Homeostasis - Biological

Biological

With regard to any given life system an organism may be a conformer or a regulator. Regulators try to maintain parameters at a constant level over possibly wide ambient environmental variations. Conformers allow the environment to determine parameters. For instance, endothermic animals (mammals and birds) maintain a constant body temperature, while ectothermic animals (almost all other organisms) exhibit wide body temperature variation.

Behavioral adaptations allow ectothermic animals to exert some control over a given parameter. For instance, reptiles often rest on sun-heated rocks in the morning to raise their body temperature. Regulators are also responsive to external circumstances, however: if the same sun-baked boulder happens to host a ground squirrel, the animal's metabolism will adjust to the lesser need for internal heat production.

An advantage of homeostatic regulation is that it allows an organism to function effectively in a broad range of environmental conditions. For example, ectotherms tend to become sluggish at low temperatures, whereas a co-located endotherm may be fully active. That thermal stability comes at a price since an automatic regulation system requires additional energy. One reason snakes may eat only once a week is that they use much less energy to maintain homeostasis.

Most homeostatic regulation is controlled by the release of hormones into the bloodstream. However, other regulatory processes rely on simple diffusion to maintain a balance.

Homeostasis includes regulation of the pH of the blood at 7.365 (a measure of alkalinity and acidity). All animals also regulate their blood glucose, as well as the concentration of their blood. Mammals regulate their blood glucose with insulin and glucagon. The human body maintains glucose levels constant most of the day, even after a 24-hour fast. Even during long periods of fasting, glucose levels are reduced only very slightly. Insulin, secreted by the beta cells of the pancreas, effectively transports glucose to the body's cells by instructing those cells to keep more of the glucose for their own use (see Dynamic equilibrium). If the glucose inside the cells is high, the cells will convert it to the insoluble glycogen to prevent the soluble glucose from interfering with cellular metabolism. Ultimately this lowers blood glucose levels, and insulin helps to prevent hyperglycemia. When insulin is deficient or cells become resistant to it, diabetes occurs. Glucagon, secreted by the alpha cells of the pancreas, encourages cells to break down stored glycogen or convert non-carbohydrate carbon sources to glucose via gluconeogenesis, thus preventing hypoglycemia. The kidneys are used to remove excess water and ions from the blood. These are then expelled as urine. The kidneys perform a vital role in homeostatic regulation in mammals, removing excess water, salt, and urea from the blood. These are the body's main waste products.

Another homeostatic regulation occurs in the gut. Homeostasis of the gut is not fully understood but it is believed that Toll-like receptor (TLR) expression profiles contribute to it. Intestinal epithelial cells exhibit important factors that contribute to homeostasis: 1) They have different cellular distribution of TLR's compared to the normal gut mucosa. An example of this is how TLR5 (activated by flagellin) can redistribute to the basolateral membrane, which is the perfect place where flagellin can be detected. 2) The enterocytes express high levels of TLR inhibitor Toll-interacting protein (TOLLIP). TOLLIP is a human gene that is a part of the innate immune system and is highest in a healthy gut; it correlates to luminal bacterial load. 3) Surface enterocytes also express high levels of Interleukin-1 receptor (IL-1R) -containing inhibitory molecule. IL-1R are also referred to as single immunoglobulin IL-1R (SIGIRR). Animals deficient in this are more susceptible to induced colitis, implying that SIGIRR might possibly play a role in tuning mucosal tolerance towards commensal flora. Nucleotide-binding oligomerisation domain containing 2 (NOD2) is suggested to have an effect on suppressing inflammatory cascades based on recent evidence. It is believed to modulate signals transmitted through TLRs, TLR3, 4, and 9 specifically. Mutation of it has resulted in Crohn's disease. Excessive T-helper 1 responses to resident flora in the gut are controlled by inhibiting the controlling influence of regulatory T cells and tolerance-inducing dendritic cells.

Sleep timing depends upon a balance between homeostatic sleep propensity, the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode, and circadian rhythms that determine the ideal timing of a correctly structured and restorative sleep episode.