Homeostasis & Feedback Mechanism	Negative & Positive Feedback Loops	Thermoregulation	Why Thermoregulate	Countercurrent Heat Exchangers	Regulating Behaviorally	Thermoregulation in Endotherms	Internal Source of Thermoregulation
Osmoregulation & Excretion	Nitrogenous Wastes	Osmotic Environment	Human	Anatomy	Microanatomy of Nephron	Control of Nephron

Homeostasis & Feedback Mechanism

• Homeostasis is defined as the tendency for physiological systems to maintain internal stability through the coordinated response of its parts to anything tending to disturb such stability.
• Homeostasis can operate through short term, immediate responses or through long term, cyclic responses.

Negative & Positive Feedback Loops

• Many homeostatic mechanisms are regulated by negative feedback loops, whereby a stimulus creates a response that in term alters or removes the stimulus, thus lessening or stopping the response.
• Positive feedback loops also occur, but they are generally a sign of physiological trouble. In this case a stimulus creates a response that in turn intensifies the stimulus, thus intensifies the response, and so on.
• Two homeostatic mechanisms in animals are thermoregulation and osmoregulation. The latter is often involved with another function, excretion, which occurs in the excretory system.

Thermoregulation

• Thermoregulation is an act of maintaining body temperatures within a certain range. Animals that maintain a relatively constant internal temperature are endothermic  (or homeothermic), while those animals those animals whose body temperatures vary with surroundings are ectothermic (or poikilothermic).

Why Thermoregulate

• Under sever cold, the alternative to thermoregulation is metabolic inactivity. With excessive heat loss a positive feedback loop begins, as cold slows the metabolic activity required to produce body heat.
• Surviving high temperatures is often more difficult that surviving low ones since the first may destroy essential enzymes while the second only temporarily inactivates them.

Countercurrent Heat Exchangers

• Excessive loss of body heat is prevented by countercurrent exchange, more specifically, countercurrent heat exchangers in the circulatory system, whereby warmed blood from deep in the body passes through vessels closely paralleling those carrying cooler blood from the extremities. The cooler blood takes up the heat as it returns, thus keeping heat out of the extremities where it would be lost.
• Countercurrent exchanges abound in the major blood vessels of the bluefin tuna and the in the rete mirable, a dense region of countercurrent vessels that keeps the muscles warm and active on spite of the cold surrounding water.

Regulating Behaviorally

• Animals actively seek out areas where temperatures are optimal. Reptiles, generally considered ectotherms, use basking behaviors, in which they expose their bodies maximally to warm up and minimally to cool down. Color changes through pigment migration in some species help in either the absorption or reflection of sunlight. Physiological mechanisms of thermoregulation in reptiles are suspected.

Thermoregulation in Endotherms

• Endotherms thermoregulate by using the metabolic heat produced by the body's oxidative processes.
• The circulatory system hastens cooling by shunting blood to the skin, where heat is lost through radiation (passage through air), conduction (passage to cooler solids) and convection (being carried away by perspiration)
• The evaporation of sweat greatly accelerates heat loss because water holds heat. Evaporative cooling fails where the relative humidity is very high.
• In retaining heat, most blood is shunted away from the surface areas, particularly into deeper veins that run parallel to the arteries. The countercurrent exchange internalizes much of the heat but decreases activity in the extremities.  Upon exposure to extreme cold, the less vital extremities freeze before the general body temperature falls to critical levels.
• Birds and mammals conserve heat through piloerection, improving insulation by fluffing the down feathers or body hair.

Internal Source of Thermoregulation

• The body also regulates metabolically - increasing or decreasing heat output by varying the rate of cell respiration.
• Temperature changes are detected by sensory neurons, and reactions may include increased heat production, shivering, piloerection, or behavioral responses.
• Heat measurements are also made by the hypothalamus of the brain.  In response it may
• stimulate the adrenal medulla to release the hormone epinephrine, which prompts the liver to release glucose for increased cell respiration and heat output;
• prompt the pituitary to release thyroid-stimulating hormone, which in turn causes the thyroid gland to release the metabolism elevating hormone, thyroxin.
• The hypothalamal thermostat may measure calcium levels rather than heat.  Suffusing the brain with calcium ions for cryogenic surgery will cause a reduction in metabolic activity and a cooling of the body.

Osmoregulation and Excretion

• Osmoregulation is the ability of animals to regulate ions and water in the body.  Osmoregulation is closely related to excretion, the removal of metabolic wastes.

Nitrogenous Wastes

• During deamination, the amine groups are removed from amino acids as toxic ammonia, NH₃, which is excreted as is by some aquatic animals.
• In insects, reptiles, and birds, the primary excretory waste is uric acid, while for many other invertebrates, and the fishes, amphibians and mammals, the primary excretory waste is urea.

