∞ generated and posted on 2018.08.21 ∞

Tissues consist of associations of cells of similar types.

This chapter provides an overview of basic biology from the perspective of human biology. This part, Section 2, considers in particular homeostasis along with the basics of metabolism.

This page contains the following terms: Homeostasis, Steady state, Equilibrium, Dynamic constancy, Set point, Negative feedback, Positive feedback, Metabolism, Catabolism, Anabolism, Basal metabolic rate, Physiological adaptation, Integrated functioning, Responsiveness.


The idea that living systems, in order to survive and prosper, must control within relatively narrow ranges various physical, chemical, and physiological parameters associated with their bodies.
Homeostasis is the key underlying basis of what body functioning is all about. Bodies are able to function in part because they set things up so that mostly they remain in a more or less constant state. In this way the cells that make up bodies remain healthy and able to perform their individual jobs towards overall body functioning.

This is equivalent to a well-functioning automobile, for example, which maintains its oil pressure at some optimal level, its battery's charge at some optimal level, its coolant temperature at some optimal level, etc. In this way the engine, etc. is able to do its job without high likelihood of failure. Furthermore, it is precisely when the automobile is no longer able to maintain this constancy that failure is most likely, such that disease typically represents some sort of perturbation away from homeostasis.

The major difference is that bodies have more components than cars and perform their various homeostatic functions mostly at microscopic scales rather than macroscopic scales.

Links to terms of possible interest: Body temperature, Cells of body, Control systems, Disease, External environment, Health, Homeostasis, Homeostatic functions, Internal environment, Macroscopic scales, Microscopic scales Physiology, Room temperature

The above video integrates a lot of ideas that have to do with homeostasis, but really serves more as an introduction to a teaching unit and as a consequence isn't as compelling as it might have been.

The above video in fact is way too complex for this point in our considerations of physiology, and is a bit crude, representing an early effort by an otherwise exemplary video maker, but it nonetheless provides a glimpse at the complexity of maintenance of homeostasis, here specifically considering body temperate regulation.

Steady state

A system for which change occurs but no net change occurs, whether or not that lack of net change requires an ongoing input of energy.
Homeostasis to a large extent can be viewed as the maintenance of series of steady states within bodies. Specifically, this means that the relative constancy that bodies preserve in order to maintain health and associated functionality occurs only because bodies are constantly putting energy into the maintenance of these steady states that we collectively call homeostasis.

For example, body temperature is maintained at a steady state, with heat generated by the body exactly balanced by heat lost from the body. If body temperature declines, then the body takes measures that involve both reducing rates of heat loss and increasing rates of heat generation. Body temperature thus is maintained at a more or less constant level through the ongoing input of energy to keep body temperature at that more or less constant level. Body temperature thus represents a steady state. So too are concentration gradients that are maintained across membranes, such as are crucial to nerve impulse conduction.

An alternative way of viewing steady states is that they represent some quality that is actively maintained at some more or less constant level away from equilibrium. The juggling of balls, for example, is literally a steady state!

Links to terms of possible interest: Dynamic equilibrium, Energy, Equilibrium, Homeostasis, Steady state

Juggling really is a steady state. It is an ongoing, more or less constant condition that is maintained by a continuous input of energy. Watch the video to learn how!

The above video discusses the concepts of equilibrium and steady states, which are not quite the same thing.


The state that a system tends to achieve especially in the absence of external input.
Equilibrium occurs when a system is left on its own to "fall" (sometimes quite literally) to a stable state, one that requires no additional components to maintain. If for example we were to stop supplying energy to bodies, such that normal body temperature may be maintained, eventually the body's temperature will decline to that of its surrounding environment. We would say then that the body is in thermal equilibrium with its environment.

Equilibrium does not mean that nothing is happening but instead that nothing is happening in net, such that for every input there is a balancing output.

Living bodies explicitly are systems which are perturbed away from equilibrium via especially an ongoing input of energy. If that energy supply is sufficiently interrupted then not just body temperature but all homeostatic body processes will decline in functionality towards whatever their equilibrium state otherwise would be.

