How does filtration through the kidney work?

There are 3 key stages in filtration through the Kidney:

1. Firstly, blood is pushed through a tubular structure called the glomerulus - imagine a bowl of noodles. The pressure of the blood in the glomerulus causes fluid to leave the blood through gaps in the blood vessel cell walls. This fluid is then re-absorbed by the bowman's capsule, which reaches round the glomerulus. 
   Ball your left hand into a fist, and cup your right hand around the left - your left hand is the glomerulus, and your right hand the bowman's capsule.
   
   
2. The Bowman's Capsule is the beginning of the kidney nephron. The nephron contains something called ultrafiltrate, which is the product of the filtration described in step 1. It is important to clearly understand the difference between blood and filtrate/ultrafiltrate. Ultrafiltrate eventually becomes urine, but before this occurs, a process called selective reabsorption must take place. During this process, important substances such as water, glucose, and soduim, are reabsorbed from the filtrate back into the blood. As you can imagine, this process is crucial for homeostasis - the kidneys control the body's water/electrolyte balance.

3) Ultrafiltrate now passes from the nephron into the cortico-collecting duct, where it travels out of the kidneys in the ureter into the bladder, where it is stored before excretion through the urethra.

FINAL THOUGHT: Molecules have to be of a certain size or smaller to be filtered through the nephron, if they are too large they will have to be metabolised by the liver until they are small enough to leave the body via the kidney (renal excretion). This shows how important both the liver and kidney are in the excretion of substances from the body.

FURTHER THOUGHT: What would happen if a patient had poorly functioning kidneys? How would this effect them and how could their condition be supported?