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Western University
Physiology 2130
Anita Woods

Renal System, Water & Electrolyte Balance Contents Section 11.1 – Objectives By the end of this section, you should be able to: • Describe homeostasis as it applies to body fluids and osmolarity. • List the functions of the kidney. • Draw a diagram of the kidney and label the major components. • Draw a diagram of the kidney blood flow. • Draw a diagram of a nephron and add arrows showing the direction for filtration, reabsorption, secretion, and excretion. • Distinguish between filtration, reabsorption, secretion, and excretion. • Describe and distinguish between renal blood flow and renal plasma flow and give values for each. • Explain glomerular filtration and list the factors that affect it. • Explain Starling Forces and list the four Starling Forces. • Describe filtered load and distinguish it from glomerular filtration. • List and describe all the tubular transport mechanisms involved in the movement of ions and fluid along the nephron. • Describe the reabsorption of sodium, glucose, amino acids, potassium, chloride and water and the secretion of hydrogen ions in the proximal tubule. • Describe the reabsorption of water in the descending limb and the reabsorption of sodium, chloride, potassium and water in the ascending limb of the loop of Henle. • Describe the reabsorption of sodium and water and the secretion of potassium in the distal tubule of the nephron. • Describe the reabsorption of sodium and water and the secretion of potassium in the collecting duct. • Describe the concept of water balance and how the kidneys regulate it. • Describe the control and release of antidiuretic hormone (ADH) and its effect on the kidneys. • Describe the renin-angiotensin system and the production of angiotensin II and its effects on the kidneys. • Describe the filtration, reabsorption, and secretion of potassium by the kidneys. Section 11.2 – Introduction • The renal system includes:  Kidneys  The principal function of the kidneys is the regulation of water balance, electrolyte levels, pH of the blood, and the long-term regulation of arterial pressure.  ureters  bladder  urethra 1 Section 11.3 – Functions of the Kidney • The basic function of the kidneys is to remove nonessential substances from the plasma, including waste metabolites, excess water, and electrolytes and to recover any essential substance like glucose. • In doing so, the kidneys play a major role in regulating the water levels, the chemical concentration of the body fluid compartments, and pH (or acidity) of the blood. • It is important to understand that the kidneys do not produce water or electrolytes but only conserve them by reducing the amount removed from the body. • The elimination of waste or foreign substances is an important function of the kidneys.  This includes the removal of drugs, food additives, and vitamins that are excreted in the urine. • The kidneys also act as an endocrine gland, producing hormones or components of hormonal systems such as erythropoietin, renin, vitamin D, and stanniocalcin. Section 11.4 - Anatomy of the Kidneys • The kidneys are roughly the size of a fist. • They consist of:  An outer renal cortex  Amiddle renal medulla  Inner calyces that drain into a central renal pelvis.  The renal pelvis then drains into the ureter. • Located within the renal pyramids are the functional units of the kidneys – the nephrons. 2  Each nephron drains through a collecting duct into a calyx. Section 11.5 – Anatomy: Blood Supply of the Kidneys • Blood flows to the kidneys through the renal artery. • This large artery branches into several interlobar arteries that, in turn, branch into arcuate arteries. • The blood in the arcuate arteries flows through the interlobular arteries to supply the nephron. • The blood supply to the nephron drains into the interlobular vein, the arcuate vein, the interlobar vein, and then into the renal vein. Section 11.6 – Anatomy: The Nephron • The nephron is the functional unit of the kidneys. • There are over 1 million nephrons in each kidney whose function is to filter the blood, reabsorb essential substances, and excrete nonessential molecules and waste. • Each nephron is composed of a highly coiled hollow tube surrounded by a complex blood supply. • The glomerular capsule (also called Bowman's capsule) surrounds a very small, highly permeable capillary bed called the glomerulus.  