Kidney Function: Urine Formation

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Questions and Answers

How does the unique structure of the glomerular filtration membrane, with its three layers, contribute to its function compared to the typical two-layered structure of normal capillaries?

The three layers (fenestrated endothelium, basement membrane, and podocytes with slit diaphragms) provide a more selective filtration barrier, preventing larger molecules like plasma proteins from being filtered while allowing efficient passage of water and small solutes.

Explain how changes in tubular reabsorption and glomerular filtration are coordinated to prevent significant fluctuations in urinary excretion.

When glomerular filtration increases, reabsorption also increases to prevent excessive loss of fluid and solutes. Conversely, when glomerular filtration decreases, reabsorption decreases to ensure adequate waste excretion. This coordination maintains a stable internal environment.

Differentiate between the terms 'reabsorption' and 'secretion' in the context of kidney function, specifically noting the direction of movement of substances in each process.

Reabsorption is the movement of substances from the tubular lumen back into the blood, while secretion is the movement of substances from the blood into the tubular lumen.

Describe how the kidneys utilize tubular reabsorption to maintain precise control over the composition of body fluids.

<p>The kidneys control the reabsorption of specific substances (e.g. water, sodium, glucose) based on the body's needs, ensuring the proper balance of electrolytes, water, and other solutes in the body fluids.</p> Signup and view all the answers

Explain the role of the sodium-potassium ATPase pump in primary active transport within the kidney tubules.

<p>The sodium-potassium ATPase pump uses ATP to transport sodium out of the cell and potassium into the cell, creating an electrochemical gradient that drives other transport processes.</p> Signup and view all the answers

How does the sodium-glucose linked transporter 2 (SGLT2) contribute to glucose reabsorption in the early part of the proximal tubule?

<p>SGLT2 uses the electrochemical gradient of sodium to transport glucose across the apical membrane into the cell against its concentration gradient, reabsorbing approximately 90% of filtered glucose.</p> Signup and view all the answers

Explain the concept of 'solvent drag' in the context of water and solute reabsorption across tight junctions in the kidney tubules.

<p>Solvent drag occurs when water moves across tight junctions via osmosis, carrying solutes along with it. This is a passive process that enhances the reabsorption of certain ions and substances dissolved in the water.</p> Signup and view all the answers

Describe how the permeability of the distal tubule and collecting ducts is regulated to control water reabsorption.

<p>Permeability is regulated by antidiuretic hormone (ADH). When ADH levels are high, the tubule permeability to water increases, enhancing reabsorption. When ADH is low, permeability decreases, resulting in less water reabsorption.</p> Signup and view all the answers

How does the reabsorption of water through the paracellular route contribute to overall water reabsorption in the kidney?

<p>Water moves through the tight junctions (paracellular route) driven by osmotic gradients. This can be a significant pathway for water reabsorption, especially when there is a high concentration of solutes in the interstitial fluid.</p> Signup and view all the answers

Differentiate between primary and secondary active transport. How do these processes contribute to tubular reabsorption?

<p>Primary active transport directly uses ATP to move substances against their concentration gradient, whereas secondary active transport uses the electrochemical gradient created by primary active transport to move other substances. They both enable reabsorption of essential solutes.</p> Signup and view all the answers

How does the reabsorption of sodium impact the reabsorption of water and other solutes in the kidney?

<p>Sodium reabsorption creates osmotic gradients that drive water reabsorption and also influences the reabsorption of other ions through mechanisms like solvent drag and secondary active transport.</p> Signup and view all the answers

Describe the role of the loop of Henle in creating a large osmotic gradient for water reabsorption.

<p>The loop of Henle establishes a medullary osmotic gradient through the countercurrent multiplier system, allowing water to be reabsorbed in the collecting duct driven by the high solute concentration in the medulla.</p> Signup and view all the answers

Explain how primary active transport contributes to the function of secondary active transport processes in the kidney tubules.

<p>Primary active transport, such as the Na+/K+ ATPase, establishes the electrochemical gradient that drives secondary active transport processes moving substances (like glucose) against their concentration gradient.</p> Signup and view all the answers

Describe the role of urea transporters in water conservation.

