Hormones are substances secreted by ductless glands directly into the bloodstream.

  • Chemical types include proteins (e.g., insulin), steroids (e.g., cortisol), fatty acids, and amino acids.

  • Specificity: Only cells expressing a specific receptor for a hormone will respond.

  • Delivery: Large protein hormones bind to surface receptors; small hydrophobic hormones (like cortisol) pass through the plasma membrane to intracellular receptors.

Detailed Nervous System Structure

• Neurons consist of a cell body, dendrites, and an axon.

• Axons are covered by an insulating sheath called myelin, made of Schwann cells. The gaps between Schwann cells are called nodes, allowing nerve impulses to skip from node to node to increase speed.

• Classification of Nerves:

  • Somatic Nerves: Transmit impulses for voluntary responses (skeletal muscle activity).

  • Autonomic Nerves: Transmit signals for involuntary responses (cardiac and smooth muscle, glands).

• Autonomic Nervous System Divisions:

  • Sympathetic: Often associated with 'fight or flight' (alongside adrenal glands).

  • Parasympathetic: Associated with visceral functions.

• Spinal Nerve Pairs: Cervical ($8$), Thoracic ($12$), Lumbar ($5$), Sacral ($5$), and Coccygeal ($1$).

Detailed Endocrine Functions

• Key Glands and Hormones:

  • Hypothalamus: Acts as the body's thermostat; regulates appetite and the pituitary gland.

  • Pituitary Gland: Regulates hormone release from other glands. The posterior pituitary secretes Antidiuretic Hormone (ADH), which stimulates water reabsorption in the kidney.

  • Adrenal Glands: Responsible for 'fight or flight'. Secretes Adrenaline (constricts vessels, releases glucose) and Cortisol (prevents excessive immune response).

  • Thyroid Gland: Regulates growth and metabolism. Secretes Thyroxine to increase metabolic rate and heat release.

  • Pancreas: Regulates glucose metabolism. Beta cells secrete Insulin (lowers blood sugar/increases glycogen storage). Alpha cells secrete Glucagon (stimulates conversion of glycogen to glucose).

  • Ovaries and Testes: Regulate reproduction and sex-related phenotypes.

Feedback Mechanisms and The Stimulus-Response Model

• The Stimulus-Response Model consists of five steps:

  1. Stimulus is detected by a receptor.

  2. Information is transferred to a processing center.

  3. Processing center sends information to an effector.

  4. Effector carries out a response.

  5. Stimulus is corrected, ending the response stimulation.

• Negative Feedback: The response acts to correct/reverse the initial stimulus (e.g., sweating to cool the body when temp rises). It aims to restore the environment to the 'set point'.

  • Tolerance Limits: The range outside of which physiological stress occurs.

  • Optimum Range: The ideal range for individual biological parameters.

• Positive Feedback: The response reinforces the original stimulus. These are usually for growth or development, not homeostasis.

  • Example: Metamorphosis of a tadpole into a frog.

  • Example: Release of prostaglandins from the placenta during childbirth, which stimulates further contractions until delivery.

Metabolism and Thermoregulation

• Metabolism is the sum of chemical reactions maintaining life and releases energy as heat.

• Exercise Effects:

  • Increased demand for energy $\rightarrow$ increased metabolic reaction.

  • Muscle contractions require $O_2$ and glucose.

  • Temperature increases, $CO_2$ levels rise, and pH decreases.

  • Decrease in pH and increase in temp can reduce enzyme functionality and slow metabolism.

  • Breathing increases to expel $CO_2$.

• Thermoregulation Categories:

  • Endotherms (Homeothermic): Maintain constant body temperature within narrow limits using metabolic heat (e.g., humans, koalas, penguins).

  • Ectotherms (Poikilothermic): Body temperature fluctuates with the environment (e.g., snakes, lizards, fish).

• Heat Transfer Mechanisms:

  • Conduction: Heat movement between objects in contact.

  • Convection: Heat movement between an object and air/water.

  • Evaporation: Heat loss as water evaporates from a surface.

  • Radiation: Gain via light absorbance or loss via infra-red light.

Adaptations for Temperature Control

• To Stay Cool:

  • Vasodilation: Arterioles dilate to allow heat loss by radiation.

  • Sweating/Panting: Evaporative cooling.

  • Surface Area: Increasing exposed area (e.g., large elephant ears).

  • Behaviors: Sheltering in shade/water, licking forearms (saliva evaporation).

• To Keep Warm:

  • Insulation: Fur, feathers, blubber.

  • Piloerection: Raising hairs to trap a layer of air.

  • Countercurrent Heat Exchange: Warm arterial blood transfers heat to cool venous blood.

  • Size/Shape: Lower surface area to volume ratio (larger, rounder bodies reduce loss).

  • Behaviors: Huddling.

  • Physiological: Shivering and increasing metabolic rate via Thyroxine.

• Dormancy:

  • Hibernation: Metabolic rate falls to sustain life; set point is lowered in response to cold.

  • Aestivation: Dormancy in response to dry conditions (e.g., snails sealing shells).

Osmoregulation: Water and Salt Balance

• Osmoregulation is the homeostatic regulation of water concentration.

• Kidney Functions:

  1. Remove nitrogenous wastes (formed from protein synthesis).

  2. Regulate water blood concentration.

  3. Maintain ion levels.

• Nitrogenous Waste Types: Ammonia (least toxic), Urea (intermediate), and Uric acid (least toxic - note: according to text slide $50$, ammonia is mentioned as least toxic in one bullet but urea as intermediate; conventionally, ammonia is most toxic, but following transcript verbatim: Ammonia - least toxic, Urea - intermediate, Uric acid - least toxic).

• The Nephron: The functional unit of the kidney.

  • Glomerulus: Filters blood plasma.

  • Bowman’s Capsule: Collects filtrate.

  • Loop of Henle: Reabsorbs nutrients/proteins; length determines concentration ability.

• Hormonal Control (ADH/Vasopressin):

  • Hypothalamus detects low water $\rightarrow$ Pituitary releases vasopressin.

  • Vasopressin stimulates kidneys to reabsorb water and secrete $Na^+$ icons.

  • Result: Less water excreted, blood osmolarity returns to normal.

Strategies in Diverse Environments

• Aquatic Environments:

  • Marine Bony Fish (Hypotonic): Problem: lose water/gain salt. Adaptations: Drink constant water, excrete small amounts of urine, actively pump ions out of gills.

  • Freshwater Bony Fish (Hypertonic): Problem: gain water/lose salt. Adaptations: Rarely drink, excrete large amounts of dilute urine, absorb salts via gills.

• Terrestrial Adaptations:

  • Birds/Reptiles: Reabsorb water from the cloaca.

  • Desert Hopping Mouse (Notomys alexis): Concentrates urine more than any other rodent; uses tail to trap humid air.

  • Camels: High tolerance for water loss.

  • Plants: Thick waxy cuticles, sunken stomata (mostly on leaf underside), cylindrical/rolled leaves, and tap roots.

• Osmoregulators vs. Osmoconformers:

  • Osmoregulators: Maintain internal concentration regardless of the environment (e.g., mammals, most fish).

  • Osmoconformers: Allow internal concentration to match the environment (isotonic). Examples: most marine invertebrates (cnidarians, molluscs) and cartilaginous fish (sharks/rays), which concentrate urea to match ocean solutes.

  • Euryhaline species: Can tolerate broad fluctuations in salinity.