Physiology of Excretion

Last Updated on Tuesday, 26 October 2010 08:29 Written by Sandesh Wednesday, 7 April 2010 10:08

Physiology of Excretion:

Formation of urine: The process of urine formation includes the filtration of blood, reabsorption of useful substances from the filtrate and secretion of unwanted substances into the filtrate. Filtration occurs in the malpighian body and reabsorption and secretion occur at the tubules.

i.      Glomerular Filtration (Ultra filtration): Ultrafiltration is the process of filtration of the glomerular capillary blood to form the capsular fluid (glomerular filtrate).

There is barrier of capillary endothelium and the epithelial cells of inner layer of Bowman’s capsule (podocytes). Podocytes have numerous slit like openings (25nm). Similarly, the capillary endothelium contains gaps or pores (0.1µm) from which constitutes of plasma can pass but blood cells and larger mol. Wt. proteins can not pass. The glomerular capillary blood is filtered through the pores of capillary endothelium and then slits of podocytes and passes down to capsular space.

As the diameter of efferent arteriole is narrower than the afferent arteriole, there is a high hydrostatic pressure in the glomerular capillaries. The glomerular hydrostatic pressure or capillary pressure is the driving force to move the fluid out of the glomerulus. This filtration under pressure is called the ultrafiltration.

The capillary hydrostatic pressure = 70mmHg

Other forces against the capillary hydrostatic pressure are,

Osmotic pressure of blood = 30mmHg

Hydrostatic pressure of filtrate = 5mmHg.

So, net filtration pressure = 70-(30+5) = 35mmHg.

ii.            Selective reabsorption: The glomerular filtrate contains both useful and harmful substances. The useful substances like water,glucose, amino acids, vitamins, minerals, salts, ions etc are reabsorbed in the blood from the tubules by the process called selective reabsorption.

Small proteins, aminoacids, glucose are reabsorbed in the PCT by passive diffusion and facilitated diffusion or active transport.

Na⁺, Cl^-, K⁺, bicarobonates, lactate, some urea, inorganic phosphate are reabsorbed in PCT.

Na⁺, Cl^-, K⁺ ions are also absorbed in loop of Henle and DCT.

Na ions reabsorption occurs also in the collevting tubules.

iii.            Tubular secretion: The blood flowing through the efferent vessels also contains unwanted substances, which are not cleared by filtration. The efferent vessels flow through the DCT, and these unwanted substances in the blood are secreted into the lumen of DCT by the process called tubular secretion. These unwanted substances include drugs, uric acids, K⁺, H⁺ etc. which are secreted by passive diffusion and facilitated diffusion or active transport.

Thus, various substances are secreted and reabsorbed in the tubules while the filtrate passes down the tubules. So the composition of urine is different from the glomerular filtrate. The urine is hypertonic (high osmotic pressure than the blood plasma).

Role of loop of Henle:

Descending portion of loop of Henle dips into the medulla of the kidney, then makes a U turn to form ascending limb which move back to cortex of the kidney and continues as DCT.

Function of loop of Henle is to concentrate NaCl in medulla of kidney. This causes osmotic flow of water from the collecting ducts and making the urine concentrated and hypertonic.

180 lt of fluid is filtered into the capsular space but only 1.5 lt kof urine is passed each day.

The descending limb of loop of Henle is permeable to water but ascending limb is impermeable to water. Na⁺ and Cl^- are actively transported out of ascending limb. So, the salt concⁿ in medullary region of kidney is maintained.

As the fluid from DCT moves into the collecting duct, it again passes through the medulla of the kidney. Here, further water is absorbed.

Hence, hypertonic urine is excreted into the renal pelvis. Finally, urine is taken to urinary bladder by the ureter.

Reabsorption by kidney is so important that if it stops reabsorbing water, then complete dehydration of the body would occur in less than 3 minutes.

2. Micturition or urination:

Micturition or urination is the process of voiding of urine temporarily stored in the urinary bladder. When the bladder is full (300ml), the pressure is generated. It causes contraction of Detrusor muscle and relaxation of the muscles of internal and external urethral sphictors. It leads to outflow of urine through urethra. Micturition is controlled by both internal and external urethral sphincters in which internal sphincter is involuntary while external sphincter is voluntary in nature.

Composition of urine:

• Urine is a pale yellow transparent fluid.
• Acidic in nature (pH is 6.0)
  • Clear yellow colour is due to urochrome pigment (produced by breakdown of Hb) and also from the degradation products of bilirubin and urobilin.
  • Urobilin is a bile pigment excreted from liver.
  • The bad smell of the urine is due to urinod(C₆H₈O).
    • Composition of urine – water(96%), urea(2%), uric acid(0.3%), creatinine, ammonia, Na⁺, K⁺,Cl^-,PO₄^-3,SO₄^-2, oxalates etc.
  • Urine gets strong odour as the urea is degraded into ammonia.