These structures are often collectively referred to as the renal corpuscle. • The tubular portion of the nephron consists of the following structures in order:  The proximal convoluted tubule (a highly coiled portion of the nephron), the descending and ascending limb of the loop of Henle, the distal convoluted tubule, and the collecting duct. Section 11.7 – Anatomy: Blood Supply of the Nephron • The blood supply of the nephron is very complex. • Blood from the renal artery eventually reaches the interlobular artery that drains into the afferent arteriole. • The afferent arteriole gives rise to the glomerulus (where filtration takes place). • The blood from the glomerulus enters the efferent arteriole. • Blood then enters the peritubular capillaries (a dense network of capillaries surrounding the tubes of the nephron), which then drains into the interlobular vein and eventually back to the renal vein. Section 11.8 – The Renal Corpuscle 3 • The renal corpuscle is made up of the glomerular capsule (also called Bowman's capsule) and glomerulus.  This is the site where the blood is filtered – a process called glomerular filtration.  The fluid that is filtered from the blood that enters the glomerular capsule (or capsular space) is called the filtrate.  Glomerular filtration, as we will see, is facilitated by a highly permeable capillary endothelium that is surrounded by podocytes.  The larger diameter afferent arteriole and smaller diameter efferent arteriole also enhance glomerular filtration. Section 11.9 - Processes along the Nephron • Each section of the nephron has different functions.  These functions can be broken into filtration, reabsorption, and secretion. • Before examining each section in detail, we must first have a good understanding of these functions. 4 Section 11.10 – Processes along the Nephron: Important Terms • Filtration:  The movement of fluid through the glomerular capillary due to hydrostatic pressures.  The filtrate is the solution created by filtration.  The filtrate is generally composed of water plus all the dissolved solutes in the blood (except for large proteins that are too big to be filtered). • Reabsorption:  The movement of a substance from the lumen of the nephron back into the blood. • Secretion:  The movement of a substance from the blood into the lumen of the nephron. • Excretion:  the removal of a substance from the body 5  Putting these together, we get the equation:  Excretion = filtration + secretion – reabsorption Section 11.11 – Glomerular Filtration • The bulk flow of fluid from the blood into the glomerular capsule. • This fluid, called the filtrate, contains the same substances as plasma with the exception of large proteins and red blood cells. • Glomerular filtration is affected by the extremely permeable capillaries, which make up the glomerulus, and Starling Forces. • Special epithelial cells called podocytes surround the capillaries.  The podocytes have large filtration slits that are formed between pedicles.  These structural features increase filtration at the glomerulus. 6 Section 11.12 - Glomerular Filtration: Starling Forces • As we saw in the cardiovascular system, Starling Forces cause the bulk movement of fluid across capillaries due to a combination of hydrostatic and colloid osmotic forces. • The glomerular capillary is similar, although the pressure of each force will be different. • The blood hydrostatic pressure is roughly 60 mmHg – almost twice that in a regular capillary, causing filtration of fluid into the glomerular capsule.  This pressure is principally due to the difference in diameter between the afferent (large) and efferent (small) arterioles. • The colloid osmotic pressure due to plasma proteins is –32 mmHg, causing reabsorption of fluid into the plasma. • The capsular hydrostatic pressure is –18 mmHg, causing the reabsorption of fluid. There is no colloid osmotic force in the glomerular capsule since very few proteins are filtered. • Therefore, the resulting net filtration pressure is 10 mmHg out of the glomerulus into the capsular space. Section 11.13 - Glomerular Filtration Rate (GFR) and Filtered Load • The kidneys filter a tremendous amount of fluid each day – roughly 180 L/day (48 gallons/day). • This glomerular filtration rate (GFR) is the volume of fluid that is filtered by the glomerulus during a certain time period. 