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Lesson 2: Homeostasis

Birds balancing on a branch

Homeostasis

By the end of this lesson, students will be able to:

  • Define homeostasis and describe the multiple levels of homeostatic maintenance in physiology.
  • Compare and contrast positive and negative feedback regarding the relationship between stimulus and response.

Introduction

Homeostasis is the body’s ability to maintain stable internal conditions despite external changes. This dynamic equilibrium is essential for survival, regulating critical physiological parameters such as temperature, pH, and glucose levels. The human body utilizes intricate mechanisms, primarily governed by the nervous and endocrine systems, to achieve homeostasis.

Regulators of Homeostasis

The two primary systems responsible for homeostasis are:

  • Nervous System: Provides rapid responses through nerve impulses.
  • Endocrine System: Regulates physiological processes via hormones.
This is a bidirectional relationship that is strongly influenced by multiple pathways, including the autonomic nervous system (ANS), enteric nervous system (ENS), hypothalamic–pituitary–adrenal (HPA), immune pathways, endocrine pathways, and neural pathways. (Suganya, Kanmani, and Byung-Soo Koo. 2020)
Figure 1. Schematic diagram showing the communication between the gut and brain. This is a bidirectional relationship that is strongly influenced by multiple pathways, including the autonomic nervous system (ANS), enteric nervous system (ENS), hypothalamic–pituitary–adrenal (HPA), immune pathways, endocrine pathways, and neural pathways. (Suganya, Kanmani, and Byung-Soo Koo. 2020)

Homeostatic Imbalance

When homeostasis is disrupted, it can lead to disease and dysfunction. A failure in homeostatic regulation may result from internal disorders or external environmental factors. Chronic imbalance can cause conditions such as diabetes, hypertension, and organ failure.

Main Symptoms of Diabetes
Figure 2. Main Symptoms of Diabetes. Overview of the most significant possible symptoms of Diabetes Mellitus.

Feedback Mechanisms

Homeostasis is maintained through feedback loops, which are categorized into negative and positive feedback mechanisms.

Negative Feedback Mechanisms

Negative feedback is the most common regulatory mechanism in the human body. It works by reversing the initial stimulus to maintain stability.

  • Example: Regulation of body temperature.
  • Example: Blood glucose regulation via insulin and glucagon secretion.
An example of a negative feedback loop is the process of increasing and decreasing glucose levels in out bloodstream
Figure 3. An example of a negative feedback loop. As we consume food, our glucose levels increase as the food is broken down causing the pancreas to release insulin. Insulin’s job is to have cells take in glucose and store it as glycogen. After this happens, less glucose is detected in our bloodstream. But if our glucose level decreases too much due to skipping a meal, the pancreas releases the glucagon hormone, which reverses insulin’s work by signaling the breakdown of glycogen to release back into the bloodstream as glucose.

Another example is blood pressure regulation through postural adjustments. When blood pressure drops due to standing up quickly, receptors signal the nervous system to constrict blood vessels and restore normal pressure.

Example of Negative Feedback: Blood Pressure
Figure 4. An example of Negative Feedback is blood pressure. A person sits up. Baroreceptors respond to a drop in blood pressure and signal to the cardiac center of the brain. The cardiac center accelerates the heart rate.

Positive Feedback Mechanisms

Unlike negative feedback, positive feedback enhances the original stimulus to drive a process to completion. This mechanism is less common in the body but plays crucial roles in specific physiological events.

  • Example: Blood clotting, where platelets continue to accumulate until a clot forms.
  • Example: Childbirth, where oxytocin increases uterine contractions until delivery.
This diagram shows the steps of a positive feedback loop as a series of stepwise arrows looping around a diagram of an infant within the uterus of a pregnant woman. Initially the head of the baby pushes against the cervix, transmitting nerve impulses from the cervix to the brain. Next the brain stimulates the pituitary gland to secrete oxytocin which is carried in the bloodstream to the uterus. Finally, the oxytocin simulates uterine contractions and pushes the baby harder into the cervix. As the head of the baby pushes against the cervix with greater and greater force, the uterine contractions grow stronger and more frequent. This mechanism is a positive feedback loop.
Figure 5. Example of positive feedback regulation. This diagram shows the steps of a positive feedback loop as a series of stepwise arrows looping around a diagram of an infant within a pregnant woman’s uterus. Initially, the baby’s head pushes against the cervix, transmitting nerve impulses from the cervix to the brain. Next, the brain stimulates the pituitary gland to secrete oxytocin, which is carried in the bloodstream to the uterus. Finally, the oxytocin stimulates uterine contractions and pushes the baby harder into the cervix. As the head of the baby pushes against the cervix with greater and greater force, the uterine contractions grow more substantial and more frequent. This mechanism is a positive feedback loop.

Summary

Homeostasis ensures that the body maintains balance through regulatory feedback systems:

  • Negative feedback is the primary mechanism, shutting off or reducing a stimulus to restore equilibrium.
  • Positive feedback amplifies a process until completion and is used in limited cases such as clotting and childbirth.
  • The nervous and endocrine systems coordinate these processes to sustain stability.

By understanding homeostasis, we gain insight into the body’s ability to self-regulate and the consequences of its disruption, reinforcing the importance of maintaining health and physiological balance.

Watch this lesson video walking you through the Module 1 Lesson 2 Homeostasis (PDF) slides.

Practice Questions

Use these practice questions to assess your knowledge before you move on to the next section.

License

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Introduction to Human Anatomy & Physiology: A Multilingual Approach Copyright © by Rachel Thwing; Hugh Jarrard; Ann DeChenne; Kiana Pigao; and Zach Ellsworth is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.

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