Ch. 30 Osmoregulation

Bird and Mammal Kidneys: Countercurrent Multiplication

"Countercurrent" refers to the opposite direction of fluid movement in the loop of Henle; fluid goes down the descending and up the ascending limb "Multiplication" describes the increasing osmotic concentration in the medulla due to ion exchange between the two limbs Descending limb of the loop of Henle is permeable to water but impermeable to solutes Ascending limb of the loop of Henle is impermeable to water Sodium chloride is actively transported from the thick portion of the ascending limb and into surrounding tissue fluid

Conservation of filtrate in the urine

(1) modification of the composition of the filtrate through tubular reabsorption and secretion, and (2) changes in the total osmotic concentration of urine through regulation of water excretion. -Most reabsorption is by active transport with hydrolysis of ATP by carrier proteins to allow movement of various electrolytes Essential function of kidney is to regulate plasma concentration of electrolytes using unique ion pumps specific for each electrolyte Water passively diffuses through the open protein channels and osmotically follows the active reabsorption of solutes.

Excretion

-Fishes excrete ammonia across the gills and the abundance of water washes it away -Terrestrial insects, nonavian reptiles, and birds convert toxic ammonia to nontoxic uric acid which is insoluble and can be excreted with little water loss -Nonavian reptiles and birds also use uric acid stored as harmless crystalline form within the amniotic sac of an egg until ready for hatching

Marine Bony fish osmoregulation

-Marine bony fishes maintain salt levels of their body fluids at about 1/3 that of seawater and are called hypoosmotic regulators -Marine fish drink seawater to compensate for water loss -Salt is absorbed in the intestine and carried by blood to gills -Special salt-secreting cells in the gills transfer the salt back to the sea -Ions like magnesium, sulfate, and calcium that remain in the intestine are excreted with feces or through the kidneys

Marine Fish & Water loss

-Marine fishes consume only enough seawater to replace water loss -Elasmobranchs like sharks and rays achieve the same osmotic balance by a different mechanism using urea and trimethylamine oxide (TMAO) -Urea compounds are metabolic wastes that animals excrete quickly -Shark kidneys conserve urea and accumulate this in the blood and body tissues until there is no osmotic difference with seawater so that water balance is maintained

How do mammals get water?

-Water is gained from food, drinking water, and metabolic water that is formed in cells via oxidation of metabolic fuel like fats and carbohydrates -Diving mammals get metabolic water from stored fats while swimming -Some desert arthropods like ticks, roaches, and mites, can absorb water vapor directly from air -In desert rodents, metabolic water constitutes most of the animal's water uptake -Humans, on the other hand, still need to drink half of the total water needed in addition to food that has high water content

Potential changes to the internal environment can be from two sources:

1. Metabolic activities require constant supply of materials like oxygen, salts, and nutrients, which have to be continuously replenished, while cellular wastes have to be expelled 2. Internal environment responds to changes in the organism's external conditions

Metanephridium

A more advanced type of nephridium is the open, or "true," nephridium (metanephridium) A type of tubular nephridium with the inner open end draining the coelom and the outer open end discharging to the exterior. found in several eucoelomate phyla such as annelids, molluscs, and several smaller phyla. A metanephridium differs from a protonephridium in two important ways. First, the tubule is open at both ends, allowing fluid to be swept into the tubule through a ciliated funnel-like opening, the nephrostome. Second, a metanephridium is surrounded by a network of blood vessels that assists in reabsorption from the tubular fluid of water and valuable materials such as salts, sugars, and amino acids -Metanephridium is a more advanced form which is an open system found in molluscs and annelids Tubule is open at both ends and allow fluid to be swept into the tubule through a ciliated funnel-like opening called the nephrostome Surrounded by a network of blood vessels that reclaim water and valuable solutes like salts The basic process of urine formation in the tubule remains the same for protonephridia and metanephridia Ensures removal of wastes from the body without loss of useful materials

Euryhaline

Able to tolerate wide ranges of saltwater concentrations.In estuaries & rivers animals must cope with large and often abrupt changes in salinity as the tides ebb and flow and mix with freshwater draining from rivers.