At death in particular, bodies achieve an equilibrium that they have spent literally a lifetime avoiding.

Links to terms of possible interest: Chemical reactions, Dynamic equilibrium, Energy, Equilibrium, Homeostatic body processes, Stable state, Static equilibrium, Steady state, Thermal equilibrium, Waste heat

A smooth surface is a surface at equilibrium. Energy put into the system, in the form of a falling droplet of water, perturbs the system away from equilibrium, but as the energy dissipates the systems returns to its original, smooth, equilibrium state.

The above video nicely introduces the concept of equilibrium from the perspective of chemical equilibrium, sort of…

There are a number of ways to present the concept of equilibrium. The above video shows this in terms of temperature.

There are a number of ways to present the concept of equilibrium. The above video shows this in terms of the interchangeability of two chemicals.

Re: dynamic equilibrium vs. steady state. These are two terms that most deservedly can be confused. The difference, though, is solely that a steady state requires energy to maintain (or at least can include such input) whereas a dynamic equilibrium does not require an input of energy to be sustained.

The difference with regard to our bodies is crystal clear. You take in energy, in the form of food, to maintain a steady state that we call homeostasis. Without this ongoing intake of energy your body loses an ability to maintain this steady state, with processes falling towards what at least briefly would be a dynamic equilibrium, but which also essentially is death.

Your car battery similarly is held at a steady state that requires an input of energy that balances that energy as it is used by your car. Equilibrium here would be a dead battery (and it is dynamic because in this case chemistry is still happening, just no net chemistry).

This is a lecture (of mine) titled Energy, Life, and Equilibrium.

Death and equilibrium and entropy.

Dynamic constancy

The idea that homeostasis actually consists of ranges in conditions that also can vary non-pathologically depending on circumstances.
Homeostasis actually is the maintenance not of precise values of various measures of body functioning and composition but instead the maintenance of ranges over which those values can normally vary. Indeed, under different circumstances it is normal for our bodies to display different levels for various physiological states.

If these levels are found outside of the normal, healthy range, then a healthy body will have the capacity not only to bring these values back to this normal range but also to modify these values, depending upon circumstances, so that the body can function optimally, such as towards running away from a predator.

These ideas that bodies are dynamic in terms various measures but nonetheless still more or less constant, particularly as averaged over longer time spans, has been dubbed as one of dynamic constancy.

Links to terms of possible interest: Body temperature, Dynamic, Dynamic constancy, Homeostasis, Normal body temperature, Parameters, Physiological, Set point

The above video provides a discussion of how body temperature varies over time, between locations of the body, and between different people. Emphasis is placed on both the need for and how one might go about figuring out what a person's normal body temperature is, as well as when abnormal body temperature should and should not be something to be terribly concerned about.

Set point

The normal level of a given homeostatic parameter.
We have set points for our blood glucose levels, our body temperature, our heart rates, our blood pressure, and even our body mass, etc. These are the values that our bodies tend to restore via negative feedback loops, that is, via homeostatic mechanisms, should our body's physiological states fall outside of their otherwise normal ranges.

A great deal of what body functioning is all about are mechanisms that the body employs to maintain the steady state associated with numerous set points. Note though that set points can vary under differing circumstances and also that the body will not be able to hold physiological states precisely at set points, hence the concept of dynamic constancy.

The above video provides a nice discussion of homeostasis including with lots of images, with a number of references to "Set point". There is additional wide-ranging but still homeostasis- (believe it or not) related discussion, but if you want to stick to homeostasis, feel free to quit around the 7:00 min point.

Negative feedback

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The tendency for perturbations of systems away from a given state to be resisted.
Homeostasis is maintained via a number of what can be described as negative feedback loops. Some of these negative feedback loops are relatively uncomplicated, involving only chemistry, while others are quite complicated, involving for example brain functioning, hormones, and the action of numerous body cells. What these processes all have in common is that the occurrence of some kind of change within the body is countered in such a way that the change is reversed back towards the norm of homeostasis (i.e., set points, resulting in dynamic constancy).

