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Biology 219 – Human Physiology Clemens Introduction to Physiology A. Core Themes of Physiology 1. Physical-chemical basis of body function 2. Structure-function relationships 3. Energy utilization 4. Homeostasis 5. Integration B. Levels of Organization molecules → macromolecules → organelles → cells → tissues→ organs → organ systems increasing complexity and integration Integration of Organ Systems Organ systems work together to support body function. Organ systems share a common internal environment; some organ systems have surfaces that contact the external environment. The nervous system and endocrine systems control other organ systems. The circulatory system transports and exchanges substances with other organ systems. C. Environments and Compartments 1. intracellular fluid (ICF) 2. extracellular fluid (ECF) = internal environment 3. external environment (EXT) includes the lumen of hollow organs that open to the outside; in the respiratory, digestive, urinary & reproductive systems) Fluid compartments: Total Body Water (TBW) ≈ 60% of body weight 1. ICF ~2/3 of TBW ICF + major solutes: potassium (K ), organic anions (A ) H2O 2. ECF ~1/3 of TBW, consists of: K+ A¯ a. interstitial fluid b. blood plasma (~ 3 L) major solutes: sodium (Na+), chloride (Cl-), some organic molecules ECF H2O Na+ Cl¯ EXT - the ECF functions as a buffer between cells and the external environment Boundaries between compartments: 1. Plasma membrane - separates ICF and ECF - maintains different composition of ICF and ECF - has selective permeability and molecular transport 2. Epithelium - forms the boundary between internal environment (ECF) and external environment a. protection b. exchange surfaces - function in absorption, secretion, gas and fluid exchange apical surface (free surface) faces the external environment or lumen basolateral surface faces the internal environment (ECF) Biology 219 – Human Physiology Clemens D. Homeostasis Claude Bernard (mid-1800’s) - constancy of the internal environment Walter Cannon (1929) - defined homeostasis and further developed principles Key points: 1. Homeostasis is a steady state process (law of mass balance) open system, input = output to maintain constant level e.g., water balance: input (food+drink+metabolism) = output (urine+feces+evaporation) 2. Energy is required to maintain homeostasis. e.g., Na-K pump uses ATP to maintain [Na+] and [K+] of the ICF and ECF 3. The ECF is regulated to maintain suitable conditions for cells 7 essential variables body temperature arterial blood pressure arterial pH arterial PO2 arterial PCO2 blood [glucose] plasma osmolarity Normal Values and Units 37ºC 120/75 mm Hg (mean = 90 mm Hg) 7.4 100 mm Hg 40 mm Hg 90 mg/dL (fasting) 290 mOsm 4. Negative feedback control maintains homeostasis Variable negative feedback Sensors input (afferent pathway) Integrating Center ↑ [set point] output (efferent pathway) Effectors Response - variables are regulated in relation to a set point - corrective response is in the opposite direction of deviation from the set point deviation: ↑ (> set point) → response: ↓ ↓ (< set point) → ↑ Example - body temperature regulation Heat balance - to maintain constant body temperature (Tb): heat gain (metabolism + input from env.) = heat loss (to env.) If heat gain > heat loss → body temp ↑; If heat loss > heat gain → body temp ↓ Negative feedback control set point: 37ºC sensors: peripheral (skin), central (hypothalamus) integrating center: hypothalamus deviation: hot (> 37ºC) effectors: sweat glands → sweat skin blood vessels → vasodilation cold (< 37ºC) skeletal muscle → shivering skin blood vessels → vasoconstriction (skin, arrector pili → piloerection) response: ↓ body temperature ↑ body temperature 5. Positive feedback - non-homeostatic; results in rapid change of a variable