Chapter 44 (4)

__:
Reabsorption in the __ is critical for the recapture of ions, water, and valuable nutrients from teh huge initaial filtrate volume. NaCl in the filtrte diffuses into the cells of teh transport epithelium, were Na+ is the actively transported into the interstitial fluid. This transfer of positivve charge out of the tubule drives the passive transport of Cl-. As salt moves from the filtrate to the interstitial fluid, water follows by osmosis. THe salt and water then diffuse from the interstitial fluid into the __. Glucose, amino acids, potassium ions, and other essential substances are also actively or passively transported from the filtrate to the interstitial fluid and then into teh __.

Processing of filtrate in the __ helps maintain a relatively constant pH in body fluids. Cells of the transport epithelium secrete H+ but also synthesize and secrete __, whichacts as abuffer to trap H+ in the form of ammonium ions.

THe more acidic the filtrate, the more __ the cells and secrete, and a mammal's urine usually contains some ammonia from this source (even though most nitrogenous waste is excreted as __). The __ also reabsorb about 90% of the buffer bicarbonate fromt eh filtratte, contributing further to pH balance in body fluids.

As the filtrate passes through the __, amterials to be excreted become concentrated. Many wastes leave the body fluids during the nonselective fitration process and remain int he filtrate while water and salts are reabsorbed. __ is reabsorbed at a much lower rate than are salt and water. SOme other toxic materials are actively secreted into filtrate from surrounding tisssues.

__:
Reabsorption of water continues as the filtrate moves into the descending limb of the loop of Henle. Here numerous water channels formed by __ proteins make the transport epithelium freely permeable to water. In contrast, there is a near ansence of channels for salt and other small solutes, resulting in avery low permeability for these substances.

For water to move out of the tubuel by osmosis, the interstitial fluid bathing the tubule must be __ to the filtrate. This condition is met along the entire length of the descending limb, because the osmolarity of the interstitial fluid increases progressively fromt eh outer cortex to the inner medulla of the kidney. As a result, the filtrate undergoes a loss of water and an accompanying increase in solute concentration at every point in its downward journey along the descending limb.

__:
The filtrate reaches the tip of the loop and tehn travels within the ascending limb as it returns to the cortex. unlike the descending limb, the ascending limb has a transport epithelium that contains ion channels, b ut not water channels. Indeed, this membrane is impermeable to water. Lack of permeability to wateris very rare among biological membranes and is critical to the function of the ascending limb.

The ascending limb has two speciaalized regions: __ and __
As filtrate ascends int he thin segment, NaCl which became concentrated int ehdescending limb, diffuses out of the permeable tubule into the interstitial fluid. THis movement of NaCl out of the tuble helps maintain the osmolarity of the interstitial fluid in the medulla. The movement of NaCl out of the filtrate continues in the thick segment of the ascending limb. Here, however, the epithelium actively transports NaCl into teh interstitial fluid. As a result of losign salt but not water the filtrate becomes progressively more dilute as it moves up to the cortex in the ascending limb of the loop

__:
THe __ plays a key role in regulating the K and NaCl concentration of body fluids. THis regulation involves variation in the amt of the K+ that is secreted into the filtrate, as well as the amount of NaCl reabsorbed fromt eh filtrate. Like the proximinal tubue, the __ contribute sto pH regulation by the controlled secretion of H+ and reabsorption of HCO3-.

__:
the _ carries the filtrate through the medulla to the renal pelvis. As filtrate passes along the transport epithelium of the collecting duct, hormonal control of permeability and transport determines the extent to which the urine becomes concentrated.

When the kidneys are conserving water, aquaporin channels in the __ allow water molecules to cross the epithelium. At the same time, the epithelium remains impermeanle to salt and in the renal cortex to urea. As the __ traverses the gradient of osmolarity in the kidney, the filtrate becomes increasingly concentrated, losing more and more water by osmosis to the hyperosmotic interstitial fluid. In the inner medulla, the duct becomes permeable to urea.

Because of the high urea concentration int he filtrate at this point, some urea diffueses out of the duct and into the interstitial fluid. Along with naCl, this urea contributes to the high __ of the interstitial fluid in the medulla. HTe net resutl is urine that is hyperosmotic to the general body fluids.

In producing __ rather than concentrated urine, the kidney actively reabsorbs salts without allowing water to floow by osmosis. At tehse times, the epithelium lacks water channels, and NaCl is actively transported out of filtrate. The state of the collecting duct epithelium is controlled by hormones that together maintain __ for osmolarity, blood pressure, and blood volume.

the mammalian kidney's ability to conserve water is akey terrestrial adaptation.
- In a mammalian kidney, hte production of __ is possible only because considerable energy is expended for the actie transport of solutes against concentration gradients. THe __ - particularly the loops of Henle- can be thought of as energy-consuming machines that produce an osmolarity gradient suitable for extracting water fromt eh filtrate int eh __. The two primary solutes affecting osmoregulatiry are NaCl, which is deposited in the __ by the loop of Henle, and urea, which passes across the epithelium of the collecting duct in the inner medulla.

Filtrate passing from Bowman's capsule to the proximinal tubule has an osmolarity of about 300 mOsm/L, the same as blood. A large amount of water and salt is reabsorbed from the filtrate as it flows throught eh proximal tubule in the __. As a result, the filtrate's volume decreases substantially, but its osmolarity remains about the same.

As the filtrate flows from cortex to medulla in the descending limb of hte __, water leaves the tubule by osmosis. Solues, including NaCl, become more concentrated, increasing the osmolarity of the fitrate. The highest osmolarity occurs at the elbow of the __. This maximizes the diffusion of salt out of the tubule as the filtrate rounds the curve and enters the ascending limb which is permeable to salt but not to water. NaCl diffusing from the ascending limb helps maintain a high osmolarity in the interstitial fluid of the renal medulla.

