Topic 16: Introduction to Human Biology

Biological Organization

- cells are the smallest unit of life

- cells are organized into tissues (epithelial, connective, muscle, nerve)

- tissues are organized into organs (eg.  stomach, heart) which contain different combinations of tissues

- organs organize to form organ systems (eg. digestive system, circulatory system)

- a collection of organ system forms a complex organism (eg. human)

Homeostasis

-  the maintenance of a constant internal environment through the coordinated activities of different organs. eg. maintaining a constant body temperature

- a homeostatic mechanism requires a sensor to detect changes in conditions and an effector mechanism that can vary the condition.  The connection between the two is called negative feedback.

Source:  http://xrislynn.blogspot.hu/2012/09/homeostasis.html

- eg. temperature regulation in endotherms:  body temperature drops (due to cold weather conditions), organism reacts by constricting peripheral blood vessels and shivering to produce extra heat (over a longer period, the animal may produce a thicker "winter" coat)

Source:  http://www.tokresource.org/tok_classes/biobiobio/biomenu/nerves_hormones_homeostasis/index.htm

- Here is another example of negative feedback

Source:  http://www.tokresource.org/tok_classes/biobiobio/biomenu/nerves_hormones_homeostasis/index.htm

-Positive feedback also exists.  In positive feedback systems, the effector enhances or increases the original stimulus.  One example is blood clotting (Bernat's favourite example).  When there is an injury to a blood vessel, platelets adhere to the site and release chemicals.  These chemicals attract more platelets, which release more chemicals, which attract more platelets until the break is completely sealed.