What is homeostasis?
Homeostasis
The term homeostasis originates from two Greek words. Homeo means “similar”. Stasis means “standing
still.” Collectively, homeostasis describes the property of a system that maintains a constant, stable internal
environment despite a widely fluctuating external environment.
All living things depend on homeostatic controls. They keep organisms alive as internal and external
conditions change. Some of these controls keep internal conditions within a tolerable range throughout the life
cycle.
Homeostasis keeps the body in a balanced state. What happens if one body process becomes out of
balance?
Homeostasis Defined
Homeostasis is the ability of an organism to maintain normal balance despite external conditions.
View Glossary Term
Negative Feedback Loops
The body maintains internal balance orhomeostasis by using a number of different negative feedback loops. What is a
negative feedback loop?
View Video Segment
Leptin Negative Feedback Control System
Leptin is a hormone involved in the control of body mass. Changes in the amount of fat tissue lead to changes in leptin
levels and thus changes in appetite. (Used with permission from Howard Hughes Medical Institute)
View Video Segment
Negative Feedback Mechanism
The negative feedback system regulates the amount of hormones in the body to maintain a stable internal environment.
Why is the system called negative feedback?
View Video Segment
Negative Feedback Loop
The most critical mechanism of homeostasis is a negative feedback loop. It is similar to the way in which a
thermostat maintains a constant room temperature. The thermostat is set to the desired temperature. If the room
is too cold, the equipment reacts by turning on the heat. If the room gets too hot, sensors prompt the air
conditioner to turn on.
This control system requires three parts:
1. A sensor that monitors the particular property of interest.
2. A processor that determines how far the measured value deviates from the set value.
3. An actuator that can change the particular property.
In the example of a thermostat, the sensor is a thermometer measuring the room temperature. The processor is
a machine or computer that compares the measured room temperature with the set value. Finally, the actuator
is a heater or an air conditioner that heats up or cools down the room.
Negative Feedback Loop.
A thermostat is a good example of how negative feedback works to maintain a constant temperature. In
this system, what happens when the temperature drops beyond a certain level?
Just like a thermostat, sensors, processors, and actuators are found throughout the human body in order to
maintain the optimum levels of pH, blood glucose, body temperature, and more. In addition, these parts must
be linked together in order for the negative feedback mechanism to function properly.
If the negative feedback system breaks down, homeostasis is not maintained. This can lead to serious diseases
like diabetes, chronic high blood pressure, or kidney disease. Homeostatic imbalance can even lead to organ
failure and death
How do different systems in the body interact to maintain homeostasis?
Hypothalamus and Pituitary Gland
The hypothalamus and pituitary gland are the main regulators of homeostasis in humans. The hypothalamus is
located in the brain just below the thalamus and above the brain stem. The pituitary gland is a protrusion from
the hypothalamus. It is connected by neural fibers.
The hypothalamus and pituitary gland are important structures in the human brain that
regulate homeostasis in humans.
The hypothalamus is part of the autonomic nervous system. It is sensitive to many properties of the internal
environment. It stimulates or inhibits the secretion of pituitary hormones into blood circulation. In turn, these
pituitary hormones act on different parts of the human body to maintain homeostasis. Together, the
hypothalamus and pituitary gland play the role of the main processor for homeostasis. They bridge the nervous
system and the endocrine system.
How Hormones Work
Endocrine glands secrete hormones into thecirculatory system. What are hormones and how do they work?
View Video Segment
Hypothalamus and Pituitary Gland
The hypothalamus and pituitary gland are responsible for regulating homeostasis in humans through the release of
hormones. Why is the pituitary gland sometimes referred to as the “master gland?”
View Video Segment
The Skin
An important function of the skin is to help maintain body temperature. What is the function of sweat glands?
View Video Segment
Control of Body Temperature
Humans maintain a core body temperature of about 37°C. A key property of heat is that it transfers from hot to
cold places. When the surrounding air is colder than the body’s temperature, the body loses energy to the
environment and needs to generate more heat. On the other hand, if the surrounding air is warmer, the body
gains heat. This heat must be dissipated.
Shivering helps the body maintain a core body temperature of 37°C. Why does this work?
Two primary mechanisms are employed to cope with these situations. In a cold environment, the body shivers.
The muscle movements require energy that eventually turns into heat. This compensates for the loss of heat to
the environment. In hot conditions, sweat is produced by sweat glands in the skin. As sweat turns into water
vapor, heat is released during the evaporation process. Sweating is done at the expense of water loss, so
prolonged heat exposure can be dangerous as it can lead to dehydration.
The hypothalamus is the main processor for body temperature. It stimulates shivering or sweating, depending
on the situation.
Exploring Homeostasis
Using the Exploration "Homeostasis," you are going to investigate the questions, “What is homeostasis?” and “How do
different systems in the body interact to maintain homeostasis?”
Control of Fluids, Electrolytes, and Waste
Human activities vary greatly throughout the day. One could exercise for half an hour in the morning then sit
in classes for several hours, eat meals, and sleep at night. Different levels of activities require different
amounts of energy and produce corresponding wastes. Moreover, the exact times of eating and drinking are
irregular. These all contribute to variations in the properties of blood circulating throughout the human body
such as acidity, electrolyte level, and waste level.
The primary sensors of these blood properties are again located in the hypothalamus. The hypothalamus is
sensitive to the osmolarity—the solute concentration—of the blood and controls the secretions of pituitary
hormones from the pituitary gland. These pituitary hormones then signal organs such as the kidneys and liver
to perform particular functions. The waste management system is the excretory system. Excretory organs
remove waste substances from the blood to maintian homeostatis. In addition, the excretory organs can get rid
of metabolic wastes produced by cellular metabolism. If too much water is ingested, the kidneys can remove
the excess water from the blood. In contrast, if the water level is low, the hypothalamus will stimulate the
pituitary gland. The pituitary gland then sends a signal to the kidneys to conserve water and produce the
sensation of thirst.
The Kidneys
Kidneys filter and clean the blood.
View Video Segment
Control of Blood Pressure
The medulla regulates heart and blood vessels to increase or decrease blood pressure.
View Image
Blood Pressure
Maintaining normal blood pressure is important to overall health. What is considered a healthy blood pressure for an
adult? What do the two numbers refer to?
View Video Segment
Control of Blood Pressure
Another critical property of the circulatory system that must be maintained is blood pressure. Blood pressure is
the pressure of circulating blood on artery walls. Normally, blood pressure is influenced by several factors
such as the rate at which the heart beats (heart rate), the volume of blood that exits the heart per beat (stroke
volume), and the amount of resistance to blood flow within the vessels. The resistance to blood flow is further
influenced by the diameters of the vessels. Larger vessels have less resistance to flow. These properties of the
heart and blood vessels are controlled by the body in order to increase or decrease blood pressure so that it
stays within healthy limits.
Blood pressure is constantly measured by receptors in the arteries and veins. These receptors then send signals
back through nerve fibers to part of the brain called the medulla. The medulla acts as the processor in the
negative feedback loop that controls blood pressure. It either increases the heart rate and stroke volume if the
pressure is too low, or decreases both when the pressure is too high. Hormones are also released to signal the
blood vessels to increase or decrease resistance in order to maintain blood pressure within an optimum range