Plants 1

Modified stems.

Modified roots.

Environmental adaptations may result in roots being modified for a variety of functions. Many modified roots are aerial roots that are above the ground during normal development.

An overview of a flowering plant.

The plant body is divided into a root system and a shoot system, connected by vascular tissue (purple strands in this diagram) that is continuous throughout the plant. The plant shown is an idealized eudicot.

Happy Teacher's Day

If children live with criticism, they learn to condemn.

If children live with hostility, they learn to fight.

If children live with fear, they learn to be apprehensive.

If children live with pity, they learn to feel sorry for themselves.

If children live with ridicule, they learn to feel shy.

If children live with jealousy, they learn to feel envy.

If children live with shame, they learn to feel guilty.

If children live with encouragement, they learn confidence.

If children live with tolerance, they learn patience.

If children live with praise, they learn appreciation.

If children live with acceptance, they learn to love.

If children live with approval, they learn to like themselves.

If children live with recognition, they learn it is good to have a goal.

If children live with sharing, they learn generosity.

If children live with honesty, they learn truthfulness.

If children live with fairness, they learn justice.

If children live with kindness and consideration, they learn respect.

If children live with security, they learn to have faith in themselves and in those about them.

If children live with friendliness, they learn the world is a nice place in which to live.

Passive transport

The diffusion of solutes across a membrane. Each of the large arrows under the diagrams shows the net diffusion of the dye molecules of that colour.

Each dye molecule wanders randomly, but there will be a net movement of the dye molecules across the membrane to the side that began as pure water. The dye molecules will continue to spread across the membrane until both solutions have equal concentrations of the dye. Once that point is reached, there will be a dynamic equilibrium, with as many dye molecules crossing the membrane each second in one direction as in the other.

We can now state a simple rule of diffusion: In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated. Put another way, any substance will diffuse down its concentration gradient . No work must be done to make this happen; diffusion is a spontaneous process. Note that each substance diffuses down its own concentration gradient, unaffected by the concentration differences of other substances.

Much of the traffic across cell membranes occurs by diffusion. When a substance is more concentrated on one side of a membrane than on the other, there is a tendency for the substance to diffuse across the membrane down its concentration gradient (assuming that the membrane is permeable to that substance). One important example is the uptake of oxygen by a cell performing cellular respiration. Dissolved oxygen diffuses into the cell across the plasma membrane. As long as cellular respiration consumes the O2 as it enters, diffusion into the cell will continue, because the concentration gradient favors movement in that direction.

The diffusion of a substance across a biological membrane is called passive transport because the cell does not have to expend energy to make it happen. The concentration gradient itself represents potential energy and drives diffusion. Remember, however, that membranes are selectively permeable and therefore have different effects on the rates of diffusion of various molecules. In the case of water, aquaporins allow water to diffuse very rapidly across the membranes of certain cells. The movement of water across the plasma membrane has important consequences for cells.

Osmosis. Two sugar solutions of different concentrations are separated by a selectively permeable membrane, which the solvent (water) can pass through but the solute (sugar) cannot. Water molecules move randomly and may cross through the pores in either direction, but overall, water diffuses from the solution with less concentrated solute to that with more concentrated solute. This transport of water, or osmosis, eventually equalizes the sugar concentrations on both sides of the membrane.

Pores in this synthetic membrane are too small for sugar molecules to pass through but large enough for water molecules. How does this affect the water concentration?

It seems logical that the solution with the higher concentration of solute would have the lower concentration of water and that water would diffuse into it from the other side for that reason. However, for a dilute solution like most biological fluids, solutes do not affect the water concentration significantly. Instead, tight clustering of water molecules around the hydrophilic solute molecules makes some of the water unavailable to cross the membrane. It is the difference in free water concentration that is imporant. But the effect is the same: Water diffuses across the membrane from the region of lower solute concentration to that of higher solute concentration until the solute concentrations on both sides of the membrane are equal. The diffusion of water across a selectively permeable membrane is called osmosis. The movement of water across cell membranes and the balance of water between the cell and its environment are crucial to organisms. Let’s now apply to living cells what we have learned about osmosis in artificial systems.

When considering the behaviour of a cell in a solution, both solute concentration and membrane permeability must be considered. Both factors are taken into account in the concept of tonicity , the ability of a solution to cause a cell to gain or lose water. The tonicity of a solution depends in part on its concentration of solutes that cannot cross the membrane (nonpenetrating solutes), relative to that in the cell itself. If there are more nonpenetrating solutes in the surrounding solution, water will tend to leave the cell, and vice versa.

