Mammalian Kidney
Countercurrent Multiplier: Thick Ascending Limb (peripheral renal medulla)
-Active transport of Cl- into the interstitial fluid; Na+ follows -Impermeable to water -Increases the interstitial fluid osmolarity
What does water permeability depend on?
-Aquaporins --Exist in many forms --Present in both apical and basolateral membranes --Abundance varies along the nephron tubule -Varying the number of aquaporins varies the membrane permeability
The cluster of capillaries within Bowman's Capsule is called?
-Glomerulus
Renal Tubule
-Processes filtrate into urine
What is the primary function of the kidney?
-Regulation of extracellular fluid (plasma and body tissue fluid) This is done by formation of urine, which is a modified filtrate of plasma. Other Functions of the Kidney: 1. Regulation of volume of plasma (relates to blood pressure) 2. Regulation of concentration of waste products in plasma 3. Regulation of concentration of electrolytes (Na+ and K+) in plasma 4. Regulation of the pH of plasma
Where does the most reabsorption of nutrients (like glucose) occur?
-The proximal convoluted tubule. -It reabsorbs 65% of important nutrients.
The fluid eliminated by the excretory system is what?
-Urine
Vasa recta
-blood vessel in medulla, parallel to Loop of Henle
Nephron
-the functional unit of the vertebrate kidney
URINE IS HYPERTONIC (COMPARED TO PLASMA)
1400 litres of blood per day --> 180 litres of glomerular filtrate per day --> 2-3 Litres of Urine per day -99% of the glomerular filtrate is reabsorbed
180 litres of glomerular filtrate per day
99% of glomerular filtrate gets reabsorbed.
Renal vein brings filtered blood into the kidney, whereas renal artery brings unfiltered blood away from the kidney.
Blood in the renal vein, compared to the renal artery, has less urea and toxins, less oxygen, less salts and ions, less water, less glucose and more carbon dioxide since it is filtered. Amount of proteins remain the same in renal vein and artery because proteins are not filtered.
Excretory Systems
Filtration: -Water and small molecules are filtered and collected in a tubule -Driven by blood pressure ______________________________________________________________________ Secretion and resorption: -Active processes that alter the filtrate composition in the tubule ______________________________________________________________________ Desired function depends upon environment: -Freshwater animals: excrete water, retain salts -Marine and land animals: excrete salts, retain water
Peritubular capillaries
The network of tiny blood vessels that surrounds the proximal and distal tubules in the kidney
Trace the flow of urine from the baby's kidneys through her urinary tract.
The urine flow through the urinary tract of the potty training baby is as follows: 1. Kidney 2. Renal pelvis 3. Ureter 4. Bladder 5. Urethra.
Explain the process of ultrafiltration.
Ultrafiltration occurs in the Bowman's capsule (aka renal capsule) of the kidney cortex. The entering afferent arteriole has a larger diameter (to allow increased blood pressure) than the leaving efferent arteriole. This creates high pressure in the capsule. Some smaller molecules are forced out of the glomerulus (in the renal capsule) because the glomerulus has basement membranes and fenestrations (pores). The glomerulus filters out water, urea, glucose, amino acids and salts. Plasma proteins, platelets and cells are too large and so they remain in the blood. The glomerular filtrate (water, urea, glucose, amino acids and salt) transports through the nephron. ______________________________________________________________________ Note: -Ultrafiltration occurs before osmoregulation. -After ultrafiltration and before osmoregulation, the proximal convoluted tubule (PCT) reabsorbs useful substances such as glucose, amino acids, vitamins, hormones and 80% of mineral ions and water. This occurs through active transport and it is selective. All the nutrients in the PCT gets passively diffused into the blood. -Arteriole is a smaller artery. -The afferent arteriole is the arteriole that enters
THE RENAL CORPUSCLE: FILTRATION
-A renal corpuscle is the blood-filtering component of the nephron of the kidney. It consists of a glomerulus - a tuft of capillaries composed of endothelial cells, and a glomerular capsule known as Bowman's capsule. -Blood enters the renal corpuscle via an afferent arteriole -Inside the corpuscle, the blood supply forms a glomerulus, a compact "tuft" of interconnecting capillary loops -Contained within a Bowman's capsule -20% of the blood plasma that enters via the afferent arteriole is filtered out in the renal corpuscle -The Bowman's capsule has two cellular layers: --Outer parietal layer --Inner visceral layer of podocytes Note: -Podocytes are cells in the Bowman's capsule in the kidneys that wrap around capillaries of the glomerulus. The Bowman's capsule filters the blood, retaining large molecules such as proteins while smaller molecules such as water, salts, and sugars are filtered as the first step in the formation of urine.
