Homeostasis

Homeostatic Process

Homeostatic processes ensure a constant internal environment by various mechanisms working in combination to maintain set points.

Learning Objectives

Give an example and describe a homeostatic process.

Key Takeaways

Key Points

  • Homeostasis is the body’s attempt to maintain a constant and balanced internal environment, which requires persistent monitoring and adjustments as conditions change.
  • Homeostatic regulation is monitored and adjusted by the receptor, the command center, and the effector.
  • The receptor receives information based on the internal environment; the command center, receives and processes the information; and the effector responds to the command center, opposing or enhancing the stimulus.

Key Terms

  • homeostasis: the ability of a system or living organism to adjust its internal environment to maintain a stable equilibrium
  • effector: any muscle, organ etc. that can respond to a stimulus from a nerve

Homeostatic Process

The human organism consists of trillions of cells working together for the maintenance of the entire organism. While cells may perform very different functions, the cells are quite similar in their metabolic requirements. Maintaining a constant internal environment with everything that the cells need to survive (oxygen, glucose, mineral ions, waste removal, etc.) is necessary for the well-being of individual cells and the well-being of the entire body. The varied processes by which the body regulates its internal environment are collectively referred to as homeostasis.

Homeostasis

Homeostasis, in a general sense, refers to stability, balance, or equilibrium. Physiologically, it is the body’s attempt to maintain a constant and balanced internal environment, which requires persistent monitoring and adjustments as conditions change. Adjustment of physiological systems within the body is called homeostatic regulation, which involves three parts or mechanisms: (1) the receptor, (2) the control center, and (3) the effector.

The receptor receives information that something in the environment is changing. The control center or integration center receives and processes information from the receptor. The effector responds to the commands of the control center by either opposing or enhancing the stimulus. This ongoing process continually works to restore and maintain homeostasis. For example, during body temperature regulation, temperature receptors in the skin communicate information to the brain (the control center) which signals the effectors: blood vessels and sweat glands in the skin. As the internal and external environment of the body are constantly changing, adjustments must be made continuously to stay at or near a specific value: the set point.

Purpose of Homeostasis

The ultimate goal of homeostasis is the maintenance of equilibrium around the set point. While there are normal fluctuations from the set point, the body’s systems will usually attempt to revert to it. A change in the internal or external environment (a stimulus) is detected by a receptor; the response of the system is to adjust the deviation parameter toward the set point. For instance, if the body becomes too warm, adjustments are made to cool the animal. If the blood’s glucose rises after a meal, adjustments are made to lower the blood glucose level by moving the nutrient into tissues in the command center that require it, or to store it for later use.

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Blood glucose homeostasis: An example of how homeostasis is achieved by controlling blood sugar levels after a meal.

Control of Homeostasis

Homeostasis is typically achieved via negative feedback loops, but can be affected by positive feedback loops, set point alterations, and acclimatization.

Learning Objectives

Discuss the ways in which the body maintains homeostasis and provide examples of each mechanism

Key Takeaways

Key Points

  • Negative feedback loops are used to maintain homeostasis and achieve the set point within a system.
  • Negative feedback loops are characterized by their ability to either increase or decrease a stimulus, inhibiting the ability of the stimulus to continue as it did prior to sensing of the receptor.
  • Positive feedback loops are characterized by their ability to maintain the direction of a stimulus and can even accelerate its effect.
  • Acclimatization is characterized by the ability to change systems within an organism to maintain a set point in a different environment.

Key Terms

  • acclimatization: the climatic adaptation of an organism that has been moved to a new environment
  • endocrine: Producing internal secretions that are transported around the body by the bloodstream.

Control of Homeostasis

When a change occurs in an animal’s environment, an adjustment must be made. The receptors sense changes in the environment, sending a signal to the control center (in most cases, the brain), which, in turn, generates a response that is signaled to an effector. The effector is a muscle or a gland that will carry out the required response. Homeostasis is maintained by negative feedback loops within the organism. In contrast, positive feedback loops push the organism further out of homeostasis, but may be necessary for life to occur. Homeostasis is controlled by the nervous and endocrine systems in mammals.

