Maintaining water balances in the body

• Homeostasis is the regulation of internal conditions inside cells or organisms to create the optimum conditions for biological function
• The kidneys are organs of the urinary system that remove excess water, salts, and urea
• Blood is transported to the kidney through the renal artery
• The kidney selectively reabsorbs useful materials such as glucose, salt ions, and water
• The kidneys produce urine to help maintain water balance
• Urine contains water, urea, and salts
• Urea is the main waste product removed in the urine, as it is not reabsorbed in the kidney
• Each kidney contains over one million microscopic filtering units called nephrons
• The nephron is responsible for 'cleaning' the blood by removing urea and excess water and mineral ions
• The kidney works in stages:

  • Stage 1: Filtration
  • Small molecules like urea, water, ions, and glucose are filtered out into the nephron tubule
  • Large molecules like blood proteins remain in the blood
  • Stage 2: Selective reabsorption
  • The kidneys selectively reabsorb molecules like glucose, water, and ions needed by the body
  • Stage 3: The formation of urine
  • Urea, excess water, and ions that are not reabsorbed continue along the nephron tubule as urine
• The kidney regulates the water and ion balance of the blood plasma and keeps the level of urea low