5.2The Mechanism of Homeostasis

The processes involved in responding to changes in the external environment in order to maintain homeostasis of the internal environment can broadly be divided into two temporal phases. One involves swift reactions in response to sudden changes, leading to evasive actions by the central nervous system, and use of hormones to convert the activity of proteins within the organism through phosphorylation and dephosphorylation. Typically, these reactions occur over a short period of time ranging from microseconds to within a few minutes. The other process refers to slower reactions that alter cells or organs through expression of novel genes. These reactions occur over a period of time spanning a few hours to a few days. Numerous tissues and organs are also involved in response or adaptations to the environment. Thus, the mechanisms underlying homeostasis are both temporally and spatially diverse.

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A Summary of Body Fluids and Their Actions as Buffers

Up to 60% or more of the body mass of most organisms is water. Body fluids, which are liquid components that make up a body, are divided into several compartments. In unicellular organisms, body fluid exists only as an intracellular fluid. However, in multicellular organisms, body fluids are divided into intracellular and extracellular fluids; extracellular fluids can be further divided into intercellular or tissue fluids permeating the spaces between cells or within tissues and intravascular fluids circulating throughout the body (blood plasma and lymph in animals) (Fig. 5-2). When there is a change in the external environment, this fluctuation is first buffered by the plasma and sequentially dampened by the intercellular fluid and then the intracellular fluid. In unicellular organisms, the extracellular fluid corresponds to the external environment; therefore, fluctuations are buffered only by the intracellular fluid. Thus, the ultimate target of homeostasis is the intracellular environment (the intracellular fluid), regardless of whether in a unicellular or multicellular organism (Fig. 5-2).

Fig. 5-2 Biological Buffering

Changes to the external environment are buffered in an organism by the plasma, intercellular fluid, and intracellular fluid in that order, and homeostasis of the internal environment is maintained within the cell.

The plasma, which is the first to respond to fluctuations in the external environment, performs material transactions with the external environment through the intestines, respiratory system, kidneys, and skin and stabilize the body fluid composition. Moreover, cells perceive changes via receptors found in the cell membrane, and thus, transportation of substances through the cell membrane and the metabolic rate in the cell are regulated to maintain homeostasis of the internal environment.

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Body Fluid Regulation: Migratory Fish an Example

Eels and salmon, which migrate between rivers and seas, are similar to humans in that the osmotic pressure of their body fluids is approximately 300 mOsm. Thus, in environments with different osmotic pressure, such as freshwater at a few mOsm or seawater at 1000 mOsm, these fishes must somehow maintain the homeostasis of their body fluids. In freshwater, ions are actively absorbed through the gills to counteract an ion deficiency, while water permeating through the gills because of the difference in osmotic pressure is discharged through the kidneys as large quantities of dilute urine. In contrast, in seawater, to recover water lost because of osmotic pressure, fish vigorously drink seawater and absorb almost all the ingested seawater along with ions into the intestines, and these ions are actively expelled by the gills and kidneys (Fig. 5-3). The ions discharged through the gills are monovalent ions such as Na+ or Cl, while those discharged through the kidneys are divalent ions such as Mg2+, Ca2+, or SO42−. When ion concentrations of seawater and body fluids are compared, a greater difference is observed in these divalent ions than in monovalent ions (Fig. 5-3). Therefore, despite a mechanism to prevent infiltration, some ions do pass in. The kidneys are responsible for discharging these ions. Therefore, the main organs regulating body fluids in fish are the gills, intestines, and kidneys.
Let us now take a look at temporal regulation in response to environmental changes. When an eel is transferred from freshwater to saltwater, within 1 min, it begins to energetically drink the seawater. The act of drinking water is triggered by nervous system regulation in response to changes in Cl− concentration of the water in the environment. Within several minutes, the gills and intestines also switch to acting as transporters of ions and water, and this early-stage regulation is performed by small peptide hormones. However, approximately 1 week after the eel is transferred to seawater, significant changes occur in its gills and intestines. In the gills, chloride cells develop that concentrate and discharge the monovalent ions (Fig. 5-4). In the intestines, epithelial cells become thin and blood vessels begin to develop on the surface. The intestinal wall therefore becomes reddish and transparent, and the inside of the intestinal tract can be observed (see Column the bottom). Further absorption of water and ions can be promoted by developing blood vessels. The effects of this long-term adaptation to changes in conditions are observed because of high-molecular-weight protein or steroid hormones. Thus, nerves and hormones have been found to have various effects both spatially and temporally.

Fig. 5-3 A Schematic showing the Intake and Output of Water and Ions in a Saltwater Fish

Divided into active transfer, which uses energy, and passive transfer through a concentration gradient. The primary ionic compositions of salt water and plasma are shown in the table.

Fig. 5-4 Differences in the Size and Activity of Chloride Cells found in Fish Gills in Freshwater and Saltwater Fish

The portions in white are chloride cells containing Na+/K+-ATPase (sodium pump) and those in red represent nuclei.

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Why Do Saltwater Fish Drink Seawater?

A human stranded at sea should never drink seawater. Because the kidneys are unable to concentrate ions in urine at the levels found in seawater, drinking seawater leads to an excessive loss of water resulting from the discharge of ions. Moreover, because seawater contains high concentrations of Mg2+ and SO42− (Fig. 5-3), the intestines basically cannot absorb these divalent ions, and therefore, an individual develops diarrhea after drinking seawater. The “nigari diet,” which involves drinking water containing MgSO4, was temporarily popular, but body weight does not increase when water absorption is suppressed. Therefore, how is it that saltwater fish absorb water even after drinking seawater?

Column Fig. 5-1 Intestines of a Saltwater Eel

Blood vessels develop and turn red to adapt to seawater, and white deposits in the intestinal lumen can be observed. SW represents the osmotic pressure of seawater.

NaCl from the ingested seawater is first selectively absorbed by the esophagus and diluted to one-half. Then, it is diluted in the stomach and proximal part of the intestines, and in the middle part of the intestines, it is reduced to one-third by having almost the same osmotic pressure as the body fluids. Here, four ions are simultaneously absorbed through the cotransporter Na-K-2Cl so that in parallel, water is absorbed into the body through aquaporin. Unlike in humans, the absorbed excessive NaCl is concentrated and discharged by the chloride cells, which is described in this chapter.

On the other hand, the intestines of saltwater fish have a mechanism that discharges bicarbonate ion HCO3 into the bowels and the concentrated Mg2+ and Ca2+ are deposited as carbonate or sulfate. Therefore, 1 week after the eel has been transferred to saltwater, white deposits can be observed in the distal part of the intestines (Column Fig. 5-1). To deposit dissolved ions in this manner, the osmotic pressure is lowered, which makes it easier to absorb water. Thus, saltwater fish can absorb 95% or more of the ingested seawater and thus avoid diarrhea.

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