Negative Feedback Mechanisms

Any homeostatic process that changes the direction of the stimulus is a negative feedback loop. It may either increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed it. In other words, if a level is too high, the body does something to bring it down; conversely, if a level is too low, the body does something to raise it; hence, the term: negative feedback. An example of negative feedback is the maintenance of blood glucose levels. When an animal has eaten, blood glucose levels rise, which is sensed by the nervous system. Specialized cells in the pancreas (part of the endocrine system) sense the increase, releasing the hormone insulin. Insulin causes blood glucose levels to decrease, as would be expected in a negative feedback system. However, if an animal has not eaten and blood glucose levels decrease, this is sensed in a different group of cells in the pancreas: the hormone glucagon is released, causing glucose levels to increase. This is still a negative feedback loop, but not in the direction expected by the use of the term “negative.” Another example of an increase as a result of a feedback loop is the control of blood calcium. If calcium levels decrease, specialized cells in the parathyroid gland sense this and release parathyroid hormone (PTH), causing an increased absorption of calcium through the intestines and kidneys. The effects of PTH are to raise blood levels of calcium. Negative feedback loops are the predominant mechanism used in homeostasis.

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Negatie feedback loop: Blood sugar levels are controlled by a negative feedback loop.

Positive Feedback Loop

A positive feedback loop maintains the direction of the stimulus and possibly accelerates it. There are few examples of positive feedback loops that exist in animal bodies, but one is found in the cascade of chemical reactions that result in blood clotting, or coagulation. As one clotting factor is activated, it activates the next factor in sequence until a fibrin clot is achieved. The direction is maintained, not changed, so this is positive feedback. Another example of positive feedback is uterine contractions during childbirth. The hormone oxytocin, made by the endocrine system, stimulates the contraction of the uterus. This produces pain sensed by the nervous system. Instead of lowering the oxytocin and causing the pain to subside, more oxytocin is produced until the contractions are powerful enough to produce childbirth.

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Positive feedback loop: The birth of a human infant is the result of positive feedback.

Set Point

Homeostasis is performed so the body can maintain its internal set point. However, there are times when the set point must be adjusted. When this happens, the feedback loop works to maintain the new setting. An example of changes in a set point can been seen in blood pressure. Over time, the normal or set point for blood pressure can increase as a result of continued increases in blood pressure. The body no longer recognizes the elevation as abnormal; there is no attempt made to return to the lower set point. The result is the maintenance of an elevated blood pressure which can have harmful effects on the body. Medication can lower blood pressure and lower the set point in the system to a more healthy level through a process of alteration of the set point in a feedback loop.

Changes can be made in a group of body organ systems in order to maintain a set point in another system. This is called acclimatization. This occurs, for instance, when an animal migrates to a higher altitude than one to which it is accustomed. In order to adjust to the lower oxygen levels at the new altitude, the body increases the number of red blood cells circulating in the blood to ensure adequate oxygen delivery to the tissues. Another example of acclimatization is animals that have seasonal changes in their coats: a heavier coat in the winter ensures adequate heat retention, while a light coat in summer assists in keeping body temperature from rising to harmful levels.

Homeostasis: Thermoregulation

Animals use different modes of thermoregulation processes to maintain homeostatic internal body temperatures.

Learning Objectives

Outline the various types of processes utilized by animals to ensure thermoregulation.

Key Takeaways

Key Points

  • In response to varying body temperatures, processes such as enzyme production can be modified to acclimate to changes in the temperature.
  • Endotherms regulate their own internal body temperature, regardless of fluctuating external temperatures, while ectotherms rely on the external environment to regulate their internal body temperature.
  • Homeotherms maintain their body temperature within a narrow range, while poikilotherms can tolerate a wide variation in internal body temperature, usually because of environmental variation.
  • Heat can be exchanged between environment and animals via radiation, evaporation, convection, or conduction processes.