If a cell without a wall, such as an animal cell, is immersed in an environment that is isotonic to the cell (iso means “same”), there will be no net movement of water across the plasma membrane. Water flows across the membrane, but at the same rate in both directions. In an isotonic environment, the volume of an animal cell is stable

 
 

The water balance of living cells. How living cells react to changes in the solute concentration of their environment depends on whether or not they have cell walls. (a) Animal cells such as this red blood cell do not have cell walls. (b) Plant cells do. (Arrows indicate net water movement since the cells were first placed in these solutions.)

Now let’s transfer the cell to a solution that is hypertonic to the cell (hyper means “more,” in this case more nonpenetrating solutes). The cell will lose water to its environment, shrivel, and probably die. This is one reason why an increase in the salinity (saltiness) of a lake can kill the animals there—if the lake water becomes hypertonic to the animals’ cells, the cells might shrivel and die. However, taking up too much water can be just as hazardous to an animal cell as losing water. If we place the cell in a solution that is hypotonic to the cell (hypo means “less”), water will enter the cell faster than it leaves, and the cell will swell and lyse (burst) like an overfilled water balloon.

A cell without rigid walls can tolerate neither excessive uptake nor excessive loss of water. This problem of water balance is automatically solved if such a cell lives in isotonic surroundings. Seawater is isotonic to many marine invertebrates. The cells of most terrestrial (land–dwelling) animals are bathed in an extracellular fluid that is isotonic to the cells. Animals and other organisms without rigid cell walls living in hypertonic or hypotonic environments must have special adaptations for osmoregulation , the control of water balance. For example, the protist Paramecium lives in pond water, which is hypotonic to the cell. Paramecium has a plasma membrane that is much less permeable to water than the membranes of most other cells, but this only slows the uptake of water, which continually enters the cell. Paramecium doesn’t burst because it is also equipped with a contractile vacuole, an organelle that functions as a bilge pump to force water out of the cell as fast as it enters by osmosis.

The contractile vacuole of Paramecium: an evolutionary adaptation for osmoregulation. The contractile vacuole of this freshwater protist offsets osmosis by bailing water out of the cell.

The cells of plants, prokaryotes, fungi, and some protists have walls. When such a cell is immersed in a hypotonic solution—bathed in rainwater, for example—the wall helps maintain the cell’s water balance. Consider a plant cell. Like an animal cell, the plant cell swells as water enters by osmosis. However, the elastic wall will expand only so much before it exerts a back pressure on the cell that opposes further water uptake. At this point, the cell is turgid (very firm), which is the healthy state for most plant cells. Plants that are not woody, such as most houseplants, depend for mechanical support on cells kept turgid by a surrounding hypotonic solution. If a plant’s cells and their surroundings are isotonic, there is no net tendency for water to enter, and the cells become flaccid (limp).

However, a wall is of no advantage if the cell is immersed in a hypertonic environment. In this case, a plant cell, like an animal cell, will lose water to its surroundings and shrink. As the plant cell shrivels, its plasma membrane pulls away from the wall. This phenomenon, called plasmolysis, causes the plant to wilt and can be lethal. The walled cells of bacteria and fungi also plasmolyze in hypertonic environments.

Let’s look more closely at how water and certain hydrophilic solutes cross a membrane. As mentioned earlier, many polar molecules and ions impeded by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane. This phenomenon is called facilitated diffusion. Cell biologists are still trying to learn exactly how various transport proteins facilitate diffusion. Most transport proteins are very specific: They transport only particular substances but not others.

As described earlier, the two types of transport proteins are channel proteins and carrier proteins. Channel proteins simply provide corridors that allow a specific molecule or ion to cross the membrane

Two types of transport proteins that carry out facilitated diffusion. In both cases, the protein transports the solute down its concentration gradient.

The hydrophilic passageways provided by these proteins allow water molecules or small ions to flow very quickly from one side of the membrane to the other. While water molecules are small enough to cross through the phospholipid bilayer, the rate of water movement by this route is relatively slow because of their polarity. Aquaporins, the water channel proteins, facilitate the massive amounts of diffusion that occur in plant cells and in animal cells such as red blood cells. Another group of channels are ion channels , many of which function as gated channels ; a stimulus causes them to open or close. The stimulus may be electrical or chemical; if chemical, the stimulus is a substance other than the one to be transported. For example, stimulation of a nerve cell by certain neurotransmitter molecules opens gated channels that allow sodium ions into the cell.

