Introduction

Meaning: A set of mechanisms and processes that regulate many factors of the internal environment (extracellular fluid) to keep it in a state of balance. Balance is key because too much of even the most essential substances, such as water and oxygen, can be harmful.

Examples of factors regulated:

• Body temperature - thermoregulation
• Blood pH
• Blood glucose concentration
• Blood pressure
• Oxygen levels
• Osmolarity (water levels) - osmoregulation
• Electrolyte concentrations

Each of these factors has a “normal” or set point, although values may vary slightly above or below it. The set point is regularly reset. For instance, body temperature fluctuates throughout the day.

How Does Homeostasis Work?

Homeostatic processes are mostly automatic and subconscious.

The main regulatory changes can be regulated in two ways:

• Negative feedback
• Positive feedback

Feedback mechanisms

Any feedback mechanism includes three components:

• Receptors (sensors) - detecting of changes
• Control centers - processing of information
• Effectors - responding to the change

Negative feedback

• Negative feedback means that a change is detected and the system tries to counteract or reverse the unwanted change.
• It allows organisms to maintain ideal and stable internal conditions, despite fluctuations in the external environment.
• Most homeostatic processes in the body rely on negative feedback.

Examples and brief explanations:

1. Regulation of blood glucose concentration

   Blood glucose concentration is kept within limits by adjusting the amount of glucose in the bloodstream. Cells in the pancreas monitor blood glucose levels and release insulin or glucagon when the level is too low or too high.

   1. In response to high blood glucose levels, insulin is released by β (beta) cells in the pancreas. Insulin causes higher levels of glucose to be converted into glycogen and influences cells to take up MORE glucose to lower blood glucose levels.
   2. In response to low blood glucose levels, glucagon is released by α (alpha) cells in the pancreas. Glucagon causes higher levels of glycogen to be broken down into glucose and released into the bloodstream. It also influences cells NOT to take up glucose.
2. Thermoregulation

   Thermoregulation is the control of core body temperature to keep it close to a set point.

   1. In response to high temperature, the body starts to sweat and blood vessels widen (vasodilation) to increase blood flow to the skin, allowing for more heat dissipation. These changes increase the loss of heat to the environment.
   2. In response to low temperature, the body starts to shiver, blood vessels narrow (to reduce blood flow to the skin), and hairs in the skin stand erect to create an insulating layer between the skin and the air.

Positive feedback

Positive feedback is traditionally NOT considered part of homeostasis, especially under the definition that homeostasis keeps the system in a state of balance.

Positive feedback mechanisms help the body to maintain appropriate conditions for survival by reinforcing the original change, rather than reversing it. They are usually used in short-term processes.

Examples:

• Blood clotting
• Childbirth (oxytocin induced contractions, and contractions increase oxytocin levels, in turn inducing more contractions)

Written by Matic Zimic