Part of the energy input required to maintain the steady states of homeostasis is towards both the maintenance and action of these negative feedback mechanisms.

When body tissues are low on oxygen, for example, various mechanisms involving hemoglobin, capillary functioning, and indeed additional breathing and cardiac output (heart functioning) conspire to make sure that specific body tissues no longer display this oxygen deficit. So too, when too much oxygen is present, these various mechanisms more or less reverse such that oxygen levels may be drawn down.

Links to terms of possible interest: Dynamic constancy, Homeostasis, Hormones, Negative feedback, Negative feedback loops, Set points, Steady states,

The above video considers feedback inhibition, a.k.a., negative feedback, as well as positive feedback.

The above video describes negative feedback using the functioning of a thermostat as an initial example, but then transitioning to talk about blood glucose levels.

Positive feedback

The tendency for perturbations of systems away from a given state to be amplified.
Positive feedback gives rise to wildly out of control situations. Normally this can be extremely dangerous for bodies. An exception however occurs when bodies need to change from one stable state to a substantially different stable state, but where the intermediate states are somewhat dangerous.

An important example of such a situation occurs with childbirth, where baby inside the mother and baby outside the mother are both relatively safe, stable situations, whereas baby being born is both quite dangerous and normally not stable at all. The body therefore employs positive feedback mechanisms which assure that once the transition has begun from inside to outside, the transition feeds back on itself, becoming stronger as it becomes more intense, such that the birth itself occurs over a relatively short time period.

The concept of feedback in music (audio feedback), by the way, is also an example of positive feedback (that is, rather than of negative feedback).

Links to terms of possible interest: Brain, Cervix, Childbirth, Negative feedback, Nerve impulses, Oxytocin, Parturition, Pituitary gland, Positive feedback, Uterus

The above video considers both negative feedback and positive feedback, describing them as loops and with reference to both set points along with various aspects of homeostasis. Included in some detail is maintenance via negative feedback of blood glucose levels and associated failures that are diabetes.

A little guitar lesson on how to get and manipulate feedback through your electric guitar. Note the admonishments to wear ear plugs!


All of the chemical reactions that take place within an organism.
Metabolism is a description of all of the chemical goings on within a body or, instead, within individual cells. As a large fraction of what life is all about are these chemical reactions, metabolism collectively describes a large fraction of what is going on inside of bodies at any given time.

Additional but also important considerations that are not quite considered under the heading of metabolism are interactions between molecules that do not involve chemical reactions, such as the holding together of plasma membranes (and other cell membranes), the interior of proteins as well as interactions between proteins, the interaction between hormones and receptor proteins, etc.

Links to terms of possible interest: Acetyl CoA, Amino acids, ATP, Carbohydrates, CO2, Citric acid cycle, e, Electron transport chain, Fats, Fatty acid spiral, Fatty acids, Glucose, Glucose-6-phosphate, Glycogen, H+, Lactic acid, Lipogenesis, NH3, Nitrogen pool, O,2 Protein, Pyruvic acid, Tissue protein, Urea, Urea cycle


Chemical reactions associated with organisms that break substances down to yield readily usable forms of energy.
Catabolism consists of all those chemical reactions that occur within bodies that serve to directly take chemical fuel – not ATP but still energy-rich compounds – and convert that fuel into a form that is more usable by the body, and especially by cells.

Important catabolic pathways include glycolysis, the Krebs citric acid cycle, and what is known as cellular respiration. The mitochondria found within cells are particularly important sites of catabolic reactions.

The primary products of catabolic reactions within our bodies are a combination of ATP, which is the primary energy carrier molecule within cells, along with water and carbon dioxide. The water liberated during cellular respiration contributes to the water that is found in our bodies and is known as metabolic water. The carbon dioxide builds up in the body and otherwise is removed via a combination of the actions of the circulatory system and the respiratory system.