Osmotic Environment

• The marine environment is a hypertonic medium in which organisms tend to lose water and gain ions.  The tissues of osmoconformers conform to the surroundings, becoming isotonic.  Osmoregulators have mechanisms for actively removing ions.
• Osmoconformers include the limpet, Acamea, along with sharks and rays and coelocanth.  The latter three maintain their isoosmotic condition by retaining urea in the blood and body fluids.  Sharks and rays also osmoregulate by actively pumping salts out of their bodies.
• Osmoregulators such as the crustacean, Artemia, and the marine bony fishes actively secrete salts out through the gill.  Such secretion permits the bony fish to drink sea water.  The nitrogen waste of bony fishes is ammonia, which excreted across the gill.  Marine birds and reptiles secrete salts from glands located near the eye, while marine mammals use the kidney for this purpose.
• The freshwater environment is hypotonic, so water tends to enter the body through osmosis.  Conversely ions tend to diffuse out of the body.  Since water conservation is not necessary, nitrogenous wastes can be readily flushed out with water.
• Some freshwater invertebrates use the flame cell system or protonephridium.  Excess fluids are transported into the flame bulb by pinocytosis, and the waving cilia push the fluids through the tubules and out through pores in the body wall.
• The kidney of the freshwater fish recovers some ions, but some must also be actively transported in through gill structures.
• The tadpole excretes ammonia, but the adult frog shifts to urea excretion.  Amphibians actively transport ions in through the skin.
• Osmoregulatory problems in the terrestrial environment primarily involve conserving water.
• The earthworm's many nephridia remove nitrogenous wastes and excess salts, releasing them through the nephridiopore, but reclaim water and other essential materials.
• Arthropods have a watertight cuticle and often rely on metabolic water.  Their Malpighian tubules collect nitrogenous wastes from coelomic fluids and produce uric acid, which is excreted into the gut.  The hindgut reabsorbs essential water, producing a semidry waste.
• All terrestrial vertebrates have specialized, water reabsorbing kidneys.  While reptiles and birds and their embryos produce uric acid, which can be excreted in a semidry state, mammals produce urea, which requires water for its excretion.

Human Excretory System

• The human excretory system consists of the kidneys, ureters, urinary bladder, urethra and renal circuit (renal arteries and renal veins).
• The kidney includes an outer cortex, middle medulla and the nephrons.  The nephrons include a capsule and a looping tubule that joins others to form the collecting ducts, making up the pyramids.  The pyramids empty into the calyces, which lead into the renal pelvis.
• The nephrons form urine, which passes from the collecting ducts to the renal pelvis.  The renal pelvis empties into the ureters, which conduct urine to the urinary bladder, and the urethra voids the urine from the body.

Microanatomy of Nephron

• The nephron begins with Bowman's capsules, which surrounds the glomerulus, a ball of capillaries arising from an afferent arteriole of the renal artery.  Leaving the glomerulus is an efferent arteriole, which forms the peritubular capillaries, where reabsorption takes place.  These spread over the nephron to later form a venule that joins others to make up the renal vein.
• Bowman's capsule leads to the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule, which joins a collecting duct.  The afferent arteriole also connects with the distal convoluted tubule, forming the juxtaglomerular complex.
• Each part of the nephron functions as follows:
• Bowman's capsule.  Force filtration in Bowman's capsule causes much of the water and ions and smaller molecules to leave the blood and enter the proximal convoluted tubule.
• The proximal tubule. the peritubular capillaries contain blood in a hyper osmotic state, so much of the water filtrate reenters the blood by osmosis.  Active transport also returns sodium (chloride following passively), glucose, and amino acids to the blood.
• The loop of Henle. The ascending loop actively transports chloride ions (sodium ions follow passively) into the surrounding area, recycling salt and creating a hyperosmotic state in the kidney medulla.  The hypertonic state is further increased by urea, which diffuses out of the collecting ducts.
• The distal tubule.  The active secretion of sodium ions occurs with chloride ions and water passively following.  Potassium ions enter the tubule.
• Collecting ducts.  Water leaves the collecting ducts in response to antidiuretic hormone (ADH), which is secreted by the posterior pituitary in response to osmotic conditions in the blood (actually detected by the hypothalamus).
• Tubular secretion forces ammonia, hydrogen ions, potassium ions, organic acids and creatine into the tubule.

Control of Nephron

• Nephron control is hormonal, with water reabsorption controlled by ADH from the posterior pituitary and sodium chloride reabsorption controlled by aldosterone from the adrenal medulla.  Sodium chloride transport is monitored by the juxtaglomerular complex.  The arteriolar cells secrete renin, which stimulates the adrenal cortex to secret aldosterone.  Aldosterone increases the absorption of sodium chloride and the excretion of potassium.