Basic Concepts in Physiology 1: Homeostasis & Body Fluids

Expected Learning Outcomes

• Define homeostasis and describe how feedback systems maintain it.
• Differentiate and provide examples of negative vs positive feedback systems.
• State the composition and volumes of all body-fluid compartments.
• Explain the principles of water movement between major fluid compartments.

Foundations: Anatomy vs. Physiology

• Anatomy
  • Science of body structures.
• Physiology
  • Science of body functions.
  • Investigates processes enabling the body to maintain homeostasis despite constantly changing environments.

Major Sub-disciplines of Human Physiology

• Neurophysiology – functional properties of nerve cells.
• Endocrinology – hormones & their control of body functions.
• Cardiovascular physiology – heart & blood vessels.
• Respiratory physiology – air passageways & lungs.
• Renal physiology – kidneys.
• Exercise physiology – cellular & organ changes due to muscular activity.

Homeostasis

• Definition: Dynamic equilibrium of the body’s internal environment produced by coordinated regulatory processes.
• Internal conditions fluctuate within limited ranges (set points) rather than remain constant.
  • Blood glucose set point ≈ 70\text{–}110\;\text{mg}/100\text{ mL}.
  • Core body temperature set point ≈ 36.5\text{–}37.5\;^{\circ}\text{C}.

Primary Regulatory Systems

• Nervous system
  • Produces rapid changes via nerve impulses.
• Endocrine system
  • Produces slower, longer-lasting changes via circulating hormones.

Feedback Systems / Loops

A self-correcting cycle: monitor → evaluate → change → re-monitor.

1. Negative feedback – reverses the initial change; dominant mechanism for homeostasis.
2. Positive feedback – amplifies the initial change; drives rapid events, usually self-terminating.

Negative Feedback – Detailed Example: Thermoregulation

• Stimulus: Increased body temperature

  • Receptors: thermoreceptors in skin & hypothalamus detect warmer blood.
  • Control center: hypothalamic heat-loss center.
  • Effectors/actions:
  • Skin arterioles dilate → ↑ skin blood flow → heat radiated.
  • Sweat glands activate → evaporative cooling.
  • Arrector pili muscles relax → hairs lie flat (↓ insulation).
  • Adrenal & thyroid secretions suppressed → ↓ metabolic heat.
  • Behavioral: removing clothing, seeking shade.
  • Result: Body temperature falls toward set point; heat-loss center shuts off.

• Stimulus: Decreased body temperature

  • Receptors: same thermoreceptors detect cooler blood.
  • Control center: hypothalamic heat-promoting center.
  • Effectors/actions:
  • Skin arterioles constrict → blood shunted away from skin.
  • Sweating inhibited.
  • Arrector pili muscles contract → piloerection, ↑ insulation.
  • Skeletal muscles shiver → heat production.
  • Adrenal (adrenaline) & thyroid (thyroxine) secreted → ↑ metabolic rate.
  • Behavioral: putting on jacket, seeking warmth.
  • Result: Body temperature rises toward set point; heat-promoting center shuts off.

Additional negative-feedback examples: regulation of arterial pressure/volume, extracellular fluid (ECF) balance, blood Ca^{2+}, blood pH, blood glucose, \text{O}2 & \text{CO}2 levels.

Positive Feedback – Key Features & Example

• Amplifies deviation; requires external brake.
• Normal rapid events: childbirth, blood clotting, protein digestion, generation of nerve APs.
• Potentially harmful if uncontrolled (e.g., runaway fever > 40^{\circ}\text{C} or severe hemorrhage).

Childbirth Loop

• Receptor: stretch-sensitive cervical mechanoreceptors.
• Control center: brain.
• Output hormone: oxytocin.
• Effector: uterine smooth muscle → stronger contractions → ↑ cervical stretch → more oxytocin.
• Loop ends only with delivery of the baby (removal of stimulus).

Body Fluids & Homeostasis

• Body fluids = body water + dissolved substances.
• Cellular function depends on the tightly regulated composition of interstitial fluid (ISF) bathing cells.

Daily Water Balance (Typical Adult)

• Intake
  • Ingested liquids & food ≈ 2100\;\text{mL·day}^{-1}.
  • Metabolic water (oxidative phosphorylation) ≈ 200\;\text{mL·day}^{-1}.
  • Highly variable with diet, activity, environment.
• Losses
  1. Insensible (skin diffusion + respiratory evaporation) ≈ 700\;\text{mL·day}^{-1}.
  2. Sweat (variable).
  3. Feces.
  4. Urine – major adjustable route; kidneys match excretion to intake.

