excretion — nephrons

• composition of urine:

  • 96% water, 2.0% urea, 1.8% mineral salts (NaCl), other nitrogenous waste
  • urine is made by ultrafiltration and reabsorption 
• bowman's capsule (ultrafiltration)

  • each nephron begins in the cortex as the bowman's capsule
  • blood enters the kidneys via the renal artery which branches into afferent arterioles which further branch into capillaries (collectively called the glomerulus) in the bowman's capsule
  • a partially permeable membrane wraps around the glomerular blood capillaries
  • only small substances can pass through the pores of the basement membrane into the bowman's capsule, forming the filtrate. large molecules are retained in the capillaries 
  • the capillaries then merge to form the efferent arteriole
• • the diameter of the efferent arteriole is much smaller than that of the afferent arteriole, creating high hydrostatic pressure in the glomerulus which is the main driving force for ultrafiltration
• proximal convoluted tubule (selective reabsorption)

  • longest and widest tubule, with microvilli to increase surface area for reabsorption 
  • reabsorption of 85% of filtered substances occurs
  • secretion by active transport occurs for drug penicillin, ammonium and hydrogen ions
  • cells on the lumen of the PCT are lined with high density of mitochondria to generate energy for active transport
  • tubules are highly convoluted and have microvilli to ensure surface area is maximised for reabsorption 
  • higher salt concentration in interstitial fluid causes osmosis of water 
• which filtered substances are reabsorbed into interstitial fluid?

  glucose, amino acids, lactic acids, water soluble vitamins, and ions
• loop of Henle

  • blood flows the opposite direction as the filtrate in the nephron to ensure exchange of ions and water
  • this maintains the concentration gradient across the entire loop of Henle
  • osmosis occurs in the descending limb as it is permeable to water molecules
  • salt concentration is highest at the bend of the loop
  • in the ascending limb, diffusion occurs at the thin segment due to high salt concentration, active transport of salt occurs at the thick segment
• distal convoluted tubule

  • when the body is dehydrated, ADH produced by the hypothalamus causes the membrane of the distal convoluted tubes to become more permeable to water
  • in the presence of ADH, more water is absorbed
  • potassium, ammonium, and hydrogen ions in blood capillaries are secreted from the blood into the filtrate to eliminate the ions from the body
• collecting duct (osmoregulation) 

  • when body is dehydrated, more ADH produced by the hypothalamus is released
  • more ADH causes the membrane of the collecting duct to become more permeable to water
  • more water is reabsorbed from the collecting duct into blood capillaries, resulting in a lower volume of urine with higher concentration
• collecting duct (osmoregulation)

  • when the body is well-hydrated, less ADH is released
  • less ADH causes the membrane of the collecting duct to become less permeable to water
  • less water is reabsorbed from the collecting duct into blood capillaries, resulting in a higher volume of urine with a lower concentration 
• which substances can be filtered through the glomerulus?

  glucose, amino acids, mineral salts, urea
• which substances cannot be filtered through the glomerulus 


  red blood cells, plasma proteins, other large molecules