Carrier proteins seem to undergo a subtle change in shape that somehow translocates the solute–binding site across the membrane. These changes in shape may be triggered by the binding and release of the transported molecule.

In certain inherited diseases, specific transport systems are either defective or missing altogether. An example is cystinuria, a human disease characterized by the absence of a protein that transports cysteine and some other amino acids across the membranes of kidney cells. Kidney cells normally reabsorb these amino acids from the urine and return them to the blood, but an individual afflicted with cystinuria develops painful stones from amino acids that accumulate and crystallise in the kidneys.

Membrane Structure

The plasma membrane is the edge of life, the boundary that separates the living cell from its nonliving surroundings. A remarkable film only about 8 nm thick—it would take over 8,000 to equal the thickness of this page—the plasma membrane controls traffic into and out of the cell it surrounds. Like all biological membranes, the plasma membrane exhibits selective permeability; that is, it allows some substances to cross it more easily than others. One of the earliest episodes in the evolution of life may have been the formation of a membrane that enclosed a solution different from the surrounding solution while still permitting the uptake of nutrients and elimination of waste products. This ability of the cell to discriminate in its chemical exchanges with its environment is fundamental to life, and it is the plasma membrane and its component molecules that make this selectivity possible.

Phospholipid bilayer cross section

More than 50 kinds of proteins have been found so far in the plasma membrane of red blood cells, for example. Phospholipids form the main fabric of the membrane, but proteins determine most of the membrane’s specific functions. Different types of cells contain different sets of membrane proteins, and the various membranes within a cell each have a unique collection of proteins.

Synthesis of membrane components and their orientation on the resulting membrane. The plasma membrane has distinct cytoplasmic and extracellular sides, or faces, with the extracellular face arising from the inside face of ER, Golgi, and vesicle membranes.

Membranes have distinct inside and outside faces. The two lipid layers may differ in specific lipid composition, and each protein has directional orientation in the membrane. When a vesicle fuses with the plasma membrane, the outside layer of the vesicle becomes continuous with the cytoplasmic layer of the plasma membrane. Therefore, molecules that start out on the inside face of the ER end up on the outside face of the plasma membrane.

The process starts with (1) the synthesis of membrane proteins and lipids in the endoplasmic reticulum. Carbohydrates (green) are added to the proteins (purple), making them glycoproteins. The carbohydrate portions may then be modified. (2) Inside the Golgi apparatus, the glycoproteins undergo further carbohydrate modification, and lipids acquire carbohydrates, becoming glycolipids. (3) Transmembrane proteins (purple dumbbells), membrane glycolipids, and secretory proteins (purple spheres) are transported in vesicles to the plasma membrane. (4) There the vesicles fuse with the membrane, releasing secretory proteins from the cell. Vesicle fusion positions the carbohydrates of membrane glycoproteins and glycolipids on the outside of the plasma membrane. Thus, the asymmetrical distribution of proteins, lipids, and their associated carbohydrates in the plasma membrane is determined as the membrane is being built by the ER and Golgi apparatus.

The biological membrane is an exquisite example of a supramolecular structure—many molecules ordered into a higher level of organization—with emergent properties beyond those of the individual molecules. The remainder of this chapter focuses on one of the most important of those properties: the ability to regulate transport across cellular boundaries, a function essential to the cell’s existence. We will see once again that form fits function: The fluid mosaic model helps explain how membranes regulate the cell’s molecular traffic.

A steady traffic of small molecules and ions moves across the plasma membrane in both directions. Consider the chemical exchanges between a muscle cell and the extracellular fluid that bathes it. Sugars, amino acids, and other nutrients enter the cell, and metabolic waste products leave it. The cell takes in oxygen for cellular respiration and expels carbon dioxide. It also regulates its concentrations of inorganic ions, such as Na+, K+, Ca2+, and Cl−, by shuttling them one way or the other across the plasma membrane. Although traffic through the membrane is extensive, cell membranes are selectively permeable, and substances do not cross the barrier indiscriminately. The cell is able to take up many varieties of small molecules and ions and exclude others. Moreover, substances that move through the membrane do so at different rates.

