Urine

• Urine Formation


  Involves two main processes:

  1. Ultrafiltration
  2. Selective Reabsorption
• Ultrafiltration

  • the afferent arteriole being wider than the efferent arteriole creates a high blood pressure in the glomerulus
  • blood plasma is forced out of the glomerular blood capillaries into the Bowman's capsule
  • the filtered blood plasma, glomerular filtrate, contains small, soluble molecules (salts, water, glucose, amino acids and waste products) that are forced out into the Bowman's capsule
• Adaptation of glomerulus

  • is a network of blood capillaries, providing large surface area for faster rate of filtration
  • blood capillaries have very thin walls that are one-cell thick, short diffusion distance.
  • tiny pores in the capillary wall to allow substances to pass through
  • blood capillaries are covered by a thin partially permeable membrane, thus only very small soluble molecules or ions can pass through. Impermeable to blood cells, platelets and large molecules such as proteins, allowing it to filter blood
• Selective Reabsorption


  • the second stage of urine formation

  • the process in which useful substances that the body needs is reabsorbed into the blood capillaries as the glomerular filtrate passes along the nephron

  • waste products such as urea, excess water and mineral salts are passed out of the nephron as urine
• Stages of Selective Reabsorption

  • At the proximal convoluted tubule, most water is reabsorbed by osmosis and all glucose and amino acids are reabsorbed by active transport. Most mineral salts are reabsorbed by diffusion and active transport

  • at the distal convoluted tubule, some water is reabsorbed by osmosis and some mineral salts are reabsorbed by active transport

  • at the loop of Henle, some water is reabsorbed by osmosis and some mineral salts are reabsorbed by active transport

  • the collecting duct reabsorbs some water by osmosis