Here are some notes on the Kidneys and Excretion for those who do not have the CGP book.
One of the main functions of the kidney is to excrete waste products, e.g. urea produced by the liver. They also regulate the body's water content. Here's an overview of how they excrete waste products.
1) Blood enters the kidney through the renal artery and then passes through capillaries in the cortex of the kidneys.
2) As the blood passes through the capillaries, substances are filtered out of the blood and into long tubules that surround capillaries. This process is called Ultrafiltration.
3) The filtered blood passes out of the kidney though the renal vein.
Blood is Filtered at the Start of the Nephrons:
The long tubules along with the bundle of capillaries where the blood is filtered are called nephrons, there's thousands of these in each kidney.
1) Blood from the renal artery enters smaller arterioles in the cortex
2) Each arteriole splits into a structure called the glomerulus - a bundle of capillaries in a cup-shaped structure called the bowman's capsule.
3) This is where Ultrafiltration takes place
4) The arteriole that takes blood into each glomerulus is called the afferent arteriole, and the arteriole that takes the filtered blood away from the glomerulus is called the efferent arteriole.
5) The efferent arteriole is smaller in diameter than the afferent arteriole, so the blood in the glomerulus is under high pressure.
6) The high blood pressure forces liquid and small molecules in the blood out of the capillary and into the renal capsule.
7) The liquid an small molecules pass through three layers to get into the renal capsule and enter the nephron tubules - the capillary wall, a membrane called the basement membrane and the epithelium of the renal capsule. Larger molecules like proteins and blood cells can't pass through and stay in the blood.
8) The liquid and small molecules, now called filtrate pass along the rest of the nephron and useful substances are reabsorbed along the way.
9) Finally, the filtrate flows through the collecting duct and passes out of the kidney along the ureter.