As glomerular filtrate moves through the ascending loop of Henle, the filtrate becomes more dilute: why?
-As glomerular filtrate moves through the ascending loop of Henle, the filtrate becomes more dilute because the ascending loop is permeable to ions but impermeable to water. hence, there's a higher concentration of ions than water, making it more dilute (?) _____________________________________________________________ Note: -The glomerular filtrate contains water and small molecules. It can become more dilute by either increasing its concentration of water or decreasing its concentrations of small molecules. -The descending loop of Henle is permeable to water; the ascending loop is impermeable.
Acid-base balance
-Equilibrium between acid and base concentrations in the body fluids -Kidneys also regulate H+ concentration --Renal tubule cells secrete H+ in exchange for Na+. --CO2 made and diffuses into tubule cells; reacts to form HCO3- --HCO3- is transported out of cell into blood --Glutamine metabolism produces NH3 --NH3 transported into tubular fluid by NH3 transporter --NH3 combines with H+ to form NH4+: excreted -Absorb H+ using bicarbonate as buffer: CO2 + H2O ⇌ H2CO3 ⇌ H+ +HCO3- -Kidneys regulate the amount of H+ and HCO3-
PROXIMAL CONVOLUTED TUBULE
-First section of the renal tubule that the blood flows through; reabsorption of water, ions, and all organic nutrients -Two-thirds of total water reabsorption from the glomerular filtrate occurs in the proximal convoluted tubule -Cuboid cell lining, villi toward the tubular lumen (large surface area), rich in mitochondria providing energy for active transport -Achieved through active Na+ transport -Na+/K+ATPase pumps move Na+ into the interstitial fluid -Cl- follows passively and water follows by osmosis -Other solutes (glucose, amino acids) are taken up by active transport
What has the ability to pass through the fenestrations of the glomerulus in a healthy individual?
-Ions (sodium, chloride, potassium), amino acids, and glucose will pass through the fenestrations. -Red blood cells, white blood cells, and large proteins will not pass through the fenestrations.
Basic Anatomy of Urinary System
-Kidneys are retroperitoneal (lie outside the abdominal cavity) -Urine flows from the kidneys into the ureters (right and left) -Both ureters discharge into the bladder -The bladder empties via urethra -Rena hilus - where ureter, renal artery and renal vein enter -Renal cortex - outer region of the kidney -Renal medulla - inner region of the kidney -Renal pelvis - collecting ducts drain into ureter
Which of the following is least likely to be present in the glomerular filtrate (the filtrate produced by the nephron before it enters the loop of Henle) of a healthy adult nephron?
-Large molecular weight protein are too large to pass through the glomerulus and into the tubular filtrate. Therefore, they are least likely to be present in the glomerular filtrate of a healthy adult nephron. Note: Electrolytes, glucose, and amino acids are all filtered from the blood into the glomerular filtrate, and subsequently reabsorbed.
Macula densa cells (which form the lining of the distal convoluted tubule) are part of the juxtaglomerular apparatus, which regulates blood pressure.
-Macula densa cells sense concentrations of sodium ion.
