Urine is a liquid waste product created by the body to remove excess water, salts, and waste substances. Its primary purpose is to help maintain the body’s internal balance by clearing soluble metabolic by-products and other compounds from the bloodstream.
Water’s Journey to the Kidneys
The journey of water to become urine begins with its consumption and subsequent absorption into the bloodstream. After water is ingested, it quickly passes through the stomach and is primarily absorbed in the small intestine. Once in the bloodstream, water, along with other absorbed substances, is transported throughout the body via the circulatory system. Eventually, a significant portion of this blood, carrying dissolved water and waste products, reaches the kidneys through the renal arteries. The kidneys receive a substantial blood supply, filtering a large volume of blood multiple times a day as part of their function.
How Kidneys Form Urine
The kidneys are the body’s main organs for urine formation, housing millions of tiny filtering units called nephrons. Each kidney contains approximately one million nephrons, which are responsible for cleansing the blood and balancing the constituents of the circulation. These nephrons perform three main functions: filtration, reabsorption, and secretion, to transform blood plasma into urine.
The first step in urine formation is glomerular filtration, which occurs within the renal corpuscle of each nephron. Here, blood enters a cluster of tiny blood vessels called the glomerulus, encased by a cup-shaped structure known as Bowman’s capsule. Blood pressure within the glomerulus pushes water and small dissolved substances, such as salts, glucose, amino acids, and waste products like urea, through a specialized filtration barrier into Bowman’s capsule. Larger molecules, including blood cells and most proteins, are typically retained in the bloodstream due to their size and electrical charge, preventing them from entering the filtrate.
Following filtration, the fluid, now called filtrate, enters the renal tubule, where tubular reabsorption takes place. This process reclaims substances the body needs from the filtrate, returning them to the bloodstream. As the filtrate moves through the loop of Henle and distal convoluted tubule, more water and ions are reabsorbed, contributing to the concentration of urine.
Simultaneously, tubular secretion involves the active transfer of certain waste products and excess substances from the blood into the filtrate within the renal tubules. This mechanism helps to remove excess ions like potassium and hydrogen, as well as certain drugs and toxins, ensuring their excretion from the body. The concentration of urine is further regulated in the collecting ducts, influenced by hormones like antidiuretic hormone (ADH). When ADH levels are high, the collecting ducts become more permeable to water, leading to increased water reabsorption and a more concentrated urine.
Urine Storage and Elimination
After its formation in the nephrons, urine flows from the collecting ducts within the kidneys. From there, it travels through two muscular tubes called ureters, which connect each kidney to the urinary bladder. Muscles within the ureter walls contract rhythmically, pushing small amounts of urine into the bladder about every 10 to 15 seconds.
The urinary bladder serves as a temporary, expandable reservoir for urine. Its walls can relax and expand to store urine, holding up to two cups for several hours in a healthy adult. When the bladder becomes sufficiently full, nerves in its walls send signals to the brain, indicating the need to urinate.
The final stage, known as micturition or urination, involves the expulsion of urine from the body. The brain signals the bladder muscles to contract, squeezing the urine out. Simultaneously, the urethral sphincter muscles relax, allowing urine to exit the bladder and pass through the urethra, a tube that carries urine out of the body.