Electrolytes 2

• Kidney Major Functions

  • Fluid/electrolyte balance (Osmotic balance)
  • Nitrogenous waste removal
  • Blood acid-base (pH) balance
  • Hormone production
  • Calcium/potassium homeostasis
  • Regulates red blood cell production
• Glomerular Filtration Rate (GFR)

  • The rate of production of filtrate through the glomerulus is called the Glomerular Filtration Rate (GFR)
• • Intrinsic (self) regulation of GFR
  • Myogenic response - smooth muscle stretching changes cause constriction/dilation of afferent arteriole
  • Tubuloglomerular feedback - macula dense cells of distal tubule control diameter of afferent arteriole
  • Mesangial control - altered permeability of glomerulus
• • Extrinsic regulation of GFR
  • Hormones circulating in the blood can regulate GFR and kidney function
• Reabsorption: Proximal Tubule

  • The filtrate leaves Bowman’s capsule and enters proximal tubule
  • The fluid inside this tubule contains water and small solutes such as urea, glucose, amino acids, vitamins, and electrolytes
  • The epithelial cells of the proximal tubule have small projections called microvilli facing the lumen
  • Functions in active transport of selected molecules out of the filtrate
  • Solutes leave the proximal tubule and enter epithelial cells - water follows the osmotic gradient
  • Valuable solutes and water are reabsorbed and returned to the body
• Proximal Tubule (Part 1)

  1. Na+/K+ - ATPase in the basolateral membranes removes intracellular Na+ into the interstitial fluid - creating a gradient favouring Na+ entry from the lumen.
  2. In the apical membrane, Na+- dependent cotransporters use the gradient to remove ions and nutrients selectively from the filtrate - movement of Na+ into the cell along its electrochemical gradient provides the means for moving other solutes against their gradients.
• Proximal Tubule (Part 2)

  3. The solutes that move into the cell diffuse across the basolateral membrane into nearby blood vessels.

  4. Water follows ions from the proximal tubule into the cell and then into the blood vessels.
• Proximal Tubule

  • Almost all nutrients, along with two-thirds of the NaCl and water, are reabsorbed through the proximal tubule
  • The osmolarity of the tubular fluid is unchanged despite the huge change in volume because the water reabsorption is proportional to solute reabsorption
• Vasa Recta

  • Water and salt that move out of the loop diffuse into the vasa recta (network of blood vessels that runs along the loop
  • As a result, water and electrolytes that are reabsorbed are returned to the bloodstream instead of being excreted in the urine
• The Distal Tubule and Collecting Duct

  • Once filtrate has passed through the loop of Henle, it enters the distal tubule
  • This fluid is slightly hyposmotic to blood, and solutes it contains are mainly urea and other waste products
  • The fluid that enters the distal tubule is always dilute
  • In contrast, the urine that leaves the collecting duct is dilute when the individual is well hydrated but concentrated when the individual is dehydrated
• ADH (Part 1)

  • Has two important effects on epithelial cells in the collecting duct:
  • Triggers the insertion of aquaporins into the apical membrane
  • Cells become much more permeable to water and large amounts of water are reabsorbed
  • Increases permeability to urea, which is reabsorbed into the surrounding fluid
  • This helps create a concentration gradient favouring water reabsorption from the filtrate
• ADH (Part 2)

  • Water leaves the collecting duct passively - following the concentration gradient maintained by the loop of Henle
  • Water is conserved and urine is strongly hyper osmotic relative to blood
  • When ADH is absent, few aquaporins are found in the collecting duct epithelium, leaving it relatively impermeable to water and resulting in hyposmotic urine
  • The collecting duct is the final place where the composition of the filtrate can be altered
  • Urine exiting the collecting ducts moves from the kidneys into ureters and then is stored in the bladder until urination
• Loop of Henle

  • In mammals, fluid from the proximal tubule enters the loop of Henle
  • In most nephrons, the loop is short and barely enters the medulla (Cortical nephrons)
  • But in about 20% of nephrons in a human kidney, the loop is long and plunges from the cortex of the kidney deep into the medulla (Juxtamedullary nephrons)
• Loop of Henle (Part 2)

  • The Loop of Henle maintains an osmotic gradient because water leaves the descending limb and salt leaves the ascending limb
  • As fluid flows down the defending limb, the fluid inside the loop loses water to the tissue surrounding the nephron - osmotic gradient
• Loop of Henle (Part 3)

  • The fluid inside the nephron loses Na+ and Cl- in the thin ascending limb - electrochemical gradient
  • Near the cortex, the osmolarity of the surrounding interstitial fluid is low - additional na+ and Cl- are actively transported out of the nephron in the thick ascending limb
  • The countercurrent flow of fluid is self-reinforcing - the presence of an osmotic gradient stimulates water and ion flows that in turn maintain the osmotic gradient
• Urine Formation

  • Under hormone control
  • Activity of the distal tubule and collecting duct is highly regulated and altered in response to osmotic stress
  • The amount of Na+, Cl-, and water the is reabsorbed in the distal tubule and in the collecting duct varies with the animal’s hydration
  • Changes in the distal tubule and collecting duct are controlled by hormones - signalling molecules in the blood
• Urine Formation

  • If Na+ levels in the blood are low, the adrenal glands release the hormone aldosterone, which leads to activation of sodium pumps and reabsorption of Na in the distal tubule
  • Aldosterone saves sodium and water
  • It stimulates the secretion of K+ and H+ from the blood into the distal tubule
  • If an individual is dehydrated, the brain releases antidiuretic hormone (ADH) - ADH saves water