<p>The urea transporters allows for the recycling of urea into the medullary space, allowing the kidney to continue to draw water out of the filtrate.</p> Signup and view all the answers

What is the role of antidiuretic hormone (ADH) in the collecting ducts, and how does it impact aquaporin channels?

<p>Antidiuretic hormone enhances water reabsorption in the collecting ducts by promoting the insertion of aquaporin channels into the apical membrane, increasing its permeability to water.</p> Signup and view all the answers

How does the transport maximum affect glucose reabsorption, and what happens when this transport capacity is exceeded?

<p>The transport maximum is the maximal amount of a substance that can be reabsorbed. When the filtered load of glucose exceeds this, the excess glucose is excreted in the urine.</p> Signup and view all the answers

Explain how the filtration and reabsorption processes contribute to overall urinary excretion.

<p>The amount of a substance excreted in the urine is the result of what is filtered minus what is reabsorbed, plus what is secreted.</p> Signup and view all the answers

Describe how the composition of fluids in the cortical and medullary regions influence water reabsorption.

<p>The cortical region is of normal osmolarity, while the medullary region exhibits a high osmotic gradient. This is especially important because the collecting duct passes through both and facilitates water reabsorption.</p> Signup and view all the answers

Describe the location of the proximal tubule and how it relates to it's high rate of reabsorption.

<p>The proximal tubule has both early and late portions that express different channels that perform different, but specific reabsorptions. These cells exhibit a high number of mitochondria to provide the ATP needed to perform these reabsorption functions.</p> Signup and view all the answers

Explain the specific action of aldosterone in controlling the reabsorption of sodium and secretion of potassium.

<p>Aldosterone increases the expression of the $Na^+/K^+$ ATPase on the basolateral membrane of the late distal tubule and collecting duct. This increases passive sodium channels on the apical membrane, thus enhancing sodium reabsorption and potassium secretion.</p> Signup and view all the answers

Explain the role of Angiotensin II in regards to sodium reabsorption.

<p>Angiotensin II stimulates sodium, chloride, and water reabsorption, thus increasing the blood volume and arterial pressure. Angiotensin II also causes the adrenal cortex to secrete aldosterone.</p> Signup and view all the answers

How do cells of the epithelium become specialized to contain more mitochondria, and how does this relate to differences in the texture of the epithelial surface?

<p>Certain cells in the nephron's epithelium have more mitochondria than others based on different functions performed by these cells relating to primary active transport that requires more ATP. The cells can also have different textures depending on the number and type of microvilli.</p> Signup and view all the answers

What is tubular fluid and what happens to it as it moves through the nephron?

<p>The tubular fluid is the fluid in the nephron consisting of water, ions, nutrients, and waste products that enters from the bowman's capsuel. As the tubular fluid flows through the nephron, its composition is altered by the reabsorption and secretion of substances along the tubular epithilium.</p> Signup and view all the answers

Differentiate between transcellular and paracellular in terms of reabsorption in the kidney.

<p>Transcellular reabsorption takes place through the cell's membranes, while paracellular reabsorption takes place between the cells.</p> Signup and view all the answers

Describe how diuretics affect tubular reabsorption and urine output.

<p>Diuretics reduce the rate of tubular reabsorption, increasing urine output. Different diuretics affect different components of reabsorption, such as water or salts, resulting in a net increase in electrolyte excretion.</p> Signup and view all the answers

How does the kidney compensate for the filtered protein?

<p>99% of protein filtered by the glomeruli is reabsorbed by pinocytosis. Proteins too large to be filtered do not enter the filtrate. Filtered proteins are reabsorbed in the proximal tubules.</p> Signup and view all the answers

Compare and contrast reabsorption in the proximal tubule and the distal tubule in terms of solute reabsorption mechanisms and regulation.

<p>In the proximal tubule, reabsorption of solutes is largely unregulated and involves co-transport mechanisms. In the distal tubule, reabsorption is more tightly regulated by hormones like aldosterone and ADH.</p> Signup and view all the answers

Explain how counter-transport (antiport) contributes to tubular reabsorption or secretion.