Key Terms

  • ectotherm: An animal that relies on external environment to regulate its internal body temperature.
  • endotherm: An animal that regulates its own internal body temperature through metabolic processes.
  • homeotherm: An animal that maintains a constant internal body temperature, usually within a narrow range of temperatures.
  • poikilotherm: An animal that varies its internal body temperature within a wide range of temperatures, usually as a result of variation in the environmental temperature.

Thermoregulation to Maintain Homeostasis

Internal thermoregulation contributes to animal’s ability to maintain homeostasis within a certain range of temperatures. As internal body temperature rises, physiological processes are affected, such as enzyme activity. Although enzyme activity initially increases with temperature, enzymes begin to denature and lose their function at higher temperatures (around 40-50 C for mammals). As internal body temperature decreases below normal levels, hypothermia occurs and other physiological process are affected. There are various thermoregulation mechanisms that animals use to regulate their internal body temperature.

Types of Thermoregulation (Ectothermy vs. Endothermy)

Thermoregulation in organisms runs along a spectrum from endothermy to ectothermy. Endotherms create most of their heat via metabolic processes, and are colloquially referred to as “warm-blooded.” Ectotherms use external sources of temperature to regulate their body temperatures. Ectotherms are colloquially referred to as “cold-blooded” even though their body temperatures often stay within the same temperature ranges as warm-blooded animals.

Ectotherm

An ectotherm, from the Greek (ektós) “outside” and (thermós) “hot,” is an organism in which internal physiological sources of heat are of relatively small or quite negligible importance in controlling body temperature. Since ectotherms rely on environmental heat sources, they can operate at economical metabolic rates. Ectotherms usually live in environments in which temperatures are constant, such as the tropics or ocean. Ectotherms have developed several behavioral thermoregulation mechanisms, such as basking in the sun to increase body temperature or seeking shade to decrease body temperature.

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Ectotherm: The Common frog is an ecotherm and regulates its body based on the temperature of the external environment.

Endotherms

In contrast to ectotherms, endotherms regulate their own body temperature through internal metabolic processes and usually maintain a narrow range of internal temperatures. Heat is usually generated from the animal’s normal metabolism, but under conditions of excessive cold or low activity, an endotherm generate additional heat by shivering. Many endotherms have a larger number of mitochondria per cell than ectotherms. These mitochondria enables them to generate heat by increasing the rate at which they metabolize fats and sugars. However, endothermic animals must sustain their higher metabolism by eating more food more often. For example, a mouse (endotherm) must consume food every day to sustain high its metabolism, while a snake (ectotherm) may only eat once a month because its metabolism is much lower.

Homeothermy vs. Poikilothermy

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Homeotherm vs. Poikilotherm: Sustained energy output of an endothermic animal (mammal) and an ectothermic animal (reptile) as a function of core temperature. In this scenario, the mammal is also a homeotherm because it maintains its internal body temperature in a very narrow range. The reptile is also a poikilotherm because it can withstand a large range of temperatures.

A poikilotherm is an organism whose internal temperature varies considerably. It is the opposite of a homeotherm, an organism which maintains thermal homeostasis. Poikilotherm’s internal temperature usually varies with the ambient environmental temperature, and many terrestrial ectotherms are poikilothermic. Poikilothermic animals include many species of fish, amphibians, and reptiles, as well as birds and mammals that lower their metabolism and body temperature as part of hibernation or torpor. Some ectotherms can also be homeotherms. For example, some species of tropical fish inhabit coral reefs that have such stable ambient temperatures that their internal temperature remains constant.

