The urinary system has four main components: the kidneys, ureters, urinary bladder, and urethra. Urine, a liquid waste product, is formed in the kidneys. From there it moves through the ureters and into the bladder, where it is stored. When the bladder gets full, urine is emptied from the body through the urethra in a process called urination.
The creation of urine is a complex process. The kidneys filter waste from the blood that passes through them, and reabsorb substances that the body requires, even though those requirements may change from moment to moment.
Each of the kidneys is composed of approximately one million subunits called nephrons. Each nephron consists of a microscopic ball of blood vessels called a glomerulus, which is connected to a twisting length of tube called the renal tubule. Because the blood vessels in the glomeruli are porous, they act as filters, removing most of the water, salt, and waste from the blood that passes through them.
As filters, the glomeruli have physical properties that prevent large cells, like red blood cells, from passing into the renal tubules. On the other hand, smaller particles, like sugar and salt, can pass easily through the glomerulus. Within the renal tubules, waste products are passed into the urine. Simultaneously, substances the body needs, such as water and salt, are reabsorbed back into the bloodstream.
The path of urine formation, reabsorption, and excretion begins at the glomerulus, continues through the renal tubules, and proceeds through a ureter into the bladder. The unique, expandable cells in the wall of the bladder stretch and become thinner as it fills. Finally, urine is excreted through the urethra.
