What is the function of the kidneys and how do they carry this out?

Broadly speaking, the main function of the kidneys is to filter out waste from the body and excrete it out in the urine. Excretion is the removal of waste (i.e. products of metabolism/ toxic products like urea) from the body. The substances that are excreted by the kidneys will be transported to the bladder via the ureters where they are eventually released in the form of urine. Kidneys carry out their function through two steps: ultrafiltration and reabsorption. Ultrafiltration is the process by which substances (ions, water, urea, glucose and amino acids) move out of the blood due to high pressure in the afferent arteriole which 'pushes' the substances through the fenestrations of the capillary, through the collagen mesh of the basement membrane and finally through the podocytes into the proximal convoluted tubule. It would be inefficient for the body to excrete useful substances like glucose, amino acids, ions and water into the urine which is why reabsorption takes place. During reabsorption, a concentration gradient is created so that glucose and water can be transported back into the blood.