1.4 describe homeostasis as the process by which organisms maintain a relatively stable internal environment

HomeostasisHomeostasis

Homeostasis (from the Greek word homoios, meaning like or the same, and stasis, meaning state) is the process by which organisms maintain a relatively stable internal environment regardless of external environmental change, and is the central theme of the topic “Maintaining a Balance”. Maintenance of a constant internal environment is important for optimal metabolic efficiency by allowing enzymes to function at optimal conditions (pH, temp, substrate concentration etc).

Such self-regulation also occurs in non-living systems eg air-conditioner thermostats maintain constant air temperature in a room. 1.5 explain that homeostasis consists of two stages:

- detecting changes from the stable state- counteracting changes from the stable state

The two stages of HomeostasisThe two stages of Homeostasis

In order to maintain a constant internal environment, the following two steps are essential:

1. Detecting change from the stable state: sensory cells (receptors) detect change in the internal environment (stimuli).

2. Counteracting change to bring body back to stable state: effector organs (such as muscles or glands) then work to reverse the change, initiating a response that that will return the body to homeostasis – its relatively constant state.

Conditions controlled by homeostasis include:Body temperature, pH, water concentration, salt concentrations, sugar levels, levels of dissolved gases eg oxygen and carbon dioxide.

Types of FeedbackTypes of Feedback

Positive FeedbackPositive Feedback

In positive feedback the body responds to an extreme condition by promoting the current direction of change (ie more of the same). Positive feedback mechanisms are uncommon in the body, since most bodily functions rely on maintaining levels within the normal range whereas positive feedback is designed to push levels outside the normal range entirely.

For example, during pregnancy as the fetus grows larger the head places pressure on the cervix. Rather than trying to alleviate this pressure (ie, achieve homeostasis), the brain is stimulated to produce oxytocin which causes the uterus to contract. In this example, positive feedback is required to allow for a "relatively" quick childbirth. Slow births are very stressful to both the mother and fetus so should be avoided.

Another example of positive feedback in the body is blood platelet accumulation, which, in turn, causes blood clotting in response to a break or tear in the lining of blood vessels.

Negative FeedbackNegative Feedback

Homeostasis is maintained as long as there is only a narrow range of fluctuation around the set point. In a negative feedback system, fluctuations in excess of the normal range trigger changes to counteract (negate) the change ie the body returns to within normal limits – a state of homeostasis.

1.6 outline the role of the nervous system in detecting and responding to environmental changes

Homeostasis and the Nervous SystemHomeostasis and the Nervous System

Homeostasis usually involves a feedback mechanism in which the response is monitored:

situation monitored changes made situation reassessed changes made

This negative feedback system is coordinated by the nervous system using a stimulus-response pathway, in which a stimulus is detected by a receptor, a message is carried by nerves to a control centre and a response is triggered:

receptorstimulus Control centre effectors

response

Detecting change: receiving stimuliDetecting change: receiving stimuliSensory cells called receptors detect stimuli (changes in the internal or external environment). In their simplest form, receptors consist of single cells, scattered over the body of the organism. In their most complex form, receptors become concentrated in particular organs to form sense organs. It is the interorecepors (internal receptors) within the body, though, that are important in detecting changes related to homeostasis – that is, internal stimuli such as changes in pH, body temperature, osmotic pressure and the chemical composition of blood.

Receptors may be named according to the type of energy to which they respond. They include thermoreceptors (which detect internal changes in temperature) and chemoreceptors (which detect the concentration of certain chemicals inside the body, eg carbon dioxide levels in the blood).

The stimulus-response pathway:The stimulus-response pathway:

change in environment stimulistimuli loud noiseloud noisedetected by

sensory cells in sense organ receptorsreceptors hair cells in earhair cells in earconvert stimuli to electrical impulses

sensory nerve carrying nerve impulses messengersmessengers auditory nerveauditory nerve

transmit impulses

brain and spinal cord CNSCNS brainbrainprocess information and trigger new impulses

motor nerve carrying nerve impulses

messengersmessengers

motor nervesmotor nerves

transmit impulsesmuscles or glands effectorseffectors muscles in neckmuscles in neck

react

reaction responseresponse head jerks and looks backhead jerks and looks back

1b gather, process and analyse information from secondary sources and use available evidence to develop a model of a feedback mechanism

Use the information below, and your own experiences, to draw a “figure of eight” feedback mechanism for thermoregulation in humans similar to that shown for control of blood glucose levels shown on the next two pages.

Draw a “figure of eight” feedback model to show the following steps in the control of blood glucose levels:Note: Normal BGL if 90 mg/100 mL

1. High blood glucose levels cause the Beta cells in the pancreas to be stimulated.2. Beta cells in the pancreas secrete insulin.3. Insulin causes the uptake of glucose by cells. Conversion of glucose to stored glycogen or fat in the liver, causing

a decrease of blood glucose levels.4. Low blood glucose levels cause the alpha cells in the pancreas to be stimulated.5. Alpha cells in the pancreas secrete glucagon. 6. Glucagon causes the breakdown of glygogen to glucose in the liver, causing a release of glucose into the blood.

1. High blood glucose levels cause the Beta cells in the pancreas to be stimulated.

2. Beta cells in the pancreas secrete insulin.

3. Insulin causes the uptake of glucose by cells. Conversion of glucose to stored glycogen or fat in the liver, causing a decrease of blood glucose levels.

4. Low blood glucose levels cause the alpha cells in the pancreas to be stimulated.

5. Alpha cells in the pancreas secrete glucagon.

6. Glucagon causes the breakdown of glygogen to glucose in the liver, causing a release of glucose into the blood.

Thermoregulation:Thermoregulation:

Draw a “figure of eight” feedback model to show the mechanisms that achieve the effects shown in the graph above.

Later in this topic we will look at the ways in which your kidneys help maintain homeostasis by adjusting water and salt balance using the action of two hormones: ADH (anti-diuretic hormone aka vasopressin) and aldosterone…

Anti-diuretic Hormone (ADH)Anti-diuretic Hormone (ADH)1. What happens to ADH levels when the blood water

concentration rises?

2. What happens to ADH levels when the blood water concentration falls?

3. What will happen to the concentration of your urine as ADH is released? What impact may this have on the colour of your urine?

4. Several factors affect the release of ADH.

a) Alcohol inhibits the release of ADH. Why does alcohol consumption cause dehydration and thirst?

b) Nicotine stimulates the release of ADH. What does this suggest about the urine of heavy smokers?

Complimentary processes regulate salt (action of aldosterone) and water balance (action of vasopressin) in your kidneys