THE KIDNEYS ARE EXCRETION ORGANS
The kidneys perform three main roles:
- removal of urea
- regulation of ion levels
- regulation of water levels
The above is done by filtering stuff out of the blood under high pressure, and the reabsorbing the useful things.
The end product is urine.
NEPHRONS ARE THE FILTERATION UNITS IN THE KIDNEYS
- A higher pressure is built up which squeezes water,urea, ions and glucose out of the blood and into the capsule.
- Membranes between the blood vessels and the capsule act like filters, so big molecules like proteins and blood cells are not squeezed out. They stay in the blood.
- All the sugar is reabsorbed.( this involvels the process of active transport)
- Sufficient ions are reabsorbed.Excess ions are not.
- Sufficient water is reabsorbed according to the level of the hormone ADH.
Release of wastes:
- urea, excess ions and excess water are not reabsorbed. These continue out of the nephrone, into the ureter and down to the bladder as urine.
WATER CONTENT IS CONTROLLED BY THE KIDNEYS
The amount of water reabsorbed in the kidney nephrons is controlled by a hormone called anti-diuretic hormone (ADH).
The brain monitors the water content of the blood and instructs the pituitary gland to release ADH into the the blood accordind to how much is needed.
The whole process of water content regulation is controlled by a mechanism called nagative feedback. This means that if the water content gets too high or too low a mechanism will be triggered that brings it back to normal.
YOUR URINE ISN'T ALWAYS THE SAME
when its hot you sweat. Sweat contains water ,so....sweating causes water loss
This means the kidney will reabsorb more water into the blood. This leaves only a small amount of excess water that needs to be got rid of - so only small amount of quite concentrated urine will be produced.
Exercise make you hot, so you sweat to cool down. This produces the same effect as heat - a concentrated, small volume urine.
Not drinking enough water will produce concetrated urine ( since there will be a little excess of water to 'dilute' the other wastes). Drinking lots of water will produce lots of dilute urine.