Countercurrent Multiplier: Thin Descending Limb (peripheral renal medulla)
-No active transport -Permeable to water -Increased interstitial fluid osmolarity removes water from the filtrate by osmosis -Equilibrates with the extracellular fluid (ECF)
Explain osmoregulation in the kidney.
-Osmoregulation is mechanisms in our body that attempt to maintain homeostatic levels of water. Osmoregulation takes place in the Loop of Henle and collecting duct, located in the medulla of the kidney. The Loop of Henle consists of a descending limb and an ascending limb. The descending limb is permeable to water and impermeable to sodium ions, whereas the ascending limb is impermeable to water and permeable to sodium ions. Sodium ions are pumped out of the ascending limb into the medulla by active transport. Some water leaves descending limb by osmosis. This creates a high concentration of solutes in the cells and fluid of the medulla, thus generating osmotic potential between nephron and medulla (in between the loop of Henle). As a result, the output of the Loop of Henle is reduced volume of filtrate (water, urea, glucose, amino acids and salts) and reduced salt concentration. When the filtrate enters the collecting duct from the distal convoluted tubule, there is a countercurrent flow of blood in capillaries and filtrate in the duct (meaning the current moves in different directions in capillaries and duct). When a person is dehydrated, the body releases a hormone called ADH, which opens aquaporins (channels) in the duct that allow the blood to absorb more water. The increased transfer of water into the blood leads to hypertonic (very concentrated) urine. ______________________________________________________________________ Note: In the descending limb, solute concentration in filtrate increases as the limb descends. However, in the ascending limb, solute concentration in filtrate decreases as the limb ascends.
Collecting Ducts
-Several nephrons share a collecting duct which serve to carry urine to the renal pelvis -Tubular fluid entering collecting ducts has the same osmolarity as glomerular filtrate, but very different composition -Major solute is now urea -The collecting ducts concentrate urine -Pass through the renal medulla with high interstitial fluid osmolarity (set up by Loops of Henle) -Water is reabsorbed from urine into interstitial fluid by osmosis -Urine becomes concentrated before reaching the renal pelvis and the ureter -Urea can leak out of collecting duct into interstitial fluid: helps increase its osmotic potential -Diffuses back into Loops of Henle -Recycling of urea contributes to concentration gradient
Glomerulus
-Site of filtration from capillaries
Podocytes
-Specialised epithelial cells in the Bowman's capsule in the kidneys that wrap around capillaries of the glomerulus -Numerous foot processes (pedicels) create filtration slits -Blood pressure forces plasma out of the capillary lumen via fenestrations, through basement membrane -The filtrate passes through filtration slits into the capsular space and into the proximal convoluted tubule -Termed the glomerular filtrate
Loop of Henle (nephron loop)
-The Loop of Henle generates a high concentration of solutes in the cells and fluid of the medulla. Output of urine is more dilute than input. -Descending Loop is permeable to water & impermeable to Na+ --As solute concentration increases in medulla, an osmotic gradient is established. --Some water leaves filtrate by osmosis. --Solute concentration in filtrate increases as loop descends -Ascending Loop is impermeable to water & permeable to Na+ --Na+ is pumped into the medulla by active transport. Cl- ions follow. --Solute concentration in filtrate decreases as loop ascends -Overall Effects --Filtrate volume decreases --Output is slightly more dilute --Large amount of salts removed -Countercurrent effect facilitates changes in the Loop of Henle -Waste products are actively secreted into filtrate from blood -Increases the osmolarity of the surrounding fluids -Countercurrent multiplier -Countercurrent: tubular fluid in the descending and ascending limbs of the loop flow in opposite directions -Multiplier: creates a concentration gradient in the renal medulla -Only cells lining the upper (thick) ascending limb are specialised for transport; cells in descending and lower (thin) ascending are not
In which ways can the body increase filtration rate?
-The body can increase filtration by increasing the afferent arteriole diameter and decreasing the efferent arteriole diameter. This is because the decreased volume of the efferent arteriole results in increased pressure and hence, an increase in filtration.
