HOMEOSTASIS

2nd Law of Thermodynamics One of the fundamental laws of nature. It states that the universe and everything in it is always moving in the direction of increasing disorder Any Other Examples

2nd Law of Thermodynamics Yet, life appears to be the exact opposite of this, for life is very organized and living organisms maintain that organization until they die. This is this property that separates life from non-life.

HOMEO – THE SAME, STASIS – TO STAY The ability of life to maintain a constant, stable, and organized internal environment is called HOMEOSTASIS. Along with evolution, this is one of the guiding principles of modern biology.

Even the simplest life form, an amoeba, maintains a distinctly ordered internal environment separate from its external environment. Why? The more complex the life form, the more elaborate and efficient are the homeostatic mechanisms they have evolved.

In humans, we have (as much as possible) physically separated all of our cells from the external environment; thus, improving the quality of our ‘internal seawater’. We have evolved ‘physiology’ which allows us to monitor and adjust our internal environment within very narrow parameters.

Homeostatic Mechanisms Sensitive receptors monitor each condition under homeostatic control For example: Temperature @ 37oC Blood pressure @ 120/80 Sugar levels @ 0.07% Osmolarity (salt levels) @ 0.1M pH @ 7

2. These monitors are linked in someway (usually by nerves or hormones) to mechanisms capable of correcting changes in the particular state (ie: warm the body temperature). 3. A linked series (of receptors and corrective mechanisms) is called a FEEDBACK LOOP.

Feedback Loops There are two types of feedback loops: Negative Feedback Loops Positive Feedback Loops

Negative Feedback Loops This type of a feedback loop occurs where the output from some body system inhibits itself (shuts itself off) at a certain set point. Example: with both a thermostat in furnace or in the brain (hypothalamus) When the heat increases, the furnace is shut off When the heat decreases, the furnace is turned on In humans temperature homeostasis is controlled by the thermoregulatory centre in the hypothalamus. It receives input from two sets of thermoreceptors: receptors in the hypothalamus itself monitor the temperature of the blood as it passes through the brain (the core temperature), and receptors in the skin monitor the external temperature. Both pieces of information are needed so that the body can make appropriate adjustments. The thermoregulatory centre sends impulses to several different effectors to adjust body temperature:

The hypothalamus maintains a set point of 37. 5 ± 0 The hypothalamus maintains a set point of 37.5 ± 0.5 °C in most mammals. The temperature is regulated within a range determined by a ‘high’ and ‘low’ set point (± 0.5 °C ) Whenever a change occurs in a system, the change automatically causes a corrective mechanism to start, which reverses the original change and brings the system back to normal.

How It Works STIMULUS

How It Works STIMULUS

How It Works STIMULUS RECEPTOR

How It Works STIMULUS RECEPTOR

How It Works STIMULUS RECEPTOR REGULATORY CENTER

How It Works STIMULUS RECEPTOR REGULATORY CENTER

How It Works STIMULUS RECEPTOR REGULATORY CENTER EFFECTOR

How It Works STIMULUS RECEPTOR REGULATORY CENTER EFFECTOR

How It Works STIMULUS RECEPTOR REGULATORY CENTER RESPONSE EFFECTOR

How It Works STIMULUS RECEPTOR REGULATORY CENTER RESPONSE EFFECTOR

How It Works STIMULUS RESPONSE EFFECTOR REGULATORY CENTER RECEPTOR

STIMULUS: Body Temperature rises above 37oC How It Works: an example STIMULUS: Body Temperature rises above 37oC

STIMULUS: Body Temperature rises above 37oC How It Works: an example STIMULUS: Body Temperature rises above 37oC

STIMULUS: Body Temperature rises above 37oC RECEPTOR: thermoreceptor How It Works: an example STIMULUS: Body Temperature rises above 37oC RECEPTOR: thermoreceptor

STIMULUS: Body Temperature rises above 37oC RECEPTOR: thermoreceptor How It Works: an example STIMULUS: Body Temperature rises above 37oC RECEPTOR: thermoreceptor

How It Works: an example STIMULUS: Body Temperature rises above 37oC RECEPTOR: thermoreceptor REGULATORY CENTER: hypothalamus

How It Works: an example STIMULUS: Body Temperature rises above 37oC RECEPTOR: thermoreceptor REGULATORY CENTER: hypothalamus

How It Works: an example STIMULUS: Body Temperature rises above 37oC RECEPTOR: thermoreceptor REGULATORY CENTER: hypothalamus EFFECTOR: smooth muscle in arterioles, sweat glands, muscles in skin, skeletal muscles, and thyroid gland

How It Works: an example STIMULUS: Body Temperature rises above 37oC RECEPTOR: thermoreceptor REGULATORY CENTER: hypothalamus EFFECTOR: smooth muscle in arterioles, sweat glands, muscles in skin, skeletal muscles, and thyroid gland

How It Works: an example STIMULUS: Body Temperature rises above 37oC RECEPTOR: thermoreceptor REGULATORY CENTER: hypothalamus RESPONSE: arterioles dilate (blood moves to extremities), sweat, thyroid gland decreases metabolism EFFECTOR: smooth muscle in arterioles, sweat glands, muscles in skin, skeletal muscles, and thyroid gland

How It Works: an example STIMULUS: Body Temperature rises above 37oC RECEPTOR: thermoreceptor REGULATORY CENTER: hypothalamus RESPONSE: arterioles dilate (blood moves to extremities), sweat, thyroid gland decreases metabolism EFFECTOR: smooth muscle in arterioles, sweat glands, muscles in skin, skeletal muscles, and thyroid gland

How It Works: an example STIMULUS: Body Temperature rises above 37oC RECEPTOR: thermoreceptor REGULATORY CENTER: hypothalamus RESPONSE: arterioles dilate (blood moves to extremities), sweat, thyroid gland decreases metabolism EFFECTOR: smooth muscle in arterioles, sweat glands, muscles in skin, skeletal muscles, and thyroid gland

FEVERS Chemicals called pyrogens are released by white blood cells to raise the set point. This causes body temp. to increase by 2-3 °C. This helps to kill bacteria (+ white blood cells work best at this temperature) and explains why you shiver even though you are hot.

Hibernation Some mammals release hormones that reduce their set point to around 5°C while they hibernate. This drastically reduces their metabolic rate and so conserves their food reserves. http://www.youtube.com/watch?v=TpY4frpheWw

Another Example: Blood Sugar Control

Positive Feedback Loops This type of a feedback loop occurs when the product of one pathway stimulates or causes another pathway to increase activity. Example: Labour (the pressure of the babies head on the uterus) stimulates the pituitary gland in the brain to release the hormone oxytocin. The oxytocin causes the uterus to contract, which leads to increased pressure. Contractions cause the pituitary to produce more oxytocin and the cycle continues until you give birth.