The final osmolarity of urine is therefore dependent on whether or not the final collecting tubules and ducts are permeable to water or not, which is regulated by homeostasis. Reabsorption throughout the nephron : A diagram of the nephron that shows the mechanisms of reabsorption. Hydrogen, creatinine, and drugs are removed from the blood and into the collecting duct through the peritubular capillary network. Tubular secretion is the transfer of materials from peritubular capillaries to the renal tubular lumen; it is the opposite process of reabsorption.
This secretion is caused mainly by active transport and passive diffusion. Usually only a few substances are secreted, and are typically waste products. Urine is the substance leftover in the collecting duct following reabsorption and secretion.
The mechanisms by which secretion occurs are similar to those of reabsorption, however these processes occur in the opposite direction. Renal secretion is different from reabsorption because it deals with filtering and cleaning substances from the blood, rather than retaining them.
The substances that are secreted into the tubular fluid for removal from the body include:. Many pharmaceutical drugs are protein-bound molecules thatDiagram showing the basic physiologic mechanisms of the kidney and the three steps involved in urine formation. Tubular secretion occurs throughout the different parts of the nephron, from the proximal convoluted tubule to the collecting duct at the end of the nephron.
The movement of these ions also helps to conserve sodium bicarbonate NaHCO 3. The typical pH of urine is about 6.
Urine that is formed via the three processes of filtration, reabsorption, and secretion leaves the kidney through the ureter, and is stored in the bladder before being removed through the urethra. At this final stage it is only approximately one percent of the originally filtered volume, consisting mostly of water with highly diluted amounts of urea, creatinine, and variable concentrations of ions. Privacy Policy. Skip to main content. Urinary System. Search for:. Physiology of the Kidneys.
Overview of Urine Formation Urine is formed in three steps: filtration, reabsorption, and secretion. Learning Objectives Summarize the steps in urine formation.
Key Takeaways Key Points Filtration involves the transfer of soluble components, such as water and waste, from the blood into the glomerulus. Reabsorption involves the absorption of molecules, ions, and water that are necessary for the body to maintain homeostasis from the glomerular filtrate back into the blood. Secretion involves the transfer of hydrogen ions, creatinine, drugs, and urea from the blood into the collecting duct, and is primarily made of water.
Blood and glucose are not normally found in urine. Key Terms urine : A liquid excrement consisting of water, salts, and urea, which is made in the kidneys then released through the urethra. Glomerular Filtration Glomerular filtration is the renal process whereby fluid in the blood is filtered across the capillaries of the glomerulus.
Learning Objectives Explain the process of glomerular filtration in the kidneys. Key Takeaways Key Points The formation of urine begins with the process of filtration. Fluid and small solutes are forced under pressure to flow from the glomerulus into the capsular space of the glomerular capsule. Blood entering the glomerulus has filterable and non-filterable components. Filterable blood components include water, nitrogenous waste, and nutrients that will be transferred into the glomerulus to form the glomerular filtrate.
Non-filterable blood components include blood cells, albumins, and platelets, that will leave the glomerulus through the efferent arteriole. Glomerular filtration is caused by the force of the difference between hydrostatic and osmotic pressure though the glomerular filtration rate includes other variables as well. Key Terms glomerulus : A small, intertwined group of capillaries within nephrons of the kidney that filter the blood to make urine.
It is the primary force that drives glomerular filtration. Learning Objectives List the conditions that can affect the glomerular filtration rate GFR in kidneys and the manner of its regulation. Example Question 2 : Excretory Physiology. Which of the following statements regarding nephron activity and environment is false? Possible Answers: The renal cortex is hypotonic. The excretion process of urine does not require energy.
Filtrate has a higher ion concentration than blood when traveling down the thin descending limb. Correct answer: The excretion process of urine does not require energy.
Explanation : ATP is needed to help power the excretion of urine through the nephron. Example Question 3 : Excretory Physiology. Possible Answers: Sodium is reabsorbed through both passive and active transport at different times within the nephron.
Sodium is actively reabsorbed in the thin ascending limb. Sodium and potassium can be co-transported or pumped in opposite directions. Correct answer: Sodium is actively reabsorbed in the thin ascending limb. Explanation : When the filtrate enters the bottom of the loop of Henle, it is at its highest concentration. Example Question 21 : Excretory System. Which section of the nephron is responsible for creating an ion gradient in the kidney? Possible Answers: Distal convoluted tubule.
Correct answer: Loop of Henle. Explanation : As the nephron dips into the medulla in the descending limb of the loop of Henle, water passively diffuses out of the filtrate. Example Question 22 : Excretory System. Possible Answers: Potassium ions. Correct answer: Water. Explanation : The loop of Henle is essential for creating an ion gradient in the renal medulla the inner part of the kidney. Example Question 23 : Excretory System. What would be the effect on urine by drugs that block sodium channel transporters in the kidney?
Possible Answers: Urine will be more dilute. Correct answer: Urine will be more dilute. Explanation : Diuretic drugs promote the production of urine.
Example Question 24 : Excretory System. Possible Answers: Sodium is concentrated in the filtrate of the ascending limb. Water reabsorption in the ascending limb is under hormonal control. Correct answer: Permeability to water. Explanation : The loop of Henle serves the crucial function of creating an ion gradient in the renal medulla by fluctuating the reabsorption of water and ions.
Example Question 25 : Excretory System. Water reabsorption occurs in which three regions of the nephron? Possible Answers: Thin descending limb, thin ascending limb, thick ascending limb.
Thin ascending limb, thick ascending limb, collecting duct. Thin descending limb, distal convoluted tubule, collecting duct. Thin descending limb, thin ascending limb, collecting duct. Correct answer: Thin descending limb, distal convoluted tubule, collecting duct.
Explanation : Water is reabsorbed at various times during the excretion process as it passes through the nephron, in order to maintain proper ion levels. Example Question 26 : Excretory System.
Which of the following is true about the nephron and urine production? Possible Answers: The osmolarity of the filtrate always matches the osmolarity of the surrounding interstitial fluid. The filtrate is at its lowest osmolarity at the bottom of the descending limb. Active transport is used to reclaim sodium in certain parts of the nephron.
Correct answer: Active transport is used to reclaim sodium in certain parts of the nephron. Explanation : Sodium is reclaimed through passive transport in the thin ascending limb and is reclaimed by active transport in the thick ascending limb, distal tubule, and collecting duct.
Example Question 27 : Excretory System. Possible Answers: Water always flows from areas of low solute concentration to high solute concentration. The descending limb is permeable to water, which is reabsorbed in this region. The thin ascending limb is impermeable to water. Correct answer: Both sodium and water are moved via active and passive transport, depending on their location in the nephron.
Reabsorption of water and specific solutes occurs to varying degrees over the entire length of the renal tubule. Bulk reabsorption, which is not under hormonal control, occurs largely in the proximal tubule. In addition, many important solutes glucose, amino acids, bicarbonate are actively transported out of the proximal tubule such that their concentrations are normally extremely low in the remaining fluid.
Further bulk reabsorption of sodium occurs in the loop of Henle. Regulated reabsorption, in which hormones control the rate of transport of sodium and water depending on systemic conditions, takes place in the distal tubule and collecting duct.
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