Amphibians osmosis

Amphibians living in water also must compensate for salt loss by actively absorbing salt from the water. They use their skin for this purpose. Pieces of frog skin continue to transport sodium and chloride actively for hours when removed and placed in a specially balanced salt solution, and frog skin became a favorite membrane system for studies of ion-transport phenomena

How do amphibians osmoregulate?

Amphibians that live in water use their skin to transport sodium and chloride

Tubular Reabsorption

Approximately 60% of the filtrate volume and virtually all of the glucose, amino acids, vitamins, and other valuable nutrients are reabsorbed in the proximal convoluted tubule. Much of this reabsorption is by active transport, in which cellular energy is used to transport materials from tubular fluid to the surrounding capillary network and thus into the blood circulation. Electrolytes such as sodium, potassium, calcium, bicarbonate, and phosphate are reabsorbed by ion pumps, which are carrier proteins driven by the hydrolysis of ATP (ion pumps are described on p. 48). Because an essential function of the kidney is to regulate plasma concentrations of electrolytes, all are individually reabsorbed by ion pumps specific for each electrolyte. The degree of reabsorption depends on the body's ability to conserve each mineral. Some materials are passively reabsorbed. Negatively charged chloride ions, for example, passively flow by diffusion through protein channels (p. 45) specific for chloride ions, drawn by the active reabsorption of positively charged sodium ions in the proximal convoluted tubule. Water, too, passively diffuses from the tubule through open protein channels, as it osmotically follows the active reabsorption of solutes. For most substances there is an upper limit to the amount of substance that can be reabsorbed. This upper limit is termed the transport maximum (renal threshold) for that substance. For example, glucose normally is reabsorbed completely by the kidney because the transport maximum for glucose is well above the amount of glucose Page 670usually present in the glomerular filtrate. Should the plasma glucose concentration exceed this threshold level, as in the disease diabetes mellitus, glucose would appear in the urine The mechanism for tubular reabsorption of glucose is like a conveyor belt running at constant speed. (A) When glucose in the glomerular filtrate are low, all is reabsorbed. (B) When glucose in the filtrate have reached the transport maximum, all carrier sites for glucose are occupied. (C) With diabetes mellitus, some glucose escapes the carriers and appears in urine (called glycosuria).

Malphighan tubules

Blind tubules opening into the hindgut of nearly all insects and some myriapods and arachnids, and functioning primarily as excretory organs. Operate in conjunction with specialized glands in the wall of the rectum

hyperosmotic (hypertonic) environment

Cell volume falls

hypoosmotic (hypotonic) environment

Cell volume rises

How do aquatic insects osmoregulate?

Clams, crayfishes, and aquatic insect larvae are also hyperosmotic regulators with similar mechanisms to freshwater fish

What is less constant than the ocean?

Conditions along coasts and in estuaries and river mouths are much less constant than those of the open ocean.

Explain negative feedback

Constant internal environment for all cells revolve around a given set point and any deviation from this physiological level will activate the negative feedback regulation process to return the system to the set point

Desert Mammals

Desert mammals have greater urine concentrating ability Camels produce urine that is 8 times more concentrated than the plasma Gerbils produce urine that is 14 times more concentrated than the plasma Australian hopping mouse is the greatest urine concentrator at 22 times Has loops of Henle extending to the tip of long renal papilla that pushes out to the mouth of the ureter

Preocess of urine formation

Despite these differences, the basic process of urine formation is the same in protonephridia and metanephridia: fluid enters and flows continuously through a tubule where the fluid is selectively modified by (1) reabsorbing valuable solutes from it and returning these to the body, and (2) adding waste solutes to it (secretion). The sequence ensures removal of wastes from the body without loss of useful materials. Kidneys of vertebrates operate in basically the same way

