Summary
Highlights
The human urinary system consists of two kidneys, two ureters, a bladder, and a urethra. Its main functions are to secrete toxic waste such as nitrogenous compounds (urea, uric acid, ammonia) and to control the volume of body fluid, ion concentration, blood osmotic pressure, and pH value. The kidney is composed of the cortex and medulla, and urine is formed within its structures.
Urine formation involves three main processes: ultrafiltration, reabsorption, and secretion. The kidney contains millions of nephrons, which are the functional units for urine formation. A nephron includes Bowman's capsule (containing the glomerulus), proximal tubules, the loop of Henle, distal tubules, and a collecting duct. These structures are surrounded by blood capillaries for exchange processes.
Ultrafiltration occurs in the glomerulus, where high hydrostatic pressure forces fluid (filtrate) from the blood into Bowman's capsule. Large molecules like red blood cells and plasma proteins do not pass through. Useful substances like ions, glucose, amino acids, and water are then reabsorbed into the blood capillaries in the proximal tubule, loop of Henle, and distal tubule. The extent of reabsorption in the distal tubule depends on the body's hydration needs.
Secretion is the process where waste products or excess substances, such as hydrogen ions, toxic drugs, and urea, are actively transported or diffused from the blood into the distal convoluted tubule to be removed. In the collecting duct, further water reabsorption occurs, with about 99% of water being reabsorbed, leaving 1% to be expelled as urine.
Osmoregulation is the process of maintaining water and mineral balance to regulate blood osmotic pressure. When excessive water is consumed, blood osmotic pressure drops, leading to less ADH (antidiuretic hormone) release, reduced water reabsorption, and increased aldosterone, which enhances mineral salt reabsorption. Conversely, when water intake is low, blood osmotic pressure rises, triggering more ADH production, increased water reabsorption, and less aldosterone, thus concentrating the urine and conserving water.
Kidney damage can result from conditions like diabetes or high blood pressure, leading to the accumulation of waste products and fluid imbalance, which can be fatal. Kidney stones, composed of uric acid, calcium oxalate, or calcium phosphate, can block the ureters, causing intense pain, blood in urine, and difficulty urinating.
For kidney failure, two main treatments are available: hemodialysis and kidney transplant. Hemodialysis involves filtering the blood using a machine, typically requiring multiple sessions per week. Kidney transplant is a surgical procedure that replaces a damaged kidney with a healthy one from a matching donor, which is a complex and time-sensitive operation.