<p>Counter-transport involves the movement of one substance across the tubular membrane in exchange for another. Some examples are hydrogen and sodium and by way of sodium-hydrogen exchangers.</p> Signup and view all the answers

Certain types of cells are selectively permeable to certain types of fluids. How does that mechanism work?

<p>Osmosis through the tubular epitheal can only take place if the cells are permeable to water. If they are not, then osmosis must take place through the connected tight junctions.</p> Signup and view all the answers

What is the descending limb of the loop of Henle's primary function, and how does this effect water reabsorption?

<p>The descending limb of the loop of Henle is highly permeable to water, but impermeable to salt. The increasing concentration of fluids in the medulla draw water our of the filtrate, moving it to the blood supply.</p> Signup and view all the answers

Flashcards

Kidney Filtration

The process by which the kidneys filter plasma in special units called glomeruli, excreting waste into urine.

Tubular Reabsorption

Movement of substances from the tubular lumen back into the blood.

Tubular Secretion

Movement of substances from the blood into the tubular lumen.

Glomerular Filtration Membrane

A filtration membrane with three layers, unlike the two layers in normal capillaries.

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Kidney Reabsorption Control

This process ensures precise control of body fluid composition.

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Reabsorption/Excretion Rate

Rate of reabsorption and excretion varies according to the body's needs.

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Reabsorption Control

Driven by hydrostatic and colloid osmotic forces.

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Active Transport

Movement of solute against an electrochemical gradient using energy from metabolism.

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Passive Transport

A non-active process where a solute or water moves down its electrochemical gradient.

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Primary Active Transport

Coupled to an energy source (ATP hydrolysis).

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Secondary Active Transport

Indirectly coupled to an energy source (e.g., energy released when another substance moves down its gradient).

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Osmosis

Occurs through tight junctions (paracellular) or tubular epithelial cells (transcellular).

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Ascending Loop of Henle

The ascending loop of Henle has low permeability, despite a large osmotic gradient.

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Urea Reabsorption

90% of urea is excreted, 50% is reabsorbed.

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Primary active transport

Driven by sodium-potassium ATPase, which creates sodium and potassium gradients.

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Secondary Active Transport of Glucose

Glucose is reabsorbed with sodium across the luminal membrane through a sodium-glucose linked transporter.

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Secondary Active Secretion

Energy for hydrogen secretion moves against concentration gradient.

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Glucose Reabsorption

Normally, all filtered glucose is reabsorbed in the proximal tubule.

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Study Notes

  • Formation of urine by the kidneys involves the tubular processing of the glomerular filtrate.

Study Goals

  • Discuss the active and passive mechanisms of tubular reabsorption
  • Understand the difference between primary and secondary active transport
  • Explain why some nephron epithelium have lots of mitochondria and different textures on the surface
  • Discuss tubular reabsorption control
  • Explain the term 'clearance'

Filtration Across the Membrane

  • Approximately 20% of the plasma that flows through the glomerulus undergoes filtration.

  • The glomerular filtration membrane has 3 layers, unlike the 2 layers found in normal capillaries.

  • A high filtration rate and prevention of plasma protein filtration is due to:

  • Split pores between podocytes possess a negative charge

  • The negative charge is contributed by collagen, and proteoglycan fibrillae large pores

  • Perforated (fenestrae) endothelium is negatively charged

  • It is important to understand hydrodynamics and different pathological effects.

Reabsorption vs Excretion

  • Reabsorption describes the movement direction of a substance from the fluid in the tubular lumen or material produced within the epithelial cell into the peritubular capillary
  • Secretion is movement of a substance from the blood or produced within the epithelial cell into the fluid within the tubular lumen.
  • Key to understand the direction of movement for reabsorption and secretion/excretion
  • Note the locations of the basolateral and apical membranes.
  • Filtration and reabsorption both contribute significantly to the excretion of a substance.
  • Changes in tubular reabsorption and glomerular filtration are coordinated to avoid large changes in urinary excretion.
  • Tubular reabsorption is highly selective, unlike filtration.
  • The excretion of substances is independently regulated.
  • The kidneys control the reabsorption of each substance.
  • Allows precise control of the composition of body fluids.