Sodium reabsorption

Distal convoluted tubules make additional adjustments of filtrate composition. Sodium reabsorbed by the proximal convoluted tubule—some 85% of the total filtered—is obligatory reabsorption; this amount is reabsorbed independent of sodium intake. In the distal convoluted tubule, however, sodium reabsorption is controlled by aldosterone, a steroid hormone secreted by the adrenal gland (p. 759). Aldosterone increases both active reabsorption of sodium and secretion of potassium by the distal tubules; the hormone thus decreases loss of sodium and increases loss of potassium in the urine. The secretion of aldosterone is regulated (1) by the enzyme renin, produced by the juxtaglomerular apparatus, a complex of cells located at the junction of the afferent arteriole with the glomerulus (see Figure 30.10), and (2) by elevated blood potassium levels. Renin is released in response to a low blood sodium level, to low blood pressure (which can occur if the blood volume drops too low), or to low sodium in the glomerular filtrate. Renin then initiates a series of enzymatic events culminating in the production of angiotensin, a blood-borne hormone that has several related effects. First, it stimulates the release of aldosterone, which acts in turn to increase sodium reabsorption and potassium secretion by the distal tubule. Second, it increases the secretion of antidiuretic hormone (vasopressin, discussed on p. 671), which promotes water conservation by the kidney. Third, it increases blood pressure. Finally, it stimulates thirst, which also is stimulated by decreased blood volume or increased blood osmolarity. These actions of angiotensin tend to reverse the circumstances (low blood sodium and low blood pressure and/or blood volume) that triggered the secretion of renin. Sodium and water are conserved, and blood volume and blood pressure are restored to normal.

Nephron

Functional unit of a vertebrate kidney, comprising a Bowman's capsule, an enclosed glomerulus, and the attached uriniferous tubule.

Osmoregulation in a saltwater fish

Gain water and salt ions from food & by drinking seawater.Excretion of salt ions from gills.Osmotic water loss through gills and excretion of salt ions & small amount of water in scanty urine from kidneys

Active Transport is used in which regulator?

Hyperosmotic;

Protonephridium

I-nvertebrates have simple tubular structures called nephridia that produce ultrafiltrate fluid secretions from the blood and then form urine -Most common design to maintain osmotic balance with flame cells system or protonephridium being the simplest arrangement for acoelomates and some pseudocoelomates -Planaria and other flatworms have protonephridial system made of two highly branched duct system to all parts of the body -Primitive osmoregulatory or excretory organ consisting of a tubule terminating internally with flame-bulb or solenocyte; the unit of a flame-bulb system. Closed system. The tubules are closed on the inner end and urine is formed from a fluid that must first enter the tubules by being transported across flame cells.

Tubular Secretion

In addition to reabsorbing materials from glomerular filtrate, the nephron can secrete materials across the tubular epithelium and into the filtrate. In this process, the reverse of tubular reabsorption, carrier proteins in the tubular epithelial cells selectively transport substances from blood in capillaries outside the tubule to the filtrate inside the tubule. Tubular secretion enables a kidney to increase the urine concentrations of materials to be excreted, such as hydrogen and potassium ions, drugs, and various foreign organic materials. The tubular epithelium is able to recognize foreign organic materials, such as ingested pharmaceutical drugs, because they are metabolized by the liver to form cationic or anionic molecules. These molecules are transported by the tubular epithelium, which has both cationic and anionic transporters in its membrane. The distal convoluted tubule is the site of most tubular secretion.

