# General Biology/Tissues and Systems/Homeostasis

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## Homeostasis

Is a very important part of everyone's and everything's lives. Defined as dynamic constancy of internal environment, maintenance of a relatively stable environment inside an organism usually involving feedback regulation.
Homeostasis is maintained in face of
* a varying external environment, or
* a non-ideal, constant external environment (as with the penguin).
Deals with temperature, pH, chemical concentrations,pressure, oxygen levels.
Occurs through negative feedback loops.
Various forms: simple thermostat in house turns off heater when above a certain temperature and on when below a certain temperature Involves stimulus, sensor,integrating center, effector and response.
More efficient control has two sensors and two effectors. Can be antagonistic to each other, such as, one cools, the other heats.
Precise control through proportional control, not all-or-none, furnace comes on a little bit if the house a bit cold. Examples in humans: vasoconstriction, change in metabolic rate, shivering. Physiological responses for high body temp: blood goes to body surface, sweating, behavioral changes (get out of sun).
Positive feedback loop: effector increases deviation from set point. Amplifies reaction. Like blood clotting process, uterine contraction during childbirth. Negative feedback must exist at some point for control.

## Osmotic environments and regulations

* Marine invertebrates
** fully marine invertebrates (not intertidal or estuarine) osmoconformers (set internal environment same as environment, no net flow of ions) in a stenohaline (narrow non-changing salt level) environment
** Coastal, intertidal, estuarine (ion levels fluctuate) invertebrates.Partly osmoconfomers, partly osmoregulators in a euryhaline (wide salt level variation) environment (ex: shore crab, regulates sometimes when salt levels in environment get real low).
* Freshwater animals. Here, environment has lower solute concentrations than do living organisms so water tends to flow in and solutes out.
** Freshwater fish (bony) dilute urine, and gills actively take up ions (NaCl)
** Freshwater invertebrates: same situation as freshwater fish but with different structures
** Freshwater amphibians: active uptake of salts across their skin
* Marine fishes: Here the environment has a higher solute concentration than does the organism so water tends to flow out and ions in.
** Bony fishes: actively secrete salts (NaCl) across gills, absorb water across gut wall, their kidney (unlike mammalian kidney) is unable to generate concentrated urine so glomerulus is reduced, active tubular secretion of MgSO$_4$
** cartilaginous fishes (and coelacanth): blood retains urea and trimethylamineoxide to increase its osmolality to that of seawater
* Terrestrial animals: here problem is loss of water to a drier environment, and regulation of salt levels.
** water loss adaptations
** concentrated exception of salts and nitrogenous wastes
Hypoosmotic: having less osmotic potential than nearby fluid
Hyperosmotic: having more osmotic potential than nearby fluid
Isoosmotic: having equal osmotic potential than nearby fluid
Glomerulus: reduces volume of kidney
Fish started in salt water, spread to fresh water, later reinvaded salt-water environment.
Terrestrial animal water sources: 1. drinking 2. moist foods 3. from breakdown of metabolic molecules like fats. (Desert kangaroo rats get 90% of their water from metabolism.)
Secretion of nitrogenous wastes: from metabolism of amino acids, amino group has to be removed in one of three basically interchangeable chemical forms: 1. ammonia (aquatic life) 2. urea (mammals) 3. uric acid (birds)
Ammonia very toxic, soluble, and cheap to produce. Easy to expel for bony fishes.
Urea: low toxicity, good solubility, more costly to lose as it contains other groups on it. Must be released in solution, water cost.
Uric acid (white part of bird poo) low toxicity, insoluble, secreted with little water loss, more costly side groups lost than the others.
Mammalian kidney: Structure: fist-sized organ in lower back. About 1/5 of blood from aorta at any time is passing through kidneys. Blood passes through kidney many times a day.
Nephron: structural and functional unit of kidney.
Bowmans capsule: funnel-like opening, contains primary filter, the glomerulus.
Proximal convoluted tubule: receives stuff from Bowmans capsule.
Loop of Henle: descends and ascends.
Vasa recta: capillaries that surround the Loop of Henle.
Glomerulus: main filter of the nephron, located within the Bowman's capsule
Kidney properties and processes important to its function
1. Active transport of solutes from one fluid to another against a concentration gradient, Na$^+$ actively transported out of filtrate by cells of the thick ascending loop of Henley into the interstitial fluid 2. Passive movement of solutes and water from one fluid to another(down a concentration gradient), movement of water and NaCl out of descending loop of Henley into interstitial fluid. 3. Differential permeability of cells in different regions of the nephron to movement of water and solutes, ascending thick look is impermeable to water, descending portion is permeable to water 4. Hormonal control of that permeability, antidiuretic hormone(ADH) increases permeability of collecting due to water, resulting in reduced volume of filtrate and thus more concentrated urine. 5. Increasing solute concentration in the interstitial fluid of the kidney, from the cortex to the deepest medulla, maintained by a countercurrent multiplier mechanism
General Biology | Getting Started | Cells | Genetics | Classification | Evolution | Tissues & Systems | Additional Material