The __ has many qualities of a countercurrent system.

In those cases, the __ involving the oloop of Henle expends energy to actively transport NaCl from teh filtrate in the upper part of hte ascending limb of the loop. Such __, which expend energy to create concentration gradients are called __. The __ involving the loop of Henle maintains a high salt cooncentration in the interior of the kidney, enabling the kidney to form concentrated urine.

The descending and ascending vesssels of the __ carry blood in opposite directions throught hte kidney's osmolarity gradient. As the descending vessel conveys blood toward the inner medulla, water is lost from the blood and NaCl gained by diffusion. These fluxes are reversed as blood flows back toward the cortex int eh ascending vessel with water reentering the blood and salt diffuisng out. Thus the __ can supply hte kidney with nutrients and other important substances carried by the blood without interfering with the osmolarity gradient that makes it possible for the kidney to excrete hyperosmotic urine.

The countercurrent-like characteristics of the __ and the __ help to generate the steep osmotic gradient between th emedulla and cortex. However, diffusion wille ventually eliminate any osmotic gradient within animal tissue unless gradient formation is supported by an expenditure of energy. In the kidney, the expenditure largely occurs in the thick segment of the ascending limb of hte __, where NaCl is actively transported out of the tbule. Even with the benefits of __, this process- along with othe rrenal active transport systems-consumes considerable ATP. Thus, for its size, the kidney has one of the highest metabolic rates of any organ.

As a result of active transport of NaCl out of the thick segment of the ascending limb, the filtrate is actually __ to body fluids by the time it reaches the distal tubule. Now the filtrate descends again toward the medulla, this time in the collecting duct, which is permeable to water but not to salt.

Therefore, osmosis extracts water from the filtrate as it passes from cortex to medulla and encounters interstitial fluid of incresaing osmolarity. This process concentrates salt, urea, and other solutes in the filtrate. Some __ passes out of teh lower portion of the collecting duct and contributes to the high interstitial osmolarity of the inner medulla.
- (this _- is recycled by diffusion into the loop of H_nle, but continual leakage from the collecting duct maintains a high interstitial urea concentrations.

When the kidney concentrates urine maximally, the rine reaches 1200 mOsm/L, the osmolarity of the interstitial fluid int he inner medulla. Although __ to the inner medulla's interstitial fluid, the urine is __ to blood and interstitual fluid elsewhere in thebody. This high osmolarity allows the solutes remaining int eh urine to be excreted fromt hebody with minimal water loss.

Vertebrate animals occupy habitats ranging from rain forests to deserts and from some of the salties bodies of water to the nearly pure waters of high mt lakes. Variations in __ structure and function equip the kidneys of different vertebrates for osmoregulation in their various habitats. THe adaptations fo the vertebrate kidney are made apparent by comparing the responses of differne tverrtebrate groups to similar environmental conditions.

The __, with its urine-concentrating features, is a key adaptation to tererstrial life, enabling mammals to get rid of salts and nitrogenous wastes without squandering water. THe remarkable ability of the mammalian kidney to produce __ urine depends on the precise arrangement of the tubules and collecting ducts in the real cortex and medulla. In this respect, the kidney is one of the clearest ex. of how the function of an organ is inseperably linked to its structure.

mammals that excrete the most hyperosmotisc urine have __ that extend ddeep into the medulla. Long loops maitntain steep osmotic gradients int he kidney, resulting in urine becoming very concentrated as it passe sfrom cortex to medulla in the collecting ducts.

In contrast, beavers,e tc. spend much of their time in fresh water adn rarely face dehydration problems have __ withr elatively short loops, resulting in a much lower ability to concentrate urine. Terrestrial mammals living in moist conditions have loops of Henle of intermediate length and the capacity to produce urine intermediate in concentration to that produced by freswater and desert mammals.

Most birds live in environments that dehydraing. Like mammals, birds have kidneys with __ that specialize in conserving water. However the nephrons of birds have loops of Henle that extend less far into the medulla than those of mammals. HTus, birds kidneys cannot concentrate urine to the high osmolarities achieved by mammalian kidneys. Although birds can produce hyperosmotic urine, their main water conservation adaptation is having __ as the nitrogen waste molecule. Since __ can be excreted as paste, it reduces urine volume.

freshwater fishes are __ to their surroundings, so they must excrete execss water continuously. In contrast to mammals and birds, freshwater fishes produce large volumes of very dilute urine. Their kidney,s, which contain many __, produce filtrate at a high rate. Freshwater fishes conserve salts by reabsoribing ions from the filtrate in their distale tubules, leaving water behind.

True or False:Amphibian kidneys function much like those of freshwater fishes. When in fresh water, the kidneys of frogs excrete dilute urine while the skin accumulates certain salts from the water by active transport. On land where dehydration is the most pressing problem of osmoregulation, frogs conserve body fluid by reabsorbingg water across the epithelium of the urinary bladder.
True False

The tissues of marine bony fishes gain excess salts from their surroundings and lose water. These environmental challenges are opposite to those faced by their freshwater relatives. Compared with freshwater fishes, marine fishes have fewer and smaller __, and the ir __ lack a distal tubule. In aiddtion, their kidneys have small __, and some lack __ entirely. In keeping with these features, filtration rates are low and very little urine is excreted.

THe main function of kidneys in marine bony fishes is to get rid of __(2+ or 2-). Marine fishes take in __ by incessantly drinking seawater. THey rid themselevs of these ions by secreting them into the proximal tubules of the nephrons and excreting them in urine. Secretion by the gills maintains proper levels monovalent ions. (1+ or 1 -)