Water Excretion

Kidneys closely regulate the osmotic pressure of blood. When fluid intake is high, the kidney excretes dilute urine, saving salts and excreting water. When fluid intake is low, kidneys conserve water by forming concentrated urine. A dehydrated person can concentrate urine to approximately four times blood osmotic concentration. This important ability to concentrate urine enables us to excrete wastes with minimal water loss. The capacity of mammalian and some avian kidneys to produce a concentrated urine involves an interaction between the loop of Henle and the collecting ducts. This interplay produces an osmotic gradient in the kidney, as shown in Figure 30.14. In the cortex, interstitial fluid is isoosmotic with blood, but deep in the medulla the osmotic concentration is four times greater than that of blood (in rodents and desert mammals that can produce highly concentrated urine the osmotic gradient is much greater than in humans). The high osmotic concentrations in the medulla are produced by an exchange of ions in the loop of Henle by countercurrent multiplication. "Countercurrent" refers to the opposite directions of fluid movement in the two limbs of the loop of Henle: down in the descending limb and up in the ascending limb. "Multiplication" describes the increasing osmotic concentration in the medulla surrounding the loops of Henle and the collecting ducts resulting from ion exchange between the two limbs of the loop.

Kidneys of Marine Fishes, Nonavian Reptiles, and Birds

Kidneys of marine fishes, nonavian reptiles, and birds have a different process than in mammals Marine bony fishes actively secrete large amounts of magnesium and sulfate, which are marine salts, from their normal osmoregulation process Nonavian reptiles and birds excrete uric acid instead of urea through the tubular epithelium Uric acid is insoluble and forms crystals in the urine such that it requires little water for removal and is an important adaptation for water conservation in marine habitats

Osmoregulation

Maintenance of proper internal salt and water concentrations in a cell or in the body of a living organism; active regulation of internal osmotic pressure.

Salty Tears?

Marine birds and turtles have a salt gland above the eyes that can secrete large amounts of salt taken in from their diet Salt glands can secrete highly concentrated sodium chloride that can be up to twice the salt concentration of seawater Marine lizards and turtles shed salt gland secretions via salty tears that have become important accessory glands since their kidneys can only produce very dilute urine and cannot manage all the salt that they take in

How do marine animals face the problem with excretion of salt?

Marine birds and turtles have evolved an effective solution for excreting large loads of salt eaten with their food. Located above each eye is a salt gland capable of excreting a highly concentrated solution of sodium chloride—up to twice the concentration of seawater. In birds the salt solution runs out the nares. Salt glands are important accessory organs of salt excretion in these animals because their kidneys cannot produce a concentrated urine, as can mammalian kidneys.

How do marine bony fish maintain homeostasis?

Marine bony fishes living today maintain the salt concentration of their body fluids similar to that of freshwater fishes

Active transport

Mediated transport in which a transmembrane protein transports a molecule across a plasma membrane against a concentration gradient; requires expenditure of energy; contrasts with facilitated diffusion. Process that requires energy because ions must be transported against a concentration gradient from a lower salt concentration (in dilute seawater) to a higher one (in blood)

Pronephros

Most anterior of three pairs of embryonic renal organs of vertebrates, functional only in adult hagfishes and larval fishes and amphibians, and vestigial in amniote embryos. In all vertebrate embryos, the pronephros is the first kidney to appear. It is located anteriorly in the body and becomes part of the persistent kidney only in adult hagfishes and a few bony fish species.

Stenohaline

Pertaining to aquatic organisms that have restricted tolerance to changes in environmental saltwater concentration. Example: Marine Spider Crab

