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Urine formation Yousaf Khan Renal Dialysis Lecturer Urine formation Urine formation involves glomerular filtration, tubular reabsorption, and tubular secretion. Glomerular Filtration - Urine formation begins when the fluid portion of the blood is filtered by the glomerulus and enters the glomerular capsule as glomerular filtrate. Glomerular filtration 1. Blood moves from the afferent arteriole into the glomerulus. Blood backs up in the glomerulus causing high pressure: ◦ The efferent arterioles (leaving the glomerulus) are smaller than the afferent arterioles (entering the glomerulus). ◦ Therefore, the blood backs up into the glomerulus and pushes against the walls of the glomerulus capillaries. Glomerular filtration 2. The high blood pressure moves any substance small enough to fit through the walls of the glomerulus into Bowman’s capsule What moves through: water plus many solutes (vitamins, amino acids, glucose, Na+, Cl-, K+, bicarbonate ions, phosphates, urea, uric acid) What is left behind: Red blood cells and large proteins are left behind to enter the efferent arteriole. Filtration Rate • The factors that affect the filtration rate are filtration pressure, glomerular plasma osmotic pressure, and hydrostatic pressure in the glomerular capsule. 180 liters of blood are filtered daily Glomerular filtration is a passive, non-selective process Fluids and solutes are forced through a membrane by hydrostatic (water) pressure, provided by the push of the heart. It is passive (no energy required) Glomerulus capillaries Afferent arteriole (Larger) H2O vitamins Uric acid Blood flow Amino acids urea glucose Electrolytes / ions Efferent arteriole (smaller) Bowman’s capsule Glomerular filtration Tubular Reabsorption Step 2- Tubular Reabsorption: Main goal: for blood to reclaim vital nutrients that left the blood during glomerular filtration. What returns to the blood in this step: water, all nutrients and most ions. Urine formation As the efferent arteriole leaves the renal corpsucle branches into the peritubular capillaries.The capillaries sit up against the nephron tubules. Water and solutes leave the Bowmans capsule and enter the proximal convoluted tubule. The urine filtrate flows through the nephron tubule. Nephrons are permeable because of: 1000’s of microvilli that line the inside and increase the surface area 1. The urine filtrate flows through the nephron tubule. This process is passive. Some substances are left in the urine filtrate- urea, uric acid, creatinine, and some of the water and few other wastes These substances stay inside the nephron tubule. 1. 2. The tubule cells then go through active transport and uses ATP to pump Na+ into the interstitial space between the tubule and the capillary. This causes the interstitial space to become hypertonic and this pulls water out of the tubule cells, along with valuable solutes This is called obligatory water reabsorption. The water is “obliged” to follow the Na+ into the interstitial fluid. 3. Now, the water and solutes are in the fluid around the capillary. The large proteins inside the capillary make the blood a hypertonic solution, so water and solutes are drawn inside the capillary. Active transport Passive transport blood Na H2O + nutrients Loop of Henle Most (70%) of the reabsorption of filtrate occurs in the proximal convoluted tubule. The way nephrons control how concentrated or dilute urine is depends on how much of the other 20% of the filtrate is reabsorbed. This takes place in the Loop of Henle, distal convoluted tubules, and collecting tubules. Urine concentration at the loop of Henle Main goal: removal of water and some ions to concentrate the urine takes place in the juxtamedullary nephrons which specialize in water homeostasis. In the descending portion of the loop of Henle, the tubule is impermeable (not open) to solutes but permeable to water. The interstitial fluid (fluid in between the tubule and the peritubular capillaries) is very concentrated with Na+ (salty) and becomes more concentrated as the loop descends into the renal medulla. STEPS IN THE URINE CONCENTRATION PROCESS: Since water is in a lower concentration in the tubule, it exits the loop of henle via osmosis and into the interstitial space and then into the peritubular capillaries. So, the filtrate in the Loop of Henle becomes more concentrated, as 10% more of the original filtrate volume returns to the blood. STEPS IN THE URINE CONCENTRATION PROCESS: The amount of water that moves out of the descending Loop of Henle is controlled by the ascending loop. If the body wants more water to move out of the tubule, the ascending loop makes the area more hypertonic. If the body wants to keep water in the tubule it makes the interstitial space less hypertonic. 2. STEPS IN THE URINE CONCENTRATION PROCESS: In the ascending portion of the loop of Henle the tubule changes permeability. It is now impermeable to water. The cells in this area of the loop have many Na+ and Cl- pumps, and these ions get pumped out of the ascending loop of Henle, so the filtrate becomes a bit more dilute again. 2. Active transport Passive transport blood cortex H2O H2O Outer medulla NaCl NaCl H2O NaCl H2O NaCl H2O H2O H2O H2O Loop of Henle Tubular Secretion Main goals: To filter out any waste that may have re-entered the blood stream during tubular reabsorption. To maintain blood pH homeostasis This step requires active transport. So waste moves from the: peritubular capillary interstial space distal convoluted tubule Urine formation Common substances that must be filtered in this step are: 1. Wastes that moved back into the blood passively by accident. 2. Artificial materials that didn’t go into the Bowman’s capsule during glomerular filtration (penicillin and pesticides) 3. Excess ions the body needs to get rid of to adjust the blood pH. How it Adjusts the blood pH: Blood will pump out excess H+ and K+ ions. These H+ ions are what give urine its acid quality. This takes place along the distal convoluted tubule. Tubular secretion: Distal Convoluted Tubule Tubule cell Nephron tubule URINE FILTRATE H2O, urea, creatinine, few other wastes To collecting tubule NEPHRON TUBULE capillary creatinine Capillary cell pesticide rbc blood H+ urea pennicillin Active transport Passive transport blood Distal convoluted tubule cortex H2O H2O Outer medulla NaCl NaCl H2O NaCl H2O NaCl H2O H2O H2O H2O Loop of Henle Wastes H+ ions Step 5: Final Urine concentration and regulation of electrolytes at the Collecting tubule Completes 2 main goals here: 1. Adjusts blood pH and salt homeostasis 2. Maintains water homeostasis. Adjusting blood pH and salt homeostasis continues: K+, H+ and HCO3 - ions that were collected in the distal convoluted tubule can be either stay in the urine for excretion or reabsorbed by the blood if the body needs them. Water Homeostasis • When the urine enters the collecting tubules they are normally almost totally impermeable to water. • At this point, the urine is still fairly dilute, and very light yellow in color. • If the body is well hydrated, nothing more happens in the collecting tubules and they remain impermeable to water and salts. • The water and salts are then excreted. Concentrated Urine If the individual needs to conserve water due to dehydration, ADH, anti-diuretic hormone is released by the posterior pituitary gland. This is a urine concentrating hormone. It causes the collecting tubules to become permeable to water (opens water transport proteins) Water moves into the interstitial space and back to the capillaries, therefore, less water stays in the collecting tubules and the urine becomes more concentrated. Thank You