Human homeostasis

Human homeostasis is the homeostasis of humans, i.e. the property to regulate the internal environment of the body so as to maintain a stable, constant condition.

Humans are regulators, in contrast to many other animals, rather than conformers, in the sense that the human body tries to maintain parameters at a constant level over possibly wide ambient environmental variations. Much of this is performed by mechanisms of negative feedback.

The kidneys are major contributors to human homeostasis, regulating in five important ways: regulation of blood water levels, reabsorption of substances into the blood, maintenance of salt and ion levels in the blood, regulation of blood pH, and excretion of urea and other wastes.

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Temperature

Main article: Human thermoregulation

Humans are warm-blooded, maintaining a constant body temperature. The regulation works by negative feedback: Its thermostat houses a thermometer, the receptor that senses when the temperature is too low. The control center, also housed in the thermostat, senses and responds to the thermometer when the temperature drops below a specified set point. Below that target level, the thermostat sends a message to the effector, the furnace. The furnace then produces heat, which warms the house. Once the thermostat senses a target level of heat has been reached, it will signal the furnace to turn off, thus maintaining a comfortable temperature - not too hot nor cold. ^[1]

Blood composition

The balance of many blood solutes belongs to renal physiology.

Sugar

See also: Blood sugar and Carbohydrate metabolism

Humans regulate their blood glucose with insulin and glucagon. These hormones are released by the pancreas.

When blood sugar levels become too high, insulin is released from the pancreas, lowering the blood sugar levels. On the other hand, when blood sugar levels become too low, glucagon is released, increasing blood sugar levels.

If the pancreas is for any reason unable to produce enough of these two hormones diabetes results.

Fats

See also: Blood lipids

Osmoregulation

Main article: Osmoregulation

Osmoregulation is the active regulation of the osmotic pressure of bodily fluids to maintain the homeostasis of the body's water content; that is it keeps the body's fluids from becoming too dilute or too concentrated. Osmotic pressure is a measure of the tendency of water to move into one solution from another by osmosis. The higher the osmotic pressure of a solution the more water wants to go into the solution.

The kidneys are used to remove excess ions from the blood, thus affecting the osmotic pressure. These are then expelled as urine.

Pressure

See also: Renin-angiotensin system

The renin-angiotensin system (RAS) is a hormone system that helps regulate long-term blood pressure and extracellular volume in the body.

Calcium

When blood calcium becomes to low, calcium-sensing receptors in the parathyroid gland becomes inactivated. This results in release of PTH, which acts to increase blood calcium, e.g. by release from bones.

On the other hand, excessive blood calcium levels causes an activation of calcium-sensing receptors in the parathyroid gland, resulting in decreased PTH release and a decrease in blood calcium.

Calcitonin works the opposite way, increasing calsium levels.

Acid-base

Main article: Acid-base homeostasis

The kidneys maintain acid-base homeostasis by regulating the pH of the blood plasma. Gains and losses of acid and base must be balanced. The study of the acid-base reactions in the body is acid base physiology.

Volume

Main article: Fluid balance

The body's homeostatic control mechanisms, which maintain a constant internal environment, ensure that a balance between fluid gain and fluid loss is maintained. The hormones ADH (Anti-diuretic Hormone, also known as vasopressin) and Aldosterone play a major role in this.

• If the body is becoming fluid-deficient, there will be an increase in the secretion of these hormones, causing fluid to be retained by the kidneys and urine output to be reduced.
• Conversely, if fluid levels are excessive, secretion of these hormones is suppressed, resulting in less retention of fluid by the kidneys and a subsequent increase in the volume of urine produced.

Hemostasis

Main article: Hemostasis

Hemostasis is the process whereby bleeding is halted. A major part of this is coagulation.

Platelet accumulation causes blood clotting in response to a break or tear in the lining of blood vessels. Another example is the release of oxytocin to intensify the contractions that take place during childbirth.^[1]

Sleep

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 which determine the ideal timing of a correctly structured and restorative sleep episode.^[2]

Extracellular fluid

The kidneys by regulating the blood composition, also controls the extracellular fluid homeostasis.

Imbalance

Much disease results from disturbance of homeostasis, a condition known as homeostatic imbalance. For instance, heart failure has been seen where negative feedback mechanisms become overwhelmed, and destructive positive feedback mechanisms then take over.^[1]

Diseases which result from a homeostatic imbalance include diabetes, dehydration, hypoglycemia, hyperglycemia, gout and any disease caused by a toxin present in the bloodstream. All of these conditions result from the presence of an increased amount of a particular substance. In these cases medical intervention is necessary to restore the imbalance or permanent damage to the organs may result.

Temperature may enter a circle of positive feedback, when temperature reaches extremes of 45ºC (113ºF), at which cellular proteins denature, causing the active site in proteins to change, thus causing metabolism stop and ultimately resulting in death.

References

1. ^ ^{a b c} Marieb, Elaine N. & Hoehn, Katja (2007). Human Anatomy & Physiology (Seventh ed.). San Francisco, CA: Pearson Benjamin Cummings.
2. ^ Wyatt, James K.; Ritz-De Cecco, Angela; Czeisler, Charles A.; Dijk, Derk-Jan (October 1999). "Circadian temperature and melatonin rhythms, sleep, and neurobehavioral function in humans living on a 20-h day". Am J Physiol 277 (4): R1152-R1163. Fulltext. Retrieved on 2007-11-25. “... significant homeostatic and circadian modulation of sleep structure, with the highest sleep efficiency occurring in sleep episodes bracketing the melatonin maximum and core body temperature minimum”