Hyperosmotic regulation scenario

Scenario: a brackish-water shore crab can resist dilution of body fluids by dilute (brackish) seawater. Although the concentration of salts in the body fluids falls, it does so less rapidly than the fall in seawater concentration.By preventing excessive dilution, and thus protecting the cells from extreme changes, brackish-water shore crabs can live successfully in the physically unstable coastal environment. Nevertheless, with only a limited capacity for osmotic regulation, they will die if exposed to greatly diluted seawater. Because a crab's body fluids are osmotically more concentrated than the dilute seawater outside, water flows into its body, especially across the thin, permeable membranes of the gills. As with the red blood cells placed in pure water, water diffuses inward because of higher solute concentration inside. For the crab, were this inflow of water allowed to continue unchecked, its body fluids would soon become diluted. The problem is solved by the kidneys (antennal glands located in the crab's head which excrete excess water as dilute urine. The second problem is salt loss. Again, because the animal is saltier than its environment, it cannot avoid loss of ions by outward diffusion across its gills. Salt is lost also in urine. To compensate for solute loss, salt-secreting cells in the gills actively remove ions from dilute seawater and move them into the blood, thus maintaining the internal osmotic concentration. This is an active transport. Organisms that are hyperosmotic tend to lose solutes to the external medium by diffusion and to gain water from the external medium by osmosis

How do sharks maintain homeostasis?

Sharks and rays (elasmobranchs) are almost totally marine, and achieve osmotic balance differently. The salt composition of shark's blood is similar to that of bony fishes, but the blood also carries large amounts of the organic compounds urea and trimethylamine oxide (TMAO). Urea is a metabolic waste product that most animals quickly excrete. The shark kidney, however, conserves urea, allowing it to accumulate in the blood and body tissues and raising the blood osmolarity to equal or slightly exceed that of seawater. With the osmotic difference between blood and seawater eliminated, water balance is not a problem for sharks and other elasmobranchs; they are in osmotic equilibrium with their environment.

What happens if a osmoconformer is exposed to dilute seawater?

Should they be exposed to dilute seawater, they absorb water by osmosis and die quickly because their body's cells cannot tolerate dilution and are helpless to prevent it. These animals are restricted to living in a narrow salinity range and are called stenohaline

Urine & Water

The amount of water reabsorbed and the final concentration of the urine depend on permeability of the walls of the distal convoluted tubule and the collecting duct. This is controlled by antidiuretic hormone (ADH, or vasopressin), which is released by the posterior pituitary gland (neurohypophysis, see p. 751). Special receptors in the brain that constantly sense the osmotic pressure of body fluids govern the release of this hormone. When the blood osmotic pressure increases or blood volume decreases, as during dehydration, the pituitary gland releases more ADH. ADH increases permeability of the collecting duct by increasing the number of water channels in the collecting duct epithelial cells. Then, as the fluid in the collecting duct passes through the hyperosmotic region of the kidney medulla, water diffuses through the channels into the surrounding interstitial fluid and is carried away by the blood circulation. Thus, urine loses water and becomes more concentrated. Given this sequence of events for dehydration, it is not difficult to anticipate how the system responds to overhydration: the pituitary stops releasing ADH, the water channels in the collecting duct epithelial cells decrease in number, and a large volume of dilute urine is excreted.

Loop of Henle

The descending limb of the loop of Henle is permeable to water but impermeable to solutes. The ascending limb is nearly impermeable to water. Sodium chloride passively moves from the lower ascending limb and is actively transported from the thick portion of the ascending limb into the surrounding tissue fluid (Figure 30.14). As the interstitium surrounding the loop becomes more concentrated with solute, water is withdrawn from the descending limb by osmosis. The tubular fluid in the base of the loop, now more concentrated, moves up the ascending limb, where still more sodium chloride diffuses or is pumped out. In this way the effect of ion movement in the ascending limb is multiplied as more water is withdrawn from the descending limb and more concentrated fluid is presented to the ascending limb (Figures 30.14 and Figure 30.15). The blood capillaries surrounding the loops of Henle, the vasa recta, are also arranged in a countercurrent fashion. Thus salt that diffuses into the blood of the vasa recta close to the ascending limb of the loop of Henle will not leave the medulla, but will diffuse from here into the blood entering the medulla in the vasa recta, so that very little salt is lost from this region. This arrangement of blood vessels is important in maintaining the osmotic concentration gradient of the medulla and cortex.

