Homeostasis

Definition

Regulating Blood Glucose LevelSource:

Raven P. and G. Johnson 1992 Biology. Third Ed. United States: Mosby Year Publishing

Homeostasis

• Mechanisms to ensure the maintenance of a constant internal condition

• “Temperature, pH, glucose concentration and oxygen must stay very constant for cell to function efficiently” (Raven, 1992)

• To maintain homeostasis the body must constantly monitor itself and act to correct any deviation = feedback loop

Feedback Loops• Feedback loops are important components

of the nerve impulse labor and delivery and resistance to disease

• Negative Feedback:– Changes that are detected and reversed

involves negative feedback mechanisms

• Positive feedback: – Occurs when a disturbance is accentuated

Negative Feedback • Prevents the departure of a controlled

variable, e.g glucose level, pH, temperature, blood pressure, from its normal value or setpoint

• In negative feedback the actual value of a controlled variable is continuously compared with the set point

• Changes in the controlled variable trigger responses that tend to oppose the change and restore the variable to its setpoint

Stimulus (input)

Receptor Integrator Effector

Response leads to change. Change is feedback. In negative feedback, the response of the system

cancels of counteracts the original stimulus

Positive Feedback• Set in motion a chain of events that intensify

an original condition, after a limited time, the intensification reverses the change

• Associated with instability in a system

Example of Positive FeedbackChildbirth (stimulus) fetus exerts pressure on the

wall of its mother’s uterus

Hypothalamus (integrator)

Production of oxytocin (effector)

Muscles contract exerting pressure on the fetus Fetus is expelled

Regulating Blood Glucose Level• Excess glucose is absorbed by liver cells

which convert it to glycogen• When blood glucose levels drop below

normal level (set point) the liver breaks down glycogen liberating glucose to the bloodstream

• Glucose level in the fluid surrounding cells changes little over time, even if the body’s intake of glucose is concentrated within a short period of time

P erip h era l N ervou s S ys tem

In ch arg e o fM oto r

F u n c tion s

C arry s ig n a lsto an d from

S ke le ta l M u sc les ,S k in an d ten d on s

S om atic S u b d ivis ion

Peripheral Nervous System

P arasym p ath e ticN erves

S ym p ath e tic N erves

V iscera l F u n c tion sN erves th a t

ca rry s ig n a ls to an d fromin te rn a l o rg an s

A u ton om ic S u b d ivis ion

The Endocrine System

• The endocrine system is a regulatory system that includes a set of glands that release chemical messengers -hormones- directly into body fluids.

• Neurosecretory cells in endocrine organs and tissues secrete hormones.

• Chemical signals bind to specific receptor proteins (plasma membrane proteins) within target cell or on their surface and initiate a metabolic pathway.

• Only target cells respond.

The Role of the Endocrine and Nervous System in Homeostasis

• Some animals have neurosecretory cells in their brain that secrete hormones into the blood.

• There are several chemicals in the body that serve as hormones of the endocrine system and as signals in the nervous system.– Epinephrine functions as the so-called “fight or

flight” hormone and as a neurotransmitter that transmits messages between neurons.

– Epinephrine is produced by the adrenal medulla, an endocrine gland. (Campbell et al, p.893-94)

Endocrine and Nervous System Interaction

Example 1: release of milkSuckling (stimulus)

stimulate sensory cells in the nipples

nervous signals to Hypothalamus

Hypothalamus stimulates the Pituitary gland

Pituitary releases Oxytocin

Example 2:

Ca level rises above set point (stimulus)

Thyroid gland releases calcitonin

Blood calcium level falls

If Ca level falls below setpoint the Parathyroid gland release PTH

Blood Ca level rises