Lesson 4 - Homeostasis, Part 1
Student Performance Objectives
1. Define homeostasis.
2. Explain the concept of the normal range of values for a population and for an individual.
3. Explain the importance of measuring parameters under standard (controlled) conditions, e.g.,
fasting, resting, and time of day.
4. Draw a diagram of a capillary bed labeling the following parts: arteriole end of the bed,
venule end of the bed, capillaries, blood, interstitial fluid, cells in the tissue space, lymphatic
channels, lymph, and intracellular fluid.
A. Homeostasis is the presence of constant satisfactory conditions within the body for continuation of life. For example the hovering of the body temperature around 37 degrees Celsius (98.6 degrees Fahrenheit) allows continuation of the body's normal metabolism. A temperature a few degrees above 37 is a fever; a few degrees below 37 may result in sluggishness or coma. Thirty-seven degrees is just right. Various physical and chemical parameters (e.g., temperature, or blood pressure) are often measured to assess the status of homeostasis. Homeostasis is maintained by, mostly, negative feedback mechanisms.
1. Physical parameters, e.g.: body temperature, blood pressure, heart rate, respiratory rate.
2. Chemical parameters, e.g.,
a. Blood levels of glucose, sodium, potassium, oxygen, carbon dioxide and hormones.
b. Urine levels of glucose, ketones, protein, and blood.
B. The normal range: All such parameters exist in each person within a range of values - the normal range for the individual. The normal range of values for a population of individuals is broader. For
example the normal population range for blood glucose is about 65-110 milligrams per 100 milliliters of blood (expressed as 65-119 mg/dl, where dl is a deciliter or 100 ml). In any individual, the daily variation in fasting blood glucose is more narrow, perhaps between 75-85 mg/dl.
1. Controlled food intake - measurements of values are most often taken after food intake has been controlled; the individual undergoes a fast for 8 or more hours depending on the tests to be performed.
2. Resting conditions - Some measurements may be made under resting conditions - e.g., heart rate (pulse).
3. Time of day - for some measurements, particularly hormonal, tests may be taken at a standard time of day since secretion from glands often follow daily (circadian) rhythms.
C. Hormonal and neural mechanisms are the main types that operate to maintain homeostasis. (Next lesson explores some mechanisms to maintain homeostasis).
D. A major arena for homeostasis: capillary beds. Hormones are secreted from cells in capillary beds and travel through blood to distant capillary beds where they influence cells in those beds.
E. The capillary bed consists of
1. Cells in the tissue spaces bathed in interstitial fluid.
2. Cells with intracellular fluid within them.
3. Nearby and surrounding capillaries containing blood with its liquid portion, the plasma.
4. Nearby lymphatic channels containing lymph.
F. Details of the relationships of these four fluids: intracellular, interstitial, plasma, and lymph, are taken up in the section on circulation.
G. Utilize this site for student practice labeling a capillary bed's parts: http://highered.mcgraw-hill.com/sites/0073031216/student_view0/exercise44/anatomy_of_a_capillary_bed.html
Normal range for an individual
Normal range for a population