7 • Since the kidneys filter many other substances, it is important to be able to calculate the amount of these substances filtered by the kidneys per day; this is called the filtered load. • The filtered load can be calculated using equation 1:  Filtered Load = GFR x Plasma Concentration of the Substance Section 11.14 - Glomerular Filtration Rate (GFR) and Filtered Load (cont’d) • Urine concentration:  The amount of the solute that is excreted per unit volume of urine (g/L). • The amount of solute excreted:  The actual amount (in grams) of solute that is excreted in the urine and can be calculated using equation 2 at right. • The amount reabsorbed:  The amount of filtered substance that is taken back up (reabsorbed) by the kidneys and can be calculated using equation 3. • The fraction excreted is calculated using equation 4. Let's try an example. 8 Section 11.15 - Try Some Calculations Yourself • You've just eaten a large bag of salt and vinegar chips.After several hours, once this food has been absorbed into the body, you urinate. Given the following data, calculate the concentration of Na+ in the urine (in g/L):  Glomerular Filtration Rate (GFR) = 180 L/day  Na+ plasma concentration = 5 g/L  Amount of Na+ reabsorbed = 626 g/day  Amount of water excreted = 1.8 L Section 11.16 – Tubular Transport Mechanisms: Introduction • Now that you have a basic understanding of glomerular filtration, it is time to look at the rest of the nephron—from the proximal convoluted tubule to the collecting duct. • Throughout these sections of the nephron, reabsorption (shown in blue) and secretion (shown in pink) occur to different substances in a regulated or non- regulated manner. • Reabsorption and secretion involve a number of transport mechanisms including active transport, secondary active transport, facilitated diffusion, simple diffusion, and osmosis (in the case of water). Section 11.7 – Tubular Transport Mechanisms: Reabsorption 9 • Over 99% of the substances filtered in the glomerulus are reabsorbed back into the circulation at different sites along the nephron. • When molecules are reabsorbed from the filtrate back into the circulation, there are two transport routes that can be taken:  paracellular transport and/or  transcellular transport. • The tubular cells are joined together by tight junctions.  Generally, these tight junctions don’t allow substances to cross between the cells.  However, along the nephron, these tight junctions vary and can be quite leaky.  As a result, some substances can diffuse between the tubular cells by a process called paracellular transport.  Paracellular transport is generally non-regulated, occurring without any hormone control. • Other substances are transported across the tubular cell membrane from the lumen into the cell, then into the interstitial fluid and into the blood.  This form of reabsorption is called transcellular (or transepithelial) transport.  In many cases, transcellular transport can be regulated (increased or decreased) by hormones.  However, most cases of transcellular transport are non-regulated and occur without any hormonal control. + + Section 11.18 – Tubular Transport Mechanism – Reabsorption: The Na /K pump • Many of the transport mechanisms along the nephron rely upon the Na /K pump + that we have seen many times before.  Recall that this pump is an active transport mechanism because it requires adenosine triphosphate (ATP) in order to move 3 Na out of the cell and 2 K into the cell.  As a result it helps establish a concentration gradient for both ions across the cell membrane – a high concentration of Na outside the cell and low inside, and a high concentration of K inside the cell and low outside.  Can power other transport mechanisms by a process called secondary active transport. Section 11.19 – Tubular Transport Mechanisms – Reabsorption: SecondaryActive Transport • In secondary active transport: + + +  The Na concentration gradient that is established by the Na /K pump is used to power other transporters. +  As Na moves into the cell down its concentrati+n gradient, other substances will either move in with the Na or will move out in exchange with the incoming Na .+ 10 +  Secondar+ ac+ive transporters include the Na /Glucose co-transporter and the Na /H exchanger, as shown at right. • The Na /Glucose co-transporter:  Located on the luminal side of tubule cells.  With this transporter, as each Na diffuses into the cell, a single glucose molecule is carried in along with it.  The Na /H exchanger, on the other hand, moves one H out of the cell for every Na that diffuses in. It is also located on the luminal side of the cells. Section 11.20 – Tubular Transport Mechanisms: Secretion • Secretion:  The process by which the kidneys remove unwanted substances from the blood into the lumen of the nephron.  