While this process of blood filtration and urine creation sounds relatively straightforward, it is actually very complex and includes the following:

• The measurement of blood oxygen levels – if it is reduced, the hormone erythropoietin is released, which stimulates red blood cell production in the bone marrow
• Electrolyte and pH balance
• Managing the body’s water levels – if this is low, antidiuretic hormone (ADH) levels increase, which causes the collecting duct to absorb more water and this in turn influences blood pressure.

The amount of water in the blood is an example of a negative feedback mechanism. If the water level is too high, perhaps because someone has been drinking a lot of water, the pituitary gland releases less ADH, which means less water is reabsorbed by the kidneys and so more is excreted in the urine making it more dilute. Too little water in the blood – for example because of excessive sweating or salt consumption – and the pituitary releases more ADH, meaning more water is reabsorbed by the kidneys and the urine becomes more concentrated and reduced in volume. 

Practice point

• The blood supply to the kidneys involves the renal artery and vein, but what’s the difference between these two types of blood vessels? Refresh your knowledge in TM's module: Body Talk: the blood.