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Homeostasis

Homeostasis keeps your body's internal environment stable and balanced. Understanding homeostasis helps us see how various body systems work together to maintain health and adapt to changes. Use this resources to explore how your body achieves stability and responds to challenges.

The hypothalamus.
Hypothalamus, by Servier Medical Art via Wikimedia Commons, licensed under CC BY 4.0

Homeostasis ("homeo" means "similar to" and "stasis" means "standing still") is the body's way of maintaining a stable internal environment despite external changes. It does this by correcting any imbalances in conditions.

The steps involved in regulating homeostasis are the same regardless of what conditions are being maintained:

  1. The stimulus (change in conditions) is detected by a sensor, such as a temperature sensor on the skin or a chemical receptor that detect the pH of blood.
  2. The sensors send signals to the hypothalamus (the "control centre" for homeostasis) or other regulatory organs which coordinate a response.
  3. Effector organs receive signals from the hypothalamus and carry out the response.

Depending on whether the response is to decrease the effect of the change or to increase it, a negative or feedback loop may be involved.

Flow charts depicting negative and positive feedback loops.

Feedback loops

Two flowcharts showing the negative and positive feedback loops.

On the left is the negative feedback loop. It shows the stimulus, then the sensor, the control centre and finally, the effector. An inhibitory arrow is shown from the effector to the stimulus. This indicates that the effector decreases the effect of the change.

On the right is the positive feedback loop. It also shows the same components, but with a regular arrow pointing from the effector to the stimulus. This indicates that the effector increases the effect of the change.

Negative feedback loops

One way that homeostasis is maintained is by negative feedback loops. Negative feedback loops reverse changes in the body’s internal environment. When a change occurs, these loops act to bring conditions back to their set points.

Some examples of internal conditions regulated by negative feedback loops are:

  • blood pressure
  • body temperature
  • blood sugar levels
  • fluid balance.

We will look at body temperature and blood glucose levels in more detail.

Regulating internal body temperature

Negative feedback loops help to keep your internal body temperature close to \(37^{\circ}\textrm{C}\) whether you're enjoying a topical holiday in \(40^{\circ}\textrm{C}\) heat or braving the frosty \(6^{\circ}\textrm{C}\) on your commute to uni one winter's morning. This process is called thermoregulation.

  1. Temperature sensors on the skin detect the increase in temperature on the outside of the body. Temperature sensors in the hypothalamus detect the increase in internal body temperature.
  2. This information is collected and processed by the hypothalamus.
  3. The hypothalamus sends signals to:
    • the sweat glands to produce more sweat, which evaporates from the skin to dissipate heat
    • cause vasodilation (the widening of blood vessels) so that heat can be lost more easily
    • the adrenal glands and thyroid to reduce metabolic rate, as metabolism generates heat
    • other regions of the brain to influence our behaviours, like making us move less, remove clothing, or decreasing our appetite.

  1. Temperature sensors on the skin detect the decrease in temperature on the outside of the body. Temperature sensors in the hypothalamus detect the decrease in internal body temperature.
  2. This information is collected and processed by the hypothalamus.
  3. The hypothalamus sends signals to:
    • the cause vasoconstriction (the narrowing of blood vessels) so that less heat is lost
    • increase the production of thyroid hormones to increase metabolic rate
    • the skeletal muscle to contract causing shivering, which produces heat
    • other regions of the brain to influence our behaviours, like making us move more, adding clothing, or increasing our appetite.

Regulating blood glucose levels

Glucose is metabolised by the body and converted into ATP, a source of energy, but the levels of glucose in the blood must be carefully maintained because fluctuations can lead to serious health issues.

Normal blood glucose levels should be between \(60\textrm{ mg/dL}\) and \(140\textrm{ mg/dL}\). To coordinate this balance, the pancreas acts as the control centre.

  1. Blood glucose sensors on beta cells in the pancreas detect an increase in blood glucose levels.
  2. The pancreas releases more insulin and less glucagon.
    • Insulin acts on cells in the liver and muscles to store glucose as glycogen. This process is called glycogenesis.
    • Insulin acts on cells in the fat tissues to store glucose as triglycerides.
    • Insulin promotes uptake of glucose into cells where they can be used for energy.
  3. Blood glucose levels decrease.

  1. Blood glucose sensors on alpha cells in the pancreas detect a decrease in blood glucose levels.
  2. The pancreas releases less insulin and more glucagon.
    • Glucagon acts on cells in the liver and muscles to break down stored glycogen into glucose, a process called glycogenolysis.
    • Glucagon causes the liver to make more glucose. This is called gluconeogenesis (where "neo" means "new").
    • Glucagon acts on cells in the fat tissues to break down triglycerides.
  3. Blood glucose levels increase.

Positive feedback loops

Did you know?

Positive feedback loops help your stomach digest a big meal. When the food arrives, protein activates a digestive enzyme called pepsin, which automatically activates even more pepsin to quickly flood your stomach and break down your dinner faster.

Positive feedback loops are less common than negative feedback loops. They involve further amplifying the change and bringing the body system further away from homeostasis.

Examples are:

  • childbirth
  • lactation
  • blood clotting
  • the menstrual cycle.

  1. During labour, contractions of the uterus push the baby's head against the opening of the cervix (the lowest part of the uterus). The increase in pressure is detected by sensory neurones in the cervix.
  2. Messages are sent to the pituitary gland in the brain.
  3. The pituitary gland produces oxytocin, which causes stronger contractions of the uterus.

  1. When a blood vessel is damaged, platelets stick to the vessel where it is damaged.
  2. The platelets release chemicals that attract more platelets to the wound.
  3. The platelets stick to the vessel and continue to signal the recruitment of more platelets until a plug forms over the damaged area.

Exercise

See how well you understand homeostasis with a quick quiz.

A blood glucose monitor.
Blood glucose monitor, by isens usa via Unsplash

Data drill

Read the scenario and use the information provided to answer the questions in the quiz.

Students enrolled in the Diploma of Nursing at RMIT are completing a course on caring for patients with diabetes. As part of their practical lab exercises, they study the body's response to glucose intake.

They analyse the blood glucose levels of three healthy volunteers who consume a standard meal. The data is collected at regular intervals over \(150\) minutes and recorded in the following table.

Time Blood glucose, Participant 1 Blood glucose, Participant 2 Blood glucose, Participant 3
\(0\textrm{ min}\) \(90\textrm{ mg/dL}\) \(85\textrm{ mg/dL}\) \(92\textrm{ mg/dL}\)
\(30\textrm{ min}\) \(140\textrm{ mg/dL}\) \(150\textrm{ mg/dL}\) \(145\textrm{ mg/dL}\)
\(60\textrm{ min}\) \(130\textrm{ mg/dL}\) \(135\textrm{ mg/dL}\) \(128\textrm{ mg/dL}\)
\(90\textrm{ min}\) \(110\textrm{ mg/dL}\) \(120\textrm{ mg/dL}\) \(115\textrm{ mg/dL}\)
\(120\textrm{ min}\) \(100\textrm{ mg/dL}\) \(105\textrm{ mg/dL}\) \(102\textrm{ mg/dL}\)
\(150\textrm{ min}\) \(95\textrm{ mg/dL}\) \(100\textrm{ mg/dL}\) \(97\textrm{ mg/dL}\)

Further resources

Homeostasis simulation

See it in action! Use this interactive to see how different behaviours and biological responses help to maintain homeostasis for a person running on a treadmill.

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