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Biology 219 – Human Physiology
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
major solutes: potassium (K ), organic anions (A )
2. ECF ~1/3 of TBW, consists of:
a. interstitial fluid
b. blood plasma (~ 3 L)
major solutes: sodium (Na+), chloride (Cl-), some organic molecules
- 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
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
120/75 mm Hg (mean = 90 mm Hg)
100 mm Hg
40 mm Hg
90 mg/dL (fasting)
290 mOsm
4. Negative feedback control maintains homeostasis
Integrating Center
[set point]
- 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