|Introduction to Organismal Biology (BIOL221) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; email@example.com; http://www.employees.csbsju.edu/ssaupe/|
I. Homeostasis - "Life Exhibits Homeostasis" - one of the general characteristics of life
An animal (or plant) is separated from its external environment by some sort of boundary. Since external and internal conditions are usually different, it shows that an organism is able to exert control over its internal conditions. Further, the internal conditions are usually remarkably stable and predictable. This stable state is termed the "set point." For example, the set points for blood pH and body temperature are 7.4 and 37 C, respectively, and they vary little from these values. The ability/result of an individual to maintain a constant internal environment within tolerable limits is termed homeostasis. Or in other words, maintaining a "constant internal milieu" in light of environmental fluctuations.
Homeostasis literally translates into "same standing."
Homeostasis occurs at various levels of biological organization (e.g., cell, organismal).
B. Control Mechanism
The basic regulatory system that allows an animal (or plant) to respond to its environment can be diagrammed: signal (or stimulus) → sensor (or receptor) → control center or integrator → effector → response
To summarize, a signal of some sort is received by a sensor, which in turn, relays the message to a control center that selects an appropriate response that is carried out by the effector.
Lets use temperature as an example. Imagine the thermostat in your home that is set for a comfortable 21 C. A thermostat (sensor) monitors the temperature (signal). The thermostat has a mechanism (integrator = control center) for turning off/on the air conditioner or furnace (effectors) as needed. Thus, if it is a hot day and the temperature in your home increases above the set point (i.e., 21 C), then the thermostat switches on the air conditioner (and the furnace off). Once the temperature drops, the thermostat turns off the air conditioner. This results in a relatively constant temperature that will fluctuate slightly around the set point since it takes the system a brief period to respond to the temperature changes. see diagram in class
This basic system is at the heart of many homeostatic regulatory systems in animals including those for temperature, weight regulation, and blood glucose.
II. Glucose Homeostasis (covered during digestion unit)
Our body maintains a fasting blood glucose concentration (set point) of approximately 70-100 mg glucose 100 mL-1. This is measured by a fasting blood test. If the level increases (hyperglycermia) or decreases (hypoglycemia) much below this level, regulatory mechanisms kick in to return the glucose level to the set point.
B. Mechanism - in class we will diagram how this system works. The basics:
C. When the system goes awry = Diabetes mellitus
III. Temperature Regulation
A. Life and Temperature
insert: graphs of body temp vs. external temp; and metabolic rate vs. extermal temp for mouse and lizard
Take Home lessons from the Graphs:
C. Mechanism of Thermoregulation
Organisms modify behavior to cool/heat body as necessary (e.g., lizards bask in sun, hide in burrow; elephants shower with water, wallow in mud/water; bees huddle in cold)
2. Body Morphology
Environmental temperature has directed evolution of body shape/size. Two major concerns:
- surface - to - volume ratios have a major effect of size, shape (flattening, stretching, cube vs. sphere vs. filament) - check out our first lecture.
- adaptations: (1) insulation - fur, feathers, fat; (2) countercurrent exchange - to move something from one pipe to another, the most efficient way is for the pipes to run in opposite directions because the diffusion gradient between the incoming and outgoing pipes never become equal. If the pipes run in the same direction, then the become equal when the incoming pipe has released 50% of its load.
3. Physiological Adaptations
- iguana - brings blood to surface, adjusts heart rate
- cold vs. hot fish - countercurrent mechanism
- increase respiratory activities (shivering, fluttering wings, muscles)
- brown fat - rich in blood vessels & mitochondria; thermogenin uncouples ATP production from electron transport - Gink & Go at the Ski Resort
D. Vertebrate Temperature Regulation
If we get too hot, blood temperature (signal) is monitored by the hypothalamus (receptor), which triggers the anterior pituitary (control center) to send a signal to the sweat glands (effector) to "chill out" (i.e., start sweating and dilate blood vessels).
If we get too cold, blood temperature triggers the hypothalamus to send a signal to the nerves to tell the blood vessels to constrict, increase shivering, and increase the rate of metabolism (done through the pituitary and thyroid glands).
Note, feedback mechanisms are important aspects of regulatory control. Feedback can be positive (increase in the signal causes an increased response) or negative (increase in the signal causes a decrease in response).
Last updated: January 28, 2009 � Copyright by SG Saupe