Dogs regulate their body temperature through panting. Have you ever wondered why dog pants on hot days? It's because dog tries to maintain homeostasis. Homeostasis is a word that biologists use to describe how a living organism regulates its internal environment to maintain stability. One very important part of homeostasis is body temperature.

Homeostasis is the property of any system especially a living organism, that regulates its internal environment so as to maintain a stable, constant condition. It is the tendency or ability of an organism or a cell to maintain internal equilibrium by adjusting its physiological processes. Body temperature in mammals, for example, is regulated by a sensor that contains the cells within the hypothalamus of the brain, sweating to reduce temperature.

Other homeostatic mechanisms include: those controlling blood glucose levels, blood acidity, and hormone secretions. It was also suggested that percentage fat composition and body weight have similar control systems. These homeostatic mechanisms involve the neural and endocrine systems of mammals.

Organization: Every organism has a definite biological organization. The external appearance of organism is invariably accompanied by an even more complex internal structure. Plants as well as animals are composed of a number of elements. Atoms in the body of an organism are again united into simple and complex molecules, which are further arranged to form different units like cells, tissues, organs.

Response to stimuli in Sunflower and Mimosa The leaves of a plant and flower heads turn towards the sun such as in Sunflower. Some plants such as Mimosa leaves close on being touched. These are the examples of physiological actions.

In physiology, a stimulus is a detectable change in the internal or external environment and what the organism expresses because of the stimuli is called response.

The ability of an organism or organ to respond to external stimuli is called sensitivity. In medical terms, stimulus excites an organism or a part of it to functional activity. It is something that incites to action or exertion or quickens action, feeling, thought, etc., substance or phenomenon that causes or changes activity of an organism.

A response can take many forms from the contraction of a unicellular organism when touched to complex reactions involving all the senses of higher animals. Responses are rather easily identified in animals than in plants. Stomata in leaves open up in response to light stimulus. Hunger and thirst are internal stimuli to which animals respond by going in search of food or water. A response is often expressed by motion, for example, the leaves of a plant turning towards the sun such as in Sunflower or an animal chasing its prey. Some plants such as Mimosa leaves close on being touched.

Living things are always connected to environment Living things are systems that tend to respond to changes in their environment, in such a way as to promote their own continuation.

Viruses and aberrant prion proteins are often considered replicators rather than forms of life, a distinction warranted because they cannot reproduce without very specialized substrates such as host cells or proteins, respectively. Also, the Rickettia and Chlamydia are examples of bacteria that cannot independently fulfill many vital biochemical processes, and depend on entry, growth, and replication within the cytoplasm of eukaryotic host cells.

Proposed definitions of life include:

1. Living things are systems that tend to respond to changes in their environment, and inside themselves, in such a way as to promote their own continuation.
2. Life is a characteristic of self–organizing, self–recycling systems consisting of populations of replicators that are capable of mutation, around most of which homeostatic, metabolizing organisms evolve.
3. Type of organization of matter producing various interacting forms of variable complexity, whose main property is to replicate almost perfectly by using matter and energy available in their environment to which they may adapt. In this definition "almost perfectly" relates to mutations happening during replication of organisms that may have adaptive benefits.
4. Life is a potentially self–perpetuating open system of linked organic reactions, catalyzed simultaneously and almost isothermally by complex chemicals (enzymes) that are themselves produced by the open system.