12 The Urinary System
Learn how the urinary system forms and transports urine while helping regulate fluid balance, blood pressure, acidity, and other body functions.
Urinary system structures
The urinary system removes wastes from the blood and helps keep the body’s internal environment stable. The kidneys work closely with the cardiovascular and endocrine systems to regulate blood volume, blood pressure, water, electrolytes, and acid–base balance.
The urinary tract consists of two kidneys, two ureters, the urinary bladder, and the urethra. The kidneys lie behind the abdominal cavity on either side of the spine. Blood enters each kidney through a renal artery and leaves through a renal vein. Urine drains from each kidney into a ureter, is stored in the bladder, and exits the body through the urethra. Muscles at the bladder outlet help retain urine until urination.
Kidney anatomy
The outer cortex of each kidney surrounds its inner medulla. Microscopic urine-forming units called nephrons extend through these regions. Each includes a , where a glomerulus filters blood within a capsule, and a tubule that leads to a collecting duct. Collecting ducts carry urine toward the renal pelvis, a funnel-shaped area that empties into the ureter.
How kidneys form urine
Urine forms through three linked processes. During , blood pressure pushes water and small dissolved substances from glomerular capillaries into the ; blood cells and most proteins normally remain in the bloodstream. During , needed water and substances such as glucose and ions return from the tubule to the blood. During , the tubule moves additional substances from the blood into the filtrate, helping remove certain wastes and regulate ions and acidity.
The fluid remaining after these processes becomes urine. It contains excess water and electrolytes, as well as wastes such as , which comes from protein breakdown, and , which comes from muscle metabolism.
Urine transport and release
Urine passes from the collecting ducts to the renal pelvis, then down the ureters to the bladder. The bladder stores urine. During urination, its muscular wall contracts and the outlet muscles relax, releasing urine through the urethra.
Fluid, electrolytes, and acidity
The kidneys adjust how much water and dissolved material the body retains or excretes. This helps stabilize blood volume and the concentrations of substances such as sodium, potassium, calcium, and phosphate. The kidneys also help control blood acidity by excreting acid and regulating bicarbonate, a base in the blood.
Hormones fine-tune these adjustments. When the body needs to conserve water, signals the later and collecting ducts to reabsorb more water, producing a smaller volume of more concentrated urine. , released by the adrenal cortex, increases sodium and potassium in parts of the . Water can follow retained sodium, linking kidney activity with the endocrine system.
After sweating during exercise, the body loses water. ADH helps the kidneys conserve water, so urine volume tends to fall and urine becomes more concentrated. This response helps preserve fluid but does not replace the need to replenish fluid losses.
Blood pressure and other body systems
The kidneys influence blood pressure by controlling the amount of sodium and water in the blood and by releasing , an enzyme that helps initiate the . release can increase when renal blood flow or sodium delivery is low. The resulting angiotensin II narrows blood vessels and promotes release. encourages sodium retention, which can increase water retention and blood volume; ADH can also support water conservation. Together, these actions help restore pressure and circulation. Conversely, when the body has excess sodium and water, the kidneys can excrete more of them, reducing blood volume.
This regulation links organ systems: the cardiovascular system supplies blood to the kidneys, while the kidneys adjust blood volume and release signals that affect blood vessels and hormones. The kidneys also produce erythropoietin, which stimulates red blood cell production, and activate vitamin D, linking them to the blood-forming and skeletal systems.