Lesson 5 - Homeostasis, Part 2
Student Performance Objectives
1. Define negative feedback.
2. Use 2 examples from the human body to explain how homeostasis is maintained mainly by
negative feedback mechanisms.
3. Explain the importance of positive feedback mechanisms in the normal functioning of the
human body and give 2 examples.
A. Review of homeostasis definition: constant satisfactory conditions for life. Homeostasis is maintained by negative feedback mechanisms.
B. Negative feedback mechanisms - maintain homeostasis. Negative feedback mechanisms return physiological parameters back to the normal range after they have left the normal range. You may read another explanation of negative feedback in the Endocrine System chapter (unit 13).
1. Body temperature regulation - hypothalamic regulatory mechanism. Review the mechanism of
sweating to cool the body, along with increased circulation of blood to the dermis which causes the radiation of heat from the body surface. To retard heat loss, the body decreases blood circulation to the skin and sweating (but not perspiration!) is slowed.
2. Blood glucose regulation - pancreatic regulation. The islets of Langerhans in the pancreas are able to secrete the hormone insulin to facilitate glucose uptake by cells which reduces circulating blood glucose after a meal. Other islet cells are able to secrete the hormone glucagon to increase breakdown of liver glycogen which increases circulating blood glucose when one is not eating. In both situations - eating or fasting - the blood glucose leaves the normal range. The pancreatic mechanism returns the blood glucose to the normal range.
3. Blood calcium regulation - parathyroid gland regulation. If the blood calcium levels drops as might occur in a pregnant woman whose baby is removing calcium from her blood to build up its own skeletal system, the parathyroid glands detect the drop and secrete parathormone. This hormone causes increased absorption of calcium from food in the digestive tract, increased absorption of calcium from fluids in the kidneys, and an increased breakdown of calcium from the mother's bones. All these functions will increase the mother's blood calcium level back up to the normal range.
4. Negative feedback concept - note that the return to the normal range from a higher or lower value is still a negative feedback process. E.g., sweating to lower body temperature or shivering to raise it are both part of negative feedback mechanisms since they return the parameter, body temperature, to the normal range.
C. Positive feedback mechanisms - Positive feedback mechanisms intensify physiological activities until some goal is attained. The physiological activity can then diminish to the normal range.
1. Birth of a baby - contractions of the uterine body result in stretching of the uterine cervix and this results in even greater contractions of the uterine body and so on eventually leading to the baby's birth. Then decreasing levels of uterine contractions occur after the baby is born and uterine stretching ceases. Imagine if the uterine contractions operated on a negative feedback principle: this would mean that when the uterine body contracted and stretched the cervix, then the uterine body would be inhibited from further contraction - the baby would never be born!
2. Lactation - the more a baby nurses, the more milk is produced by the mammary glands over a
period of days and weeks. Eventually when the baby is weaned, the less the baby nurses, the less milk that will be produced.
3. Force of contraction of a healthy heart following Starling's Law: when more blood returns to the heart, more blood is pumped out of the heart. If less blood returns to the heart, the heart pumps out less. This underlies the concept of "shock". If you have internal or external bleeding, for example, each beat of the heart results in blood exiting the circulatory system. The result is less and less blood returning to the heart as less and less blood is pumped out of the heart.
Islets of Langerhans