Flame cell system

The flame-cell system is extensively branched throughout a flatworm's body. Thus, these animals have no necessity for a circulatory system to deliver wastes to a centralized excretory system (such as the kidneys of vertebrates and many invertebrates). -Fluid enters the system through specialized "flame cells" where rhythmical beating of a flagellar tuft creates a negative pressure that draws fluid into the tubes -As fluids move through the tubes, water and metabolites are recovered by reabsorption and wastes are expelled through excretory pores -Nitrogenous wastes, mainly ammonia, diffuse across the surface of the body -Flatworms have no circulatory system so the flame cell system is extensively branched throughout the animal -Protonephridium is a closed system since tubules are closed on the inner end of tubules

Mesonephros

The middle of three pairs of embryonic renal organs in vertebrates. Functional kidney of embryonic amniotes; its collecting duct is a Wolffian duct. In all other vertebrates the pronephros degenerates during development and is replaced by a more centrally located mesonephros. The mesonephros is the functional kidney of embryonic amniotes (nonavian reptiles, birds, and mammals).

What role does the Nephridium play in homeostasis?

The most common type of invertebrate excretory organ is the nephridium, a tubular structure designed to maintain appropriate osmotic balance. One of the simplest arrangements is the flame cell system (or protonephridium) of acoelomates (for example, flatworms) and some pseudocoelomates.

Nephron characteristics

The nephron, with its pressure filter and tubule, is intimately associated with blood circulation. Blood from the aorta enters each kidney through a large renal artery, which divides into a branching system of smaller arteries. The arterial blood reaches the glomerulus through an afferent arteriole and leaves by an efferent arteriole. From the efferent arteriole the blood travels to an extensive capillary network that surrounds and supplies the proximal and distal convoluted tubules and the loop of Henle. This capillary network provides a means for the pickup and delivery of materials that are reabsorbed or secreted by the kidney tubules. Page 669From these capillaries blood collects in veins that unite to form the renal vein. This vein returns blood to the posterior vena cava.

Vertebrate Kidney Function

The organization of kidneys differs somewhat in different groups of vertebrates, but in all the basic functional unit is the nephron. This vast blood flow is channeled to approximately 2 million nephrons, which form the bulk of the two kidneys. Each nephron begins with an expanded chamber, the Bowman's capsule, containing a ball of intertwined capillaries, the glomerulus, which together form the renal corpuscle. Blood pressure in the glomerular capillaries forces an almost protein-free filtrate into a Bowman's capsule and along a renal tubule, consisting of several segments that perform the functions of reabsorption and secretion in the process of urine formation. The filtrate passes first into a proximal convoluted tubule, then into a long, thin-walled loop of Henle, which may extend deep into the inner portion of the kidney (the medulla) before returning to the outer portion (the cortex) where it becomes a distal convoluted tubule. From the distal tubule the fluid empties into a collecting duct, which drains into the renal pelvis. Here the urine collects before being carried by the ureter to the urinary bladder.

What problem does excretion of wastes present in terrestrial organisms?

The primary end product of protein breakdown is ammonia, a highly toxic material. Fishes easily excrete ammonia by diffusion across their gills, since there is an abundance of water to wash it away. Terrestrial insects, nonavian reptiles, and birds have no convenient way to rid themselves of toxic ammonia; instead, they convert it into uric acid, a nontoxic, almost insoluble compound. This conversion enables them to excrete a semisolid urine with little water loss. Uric acid as an end product has another important benefit. Nonavian reptiles and birds lay amniotic eggs enclosing their embryos, together with their stores of food and water, and waste products that accumulate during development. By converting ammonia to uric acid, a developing embryo's waste can be precipitated into solid crystals, which are stored harmlessly within the amniotic sac, inside the egg until hatching.

