The nephron is the functional unit of the kidney. It does the job of the urinary system. The primary function of the nephron is to remove waste products from the body before they build up to toxic levels.

The nephron does its job of getting rid of metabolic wastes through filtration and secretion. Useful substances are reabsorbed back into the blood. 

Filtration

Blood enters the glomerulus via the afferent arteriole (branches from the renal artery), and leaves via the efferent arteriole. The efferent arteriole is narrower than the afferent arteriole which helps in building up a hydrostatic pressure. The flow of blood in the glomerulus creates hydrostatic pressure in the glomerulus which forces molecules through the glomerular filtration membrane. This process is called filtration.

Secretion and Reabsorption

Capillary beds surround the loop of henle, the proximal and distal convoluted tubules. As the filtrate flows through the nephron, elements of the blood get added to or removed from the nephron. Generally, more elements get added into the nephron to be excreted then they get out of the nephron.

The movement of elements from the nephron back into the blood is known as reabsorption while the movement of elements from the blood into the nephron is known as secretion