Hydrophobic (nonpolar) molecules, such as hydrocarbons, carbon dioxide, and oxygen, can dissolve in the lipid bilayer of the membrane and cross it with ease, without the aid of membrane proteins. However, the hydrophobic core of the membrane impedes the direct passage of ions and polar molecules, which are hydrophilic, through the membrane. Polar molecules such as glucose and other sugars pass only slowly through a lipid bilayer, and even water, an extremely small polar molecule, does not cross very rapidly. A charged atom or molecule and its surrounding shell of water find the hydrophobic layer of the membrane even more difficult to penetrate. Fortunately, the lipid bilayer is only part of the story of a membrane’s selective permeability. Proteins built into the membrane play key roles in regulating transport.

Cell membranes are permeable to specific ions and a variety of polar molecules. These hydrophilic substances can avoid contact with the lipid bilayer by passing through transport proteins that span the membrane. Some transport proteins, called channel proteins, function by having a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane. For example, the passage of water molecules through the membrane in certain cells is greatly facilitated by channel proteins known as aquaporins . Other transport proteins, called carrier proteins, hold onto their passengers and change shape in a way that shuttles them across the membrane. In both cases, the transport protein is specific for the substance it translocates (moves), allowing only a certain substance (or substances) to cross the membrane. For example, glucose carried in blood and needed by red blood cells for cellular activities enters these cells rapidly through specific transport proteins in the plasma membrane. This “glucose transporter” is so selective as a carrier protein that it even rejects fructose, a structural isomer of glucose.

Thus, the selective permeability of a membrane depends on both the discriminating barrier of the lipid bilayer and the specific transport proteins built into the membrane. But what determines the direction of traffic across a membrane? At a given time, will a particular substance enter or leave the cell? And what mechanisms actually drive molecules across membranes? We will address these questions next as we explore two modes of membrane traffic: passive transport and active transport.

Avoiding Teacher Burnout

Objectively, teaching has got to be one of the top 5 most stressful careers in the world. In a single hour, we can play many different roles: nurse, babysitter, counselor, administrator, parental doormat, paper pusher, and maybe, if we're lucky, educator.

After my first year of teaching, what surprised me most was the fact that the least of my problems are actually in the classroom dealing with the kids. That's the easy part. The before school, extra classes, and afternoon meetings, not to mention the obnoxious amounts of paperwork, are enough to drive even the sanest teacher to the sanitarium. For these reasons and many more, teaching has the highest degree of career turnover of any profession.

But, how can we quit our complaining and attempt to avoid serious burnout? Try these strategies for avoiding teacher burnout and make concrete improvements in your attitude and outlook in the classroom.

Ask for Help - This is a really hard one for me to do. Often, it seems far easier to just do it myself than to explain how something should be done. Parents, friends, and students can be a valuable time-saving resource in your classroom, but only if you take the time to ask them. With a little planning and time invested up front, you can set up routine times and duties for the people available around you.

Don't Sweat the Small Stuff - This is a big piece of advice that applies in all areas of life. But, in teaching, we really need to put things in perspective. Is any one but you really going to care if the border on that bulletin board is crooked? Do you stage a Broadway-style dramatic production each year for yourself more than anyone else? Let's face it, we're there to teach the children. Some things that don't fall into the teaching/imparting knowledge category just may not be worth a disproportionate amount of time and effort. So, put down those fancy scissors that make the cool edge designs and go back to the essentials. Just something to think about.

Don't Play the Teacher at Home - When I first started student teaching, I was appalled to find that some of my new teacher habits were making their way into my home and marriage. For example, if my wife would do something annoying, I found myself giving her my evil "teacher look." You know which one I mean! Or, I would say something like, "Let's think about our choices." When you're at home, give it up! Don't scold, don't correct, and don't try to be the model of perfection. I've found that many people expect teachers to know everything and do everything perfectly. Don't fall for that trap. You're human so act like it at home. When I lock my classroom door, that's it.

Take Time for Yourself - Watch a stupid sitcom, listen to "un-teachery" music, talk to an old friend on the phone, forget about the papers that need to be graded that evening. Obviously, we can't do these things all the time. But, try not to beat yourself up over it if you do something fun once in awhile. I try to do something purely for pleasure each day. It really does keep me sane. Some nights, I get into bed early so I can read a book for fun. On weekend nights, I watch movies and I don't apologise for it. A little time invested in joyful activities can go a long way towards avoiding serious burnout.

