Homeostasis is the condition of equilibrium (balance) in the body’s internal environment due to the constant interaction of the body’s many regulatory processes. Homeostasis is a dynamic
condition. In response to changing conditions, the body’s equilibrium can shift among points in a narrow range that is compatible with maintaining life. For example, the level of glucose in blood normally stays between 70 and 110 milligrams of glucose per 100 milliliters of blood. Each structure, from the cellular level to the systemic level, contributes in some way to keeping the internal environment of the body within normal limits.
An important aspect of homeostasis is maintaining the volume and composition of body fluids, dilute, watery solutions containing dissolved chemicals that are found inside cells as well as surrounding them. The fluid within cells is intracellular fluid , abbreviated ICF. The fluid outside body cells
is extracellular fluid , abbreviated ECF. The ECF that fills the narrow spaces between cells of tissues is
known as interstitial fluid . As you progress with your studies, you will learn that the ECF differs depending on where it occurs in the body: ECF within blood vessels is termed blood plasma, within lymphatic vessels it is called lymph, in and around the brain and spinal cord it is known as cerebrospinal fluid, in joints it is referred to as synovial fluid, and the ECF of the eyes is called aqueous humor and vitreous body.
The proper functioning of body cells depends on precise regulation of the composition of the interstitial fluid surrounding them. Because of this, interstitial fluid is often called the body’s internal environment. The composition of interstitial fluid changes as substances move back and forth between it and blood
plasma. Such exchange of materials occurs across the thin walls of the smallest blood vessels in the body, the blood capillaries. This movement in both directions across capillary walls pro-
vides needed materials, such as glucose, oxygen, ions, and so on, to tissue cells. It also removes wastes, such as carbon dioxide, from interstitial fluid.
Homeostasis in the human body is continually being disturbed. Some disruptions come from the external environment in the form of physical insults such as the intense heat of a Texas summer or a lack of enough oxygen for that two-mile run. Other disruptions originate in the internal environment, such as a blood glucose level that falls too low when you skip breakfast.Homeostatic imbalances may also occur due to psychological stresses in our social environment the demands of work and school, for example. In most cases the disruption of homeostasis is mild and temporary, and the responses of body cells quickly restore balance in the internal environment. However, in somecases the disruption of homeostasis may be intense and prolonged, as in poisoning, overexposure to temperature extremes,
severe infection, or major surgery. Fortunately, the body has many regulating systems that can
usually bring the internal environment back into balance. Most often, the nervous system and the endocrine system, working together or independently, provide the needed corrective mea-
sures. The nervous system regulates homeostasis by sending electrical signals known as nerve impulses (action potentials) to organs that can counteract changes from the balanced state. The endocrine system includes many glands that secrete messenger molecules called hormones into the blood. Nerve impulses typically cause rapid changes, but hormones usually work more slowly. Both means of regulation, however, work toward the same end, usually through negative feedback systems.
The body can regulate its internal environment through many feedback systems. A feedback system or feedback loop is a cycle of events in which the status of a body condition is monitored, evaluated, changed, remonitored, reevaluated, and so on. Each monitored variable, such as body temperature, blood pressure, or blood glucose level, is termed a controlled condition. Any disruption that changes a controlled condition is called a stimulus. A feedback system includes three basic components a receptor, a control center, and an effector .
1. A receptor is a body structure that monitors changes in a controlled condition and sends input to a control center. Typically, the input is in the form of nerve impulses or chemical signals. For example, certain nerve endings in the skin sense temperature and can detect changes, such as a dramatic drop in
temperature.
2. A control center in the body, for example, the brain, sets the range of values within which a controlled condition should be maintained, evaluates the input it receives from receptors, and
generates output commands when they are needed. Output from the control center typically occurs as nerve impulses, or hormones or other chemical signals. In our skin temperature example, the brain acts as the control center, receiving nerve impulses from the skin receptors and generating nerve impulses as output.
