Nephrons

• The long tubules along with the bundle of capillaries where blood is filtered are called Nephrons - there are around one million nephrons in each kidney.
• Blood from the renal artery enters into smaller arterioles in the cortex. Each arteriole splits into a structure called a glomerulus - a bundle of capillaries looped inside a hollow ball called the bowman's capsule.
• Ultrafiltration takes place in Bowman's Capsule.
• The arteriole that takes blood into each glomerulus is called the afferent arteriole, and the arteriole that takes the filtered blood away from the glomerulus is called the efferent arteriole.
• The efferent arteriole is smaller in diameter than the afferent arteriole, so the blood in the glomerulus is under high pressure.
• The high pressure in the efferent arteriole forces liquid and small molecules in the blood out of the capillary and into bowman's capsule.
• The liquid and smallmolecules pass through three layers to get into the Bowman's capsue and enter the nephron tubule - the capillary wall, a (basement) membrane and the epithelium of the bowman's capsule. Larger molecules such as protein and blood can't pass through this.
• Selective reabsorption takes place as the filtrate flows along the proximal convoluted tubule, through the loop of henle, and along the distal convoluted tubule.
• Useful substances leave the tubules of the nephrons and enter the capillary network that's wrapped around them
• The epithelium of the wall of the PCT has microvilli to provide a large surface area for the reabsorption of useful matericals from the filtrate into the blood.
• Useful solutes like glucose, amino acids, vitamins and some salts are reabsorbed along the PCT by active transport and facilitated diffusion.
• Water enters the blood by osmosis because the water potential of the blood is lower than that of the filtrate. Water is reabsorbed from the loop of Henle, DCT and the collecting duct.
• The filtrate that is left after reabsorbtion is urine, which passes along the ureter to the bladder.
• Urine is usually made up of:

  • Water and dissolved salts
  • Urea
  • Other substances; Hormones and excess vitamins ect.
• Urine doesn't usually contain:

  • Proteins and blood cells - too big to be filtered out of the blood.
  • Glucose and amino acids - actively reabsorbed into the blood.