Glomerular Filtration

The process of urine formation begins in the glomerulus. The glomerulus acts as a specialized mechanical filter, which produces an almost protein-free filtrate of the plasma in the fluid-filled space of the Bowman's capsule as a result of high blood pressure across glomerular capillary walls. The diameter of the afferent arteriole entering the glomerulus is greater than that of the exiting efferent arteriole, providing the high hydrostatic pressure that allows formation of the glomerular filtrate. Solute molecules small enough to pass through the filtration slits of the capillary wall are carried through by the filtrate in which they are dissolved. Red blood cells and almost all plasma proteins, however, are withheld because they are too large to pass through these pores.

What role does the contractile vacuole play in homeostasis?

The tiny, spherical, intracellular vacuole of unicellular eukaryotes and freshwater sponges is not a true excretory organ, since ammonia and other nitrogenous wastes of metabolism readily diffuse into the surrounding water. The contractile vacuole of freshwater unicellular eukaryotes is an organ of water balance that expels excess water gained by osmosis. As water enters the cell, a vacuole grows and finally contracts and empties its contents through a pore on the surface. The cycle is repeated rhythmically. Although the mechanism for filling the vacuole is not fully understood, recent research suggests that contractile vacuoles have numerous proton pumps located within their membrane (proton pumps are described in connection with the electron transport chain in Chapter 4, p. 64 and following). Proton pumps create H+ and HCO− gradients that draw water into the vacuole, forming an isoosmotic solution. These ions are excreted when the vacuole empties (see Figure 11.15, p. 226). Contractile vacuoles are common in freshwater unicellular eukaryotes, sponges, and radiate animals (such as hydra), but rare or absent in marine forms of these groups, which are isoosmotic with seawater and consequently neither lose nor gain too much water.

Arthropod Kidneys

There are two basic types of arthropod kidneys, which are uniquely used for excretion Paired antennal glands of crustaceans located in the ventral part of the head are an elaboration of the basic nephridia Lack open nephrostomes; hydrostatic pressure of the blood forms a protein-free filtrate in the end sac In tubular portion, certain salts are selectively reabsorbed or actively secreted into the filtrate This system is similar to the vertebrate system in sequence of urine formation Malpighian tubules are used by insects and spiders in conjunction with glands in the walls of the rectum Thin, elastic, blind Malpighian tubules are closed and lack arterial supply Urine is not produced by filtration of body fluids as in nephridium but by tubular secretion mechanisms Cells lining the Malphighian tubules lined with hemolymph (blood) Process is initiated by active transport of hydrogen ions into the tubular lumen Protein transporters move hydrogen ions back into cells and exchanged for sodium or potassium ions, with chloride ions following

Importance of Malphighan tubules

These thin, elastic, blind Malpighian tubules are closed and lack an arterial supply. Urine formation does not occur by filtration of body fluids as is the case in the nephridium, but is produced by tubular secretion mechanisms by the cells lining the Malpighian tubules that are bathed in hemolymph (blood). This process is initiated by active transport of hydrogen ions into the Malpighian tubular lumen. These hydrogen ions are then moved back into the cells lining the tubule using protein transporters that move hydrogen ions in exchange for sodium or potassium ions, and chloride ions follow passively. Herbivorous and omnivorous insects secrete primarily potassium into the tubular lumen. Carnivorous insects, such as blood-sucking mosquitoes, initially secrete a fluid high in sodium that reflects the high salt content of a blood plasma meal. As the red blood cells are digested, the fluid content of sodium falls and becomes rich in potassium ions. The secretion of ions creates an osmotic pressure that draws water, solutes, and nitrogenous wastes, especially uric acid, into the tubule. Uric acid enters the blind-ending distal segment of the tubule as soluble potassium urate, which precipitates as insoluble uric acid in the proximal end of the tubule. Once the formative urine drains into the rectum, water and salts may be reabsorbed by specialized rectal glands, leaving behind uric acid, excess water, salts, and other wastes, which are expelled in feces. Rectal glands of aquatic insect larval stages absorb solute, but little water, whereas blood-sucking insects can alter the amount of salt and water reabsorption during and between meals. The feces of bloodsucking insects will be high in water and excess salts during and immediately after a blood meal, but low in salts and water between meals. The Malpighian tubule excretory system is ideally suited for life in dry environments and has contributed to the adaptive diversification of insects on land.

