5.1The External Environment and Homeostasis of the Internal Environment
The terrestrial environment is inhospitable to life. The temperature range from the equator to the poles is greater than 100°C, and the humidity range approaches 100% from the deserts to the tropical rainforests. The effect of gravity is also a major constraint on the movement of terrestrial animals (quadrupeds), and its influence is also felt by astronauts returning to Earth. On the other hand, the environment for aquatic animals (fish) is stable. Water experiences little fluctuation in temperature because of its high specific heat, and the effects of gravity are almost negated by the buoyancy created from its high density. However, the high specific heat makes it difficult for endotherms to maintain their body temperature. The passengers of the Titanic, which sank in the North Atlantic sea, could only survive for a short period in the frigid sea. The high density of water presents a challenge in the form of hydraulic pressure for deep-sea fish (the pressure reaches 101 atm at a water depth of 1000 m). The enormous amount of nutritive salts (organic salts) dissolved in water, an excellent solvent, is an abundant nutrient source for aquatic organisms. However, the large quantities of dissolved inorganic salts elevate the osmotic pressure of seawater. Therefore, migratory fishes such as eels and salmon, which migrate in and out of rivers and oceans, struggle for survival against an excess of water and insufficiency of salts in freshwater, and conversely, struggle for survival against dehydration and an excess of salt in seawater. Moreover, light cannot reach water depths greater than 200 m; therefore, photosynthetic plants cannot survive at such depths. Thus, even when only considering physical factors such as temperature, pressure, osmotic pressure, and light, external environments inhabited by organisms are diverse, and each of these factors has been found to fluctuate with time.
In 1878, Claude Bernard presented the idea that an organism’s environment is divided into the external environment surrounding an organism and the internal milieu (inside the organism). The ability to maintain the internal milieu as almost constant, functioning against the changes in the external environment, is called homeostasis. In humans, body temperature is maintained at approximately 36°C, mean blood pressure at approximately 100 mmHg, and blood plasma osmotic pressure at approximately 300 mOsm, such that some fluctuation in the variable factors of the physical environment scarcely affect these factors. The concept of homeostasis was first used by Walter Cannon in 1926. It refers to a dynamic state by referring to a similar (“homeo-”) rather than same (“homo-”) condition (“-stasis”). The nervous and endocrine systems play an important role in maintaining homeostasis (Fig. 5-1). Recently, it is also believed that biological defense systems, such as the immune system, also play a leading role in maintaining homeostasis. Changes in the external environment that affect an organism are widely considered to be stresses such as physical or chemical stresses as well as biological stresses such as exposure to bacteria. The mechanism underlying the functioning of the immune system will be explained in greater detail in Chapter 21.
The nervous system exerts the first response against fluctuations in the external environment. Fluctuations in external physical factors are perceived as signals by receptors such as those for temperature, pressure, and osmotic pressure. These signals are then transmitted to the central nervous system by sensory nerves and are quickly regulated by autonomic nerves. However, the endocrine system plays an important role, beyond the role of the nervous system, in maintaining homeostasis. Hormones, which are the messengers of the endocrine system, are of many different types and are immediately secreted to act on environmental changes and provide fortification in order to adapt to new environments over a long period (to be continued).