The human body continuously processes nutrients and produces waste. The urinary system filters unwanted substances from the blood, transforming them into urine. This system ensures excess water and metabolic waste, such as urea, are effectively removed, maintaining overall balance. The process begins with blood filtration and culminates in the regulated expulsion of urine.
Urine Creation in the Kidneys
Urine formation begins within the kidneys, specifically in microscopic filtering units known as nephrons. Each kidney contains approximately 1 to 1.5 million nephrons, which process blood and create urine through a series of steps. The initial step is glomerular filtration, where blood enters a specialized capillary network within the nephron called the glomerulus. Here, blood pressure pushes water and small solutes from the capillaries into a cup-shaped structure known as Bowman’s capsule. Large components of the blood, such as blood cells and proteins, are retained and do not pass into the capsule.
Following filtration, the fluid, now called glomerular filtrate, moves into the renal tubule, where most of its contents are reclaimed by the body through reabsorption. Cells lining the tubule actively transport useful substances, including nearly all the filtered water, glucose, and certain salts, back into the bloodstream. About 99% of the filtered water is recovered daily.
The final step in urine creation is secretion, essentially the reverse of reabsorption. During secretion, additional waste products, excess ions like potassium and hydrogen, and certain drugs are actively transported from the blood in surrounding capillaries directly into the filtered fluid within the tubule. This process ensures that substances not initially filtered or present in insufficient amounts are effectively removed. The modified fluid collects in the renal pelvis, a funnel-shaped area at the center of the kidney.
The Journey to the Bladder
Once urine is formed in the kidneys, it travels to the bladder through two slender tubes called ureters. These muscular tubes, 20 to 30 centimeters (8 to 12 inches) long and 3 to 4 millimeters in diameter in adults, connect each kidney to the bladder. The transport of urine through the ureters is not a passive flow driven by gravity.
Instead, the smooth muscle walls of the ureters undergo rhythmic, wave-like contractions, a process known as peristalsis. These contractions push small spurts of urine downward, away from the kidneys and into the bladder. This continuous, active movement occurs approximately every 10 to 15 seconds, preventing urine from backing up and helping to protect the kidneys from infection.
Expulsion from the Body
The bladder serves as a temporary reservoir for urine. This hollow, muscular, and elastic organ can store a considerable volume of urine; a healthy adult bladder holds up to 300 to 500 milliliters (about 1.3 to 2 cups) for several hours. As the bladder fills, its walls stretch, activating specialized stretch receptors within the muscle layers. These receptors send signals to the brain, which are perceived as the urge to urinate.
When a person decides to urinate, a coordinated process called micturition occurs. The brain sends signals to the detrusor muscle, which forms the main wall of the bladder, causing it to contract and squeeze the urine out. Simultaneously, the brain signals the internal urethral sphincter, an involuntary muscle at the bladder’s exit, to relax. The external urethral sphincter, which is under voluntary control, must also relax to allow urine to pass.
Finally, urine exits the body through the urethra, a tube that extends from the bladder to the outside. The length of the urethra differs between biological males and females; it is significantly shorter in females.