Part 1: excretion

Excretion is one of 7 (or 8 once you include homeostasis) life processes. Excretion is the removal of metabolic waste from the body. This can be done in different ways depending on which organ you're looking at, and as all living things carry out all 8 life processes, it is also done by plants as well as animals:
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• Skin
• Kidneys
• Lungs
• Leaves

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The skin is an organ, and it is an excretory organ at that. The skin is used to excrete excess salt/ ions and water as well as waste heat from respiration. This is done by secreting water from the skin and allowing it to evaporate. The skin has adapted to do this by having a large surface area and specialised glands which secrete sweat (containing the aforementioned water, salts/ ions and heat)

The kidneys are two highly adapted excretory organs which I will go into in more detail later, but for now here's the basics: the waste products from the kidneys are urea, excess water and excess salts/ ions. They are transported to the bladder through the ureter after being filtered from the blood via the nephrons. Kidneys have adapted to have a good blood supply, millions of nephrons and high pressure to enable them to carry out their function of filtration quickly and efficiently.

The lungs are in fact an excretory organ - they excrete CO2 and water vapour. These are metabolic waste products as they are both produced by respiration in the cells. The lung has adapted for its role due to its alveoli having a large surface area, the alveoli being 1 cell thick, good blood supply and a moist alveoli lining.

Much like the skin, the leaves are not often though of as excretory organs, or even as organs for that matter. The leaves remove O2 as the waste product from photosynthesis and CO2 as the waste product of respiration. They do this by diffusing the waste gases through the stomata in the leaf. The leaf's adaptations consist of a broad, flat surface creating a large surface area, they are very thin and there are many stomata making the gas exchange quicker.

Part 2: the urinary system

The urinary system is an important system in the body - it gets rid of unnecessary, metabolic waste from the blood. The urinary system consists of the two kidneys, your two ureters (one from each kidney), the bladder and the urethra. Urine contains excess water, the amount of which can be changed as you'll see later; excess salts/ions and urea.

Urea is a more stable form of nitrogenous waste - so called nitrogenous due to it being a nitrogen-based compound.

The reason why the urinary system is so important to the body is that when proteins are broken down into amino acids, nitrogen, which is one of the four components of amino acids (the other three being Carbon, Hydrogen and Oxygen), is released and forms ammonia which is toxic to our cells. As a result, our liver converts this ammonia into the non-toxic urea. The urea is then transported through the blood to the kidneys to be put through the urinary system. The removal of the excess salts/ions and water is important because it helps with osmoregulation (the control of the water concentration of the blood). 
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This diagram is about as detailed as you'll need for GCSE level.
The only major thing to note is that the medulla refers to all the renal pyramids together.
When looking at the kidney, the pinker an area is, the higher the blood supply is - as you can see, the renal pyramids has a very high blood supply which is obvious once we get to the next part.

This is the nephron. This is where filtration occurs. (Just as a note, where it says afferent and efferent arteriole, it should say capillary instead of arteriole). 
   As you can see, the nephron starts when a capillary loops around itself to create a dense knot, called the glomerulus. The glomerulus is important because it creates very high pressure in the bloodstream. Where the glomerulus and the Bowman's capsule meet, a process called ultrafiltration occurs. Ultrafiltration is the removal of small molecules from the blood/plasma at high pressure. When ultrafiltration takes place, it creates a glomerular filtrate. The contents of this filtrate are: plasma, glucose, salts/ions, and urea. The other contents of the blood are too large to get through with the exception of some white blood cells when an infection is in the nephron.
   At the proximal convoluted tubule, a process called selective reabsorption occurs. In actual fact, the nephron is wrapped in capillaries and because of this, glucose is reabsorbed into the bloodstream by active transport, though not all of it will be reabsorbed.
   The next part of filtration is the loop of Henle. The loop of Henle is the first part of filtration where osmosis occurs. Here, some of the water is diffused back into the blood,p and some salts/ions are also reabsorbed by active transport.
   Then we come to the distal convoluted tubule. Here, more diffusion takes place.
   Finally, we arrive at the collecting duct. This is the last stage of filtration and what it does can vary - the collecting duct contains cells which, when stimulated by the hormone ADH (Antidiuretic Hormone) become more permeable to reabsorb more water. The level of water reabsorbed depends on the amount of ADH released which is controlled by the pituitary gland.
   Then the urine goes down the ureter to the bladder.
Because of how important filtration is, there are millions of nephrons per renal pyramid.

The last thing which will be said on this are the steps of osmoregulation during filtration:
when you're dehydrated:

• The hypothalamus detects too little water in the blood
• The hypothalamus "tells" the pituitary gland to release ADH
• The ADH travels through the bloodstream to the kidney nephrons
• The collecting ducts become more permeable to water meaning more water is reabsorbed
• Urine becomes more concentrated (lower in water volume, higher in urea concentration

When you're hydrated:

• The hypothalamus detects too much water in the blood
• The hypothalamus "tells" the pituitary gland not to release ADH
• No ADH travels to the kidney nephrons
• The collecting ducts become less permeable to water so less water is reabsorbed
• Urine becomes less concentrated (higher in water volume, lower in urea concentration)