Stages Of Urine Formation

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Urine Formation


Urine formation is a process by which waste product is excrete from the blood to produce urine. Urine consist of 95% water, 5% of waste product such as creatinine, ammonia, uric acid, and other electrolytes as sodium, potassium, calcium etc.


Kidney receive an average of 20% of cardiac output from heart. Oligouria is define as low urine formation less than 400 ml per day, anuria is define as urine formation less than 100 ml per day. Polyuria is defined as the formation of excess urine usually greater than 2.5 L/ day.


Urine formation occur in nephron See: Structure of nephron is consist of proximal convoluted tubule (PCT), distal convoluted tubule (DCT), and loop of henle. There are three stages through which urine is formed,

  1. Glomerular Filtration 
  2. Tubular Reabsorption 
  3. Tubular Secretion 


Glomerular Filtration


Glomerular filtration is the process of filtration of blood in glomerulus. In the glomerulus, 90% of water is reabsorbed it is termed as glomerular filtration. The rate at which this filtration occurs is called the Glomerular filtration rate. 


Blood filtration occurs due to the difference between blood pressure of two arterioles i.e. efferent and afferent arteriole. Hydrostatic and osmotic pressure also facilitates filtration of electrolyte and water molecule. Glomerular filtration is regulated by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone. 


The filtered blood is then sent to lungs for oxygenation and waste products form are send further for tubular reabsorption and secretion. During filtration, only protein molecules are not filtered due to their large size, it is also called as ultrafiltration.


Tubular Reabsorption



Tubular reabsorption is filtration of blood occur in DCT and PCT tubule.The essential component present in the glomerular filtrate is reabsorbed in the body through microvilli present in the proximal and distal convoluted tubule. Substance reabsorbed in the tubular portion are salts, hormones, sodium chloride, bicarbonates, vitamin, glucose, and amino acid. Some substances occur in urine when they increased more than the threshold limit of filtration ex, in DM, sugar is present in urine.


Substance reabsorbed in the peritubular area are reabsorbed by sodium transport mechanism NA+/K ATPase. Substances that doesn’t reabsorb is termed as non-reabsorbed filtrate as it moves towards collecting duct.


Tubule Secretion


Tubular secretion involve synthesis of hydrogen and potassium ions that further mix with waste products to form urine. It maintain acid base balance of the urine. Further the urine is send to renal calyces via collecting duct of nephron. Thereafter by ureter it is reach to the bladder.


Composition of Urine


Urine is consist of water, urea and other solutes such as creatinine, ammonia, sodium, potassium, chloride, phosphate, sulfate and oxalates etc. Presence of bile pigment responsible for yellow coloration of urine. Urine ph is ranges from 4.5 to 8ph. The normal urine output is 1 to1.5 liters per day . Kidney receive about 1,300 ml of blood from heart and perform filtration, unwanted substances along with water form as urine in renal calyces which is then excreted through urination.


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