Role of Kidneys

• Adjust excretion rates of water and electrolytes to maintain balance.

Fluid Compartments & Barriers (70-kg Reference Male)

• Total body water (TBW): 42\;\text{L} (≈ 60\% body mass).
  • Intracellular fluid (ICF): 28\;\text{L} (≈ 40\% body mass).
  • Extracellular fluid (ECF): 14\;\text{L} (≈ 20\% body mass).
  • Interstitial fluid (ISF): 11\;\text{L}.
  • Plasma: 3\;\text{L}.
• Barriers
  1. Plasma membrane – separates ICF from ISF; semi-permeable.
  2. Capillary endothelium – separates ISF from plasma; low permeability to plasma proteins.

Sub-Compartments of ECF

• Lymph, cerebrospinal fluid (CSF), synovial fluid, pleural/pericardial/peritoneal fluids, aqueous/vitreous humors, bile, fluids in GI/urinary/respiratory tracts.

Composition of Major Fluids

Intracellular Fluid (ICF)

• High \text{K}^+ & \text{HPO}_4^{2-} (phosphate).
• High protein concentration (~4× plasma).
• Low \text{Na}^+ & \text{Cl}^-.
• Negligible \text{Ca}^{2+}.
• Total osmolarity ≈ 300\;\text{mOsm·L}^{-1} (corrected activity ≈ 281\;\text{mOsm·L}^{-1}).

Extracellular Fluid (Plasma & ISF)

• Principle cation: \text{Na}^+; principle anion: \text{Cl}^-.
• Plasma contains many negatively charged proteins (limited entry into ISF).
• Total osmolarity ≈ 300\;\text{mOsm·L}^{-1}.
• Na^+–K^+ pump maintains high extracellular \text{Na}^+ / high intracellular \text{K}^+.

Representative Osmolar Table (excerpt)

• Plasma \text{Na}^+: 142\;\text{mOsm·L}^{-1}, ISF 139, ICF 14.
• Plasma \text{K}^+: 4.2, ISF 4.0, ICF 140.
• Plasma protein 1.2, ISF 0.2, ICF 4.
• Corrected osmolar activity uniform (~281\;\text{mOsm·L}^{-1}) ensuring osmotic equilibrium.

Principles of Water Movement

• Osmosis: water diffusion through semi-permeable membrane from lower → higher solute concentration.
  • Driven by osmotic pressure ((\pi)) proportional to solute particle number.
• Capillary filtration: hydrostatic pressure forces plasma water into ISF.
• Lymphatic return: excess ISF returns to bloodstream.
• Cellular uptake: water can enter cells osmotically.

Regulation Between ICF & ECF

• Distribution governed primarily by small, effective osmoles (mainly \text{Na}^+, \text{Cl}^-, other electrolytes) in ECF.
• Cell membranes: high water permeability, low solute permeability → rapid equilibration.
• Scenarios
  1. ↑ ECF osmolarity (hypertonic ECF) → water leaves cells → cell shrinkage.
  2. ↓ ECF osmolarity (hypotonic ECF) → water enters cells → cell swelling.
• Complete osmotic equilibrium achieved within seconds–minutes; e.g., 30 min after drinking water.

Summary Pathway of Water Fluxes

\text{Digestive tract} \xrightarrow[osmosis]{} \text{Blood plasma} \xrightarrow[filtration]{} \text{ISF} \xrightarrow[uptake]{} \text{Cells / Lymph} \rightarrow \text{Blood}.

Key Takeaways & Integration

• Homeostasis relies chiefly on negative feedback; positive feedback provides rapid, purposeful amplification when necessary.
• Set points are target averages around which variables fluctuate.
• Body-fluid balance (volume & composition) is crucial for cellular function; kidneys and membrane transport processes are central regulators.
• Rapid water shifts maintain equal osmolarity for ICF and ECF, protecting cells from extreme volume changes.

Practical & Clinical Relevance

• Understanding feedback loops guides clinical interventions (e.g., treating fever, managing blood pressure, hormone replacement).
• Fluid compartment knowledge is essential for interpreting lab values, IV fluid therapy, and understanding edema or dehydration.
• Disruption of Na^+–K^+ pump (e.g., hypoxia) or uncontrolled positive feedback (e.g., DIC in sepsis) can be life-threatening.