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

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 the homeostatic mechanisms they have evolved are.

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 some way (usually by nerves or hormones) to mechanisms capable of correcting changes in the particular state (ie: warm the body temperature). 3. A linked seriesof 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 the furnace at home 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

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.

Positive Feedback Loops Positive feedback is a mechanism that brings about an ever greater change in the SAME DIRECTION. This type of loop helps the body to complete a process that has a definite cut off point, such as: a. Blood clotting b. Digesting proteins in the stomach c. Birth d. Body temperature during a fever The feedback loop keeps going until the stimulus that initiated the loop stops.

Positive Feedback Loops Ex: during a fever, a presence of a pathogen is the stimulus that starts the positive feedback loop. Chemicals called pyrogens are released by the body These cause the body temperature to increase White blood cells work better under warmer temperatures, so they can do a better job of killing off the pathogen Pyrogens will keep being released (and body temperature keeps rising) until the stimulus (pathogen) is gone. Note: your body will also keep doing negative feedback at the same time, which is why you sweat so much when you have a fever: negative feedback is trying to cool you down!!