Means of Heat Transfer

Heat can be exchanged between an animal and its environment through four mechanisms: radiation, evaporation, convection, and conduction. Radiation is the emission of electromagnetic “heat” waves. Heat radiates from the sun and from dry skin the same manner. When a mammal sweats, evaporation removes heat from a surface with a liquid. Convection currents of air remove heat from the surface of dry skin as the air passes over it. Heat can be conducted from one surface to another during direct contact with the surfaces, such as an animal resting on a warm rock.

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Mechanisms for heat exchange: Heat can be exchanged by four mechanisms: (a) radiation, (b) evaporation, (c) convection, or (d) conduction.

Heat Conservation and Dissipation

Animals have processes that allow for heat conservation and dissipation in order to maintain a homeostatic internal body temperature.

Learning Objectives

Describe some of the changes animals use in order to maintain body temperature

Key Takeaways

Key Points

  • Heat conservation is characterized by the ability to ensure blood remains in the core by undergoing vasoconstriction, reducing blood flow to the periphery (also known as peripheral vasoconstriction).
  • Heat dissipation is characterized by the ability to undergo vasodilation which increases blood flow to the periphery, resulting in evaporative heat loss.
  • Endothermic animals are defined by their ability to utilize both vasoconstriction and vasodilation to maintain internal body temperature.
  • Ectothermic animals are defined by their change in behavior (lying in sunlight to warm up, hiding in shade to cool down) to regulate body temperature.

Key Terms

  • endotherm: a warm-blooded animal that maintains a constant body temperature
  • ectotherm: a cold-blooded animal that regulates its body temperature by exchanging heat with its surroundings

Heat Conservation and Dissipation

Animals conserve or dissipate heat in a variety of ways. In certain climates, endothermic animals have some form of insulation, such as fur, fat, feathers, or some combination thereof. Animals with thick fur or feathers create an insulating layer of air between their skin and internal organs. Polar bears and seals live and swim in a subfreezing environment, yet they maintain a constant, warm, body temperature. The arctic fox uses its fluffy tail as extra insulation when it curls up to sleep in cold weather. Mammals have a residual effect from shivering and increased muscle activity: arrector pili muscles create “goose bumps,” causing small hairs to stand up when the individual is cold; this has the intended effect of increasing body temperature. Mammals use layers of fat to achieve the same end; the loss of significant amounts of body fat will compromise an individual’s ability to conserve heat.

Endotherms use their circulatory systems to help maintain body temperature. For example, vasodilation brings more blood and heat to the body surface, facilitating radiation and evaporative heat loss, which helps to cool the body. However, vasoconstriction reduces blood flow in peripheral blood vessels, forcing blood toward the core and the vital organs found there, conserving heat. Some animals have adaptions to their circulatory system that enable them to transfer heat from arteries to veins, thus, warming blood that returns to the heart. This is called a countercurrent heat exchange; it prevents the cold venous blood from cooling the heart and other internal organs. This adaption, which can be shut down in some animals to prevent overheating the internal organs, is found in many animals, including dolphins, sharks, bony fish, bees, and hummingbirds. In contrast, similar adaptations (as in dolphin flukes and elephant ears) can help cool endotherms when needed.

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Control of body temperature: In endotherms, the circulatory system is used to help maintain body temperature, either by vasodilation or vasoconstriction.

Many animals, especially mammals, use metabolic waste heat as a heat source. When muscles are contracted, most of the energy from the ATP used in muscle actions is wasted energy that translates into heat. In cases of severe cold, a shivering reflex is activated that generates heat for the body. Many species also have a type of adipose tissue called brown fat that specializes in generating heat.

Ecothermic animals use changes in their behavior to help regulate body temperature. For example, a desert ectothermic animal may simply seek cooler areas during the hottest part of the day in the desert to keep from becoming too warm. The same animals may climb onto rocks to capture heat during a cold desert night. Some animals seek water to aid evaporation in cooling them, as seen with reptiles. Other ectotherms use group activity, such as the activity of bees to warm a hive to survive winter.