What happens to our bodies on a cold, winter day? If we are not wearing enough layers of warm clothing, chances are we will start to shiver. By shivering, our body works to maintain a constant temperature when the external temperature is low.
Changes in the outside temperature are detected by our nervous system, which includes receptors in our skin. These receptors send a signal to the hypothalamus portion of our brain. The hypothalamus sends a signal to the muscular system so that muscles rapidly contract and relax, causing us to shiver in an attempt to produce body heat. In addition, smooth muscles in the circulatory system contract to constrict blood vessels, retaining body heat so the skin stays warm. The nervous system also sends signals so that we instinctively curl up as much as possible to bring all our body parts closer together. This behavior serves to expose less surface area so that less heat escapes our body.
If anything happens to or within a living organism that affects its normal state, processes to restore the normal state begin. The regulation of an organism’s internal environment to maintain and balance the conditions needed for life is called homeostasis. Through homeostasis, body systems in organisms maintain characteristic conditions. Parts of the nervous system and endocrine systems are dedicated to maintaining homeostasis, and their action is coordinated by the hypothalamus. Conditions that are regulated to maintain homeostasis include the following:
- body temperature
- heart rate
- breathing rate
- blood glucose levels
- blood pH
Responses to Stimuli
A stimulus (plural, stimuli) is anything that causes a reaction or a change in a living organism. An increase in temperature in an environment is a stimulus to the human body, but the act of sweating to cool the body down is a response, or reaction to the stimulus. When sensory receptors in the nervous system detect stimuli, they send a message to the hypothalamus. The hypothalamus interprets the information and sends out a signal indicating how the body should respond to the stimuli.
A particular response to a stimulus is often recognized by a certain behavior or action. For example, a hunger response is triggered when our stomach is empty. The common behavior in response to hunger is to eat a snack or a meal. Behavior patterns of an organism are usually connected to its constant struggle to survive or reproduce. Other responses include an increased heart rate in response to fear, and an increase in white blood cell production in response to an infection.
External stimuli come from outside an organism and influence the organism’s behavior. Temperature, sound, light, and other organisms act as external stimuli and cause different responses in organisms. Oxygen is an external stimuli required to sustain life. Low levels of oxygen in our immediate surroundings trigger a behavioral response in humans. If we cannot take in enough oxygen from our environment, we will gasp for air, breathing faster and more deeply, trying to bring more oxygen to our blood cells. This behavior is due to the response in our body in which cells sense the lack of oxygen.
Internal stimuli such as hunger or thirst come from inside an organism’s body and also influence behavior patterns. When humans lose water on the surface of the skin in the form of sweat, the brain triggers specific hormones that let us know our body needs more water, and we feel thirsty. Other examples of internal stimuli include fatigue, a full stomach or a full bladder, and internal pain.
In general, the human body remains in homeostasis when it is functioning properly and all basic needs are met. Every body system contributes to maintaining homeostasis in some way, and this maintenance is similar to a very complex balancing act. Variables such as body temperature, heart rate, blood pressure, blood glucose, blood pH, and hydration levels all have a normal range. If these values deviate significantly from their norm, body systems work together to restore the normal value. Feedback mechanisms are systems set up to respond to changes in these variables. The diagram below provides a visual explanation as to how homeostasis is maintained.
Effect of change returns variable to normal range (homeostasis restored) When a condition in the body, such as temperature, deviates from its normal value, or set point, due to an internal or external stimulus, the body activates certain systems or chemicals. These systems or chemicals help reverse the change and return the value to its set point. This process is called a negative feedback mechanism. For example, when the internal body temperature in humans gets too high due to high external temperatures, a signal is sent from the brain to dilate blood vessels. Dilated blood vessels are closer to the skin’s surface, so excess heat can escape. Another signal from the brain activates sweat glands to produce sweat, which also cools the body. Both of these systems work to return the body’s temperature to its set point.
If the negative-feedback mechanism is not activated, homeostasis cannot be restored and the body temperature will continue to rise. This can lead to hyperthermia, which can cause dehydration, dizziness, and nausea. It is often called heat exhaustion. More severe hyperthermia leads to heat stroke, brain damage, organ-system shutdown, or possibly death. Blood glucose levels are also controlled by a negative-feedback mechanism. After a meal, especially one high in carbohydrates, blood glucose levels rise. Receptors in the body sense the change and the pancreas, which acts as a control center for this feedback mechanism, releases a chemical called insulin into the blood. Insulin acts to lower the level of glucose in the blood and return it to its normal range, or set point.
In some cases, the body may activate a positive-feedback mechanism. A positive-feedback mechanism is a system set in motion to accelerate a change in an effort to maintain homeostasis. A fever is the result of a positive feedback mechanism that acts to kill an infecting virus or bacteria. During an infection, a positive-feedback mechanism in the body accelerates an increase in internal body temperature in an effort to kill the infecting agent. Once the infecting agent is killed, the temperature returns to its set point.
The process of blood clotting when you are bleeding from an injury is also controlled by a positive-feedback mechanism. When a blood vessel is damaged, platelets in the blood cling to the injured site and release chemicals that attract even more platelets to the injured site. The platelets continue to accumulate until they form a clot, which stops the bleeding at the site.