Freshwater Fishes

They must keep the salt concentration of their body fluids higher than that of the water in which they live. Water enters their bodies osmotically, and salt is lost by outward diffusion. Unlike the habitat of the brackish-water shore crab, freshwater is much more dilute than are coastal estuaries, and there is no retreat, no salty sanctuary into which a freshwater animal can retire for osmotic relief. Freshwater organisms maintain their internal osmotic concentrations lower than marine organisms. This represents a balance between solute requirements and the amount of work it takes to keep water out.

How do terrestrial animals maintain water loss?

They replace such losses by consuming water in food, drinking water when available, and retaining metabolic water formed in cells by oxidation of metabolic fuel molecules, such as carbohydrates and fats

How does a marine bony fish compensate for water loss?

To compensate for water loss a marine bony fish drinks seawater. This seawater is absorbed from the intestine, and the major sea salt, sodium chloride, is carried by the blood to the gills, where salt-secreting cells transport it back into the surrounding sea. Ions remaining in the intestinal residue, especially magnesium, sulfate, and calcium, are voided with feces or excreted by kidneys. In this indirect way, marine fishes rid themselves of excess sea salts and replace water lost by osmosis. Samuel Taylor Coleridge's ancient mariner, surrounded by "water, water, everywhere, nor any drop to drink" undoubtedly would have been tormented even more had he known of the marine fishes' ingenious solution to thirst. A marine fish regulates the amount of seawater it drinks, consuming only enough to replace water loss and no more.

Osmoregulation in a freshwater fish

Uptake of water & some ions in food, uptake of salt ions by gills, Osmotic water gain through gills and excretion of large amounts of water in dilute urine from kidneys.

Metanephros

Vertebrate kidney formed from the most posterior of three embryonic regions capable of forming renal organs; functional kidney of adult amniotes; drained by ureter.The mesonephros and metanephros, together called the opisthonephros, form the adult kidney of most fishes and amphibians.

Osmosregulation

Water that enters by osmosis across the gills is pumped out by the kidney as very dilute urine. Salt-absorbing cells in the gills move salt ions from water to blood. Salt present in food also replaces any salt that is lost by diffusion. Freshwater fish uses little energy for osmoregulation due to its efficiency.

Ancestral kidney

archinephros

Kidney Evolution

biologists infer that the kidney of the earliest vertebrates extended the length of the coelomic cavity and was composed of segmentally arranged tubules, each resembling an invertebrate nephridium. Each tubule opened at one end into the coelom by a nephrostome and at the other end into a common archinephric duct. -Ancestry and embryologic studies of the earliest vertebrate kidney show an extended length of the coelomic cavity Composed of segmentally arranged tubules similar to an invertebrate nephridium Each tubule opens into the coelom at a nephrostome and the other end leads into a common archinephric duct This ancient kidney is called an archinephros, and is similar to segmented kidney found in embryos of hagfishes and caecilians

Unirne formation 3 processes

filtration, reabsorption, and secretion.

Marine Invertebrates

have body surfaces permeable to water and salts; body fluid concentration rises or falls in conformity with changes in concentration of seawater

Hypoosmotic Regulators

maintain body fluids less concentrated than the surrounding water

Hyperosmotic regulators

maintains its body fluids more concentrated (hence hyper-) than the surrounding water.

Osmoconformer

marine organisms that maintain an internal environment that is osmotic to their external environment. This means that the osmotic pressure, or osmolarity, of the organism's cells is equal to the osmotic pressure of their surrounding environment.

Excretion gets rid of

metabolicwastes

The ocean is a

osmotically stable environment

3 Kidney Stages

pronephros, mesonephros, and metanephros