Remember Why You Teach - Look past all of the annoyances and hassles, both big and small, and remember why you became a teacher in the first place. I quit a job in an office. Some days, I do question my sanity. But, most of the time, I just have to think about how useless and uninspired I felt behind that desk, staring at a boring spreadsheet, and I can remember why I teach. I teach in order to make a difference for children and to share myself with the world. Keep your reasons for teaching close to your heart and you'll realise that all of the stress really can be worth it.

After writing all of these anti-burnout tips, I feel a little more relaxed already! I hope you do, too.

Mammalian Kidney

The excretory system of mammals centres on the kidneys, which are also the principal site of water balance and salt regulation. Mammals have a pair of kidneys. Each kidney, bean–shaped and about 10 cm long in humans, is supplied with blood by a renal artery and drained by a renal vein.

Blood flow through the kidneys is voluminous. In humans, the kidneys account for less than 1% of body weight, but they receive about 20% of resting cardiac output. Urine exits each kidney through a duct called the ureter, and both ureters drain into a common urinary bladder. During urination, urine is expelled from the urinary bladder through a tube called the urethra, which empties to the outside near the vagina in females or through the penis in males. Sphincter muscles near the junction of the urethra and the bladder, which are under nervous system control, regulate urination.

The mammalian kidney has two distinct regions, an outer renal cortex and an inner renal medulla. Packing both regions are microscopic excretory tubules and their associated blood vessels. The nephron—the functional unit of the vertebrate kidney—consists of a single long tubule and a ball of capillaries called the glomerulus (Figure 44.13c and d). The blind end of the tubule forms a cup–shaped swelling, called Bowman′s capsule, which surrounds the glomerulus. Each human kidney contains about a million nephrons, with a total tubule length of 80 km.

Filtration of the Blood

Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman′s capsule. The porous capillaries, along with specialised cells of the capsule called podocytes, are permeable to water and small solutes but not to blood cells or large molecules such as plasma proteins. Filtration of small molecules is nonselective, and the filtrate in Bowman′s capsule contains salts, glucose, amino acids, and vitamins; nitrogenous wastes such as urea; and other small molecules—a mixture that mirrors the concentrations of these substances in blood plasma.

Pathway of the Filtrate

From Bowman′s capsule, the filtrate passes through three regions of the nephron: the proximal tubule; the loop of Henle, a hairpin turn with a descending limb and an ascending limb; and the distal tubule. The distal tubule empties into a collecting duct, which receives processed filtrate from many nephrons. This filtrate flows from the many collecting ducts of the kidney into the renal pelvis, which is drained by the ureter.

In the human kidney, approximately 80% of the nephrons, the cortical nephrons, have reduced loops of Henle and are almost entirely confined to the renal cortex. The other 20%, the juxtamedullary nephrons, have well–developed loops that extend deeply into the renal medulla. Only mammals and birds have juxtamedullary nephrons; the nephrons of other vertebrates lack loops of Henle. It is the juxtamedullary nephrons that enable mammals to produce urine that is hyperosmotic to body fluids, an adaptation that is extremely important for water conservation.

The nephron and the collecting duct are lined by a transport epithelium that processes the filtrate to form the urine. One of this epithelium′s most important tasks is reabsorption of solutes and water. Between 1,100 and 2,000 L of blood flows through a pair of human kidneys each day, a volume about 275 times the total volume of blood in the body. From this enormous traffic of blood, the nephrons and collecting ducts process about 180 L of initial filtrate, equivalent to two or three times the body weight of an average person. Of this, nearly all of the sugar, vitamins, and other organic nutrients and about 99% of the water are reabsorbed into the blood, leaving only about 1.5 L of urine to be voided.

Blood Vessels Associated with the Nephrons

Each nephron is supplied with blood by an afferent arteriole, a branch of the renal artery that subdivides into the capillaries of the glomerulus. The capillaries converge as they leave the glomerulus, forming an efferent arteriole. This vessel subdivides again, forming the peritubular capillaries, which surround the proximal and distal tubules. More capillaries extend downward and form the vasa recta, the capillaries that serve the loop of Henle. The vasa recta also form a loop, with descending and ascending vessels conveying blood in opposite directions.

