Paul Andersen reviews the major concepts within the fourth unit of the new AP Biology framework. He begins by differentiating between negative and positive feedback loops. He explains how a stable internal environment is maintained through behavior and physiology. He details thermoregulation, osmoregulation and blood glucose regulation. He also describes how natural selection has shaped behavior.

 

Paul Andersen explains how hugs between tissues can help maintain homeostasis. Countercurrent heat exchange allows heat to stay within the core of the body. Close contacts between the capillaries and alveoli allow our body to maintain the correct concentration of oxygen. Capillaries also hug the tubules in the nephron to maintain osmolarity and filter the blood. Hugs in the gills of a fish allow them to maintain an adequate amount of oxygen in an oxygen-poor environment.

Paul Andersen defines the major elements of feedback loops. The receptors and effectors both sense and respond to changes in their environment. The following examples are used to illustrate the importance of feedback loops in maintaining homeostasis: speed signs, thermostats, thermoregulation, and blood glucose maintenance.