Reabsorption

  • Almost completely reabsorbed products:
  • Glucose
  • Amino acids
  • Reabsorption and excretion rates vary based on the body's needs.
  • Sodium
  • Chloride
  • Bicarbonate ions
  • Poorly reabsorbed products:
  • Waste products
  • Urea
  • Creatinine
  • Reabsorption is controlled by hydrostatic and colloid osmotic forces.

Active vs Passive Reabsorption

  • Active transport involves moving a solute against an electrochemical gradient, using energy from metabolism.
  • Primary active transport directly couples to an energy source, like ATP hydrolysis via the Sodium-potassium ATPase pump
  • Secondary active transport indirectly links to an energy source
  • Energy is released when another substance moves across a membrane down its electrochemical gradient (e.g., glucose reabsorption).
  • Passive transport involves a solute or water moving down its electrochemical gradient, and does not require energy.
  • Osmosis happens through tight junctions or through the tubular epithelial cells
  • Primary or secondary active reabsorption of solutes creates a concentration difference driving osmosis of water into the renal interstitium.

Solvent Drag

  • As solutes move together with water, they move from osmosis across tight junctions
  • Reabsorption often relates to Na+
  • Na+ reabsorption can influence water and other ion reabsorption
  • In the nephron(Loop of Henle to the collecting duct) permeability decreases:
  • Tight junctions less permeable
  • Epithelial cells have a decreased membrane surface area
  • Osmosis can occur when the tubular epithelial cells are permeable to water.
  • Proximal tubule: highly permeable
  • Ascending loop of Henlé: low permeability despite a large osmotic gradient
  • Distal and collecting tubules: Permeability determined by antidiuretic hormone (ADH)

Chloride

  • Reabsorption occurs through:
  • Secondary active transport (co-transport with sodium across the luminal membrane)
  • Passive diffusion

Urea

  • Approximately ninety-percent of the nitrogen generated by the liver gets excreted via the urine as urea
  • Only fifty-percent of filtered urea is reabsorbed Reabsorption occurs through:
    • Passive diffusion
    • Tubules are less permeable for urea than for water
    • Passive reabsorption is facilitated by urea transporters in the inner medullary collecting duct

Active Transport

  • Sodium-potassium, hydrogen, hydrogen-potassium, and calcium ATPases are types of ATP-ase pumps in the kidneys

  • Sodium reabsorption from the tubular lumen back into the blood includes:

  • Sodium diffusing across the luminal membrane into the tubular epithelial cell down an electrochemical gradient by a Na+-K+ATPase pump in the basolateral membrane

  • Sodium being transported against an electrochemical gradient by the Na+-K+ATPase pump

  • Sodium, water, and other substances being reabsorbed into the peritubular capillaries from the interstitial fluid by ultrafiltration

  • Sodium moves downhill across the apical membrane

  • Differences exist between the segments

  • Some steps are similar in all segments

Glucose Reabsorption

  • The energy released as one substance diffuses with electrochemical gradient, is used to move another substance against gradient.
  • Glucose undergoes secondary active transport in the proximal tubule:
  • In the first part of the proximal tubule:
  • Ninety-percent of glucose transport
  • Transport happens against its concentration gradient across the apical membrane
  • Transport depends on utilizes a sodium-glucose linked transporter-2 (SGLT2)
  • Latter part of the proximal tubule:
  • Ten-percent of glucose transport happens across the apical membrane via SGLT1
  • Glucose moves into the peritubular capillaries via mass filtration, which involves ultrafiltration

Key points

  • Secondary active transport helps move virtually all the glucose and amino acids the tubular lumen.
  • Glucose undergoes active secretion.
  • Counter transport of hydrogen and sodium occurs via sodium-hydrogen exchangers
  • Sodium reabsorption provides energy for hydrogen secretion against its concentration gradient.
  • Under normal conditions, all filtered glucose is reabsorbed in the proximal tubule.
  • Transport maximum is 375 mg/min.
  • When filtered load > Transport maximum, there is excess glucose excreted.

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