Consequences of solute recovery

• As the filtrate leaves the renal corpuscle, it is iso-osmotic to the plasma and has approximately the same composition
• Proximal tubule/proximal convoluted tubule (PCT) is the mechanism that pulls $Na^+$ through to the plasma, glucose, amino acids, phosphate etc. are pulled through by the $Na^+$ concentration gradient, $K^+$ is dumped into the tubule lumen, again due to the basal pump, $HCO_3^-$ is recovered, with $H^+$ cycling (powered by $Na^+$ gradient), and all of this solute recovery tries to lower the osmolarity of the tubule, so water flows passively from the tubule to counteract this.
• Chloride also leaves the lumen passively to stop its concentration rising in the tubule.
• Proximal tubule/proximal convoluted tubule (PCT) has microvilli and a long winding tube to increase surface area, and a “leaky” tight junction which allows for the water to passively enter the plasma.
• Proximal tubule/proximal convoluted tubule (PCT) recovers about 65% of the sodium, chloride, phosphate and calcium ions, recovers around 65% water, and doesn’t change the osmolarity or the concentration of the urine (still roughly iso-osmotic).
• Proximal tubule doesn’t have a major affect on the control of the acid/base.
• The proximal tubule is a long winding tube (increased surface area) which has microvilli and 'leaky' tight junctions (allow for passive flow of water into the plasma)
• sodium is pulled through to the plasma by the basal pump in the proximal tubule, and the concentration gradient created is used to resorb glucose, amino acids, phosphate
• potassium is dumped into the lumen due to the basal pump
• bicarbonate is recovered with proton cycling (powered by sodium gradient)
• all solute recovery creates a gradient, which drives passive water recovery
• chloride the leaves the lumen passively to stop its concentration rising in the tubule
• The proximal tubule recovers about 65% of the sodium, chloride, phosphate and calcium ions, as well as around 65% water
• The resorption in the proximal tubule doesn’t change the osmolarity or the concentration of the urine (still roughly iso-osmotic), and doesn’t have major affect of the control of the acid/bace
• The loop of Henle has "non-leaky" tight junctions which restrict water movement (allows for creation of an area with high concentration gradient, used for increased water resorption)
• The loop of Henle is a thin limbed structure with three areas

  • thin descending limb
  • thin ascending limb
  • thick ascending limb
• The thin descending limb of the loop of henle goes to the medulla of the kidney and is permeable to water but impermeable to ions and urea
• The thin ascending limb of the loop of Henle returns back to the cortex of the kidney and is impermeable to water but permeable to ions an urea
• The thick ascending limb of the loop of henle active recovers ions
• the loop of Henle recovers about 10% of filtered water and 25% ions
• Due to the ion concentration in the medulla water passively flows out of the thin descending tubule and into the surrounding, making the inside of the tubule more concentrate compared to the renal corpuscle
• Ion and urea pumps in the thin ascending and thick ascending tubule release the ions into the surrounding tissue of the renal medulla, increasing ion concentration outside the tubules (causes water flow from the thin descending tubule) and decreasing the concentration in the tubule to 0.1 osmmol/kg
• osmolality in the proximal tubule is around 0.29 osmmol/kg, and 1.4 osmmol/kg in thin descending tubule
• The artery which supplies the renal corpuscle also supplies the loop of Henle
• in order to maintain the concentration gradient in the renal medulla:

  • the water is drawn out of the descending artery, and ions are drawn into the plasma
  • the artery turns into a secondary capillary system (vasa recta) which encapsulates the LoH
  • as the capillaries join to form a rising venule the water is brought back into the plasma and the ions are deposited in the the renal medulla
• The distal tubule recovers more ions but no water
• by the distal tubule, 95% of the ions are recovered and 75% of the water
• In the distal tubule the concentration osmolarity further decreases to 0.08 osmmol/kg, and there are water channels before the collecting duct which return the osmolarity to 0.29 osmmol/kg
• In the collecting duct there are further sodium pumps which deposit the sodium in the renal medulla leading to the final osmolarity of the urine reaching 1.4 osmmol/kg (allows for further water resorption through ADH)
• In the collecting duct there are urea channels which can leak urea (regulated by ADH)
• There is a separation between the normal zone superficially and hypertonic zones deep in the kidney.
• The oxygen sensing system (EPO producing system) is present at the bottom of the loop of Henle
• the medullary zones are grouped in renal pyramids, around a central urine collecting area (renal pelvis) which collects into the ureter and then into the bladder
• The arcuate artery/vein supplies each individual renal pyramid, and the long parallel blood vessels for the LoH mean that the oxygen supply gets shunted before the blood enters the capillaries.
• The blood system follows the branched collecting system of the kidney to allow for sufficient circulation.
• The blood vessels formation makes the kidneys particularly sensitive to ischaemia.
• ADH increases the permeability of the collecting duct to allow water to be absorbed from the lumen into the interstitial fluid
• control of filtration through glomerular blood flow:

  • control of the systemic blood pressure
  • decreased glomerular pressure through afferent arteriole constriction
  • increased glomerular pressure through efferent arteriole constriction (can occur due to diabetes)
• sodium recovery in the distal tubule, urea recovery, water permeability and acid-bace balance in the collecting duct all affect the blood pressure and can thus be used to control the glomerular filtration rate