During urine formation, the nitrogenous waste materials are removed as urine from the blood without losing the valuable molecules of blood in the process. It involves three main stages namely.
1. Ultra filtration (Glomerular filtration)
2. Reabsorption (tubular)
3. Tubular secretion.
1. Ultra filtration: Involves the removal or filtration of substances from blood of the glomerular lumen into the Bowman’s capsular lumen through the Ultra fine endothelia capsular membrane. Glomerular filtration is caused due to changes in blood pressure, the pressure of the Bowman’s capsular fluid and osmotic pressures excreted by contents in glomerular and Bowman’s lumen. The glomerular filtrate resembles the blood plasma in its chemical composition except for the molecules of high molecular weight.
2. Tubular reabsorption: This is the second step or stage in urine formation and involves the reabsorption of physiologically important substances into the surrounding capillaries when the primary urine moves down route in the nephron tubules. The reabsorption may be passive cr active at various regions of the convoluted tubule. During this process glucose, amino acids, inorganic salts and some water is absorbed. Some of the absorptions are under the influence of hormones.
3. Tubular secretion: (Active secretion) This is the last stage or step in the urine formation at the end of which a hypertonic or highly concentrated urine is formed for excretion. During this stage, the capillaries surrounding the convoluted tubule actively secrete certain unwanted substances which could not be the filtered in the Bowman’s capsule by the glomerulus into the filtrate of the tubular lumen.
As a result of these 3 processes or stages .in urine formation, homeostasis is blood is restored and waste products remain in the tubular fluid forming concentrated urine for excretion from urine. Two hormones come into play during the process namely ADH – for reabsorption of water when the body has the deficit of water and Aldosterone helping in reabsorption of ions like K+ when the body faces the deficit of these ions.
