According to Scottish scientist A. R. Cushney, urine is produced in 3 stages in the human body. namely-
1. Ultrafiltration: The process by which nitrogenous wastes are filtered by the Malpighian body is called ultrafiltration. Blood enters the glomerulus through the afferent arteriole. Water, salt, sugar, urea, uric acid, creatinine, hormones, metal ions etc. are filtered from glomerulus under hydrostatic pressure. All these filtrates are called glomerular filtrate or primary urine. Glomerular filtrate accumulates in the capsular space. The hydrostatic pressure of the glomerulus is 70 mm/Hg. Other pressure against this pressure is 35 mm/Hg. As a result, the glomerular filtrate is filtered as the actual filtration pressure (70-35 = 35) is 35 mm/Hg.
For filtration, the capillary wall of the glomerulus and the wall of Bowman’s capsule together form a three-layer filter. The layers that make up the filter are the endothelium layer of the glomerular blood vessels, the basement membrane layer of the glomerular blood vessels and the endothelium layer of Bowman’s capsule. Two kidneys in the human body pump about 1,200 ml of blood per minute and filter about 125 cc of glomerular filtrate.
2. Selective reabsorption: As the glomerular filtrate flows through the renal tubules, various substances are reabsorbed by the cells of the tubular wall and enter the blood. This is called selective reabsorption. 80% of the required material is reabsorbed from the glomerular filtrate.
(i) Proximal tubular reabsorption: The proximal tubule reabsorbs 60% of the glomerular filtrate. It reabsorbs water, glucose, amino acids, vitamins, hormones, sodium, potassium, chloride, phosphate, bicarbonate, urea etc. These substances are called high quality storable substances.
(ii) Reabsorption of Henle’s loop: Water, sodium, potassium, magnesium, calcium, chloride etc. are reabsorbed by the loop of Henle.
(iii) Distal tubular reabsorption: Water, sodium, potassium, hydrogen and chlorine ions are reabsorbed by the distal tubule. These substances are called intermediate quality storable substances.
(iv) Reabsorption of collecting ducts: Water, urea, sodium and chlorine ions are reabsorbed by collecting ducts.
After 4-5 minutes the blood of the body passes through the kidneys to filter it. Human body filters 170 liters of water in 24 hours. Of this, 99% or 168.50 liters of water is reabsorbed by the renal tubules. The remaining 1.5 liters of water is excreted as urine. 80% of water is reabsorbed by the proximal convoluted tubule and 20% by the loop of Henley and distal convoluted tubule. Sulfate and creatinine are not reabsorbed. For this reason, they are called non-preservable substances.
3. Active secretion: Creatinine and some urea are secreted from the capillary network of the proximal tubule. These substances mix with the glomerular filtrate and are carried in the urine. Hydrogen ions, potassium ions, ammonium ions, serotonin, choline, histamine etc. are secreted from the distal tubule. All these substances mix with the filtrate and become urine.