Secretion is generally a hormonally regulated process but, in some cases, it can occur without any hormone control (non-regulated).  Most substances that are secreted are eventually excreted in the urine. + +  Secreted substances include H and K . Section 11.21 - Tubular Transport Mechanisms • We are now ready to look at the reabsorption and secretion of different molecules along each section of the nephron. Below is a brief list of substances and at right is a basic diagram of the nephron. You may wish to print the diagram at right and make notes on it as we work through each section. • Keep in mind the following: +  Na reabsorption:  Takes place in the proximal tubule, ascending limb of the loop of Henle, and early distal tubule – mostly by non-regulated mechanisms.  However, Na reabsorption in the proximal tubule can also be regulated by the hormone angiotensin II, and by the hormone aldosterone in the late distal tubule and collecting duct. • In a healthy individual, all the glucose that is filtered at the glomerulus is reabsorbed in the proximal tubule. • Amino acids, the building blocks of proteins, are also reabsorbed in the proximal tubule. • Water reabsorption:  Takes place in the proximal tubule and descending limb of the loop of Henle through non-regulated mechanisms. 11  No water is reabsorbed in the ascending limb of the loop of Henle. Water reabsorption is regulated by antidiuretic hormone (ADH) in the late distal tubule and collecting duct. • K reabsorption:  Takes place in the proximal tubule and the ascending limb of the loop of Henle. +  Secretion of K occurs in small amounts in the ascending limb of the loop of Henle.  Larger amounts of this ion are secreted in the late section of the distal tubule and collecting duct under the influence of the hormone aldosterone. + • H secretion:  Occurs in the proximal tubule and the ascending limb of the loop of Henle – it can be both regulated and non-regulated.  Hydrogen ions are also secreted in the late distal tubule and collecting duct but these complex mechanisms will not be covered on this CD. Section 11.22 – Proximal Convoluted Tubule: Reabsorption of Na , Glucose, and AminoAcids • The proximal convoluted tubule reabsorbs roughly 66% of the total filtrate. 12 + + • One of the most important transport mechanisms of the nephron is the Na /K pump. • Due to the Na concentration gradient established by the pump, Na can be + reabsorbed into the tubule cells by simple diffusion, the Na /glucose co- transporter, or the Na /H exchanger. + + • We will see later that the Na /H exchanger can be regulated by the hormone angiotensin II. • AminoAcids, the building blocks of proteins, are also reabsorbed along this + sec+ion of the nephron by a Na /AminoAcid co-transporter similar to the Na /glucose co-transporter. • In a healthy individual, the Na /glucose co-transporter reabsorbs all of the glucose in the filtrate. This is not the case in people with diabetes mellitus. Section 11.23 – Proximal Convoluted Tubule: Diabetes Mellitus • Diabetes mellitus:  Adisease that affects the pancreas' ability to produce the hormone insulin.  Insulin is essential for cells to take up and store glucose after a meal.  Therefore, without insulin, glucose concentrations build up in the blood. o Large quantities of glucose are filtered by the glomerulus, and as a result, the Na /glucose co-transporters cannot reabsorb all of it. o Consequently, some is excreted in the urine. - This is one of the important symptoms of diabetes mellitus – glucose in the urine. • Since this co-transport is a form of secondary active transport, it can be saturated. Section 11.24 - Proximal Convoluted Tubule: Reabsorption of Water + • Now that Na , glucose, and amino acids have been reabsorbed, the filtrate will have a lower solute concentration (and higher water concentration) compared to the cell and interstitial fluid. • With this osmotic gradient and the presence of special water channels (called aquaporins), water will move down its concentration gradient by osmosis causing it to be reabsorbed. • Water will be reabsorbed by both paracellular transport between the cells and by transcellular transport across the cells. • It is extremely important to note that water reabsorption takes place onlyAFTER + solutes have been reabsorbed – particularly Na . 13 Section 11.25 - Proximal Convoluted Tubule: Potassium and Chloride • 65% of all the filtered K and Cl are reabsorbed in the proximal tubule.  