3. An effector is a body structure that receives output from the control center and produces a response or effect that changes the controlled condition. Nearly every organ or tissue in the body can behave as an effector. When your body temperature drops sharply, your brain (control center) sends nerve impulses (output) to your skeletal muscles (effectors). The result is shivering, which generates heat and raises your body temperature. A group of receptors and effectors communicating with their control center forms a feedback system that can regulate a controlled condition in the body’s internal environment. In a feedback system, the response of the system “feeds back” information to
change the controlled condition in some way, either negating it (negative feedback) or enhancing it (positive feedback).
condition. In response to changing conditions, the body’s equilibrium can shift among points in a narrow range that is compatible with maintaining life. For example, the level of glucose in blood normally stays between 70 and 110 milligrams of glucose per 100 milliliters of blood. Each structure, from the cellular level to the systemic level, contributes in some way to keeping the internal environment of the body within normal limits.
Homeostasis and Body Fluids:
An important aspect of homeostasis is maintaining the volume and composition of body fluids, dilute, watery solutions containing dissolved chemicals that are found inside cells as well as surrounding them. The fluid within cells is intracellular fluid , abbreviated ICF. The fluid outside body cells
is extracellular fluid , abbreviated ECF. The ECF that fills the narrow spaces between cells of tissues is
known as interstitial fluid . As you progress with your studies, you will learn that the ECF differs depending on where it occurs in the body: ECF within blood vessels is termed blood plasma, within lymphatic vessels it is called lymph, in and around the brain and spinal cord it is known as cerebrospinal fluid, in joints it is referred to as synovial fluid, and the ECF of the eyes is called aqueous humor and vitreous body.
The proper functioning of body cells depends on precise regulation of the composition of the interstitial fluid surrounding them. Because of this, interstitial fluid is often called the body’s internal environment. The composition of interstitial fluid changes as substances move back and forth between it and blood
plasma. Such exchange of materials occurs across the thin walls of the smallest blood vessels in the body, the blood capillaries. This movement in both directions across capillary walls pro-
vides needed materials, such as glucose, oxygen, ions, and so on, to tissue cells. It also removes wastes, such as carbon dioxide, from interstitial fluid.
Control of Homeostasis:
Homeostasis in the human body is continually being disturbed. Some disruptions come from the external environment in the form of physical insults such as the intense heat of a Texas summer or a lack of enough oxygen for that two-mile run. Other disruptions originate in the internal environment, such as a blood glucose level that falls too low when you skip breakfast.Homeostatic imbalances may also occur due to psychological stresses in our social environment the demands of work and school, for example. In most cases the disruption of homeostasis is mild and temporary, and the responses of body cells quickly restore balance in the internal environment. However, in somecases the disruption of homeostasis may be intense and prolonged, as in poisoning, overexposure to temperature extremes,
severe infection, or major surgery. Fortunately, the body has many regulating systems that can
usually bring the internal environment back into balance. Most often, the nervous system and the endocrine system, working together or independently, provide the needed corrective mea-
sures. The nervous system regulates homeostasis by sending electrical signals known as nerve impulses (action potentials) to organs that can counteract changes from the balanced state. The endocrine system includes many glands that secrete messenger molecules called hormones into the blood. Nerve impulses typically cause rapid changes, but hormones usually work more slowly. Both means of regulation, however, work toward the same end, usually through negative feedback systems.
Feedback Systems
1. A receptor is a body structure that monitors changes in a controlled condition and sends input to a control center. Typically, the input is in the form of nerve impulses or chemical signals. For example, certain nerve endings in the skin sense temperature and can detect changes, such as a dramatic drop in
temperature.
2. A control center in the body, for example, the brain, sets the range of values within which a controlled condition should be maintained, evaluates the input it receives from receptors, and
generates output commands when they are needed. Output from the control center typically occurs as nerve impulses, or hormones or other chemical signals. In our skin temperature example, the brain acts as the control center, receiving nerve impulses from the skin receptors and generating nerve impulses as output.
3. An effector is a body structure that receives output from the control center and produces a response or effect that changes the controlled condition. Nearly every organ or tissue in the body can behave as an effector. When your body temperature drops sharply, your brain (control center) sends nerve impulses (output) to your skeletal muscles (effectors). The result is shivering, which generates heat and raises your body temperature. A group of receptors and effectors communicating with their control center forms a feedback system that can regulate a controlled condition in the body’s internal environment. In a feedback system, the response of the system “feeds back” information to
change the controlled condition in some way, either negating it (negative feedback) or enhancing it (positive feedback).
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