Although the excretory tubules and their surrounding capillaries are closely associated, they do not exchange materials directly. The tubules and capillaries are immersed in interstitial fluid, through which various substances diffuse between the plasma within capillaries and the filtrate within the nephron tubule. This exchange is facilitated by the relative direction of blood flow and filtrate flow in the nephrons.

Proximal tubule. Secretion and reabsorption in the proximal tubule substantially alter the volume and composition of filtrate. For example, the cells of the transport epithelium help maintain a relatively constant pH in body fluids by the controlled secretion of H+. The cells also synthesize and secrete ammonia, which neutralizes the acid and keeps the filtrate from becoming too acidic. The more acidic the filtrate, the more ammonia the cells produce and secrete, and the urine of a mammal usually contains some ammonia from this source (even though most nitrogenous waste is excreted as urea). The proximal tubules also reabsorb about 90% of the important buffer bicarbonate (HCO3−). Drugs and other poisons that have been processed in the liver pass from the peritubular capillaries into the interstitial fluid, and then are secreted across the epithelium of the proximal tubule into the nephron′s lumen. Conversely, valuable nutrients, including glucose, amino acids, and potassium (K+), are actively or passively transported from the filtrate to the interstitial fluid and then are moved into the peritubular capillaries.

One of the most important functions of the proximal tubule is reabsorption of most of the NaCl (salt) and water from the huge initial filtrate volume. Salt in the filtrate diffuses into the cells of the transport epithelium, and the membranes of the cells actively transport Na+ into the interstitial fluid. This transfer of positive charge is balanced by the passive transport of Cl− out of the tubule. As salt moves from the filtrate to the interstitial fluid, water follows by osmosis. The exterior side of the epithelium has a much smaller surface area than the side facing the lumen, minimizing leakage of salt and water back into the tubule. Instead, the salt and water now diffuse from the interstitial fluid into the peritubular capillaries.

Descending limb of the loop of Henle. Reabsorption of water continues as the filtrate moves into the descending limb of the loop of Henle. Here the transport epithelium is freely permeable to water but not very permeable to salt and other small solutes. For water to move out of the tubule by osmosis, the interstitial fluid bathing the tubule must be hyperosmotic to the filtrate. The osmolarity of the interstitial fluid does in fact become progressively greater from the outer cortex to the inner medulla of the kidney. Thus, filtrate moving downward from the cortex to the medulla within the descending limb of the loop of Henle continues to lose water to interstitial fluid of greater and greater osmolarity, which increases the solute concentration of the filtrate.

Ascending limb of the loop of Henle. The filtrate reaches the tip of the loop, deep in the renal medulla in the case of juxtamedullary nephrons, then moves back to the cortex within the ascending limb. In contrast to the descending limb, the transport epithelium of the ascending limb is permeable to salt but not to water. The ascending limb has two specialized regions: a thin segment near the loop tip and a thick segment adjacent to the distal tubule. As filtrate ascends in the thin segment, NaCl, which became concentrated in the descending limb, diffuses out of the permeable tubule into the interstitial fluid. This movement increases the osmolarity of the interstitial fluid in the medulla. The exodus of salt from the filtrate continues in the thick segment of the ascending limb, but here the epithelium actively transports NaCl into the interstitial fluid. By losing salt without giving up water, the filtrate is progressively diluted as it moves up to the cortex in the ascending limb of the loop.

Distal tubule. The distal tubule plays a key role in regulating the K+ and NaCl concentration of body fluids by varying the amount of the K+ that is secreted into the filtrate and the amount of NaCl reabsorbed from the filtrate. Like the proximal tubule, the distal tubule also contributes to pH regulation by the controlled secretion of H+ and reabsorption of bicarbonate (HCO3−).

Collecting duct. The collecting duct carries the filtrate through the medulla to the renal pelvis. By actively reabsorbing NaCl, the transport epithelium of the collecting duct plays a large role in determining how much salt is actually excreted in the urine. Though its degree of permeability is under hormonal control, the epithelium is permeable to water. However, it is not permeable to salt or, in the renal cortex, to urea. Thus, as the collecting duct traverses the gradient of osmolarity in the kidney, the filtrate becomes increasingly concentrated as it loses 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 in the filtrate at this point, some urea diffuses out of the duct and into the interstitial fluid. Along with NaCl, this urea contributes to the high osmolarity of the interstitial fluid in the medulla. This high osmolarity enables the mammalian kidney to conserve water by excreting urine that is hyperosmotic to the general body fluids.