Short notes on the process of formation of urine in man

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Short notes on the process of formation of urine in man

The formation of Urine involves three processes. These are filtration, selective reabsorption and secretion.

Filtration (Ultra Alteration):

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The Bowman’s capsules of kidney act as ultra-filters. The blood flowing though the afferent arterioles contains urea, water, several salts and blood proteins dissolved in plasma. It is separated from the cavity of renal sule only by two thin membranes – the endot­helial layer of blood cap­illaries and the epithelial layer of Bowman’s cap­sule. The diameter of afferent arterioles is more and lies in close contact.

The diameter of afferent arteriole is more than the efferent arteri­ole. Therefore, the amount of blood which enters the afferent arteriole in a definite time is not able to be drained out and the hydrostatic pressure of blood in the capillary network of glomeru­lus increases. The capillary pressure is about 75 mm Hg. This pressure has to overcome the osmotic pressure of the plasma, which works to retain constituents within capillary walls. The osmotic pressure of plasma is about 30 mm Hg.

The intestinal pressure on the capillaries themselves, together with the resistance to flow in the uriniferous tubules contributes a pressure about 20mm Hg. This also works against the capillary pres­sure. Therefore, the net filtration pressure supplying energy for the filtra­tion is about 25mm Hg. As a result, water and diffusible solute molecules are forced across the membrane from the blood plasma into the Bowman’s capsule. This process is known as ultra-filtration. The effective filtration pressure (EFP) can be calculated as follows;

Glomerular pressure = 75 mm Hg.

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Osmotic pressure of blood capillaries+ capsular pressure

= 25 mm Hg.

EFP = 25 mm Hg.

Tubular Reabsorption:

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The nephron functions like an exclusive filter and as such it filters all the substances including those that are to be re­tained in the body and are necessary for metabolism. These substances are glucose, amino acids inorganic salts etc., The filterate also consists of urea, creatin and uric acid which are to be removed from the body. In selective reabsorption the essential constituents are reabsorbed into the body during its passage through the kidney tubules. This is necessary to maintain the fluid and electrolyte balance and also of the alkalinity of the blood. (pH 7.4)

Every minute about 125ml of the filterate enters the tubule of which about 124 ml is reabsorbed. Inorganic salts like sodium chloride, water, glu­cose, amino acids etc., first difuse passively into the epithelial cell from the tubular lumen. Then glucose and amino acids are actively transported (using metabolic energy) and are reabsorbed.

The different parts of the tubule absorb different substances. The Henle’s loop and the distal con­voluted tubule reabsorb water. It has been estimated that about 80% of the water is reabsorbed in the proximal tubule by a passive process and is referred to as obligatory reabsorbtion.

Secretion (active secretion): While tubular reabsorbtion removes the necessary substances from the filterate into the blood, secretion removes materials from the blood and adds to the filterate. Metabolic products present in the blood but not required by the body and in fact are injurious or secreted out from the blood to the filterate. The tubular epithelium plays an important role in this. The substances that are secreted into the filterate include potassium, hydrogen, ammonia, creatine, uricacid and drugs like penicillin. These secretions mix with the filterate of the neph­rons and constitute the urine.

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Tubular secretion has two main roles – it rids the body of unwanted mate­rials and it also controls the pH of the blood.

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