Physiology of Urine Formation.

 

Introduction.

  • Urine is a waste product formed by the kidneys after filtration of the blood.

Composition of Urine:

  • Color: Yellow / Pale yellow.

  • pH: 6 to 7.5

  • Volume: 1 to 2 Liters / Day.

  • Odor: Aromatic, Strong ammoniacal on standing or concentrating.

  • Composition:

Normal Ingredients:

  • Water, Urea, Uric Acid, Creatinine, Ammonia, Sodium, Potassium, Chlorides, Sulphates, Phosphates etc.

Abnormal Ingredients:

  • Glucose: Diabetes mellitus.

  • Proteins: Kidney infection.    

  • Blood Components: Kidney Infection.

  • Ketone bodies: Ketosis.

  • Bile Pigments: Liver disorder, heavy RBC destruction.

  • Urine production is lowered during sleep and exercise. 

Physiology of Urine Formation

  • The main function of the kidney is to clean the plasma, the waste product formed in the process is called “Urine”.

  • The urine formed in kidneys is transported via ureters to the urinary bladder, the temporary urine storage site, urine is then excreted from the body by a process called “Micturition”.

  • Urine is formed in the kidney in the following three steps,

  1. Glomerular Filtration

  2. Selective Reabsorption.

  3. Tubular Secretion

  1. Glomerular Filtration:

  • First step in the process of urine formation.

  • It occurs in the ‘renal corpuscle’ at the ‘filtration membrane’.

  • Afferent arteriole has a large diameter and efferent arteriole has a smaller diameter; this difference between diameters causes an increase in pressure inside glomerulus which facilitates filtration.

  • The blood cells, plasma proteins and any other larger molecules normally can not be filtered due to their larger size. 

  • The fluid that passes into the capsular space of the bowman's capsule is called “Filtrate”.

  • Glomerular Filtration Rate:

    • The amount of filtrate formed in all renal corpuscles of both the kidneys per minute is called “Glomerular Filtration Rate (GFR)”.

    • Normal GFR = 125 ml/min (125x60x24. 180 lit. per day)

  1. Selective Reabsorption:

  • The filtrate after entering the “renal tubule” gets reabsorbed.

  • About 99% of the filtrate is reabsorbed and enters the blood, while only 1% filtrate forms the urine.

  • The movement of water and other solutes from filtrate to peritubular capillaries is called “Selective Reabsorption.”

  •  The term “Selective” is used as only selected substances are reabsorbed e.g. glucose, amino acids, ions like sodium, chloride, potassium, bicarbonate, phosphate etc.

  • The reabsorption is active as well as passive, involving energy as well as without involving energy.

  • Selective reabsorption is influenced by various hormones like,

    • Parathyroid hormones (Parathyroid Gland): ↑ reabsorption of calcium and phosphate ions.

    • AntiDiuretic Hormone, ADH (Pituitary Gland): ↑ reabsorption of water.

    • Aldosterone (Adrenal Gland): ↑ Sodium and water reabsorption.

    • Atrial Natriuretic Peptide (Cardiac Atria): ↓ reabsorption of sodium and water from PCT.

  1. Tubular Secretion:

  • It is the third and last step in urine formation.

  • In this process various substances are added in filtrate, e.g. Potassium ions, Hydrogen ions, urea, ammonium ions, creatinine, certain drugs like Penicillin etc.

  • Tubular secretion of “Hydrogen Ions” is an important thing in relation to body pH maintenance.

Commonly asked Questions.

  1. What is Urine? Discuss physiology of formation of urine.

  2. With a well labelled diagram of Nephron discuss physiology of formation of urine.

  3. Draw a well labelled diagram of L. S. of kidney and discuss in detail the physiology of formation of urine.

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