How is urine formed?
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Urine formation is a complex and vital process carried out by the kidneys to filter waste products from the blood and maintain the body's fluid and electrolyte balance. This process occurs in three main steps within the nephrons, the functional units of the kidneys: * Glomerular Filtration (UltrafilRead more
Urine formation is a complex and vital process carried out by the kidneys to filter waste products from the blood and maintain the body’s fluid and electrolyte balance. This process occurs in three main steps within the nephrons, the functional units of the kidneys:
See less* Glomerular Filtration (Ultrafiltration):
* Blood enters the glomerulus, a network of tiny capillaries within Bowman’s capsule (part of the nephron).
* Due to the difference in diameter between the afferent arteriole (bringing blood in) and the efferent arteriole (carrying blood out), a high hydrostatic pressure is created in the glomerulus.
* This pressure forces water and small solutes (like ions, glucose, amino acids, urea, uric acid, creatinine) from the blood through a specialized filtration membrane into Bowman’s capsule.
* Larger molecules, such as blood cells and large proteins, are too big to pass through the membrane and remain in the bloodstream.
* The fluid collected in Bowman’s capsule is called the glomerular filtrate or nephric filtrate. Roughly 180 liters of filtrate are produced daily, but only a small fraction of this becomes urine.
* Tubular Reabsorption (Selective Reabsorption):
* The glomerular filtrate contains many substances the body still needs, not just waste.
* As the filtrate flows through the renal tubule (including the proximal convoluted tubule, loop of Henle, and distal convoluted tubule), essential substances are reabsorbed back into the bloodstream in the peritubular capillaries surrounding the tubules.
* This reabsorption is highly selective and involves both active transport (requiring energy) and passive diffusion.
* In the proximal convoluted tubule (PCT): A large percentage of water, glucose, amino acids, and essential ions (like sodium, potassium, chloride, bicarbonate) are reabsorbed. Glucose and amino acids are typically reabsorbed almost completely.
* In the loop of Henle: This section plays a crucial role in concentrating urine. The descending limb is permeable to water, allowing water to be reabsorbed. The ascending limb is permeable to ions (especially sodium and chloride) but not water, helping to create a concentration gradient in the kidney medulla.
* In the distal convoluted tubule (DCT) and collecting duct: Further reabsorption of water and ions occurs, largely regulated by hormones like Antidiuretic Hormone (ADH) for water and aldosterone for sodium. This allows the body to fine-tune the final volume and concentration of urine based on its hydration needs.
* Tubular Secretion:
* While reabsorption takes substances out of the filtrate and puts them back into the blood, tubular secretion does the opposite: it adds specific waste products and excess ions from the blood into the filtrate within the renal tubule.
* This process primarily occurs in the proximal and distal convoluted tubules and the collecting duct.
* Substances secreted include hydrogen ions (H^+), potassium ions (K^+), creatinine, certain drugs, and metabolic wastes.
* Tubular secretion is crucial for:
* Eliminating substances not adequately filtered in the glomerulus.
* Maintaining the body’s acid-base balance by secreting H^+ ions when the blood is too acidic and reabsorbing bicarbonate.
* Excreting excess K^+ ions.
After these three processes, the fluid that remains in the collecting ducts is now called urine. This urine then flows from the collecting ducts into the renal pelvis, down the ureters to the urinary bladder for storage, and finally is expelled from the body through the urethra.