Urinary system
Revised 2025-08-28 1:04:50 PM EDT
The urinary system is a group of organs responsible for removing metabolic waste from the body while maintaining essential substances in the bloodstream. In addition to waste elimination, this system helps regulate blood volume, blood pressure, and pH balance by selectively controlling the amount of water and ions – such as sodium (Na⁺) and potassium (K⁺) – retained or excreted. The major components of the urinary system include the kidneys, ureters, urinary bladder, and urethra.
Kidneys: filters of the blood
The kidneys are the primary organs of this system. These two reddish-brown, bean-shaped structures measure approximately 5 inches in length, 3 inches in width, and 1 inch in thickness. They are located just above the waist, nestled against the posterior abdominal wall and partially shielded by the 11th and 12th ribs. Positioned behind the peritoneum, they are classified as retroperitoneal organs.
Each kidney is encased in three distinct layers for protection and structural integrity.
A notch on the inner border of each kidney, called the hilum, serves as the entry and exit point for the renal artery, renal vein, and ureter. The renal artery delivers unfiltered blood to the kidney, while the renal vein carries filtered blood back into circulation. The ureter transports the resulting urine toward the bladder.
Kidney structure: cortex, medulla, and pelvis
Internally, the kidney is divided into several functional regions. The outer layer, or renal cortex, contains microscopic units known as nephrons, which are responsible for filtering the blood. There are roughly 1 million nephrons per kidney, each contributing to the production of urine.
Beneath the cortex lies the renal medulla, where collecting ducts gather the filtered fluid from nephrons. These ducts converge in cone-shaped structures called renal pyramids, whose pointed ends (papillae) direct the urine into small chambers known as calyces. From there, urine flows into the central cavity of the kidney – the renal pelvis – which acts as a funnel, channeling urine into the ureter for transport to the bladder.
Between the pyramids are renal columns, which contain blood vessels that support the nephrons. This vascular arrangement is essential for maintaining a steady supply of blood to each nephron and facilitating the reabsorption of water and solutes.
The nephron: functional unit of the kidney
The nephron is the microscopic structure where blood filtration and urine formation occur. Each nephron begins with a cup-shaped structure called Bowman’s capsule, which surrounds a tangled bundle of capillaries known as the glomerulus. Blood enters the glomerulus through an afferent arteriole, where high pressure forces water and small solutes out of the bloodstream and into Bowman’s capsule. This filtered fluid, called filtrate, excludes larger molecules such as blood cells and plasma proteins, which remain in the capillaries.
After the filtration process, the remaining blood exits the glomerulus via an efferent arteriole, which branches into a second capillary network that wraps around the rest of the nephron—particularly the renal tubule. This tubule is composed of three main segments: the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule.
As the filtrate flows through these tubules, a process of reabsorption and secretion fine-tunes the composition of the final urine. Water, ions, and nutrients needed by the body are reabsorbed into the blood, while additional waste products and excess ions may be secreted into the tubule. These exchanges maintain the body’s internal equilibrium, or homeostasis.
By the time the fluid leaves the renal tubule, it has been transformed into urine, which is composed primarily of water – typically over 90% – along with dissolved waste products. This urine drains into the collecting ducts located in the renal medulla, which in turn empty into the renal pelvis.
Ureters: transport channels
Once formed, urine is carried away from the kidneys by the ureters, two slender tubes each measuring about 10 to 12 inches in length. Each ureter is composed of three tissue layers: an inner mucosa, a middle muscularis layer, and an outer connective tissue layer. The mucosa contains transitional epithelium, which allows for stretching, while the muscularis enables rhythmic contractions known as peristalsis. These wave-like motions propel urine from the renal pelvis toward the bladder.
An important functional feature of the ureters is their entrance into the bladder. As the bladder fills with urine, it presses against the ends of the ureters, creating a crimped or kinked effect that helps prevent urinary reflux, which could otherwise allow urine to back up into the kidneys.
The urinary bladder: holding urine
The urinary bladder serves as a temporary holding chamber for urine before it is eliminated from the body. Located in the pelvic cavity, the bladder sits just in front of the rectum in men and beneath the uterus in women. It consists of three main tissue layers: an inner mucosal layer lined with transitional epithelium, a middle muscular layer called the detrusor muscle, and an outer serosal layer.
When empty, the inner lining of the bladder forms folds known as rugae, which allow it to expand significantly as it fills. Though the bladder can hold up to 700-800 milliliters of urine, the urge to urinate is typically triggered when the bladder reaches about one-third of its capacity.
Urethra: the final pathway
Urine exits the bladder through the urethra, a tubular structure that connects the bladder to the external environment. The urethra differs significantly in length and function between men and women. In females, it is relatively short and serves solely to eliminate urine. In males the urethra is longer and also passes through the prostate gland and penis, acting as a shared conduit for both urine and semen.
Micturition: releasing the bladder
The act of releasing urine from the bladder is known by several terms: urination, voiding, or micturition. This process involves both involuntary and voluntary control mechanisms. Two urethral sphincters regulate the outflow of urine:
When the bladder fills and stretches, it stimulates stretch receptors in the bladder wall. This activates the micturition reflex, prompting the detrusor muscle to contract and the internal sphincter to relax. Conscious relaxation of the external sphincter allows urine to pass through the urinary meatus at the end of the urethra and exit the body.