Homeostasis

Glossary Maintain – keep up. Constant – the same. Internal – inside the body. Environment – surroundings of the body.

What is Homeostasis? Body cells work best if they have the correct Temperature Water levels Glucose concentration Your body has mechanisms to keep the cells in a constant environment.

What is Homeostasis? The maintenance of a constant environment in the body is called Homeostasis

Homeostasis In order to maintain homeostasis the body uses feedback loops. There are 2 types of feedback loops: Negative Positive

Negative Feedback Negative feedback is when the response is opposite to the stimulus. This allows the body to stay constant (maintain homeostasis)

Graph of negative feedback

Negative Feedback A good example of negative feedback is the cruise control in a car. The cruise control has a sensor that senses the speed of the car as well as a control mechanism that processes the information from the sensor. It then adjusts the speed of the car by manipulating the accelerator. Okay but where is the negative part?

Negative Feedback Continued If I go downhill the car naturally speeds up a bit. The sensor senses this and the controller decreases the speed by easing up on the accelerator. Can you see that the response (slowing down) is opposite to the stimulus (speeding up)?

Negative Feedback Continued Likewise as the car goes uphill and slows down a bit. The cruise control works the accelerator to increase the speed. Again the response (speeding up) is opposite of the stimulus (slowing down). This is negative feedback in action.

Negative Feedback Loop: Home heating system Thermostat is set at a desired temp. Furnace turns on and heats the house to desired temp When desired temp is reached the furnace turns off. The house cools & temp drops below the desired temp. the furnace turns on & the house warms back up to desired temp.

Controlling body temperature All mammals maintain a constant body temperature. Human beings have a body temperature of about 37ºC. E.g. If your body is in a hot environment your body temperature is 37ºC If your body is in a cold environment your body temperature is still 37ºC

What mechanisms are there to cool the body down? Sweating When your body is hot, sweat glands are stimulated to release sweat. The liquid sweat turns into a gas (it evaporates) To do this, it needs heat. It gets that heat from your skin. As your skin loses heat, it cools down.

Sweating The skin

What mechanisms are there to cool the body down? Vasodilation Your blood carries most of the heat energy around your body. There are capillaries underneath your skin that can be filled with blood if you get too hot. This brings the blood closer to the surface of the skin so more heat can be lost. This is why you look red when you are hot!

This means more heat is lost from the surface of the skin If the temperature rises, the blood vessel dilates (gets bigger).

What mechanisms are there to warm the body up? Vasoconstriction This is the opposite of vasodilation The capillaries underneath your skin get constricted (shut off). This takes the blood away from the surface of the skin so less heat can be lost.

This means less heat is lost from the surface of the skin If the temperature falls, the blood vessel constricts (gets shut off).

What mechanisms are there to warm the body up? Body Hair When the hairs on your skin “stand up” . It is sometimes called “goose bumps” The hairs trap a layer of air next to the skin which is then warmed by the body heat The air becomes an insulating layer.

Controlling Glucose levels Your cells also need an exact level of glucose in the blood. This is regulated by 2 hormones (chemicals) from the pancreas called: Insulin Glucagon

Glucose levels rise after a meal. Insulin is produced and glucose levels fall to normal again. Glucose Concentration Normal Time Meal eaten

Meal eaten Glucose levels rise after a meal. Diabetic Concentration Diabetic Insulin is not produced so glucose levels stay high Time Meal eaten

Positive Feedback Mechanisms Positive feedback is where the response is the same as the stimulus. In positive feedback the response can be magnified.

Positive Feedback Mechanisms A good example of positive feedback is the feedback you hear from sound systems in concerts. In this example the stimulus (sound going into microphone) is processed to produce a magnified response (sound coming out of the speakers). Sometimes the microphone picks up sound from the speakers and continues to magnify it until it is out of control (the feedback that hurts your ears).

Positive Feedback One example of a biological positive feedback loop is the onset of contractions in childbirth. When a contraction occurs, the hormone oxytocin is released into the body, which stimulates further contractions. This results in contractions increasing in amplitude and frequency. http://en.wikipedia.org/wiki/Positive_feedback_loop#In_biology

Positive Feedback Graph http://en.wikipedia.org/wiki/Positive_feedback_loop#In_biology

Positive Feedback Another example is blood clotting. The loop is initiated when injured tissue releases signal chemicals that activate platelets in the blood. An activated platelet releases chemicals to activate more platelets, causing a rapid cascade and the formation of a blood clot. http://en.wikipedia.org/wiki/Positive_feedback_loop#In_biology