The Renal System.

The kidney uses the process of ultrafiltration and selective reabsorbtion. Ultrafiltration is where the body sorts out what is in the system and what it wants to keep and get rid of. When the body takes back what it wants to use this is called selective absorption. These two processes combined means that the kidneys are able to get rid of almost anything that enters the body, even if the body doesn’t recognise it this could be a drug for example. The body gets rid of anything that enters it through ultrafiltration. Once the body recognises something (chemicals) that is useful to it they will be taken back to the kidneys through selective re-absorption. The kidneys use millions of tubes, called nephrons to carry out all these tasks.  Each kidney contains nephrons, over one million, combined their total length is over 100km. Approximately 1,300ml of blood are processed a minute by the nephrons, which make 1ml of urine and return 1,299ml of clean blood back to the body.

Selective Re-absorption:

In a healthy person the urine should contain no glucose or protein as these useful substances have been

Urine can be used to determine the state of someone’s health. If any unrecognised chemicals are in the body, they will show up very quickly in the urine. This is one reason for athletes (track runners) to be tested through urine tests as the drugs show up very quickly, and cheating can be quickly established. Medical professionals, for example nurses, and doctors, regularly use urine tests for their patients.

Homeostasis – Osmoregulation:

Osmoregulation is the control of the body’s water levels. Chemical reactions in the body’s cells work in solutions and any changes to the concentrations (strengths) of the solutions can result in the reactions being quicker or slower. No matter how small the change in the concentration is, it can still have a major effect.  The body needs water to:

• Keep substances in solution so that they can perform reactions
• Move substances around the body dissolved in blood
• Act as a ‘heat store’
• Produce sweat to cool the body
• Carry waste products out of the body via the urine (dissolved).

Water levels in the body are controlled by the hypothalamus and the pituitary gland in the brain and the kidneys. Any changes in the water levels, for example if they decrease the hypothalamus signals to the pituitary gland to produce a hormone called ADH, Antidiuretic Hormone. This enables the kidneys to secrete small amounts of concentrated urine, which therefore leaves more water to be stored within the body. ADH is stopped being produced if the body’s water levels are high, and the kidneys produce a lot of dilute urine, which carries any excess water out of the body.  Osmoreceptors are sensory receptors which sense and determine the osmotic pressure, for example in stomach or small intestine. Osmoreceptors situated in the hypothalamus monitor the concentration of chemicals in the blood. Sodium ions are especially monitored so that a decision can be made when to turn the receptors on. Receptors in the aorta measure the volume of blood in the body. If the receptors detect a change in the blood levels, for example a fall in the levels then signals are passed out to the hypothalamus which stimulates the pituitary gland to secrete ADH. If a person’s body looses a large amount of blood, (and does so quite quickly) then pressure receptors also seem to play a part at this time.  

Diseases of the Renal System:

Severe diarrhoea is a disease of the renal system.