Hormones & Homeostasis Endocrine System Hormones & Homeostasis 2009-2010

Homeostasis Homeostasis maintaining internal balance in the body organism must keep internal conditions stable even if environment changes also called “dynamic equilibrium” example: body temperature humans: too cold = shiver too warm = sweat lizard: too cold = bask in sun too warm = hide in shade

Regulation How we maintain homeostasis nervous system endocrine system nerve signals control body functions endocrine system hormones chemical signals control body functions

Controlling Body Temperature Nervous System Control Feedback Controlling Body Temperature nerve signals brain sweat dilates surface blood vessels high body temperature low constricts surface blood vessels shiver brain nerve signals

Hormones Why are hormones needed? chemical messages from one body part to cells in other parts of body communication needed to coordinate whole body maintaining homeostasis growth hormones

Endocrine System Endocrine system releases hormones glands which secrete (release) chemical signals into blood chemicals cause changes in other parts of body growth hormones sex hormones response hormones metabolism hormones and more…. Hormones coordinate slower but longer–acting responses to stimuli such as stress, dehydration, and low blood glucose levels. Hormones also regulate long–term developmental processes by informing different parts of the body how fast to grow or when to develop the characteristics that distinguish male from female or juvenile from adult. Hormone–secreting organs, called endocrine glands, are referred to as ductless glands because they secrete their chemical messengers directly into extracellular fluid. From there, the chemicals diffuse into the circulation.

Responding to hormones Lock and key system hormone fits receptor on “target” cell target cell secreting cell non- target cells can’t read signal can’t read signal

Glands Pineal Pituitary Thyroid Adrenal Pancreas Ovary Testes melatonin Pituitary many hormones: master gland Thyroid thyroxine Adrenal adrenaline Pancreas insulin, glucagon Ovary estrogen Testes testosterone

Positive vs. Negative Feedback Positive- Causes the variable to accelerate the change in the same direction of the initial disturbance. or Negative- Causes the variable to change in a direction opposite to that of the initial change.

Maintaining homeostasis Feedback Maintaining homeostasis hormone 1 gland lowers body condition high specific body condition low raises body condition gland hormone 2

specific body condition Negative Feedback Response to changed body condition if body is high or low from normal level signal tells body to make changes that will bring body back to normal level once body is back to normal level, signal is turned off hormone 1 gland lowers body condition high specific body condition

Regulation of Blood Sugar Endocrine System Control Feedback Regulation of Blood Sugar insulin body cells take up sugar from blood liver stores sugar reduces appetite pancreas liver high blood sugar level (90mg/100ml) low liver releases sugar triggers hunger pancreas liver glucagon

Positive or Negative Feedback? Oxytocin Production during birth Baby pushes on the cervix Nervous system sends a signal to the hypothalamus Hypothalamus manufactures oxytocin Oxytocin is transported to the posterior pituitary gland and released Oxytocin stimulates uterine contraction Loop stops when the baby leaves the birth canal

Positive or Negative Feedback? Thyroid Gland and TRH production Hypothalamus senses that the thyroxine level in the blood is low Hypothalamus secretes thyrotropin-releasing hormone (TRH) TRH stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH) TSH stimulates the release of thyroxine by the thryoid gland High levels of thyroxine in the blood inhibit the secretion of TRH and TSH (stops the release of additional thyroxine) *This loop keeps the level of thyroxine in the blood relatively constant.

Feedback Suppose the secretion of a certain hormone causes an increase in the concentration of substance X in the blood. A low concentration of X causes the hormone to be released. What is the effect on the rate of hormone secretion if an abnormal condition causes the level of X in the blood to remain very low?