Urine formation in the kidneys involves three key processes: glomerular filtration, tubular reabsorption, and tubular secretion. These processes work together to remove waste products and excess water from the blood while retaining essential substances.
1. Glomerular Filtration:
Blood enters the kidney through the afferent arteriole and flows through the glomerulus, a network of capillaries.
Blood pressure forces water and small solutes (like waste products and electrolytes) through the filtration membrane into Bowman’s capsule, forming the initial filtrate.
Large molecules like proteins and blood cells are too big to pass through the membrane and remain in the bloodstream.
The filtrate then enters the proximal convoluted tubule.
2. Tubular Reabsorption:
As the filtrate moves through the renal tubules, essential substances are reabsorbed back into the bloodstream.
This process involves both active and passive transport mechanisms.
Water, glucose, amino acids, and essential electrolytes are reabsorbed from the filtrate back into the blood.
This reabsorption occurs primarily in the proximal convoluted tubule, but also in the loop of Henle and distal convoluted tubule.
The reabsorbed substances are then transported by the peritubular capillaries to the general circulation.
3. Tubular Secretion:
Certain substances that are not initially filtered (like hydrogen ions and some drugs) are actively secreted from the blood into the filtrate in the renal tubules.
This process helps regulate pH, electrolyte balance, and the excretion of certain waste products.
Tubular secretion also occurs in the proximal convoluted tubule, distal convoluted tubule, and collecting duct.
4. Excretion:
The final urine, containing waste products and excess water, is then collected in the collecting duct and flows into the renal pelvis, ureter, and bladder for excretion.