Regulating Blood Water Content (WJEC GCSE Biology)

• The regulation of blood water content involves maintaining water and salt concentrations within the blood; this is an example of homeostasis
  • This is vital to prevent harmful changes occurring to cells of the body as a result of osmosis
• The kidneys regulate the water content of the blood by removing water from the body in the urine
• The volume of urine varies depending on the blood water content
  • When blood water content is high a large volume of dilute urine is produced
  • When blood water content is low a small volume of concentrated urine is produced
• The colour of urine can be an indicator of concentration:
  • Dilute urine is usually pale in colour because it contains a lot of water in relation to solutes
  • Concentrated urine is darker in colour because it contains little water in relation to solutes

Higher Tier Only

• The control of water reabsorption by the tubules is an example of negative feedback
• Water reabsorption is controlled by the hormone ADH, which affects the permeability of the tubules to water

Low blood water content

• If the water content of the blood is too low:
  • The brain detects the decreased water content of the blood
  • The pituitary gland in the brain releases more ADH 
  • ADH causes the kidneys tubules to reabsorb more water
  • The kidneys produce a small volume of concentrated urine

ADH and low blood water content diagram  

When blood water content is low the brain releases more ADH; this causes increased water reabsorption by the kidneys

High blood water content

• If the water content of the blood is too high:
  • The brain detects high blood water content
  • The pituitary gland releases less ADH
  • ADH causes the kidneys to reabsorb less water
  • The kidneys produce a large volume of dilute urine

ADH and high blood water content diagram

When blood water content is high the brain releases less ADH; this causes decreased water reabsorption by the kidneys