Describe the position of the Nephron.
-The nephron is the urine-producing part of the kidney -Glomeruli, proximal and distal convoluted tubules are in the renal cortex -Loops of Henle & collecting ducts are in the renal medulla *NEPHRONS ARE SURROUNDED WITH BLOOD VESSELS
Collecting Duct
-The portion of the nephron where water reabsorption is regulated via antidiuretic hormone (ADH). -Several nephrons empty into each collecting duct, and this is the final region through which urine must pass, on its way to the ureter. -Collecting ducts deliver urine to the renal pelvis to be passed on to the ureter.
Presence of which of the following in the urine, even in small amounts, is indicative of a possible problem in the renal system of a patient?
-The presence of glucose in the urine, even in small amounts, is indicative of a possible problem in the renal system of a patient. _____________________________________________________________ Note: -Sodium, urea, and creatinine are all eliminated by the kidney through urination in various quantities over time in order to maintain homeostatic balance in the milieu interieur of the body. -Any glucose filtered out of the blood by the nephron is normally completely reabsorbed during tubular reabsorption.
The process that returns useful substances in the filterate to the bloodstream is called?
-Tubular Reabsorption
Countercurrent Multiplier: Thin Ascending Limb (central renal medulla)
-Tubular fluid becomes increasingly concentrated passing through the renal medulla -Impermeable to water; permeable to Na+ and Cl- -Na+ and Cl- diffuse out down their concentration gradients, raising the interstitial fluid osmolarity
Distal Convoluted Tubule (DCT)
-Tubular fluid entering the distal convoluted tubule has a lower osmolarity than the interstitial fluid (NaCl has been reabsorbed) -First part of the distal convoluted tubule: -- Similar to loop of Henle: active NaCl transport, water impermeable --The tubular fluid becomes increasingly dilute -Second part of the distal convoluted tubule: -- No active NaCl transport, water permeable --Water reabsorption into ECF begins by osmosis -Cuboidal cells with very few microvilli -Function more in secretion than reabsorption -Confined to cortex -Fine-tunes the composition of urine --Eg: K+ & Ca2+ level regulated here
Blood Vessels
-arteries, veins, capillaries -tube-like structures that carry blood throughout the body -Most peritubular capillaries lie in the cortex -Some run parallel to Loop of Henle into medulla forming the vasa recta
Which of the following has a rate of urinary excretion (units of solute per unit time) that is almost always identical to its rate of glomerular filtration in a healthy adult? A. Sodium B. Glucose C. Chloride D. Creatine
D. Creatine Creatinine has a rate of urinary excretion that is almost always identical to its rate of glomerular filtration in a healthy adult. -The rate of urinary excretion should equal the rate of glomerular filtration for molecules that the body does not want to reabsorb under any circumstance (waste products). -Creatinine is a waste product, eliminated by the kidney. Sodium, chloride, and glucose are all molecules that the body may want to eliminate or reabsorb, depending on the situation.
Which of the following has a rate of urinary excretion (units of solute per unit time) that is always much lower than its rate of glomerular filtration in a healthy adult? A. Urea B. Potassium C. Sodium D. Glucose
D. Glucose The rate of urinary excretion for glucose will always be much lower than its rate of glomerular filtration in healthy adults. -The rate of urinary excretion will be always much lower than the rate of glomerular filtration for molecules that are filtered by the kidney but almost always reabsorbed in healthy adults. -Glucose is filtered by the kidney, but almost always reabsorbed during tubular reabsorption.
Thick Ascending Loop of Henle: Na-2Cl-K symporter is an integral protein found specifically in the kidney, where it serves to extract sodium, chloride and potassium from the urine through the process of active transport, so that they can be reabsorbed into the blood.
Note: a symporter is an integral membrane protein that is involved in the transport of many differing types of molecules across the cell membrane. Molecules are transported across the cell membrane at the same time and therefore, symporters are a type of cotransporter.