All of this reabsorption is through two types of paracellular transport – both of which are not regulated. • Recall that the tubule cells are joined together by tight junctions and that these junctions can be leaky – particularly in the proximal tubule. • Chloride ions are also reabsorbed in the proximal tubule by transcellular transport that uses a complex mechanism (won’t be covered on this CD). Section 11.26 - Proximal Convoluted Tubule: Potassium and Chloride Reabsorption + − • As mentioned on the previous page, reabsorption of K and Cl occurs by two paracellular mechanisms: 1. Solvent drag +  Involves the reabsorption of K with the movement of water.  This water, as it moves between the cells, also carries with it some of the dissolved substances in the filtrate – including K . +  This movement of a solute (the K ) in a solvent (the water) is called solvent drag. 14 2. Simple diffusion  The other paracellular route of K reabsorption is by simple diffusion down its concentration gradient.  Recall that water is reabsorbed by osmosis by both paracellular and transcellular mechanisms. +  As water is reabsorbed through the tubule cells, K remain in the filtrate.  As more water is reabsorbed alone, the filtrate becomes more and + more concentrated with K .  As a result the K concentration gradient increases to the point where K can move into the interstitial space between the tubule cells (paracellular route) via simple diffusion. • We have already seen that water is reabsorbed by osmosis between the tubule cells. Section 11.27 - Reabsorption of Filtrate Back into the Circulation • We have seen that the cells lining the proximal tubule reabsorb a great deal of material, including Na , glucose, and water from the lumen of the nephron. • But how is all this material returned to the circulation?  This material must first leave the cells and enter the interstitial space. + + +  Na will leave the tubule cells by the Na /K pump. Glucose and amino acids are transported across the basal membrane of the cells by specific facilitated diffusion transporters. +  Remember, K are reabsorbed by paracellular transport so they are already in the interstitial fluid. • Once in the interstitial fluid, all these substances are reabsorbed into the circulation by Starling Forces. Section 11.28 – Reabsorption of Filtrate Back into the Circulation • Reabsorption of water and all dissolved molecules from the interstitial space back into the peritubular capillaries occurs due to Starling Forces • The pressures for these Starling Forces around the tubule cells are much different than those around a typical capillary.  In the kidney, the capillary hydrostatic force (P ) = c13 mmHg,  The interstitial hydrostatic force (P ) = ~6 mmHg, IF  Osmotic force due to proteins in the plasma (Π ) = ~32pmmHg  The interstitial osmotic force (Π ) = IF5 mmHg.  As a result, the Net Filtration Pressure moving the fluid, which is equal to (P − P ) − (Π − Π ), is calculated to be −10 mmHg, or simply 10 mmHg c IF P IF back into the capillary. • You should note the very large osmotic pressure caused by the plasma proteins. 15  This pressure is large because during glomerular filtration, almost all substances were filtered except large proteins. Section 11.29 – Proximal Convoluted Tubule—Concentration of Filtrate • After reabsorbing Na , glucose, amino acids, K , and water, the concentration of the filtrate leaving the proximal tubule will not have changed significantly from what it was at the beginning of the tubule.  It is still roughly 290–300 mOsm/kg water. • Essentially, this means that the same proportion of solutes and water are being reabsorbed in the proximal tubule. Section 11.30 - The Loop of Henle: Concentration Gradient in the Medulla of the Kidneys • The loop of Henle consists of:  Adescending section that extends deep into the medulla of the kidneys  An ascending section that loops back into the cortex.  About 15% of the filtered water and roughly 20% of the filtered sodium is reabsorbed in these regions. • There is a dramatic change in the concentration of the interstitial fluid within the medulla.  At the junction between the cortex and the medulla, the concentration of the interstitial fluid is 300 mOsm/kg water.  As the nephron descends into the medulla, the concentration progressively increases to 1200 mOsm/kg water. Section 11.31 – Loop of Henle: Descending Limb – Reabsorption of Ions and Water • The descending loop of Henle is very permeable to water a
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