Links to terms of possible interest: ADP, Anabolic reactions, Anabolism, ATP, Carbon dioxide, Catabolic reactions, Catabolism, Cellular respiration, Chemical reactions, Circulatory system, Dehydration synthesis, Energy, Energy coupling, Energy-rich compounds, Glycolysis, Hydrolysis of ATP, Inorganic phosphate, Metabolic water, Mitochondria, Pi, Respiratory system


Chemical reactions associated with organisms that are energy requiring and which build molecules up.
The building up of bodies or simply cells during growth or repair is an anabolic process. Consistently, we describe artificial hormones that can result in a building up of bodies, in terms of muscle, as anabolic steroids.

The process of anabolism requires both raw materials and energy. The raw materials are either synthesized within the body or otherwise derived from the molecules found within the food we eat. The energy in turn also is derived from the food we eat, but is processed by catabolic reactions to generate especially ATP.

Links to terms of possible interest: ADP, Anabolic process, Anabolism, ATP, Catabolic reactions, Catabolism, Chemical reactions, Energy, Energy source, Food Homeostasis, Metabolism, Muscle

Basal metabolic rate

Energy consumption observed by an organism that is at rest and otherwise not participating in any extraneous activities.
Basal metabolic rate (BMR) is a measure of one aspect of a body's metabolism, that is of the minimum ongoing energy usage that is required to keep the body going. This energy is supplied by food, with that food converted via cellular respiration largely to ATP to maintain basal metabolism.

Underlying this energy consumption is metabolism more generally, that is, the entirety of chemical reactions that occur within bodies on an ongoing basis. That is, even at rest a body is constantly using up energy. Even after fasting – which is when BMR is traditionally measured such that food is not being digested – the body is still constantly using up energy, even at rest or while sleeping.

The above video describes, very quickly, the degree to which Calorie usage declines with age.

Physiological adaptation

Particularly shorter-term changes that organisms display that serve to enhance organism functionality especially in response to new environmental conditions.
Physiological adaptations represent changes in organism metabolic processes so that an organism can maintain homeostasis despite changes to its environment or instead so that an organism can take advantage of resources that have become available in the environment.

This contrasts with behavioral responses, which can occur over even shorter time spans (e.g., seconds) than can physiological adaptations (e.g., minutes). This also contrasts with morphological adaptation or acclimatization, both of which tend to occur over much longer time frames (days or longer) than do physiological adaptations. We adapt physiologically, for example, to the consumption of food, excessive or otherwise, as well as to the absence of consumption of food, etc.

Physiological adaptation also contrasts with evolutionary adaptation, which involves changes in frequencies of genes (actually, of alleles) across populations of organisms over time, especially in response to environmental change, rather than changes in the functioning of an individual organism in response to environmental change.

Links to terms of possible interest: Adaptation, Arterioles, Bladder, Constriction, Consumption of food, Dilation, Ejaculation, Environmental conditions, Erection, Evolutionary adaptation, Homeostasis, Intestinal motility, Metabolic processes, Morphological adaptation, Organisms, Pancreas, Parasympathetic division, Peripheral nervous system, Physiological adaptation, Physiology, Pupil, Salivary flow, Stomach, Sympathetic division, Tear glands, Vasoconstriction, Vasodilation

Integrated functioning

Contribution of multiple interacting systems to the maintenance of homeostasis.
"Integrated functioning" is another way of saying that the functioning of bodies involves multiple components and that those components do not operate in isolation. Indeed, it is in part the interaction of these numerous components that can make body functioning somewhat difficult to appreciate.

This coordination of body systems towards homeostasis requires communication between different components, which in turn is mediated by a combination of hormonal communication (endocrine system), nerve impulses (nervous system), and ongoing mixing in the makeup of blood across much of the body. In this way, different systems found in different locations within the body can contribute in different but coordinated ways to body functioning.


Detection of changes by bodies thus leading to behavioral, physiological, or morphological change in body functioning.
Responsiveness is a basic characteristic of living organisms. These responses can be described in many cases as adaptive, meaning that an organism responds to signals emanating from within or about its body in such a way that serves to enhance an organism's ability to survive, gather resources, and/or reproduce.

Important responses are especially those that lead to modifications in body functioning in such a way that homeostasis is maintained.