PHYSIOLOGY OF EXCRETION
Man is ureotelic. Urea is formed mainly in the liver and in small amounts in the brain and kidneys, is released to the blood stream, and is removed by Kidneys in urine. The excretory proses may, thus, be divided into two men. Events urea formation and urine formation.
Urea Formation: -
Urea is formed in the liver by a cyclic process called urea cycle, or ornithine cycle, or Krebs-Henselelt cycle.
The amino acids not needed in the body are deaminated by an enzyme oxidase, producing ammonia, NH3. Ammonia, being toxic, is quickly changed to urea. Three amino acids participate in the process -
(i) Ornithine combines with ammonia and carbon dioxide to form citrulline and water.
(ii) Citrulline combines with more ammonia to form arginine and water.
(iii) Arginine then decomposes to form urea and ornithine in the presence of enzyme arginase and water. Ornithine is set free for reuse in the urea cycle.
Urine Formation (Uropoiesis):-
Urine formation occurs in the kidneys. It involves three processes: glomerular filtration, tubular reabsorption and tubular secretion
1. Glomerular Filtration (Fig. 20.16). Walls of glomerular capillaries and Bowman's capsule are very thin and are semipermeable due to the presence of pores in the former and slit pores in the latter. They Allow water and small molecule in the blood to pass through them. Fluid containing these materials is forced out of the glomerular capillaries into the Bowman's capsule by the high pressure of the blood in the Glomerular capillaries. The pressure is high because the glomerular capillaries are narrower than the afferent artery this pressure is about 75 mm. Hg. in man, the fluid tends to move in the reverse direction osmotic pressure of plasma proteins in the glomerular capillary and hydrostatic pressure Or the Fluid in the urinary tubule. These pressures in man are about 30 mm. Hg. and 20 mm. H8. Respectively. The net force moving the fluid from the glomerular capillaries. called the filtration pressure, is />0+20) or mm. Hg. The separation of small molecules and ions from large molecules and cells in the blood is entered ultrafiltration. The filtered-out fluid is known as glomerular filtrate. or capsular nitrate, or ultrafiltrate.
1) Glomerular Filtration Rate. :-
About 1100-2000 litters of blood flows through the human kidney each day. This is about 275 times the total volumes of blood in the body. The glomerular filtration rate in a normal adult human being is about 125 ml. per minute, and some 180 litters of filtrate is produced daily. This is about four and a half times the amount of fluid in the whole body. Kidneys excrete only about 1.5 litters of urine in a day. The ultrafiltrate contains sodium. potassium. and chloride 1ons. glucose. amino acids. along with urea, uric acid, creatinine*, ketone bodies, and a large amount of water. The blood is left with only corpuscles, and plasma proteins (albumen. globulins). The concentrations of various materials in the glomerular filtrate are nearly equal to their respective concentrations in the plasma The filtrate. therefore, almost resembles the protein-free and cell-free plasma in composition and osmotic pressure.
2.) Tubular Reabsorption.
From the Bowman's capsule, the glomerular filtrate passes in ten. Tubule and flows through it to the collecting duct. During this course, its composition, osmotic pressure and pH Change due to selective reabsorption of materials from it and secretion of more waste materials into it.
i)Proximal convoluted tubule: - The cells lining the PCT are well adapted for reabsorption of materials from the filtrate. They have abundant mitochondria and bear numerous microvilli on the free side. Mitochondria power the active transport of nutrient molecules back into the blood. Microvilli increase the Surface for reabsorption. The cells reabsorb entire glucose. amino acids, most of the inorganic 1ons (Na*, C, much of the water as well as some urea from the filtrate. Reabsorption takes place as under.
A) Glucose, amino acids and Na+, K+, ions are reabsorbed by active transport. Glucose reabsorption is so efficient that appearance of only a trace of glucose in urine suggests a possible presence of a disease named diabetes mellitus.
(B) Cl- are reabsorbed by passive transport following the positively charged ions.
(C) Active uptake of ions reduces the concentration of the filtrate, and an equivalent amount of water passes into the peritubular capillaries by osmosis.
(D) Most of the important buffer bicarbonate (HCO;) is also reabsorbed from the filtrate.
(E) Some urea is reabsorbed by diffusion. The rest remains in the filtrate for removal in the urine.
(ii) Henle's Loop. The following events occur in the Henle's loop.
(a) The first wide part of the descending limb is impermeable to ions, urea and water. It merely transfers the nearly isotonic filtrate from the PCT to the narrow region of the descending limb.
(b) The second (narrow) part of the descending limb is around freely permeable to water. The interstitial fluid it has a high osmotic Therefore, water pressure due to a high concentration of sodium chloride and urea in it. is drawn out of the filtrate by osmosis.
(c) The exit of water makes the filtrate hypertonic by the time il reaches the turn of the loop the ascending limb.
(d) The ascending limb is impermeable to water along its entire length Its first (narrow part is permeable to inorganic ions (Na", K", U) and urea. lons leave the filtrate by diffusion and the urea enters the filtrate by diffusion (secretion).
(e) The cells of the second (wide) region of the ascending limb pass inorganic ions out of the filtrate by active transport into the renal medulla whicn Decomes concentrated. This helps the process given in (b)
(f) The filtrate becomes hypotonic to plasma due to loss of inorganic ions and passes into the DCT.
(iii) Distal Convoluted Tubule, Collecting Tubule and Collecting Duct. Following events occur in these regions.
(a) When the level of plasma waterfalls, the posterior pituitary lobe releases the antidiuretic hormone (ADH) which increases the permeability of the distal convoluted tubule, collecting tubule and the collecting duct to water. Water is reabsorbed from the filtrate by osmosis, and a reduced amount of concentrated urine is produced. When the level of plasma water becomes normal, ADH is not secreted, permeability of DCT collecting tubule and collecting duct to water decreases, less water is reabsorbed, and abundant dilute urine is produced.
(b) The distal convoluted tubule, collecting tubule and the collecting duct actively reabsorb sodium from the filtrate under the influence of the adrenal hormone aldosterone which makes their walls permeable to ions. The reabsorption of sodium brings about the uptake of an osmotically equivalent amount of water. Lack of aldosterone makes the DCT, collecting tubule and he collectingg duct impermeable to ions.
(c) Some urea diffuses from the last part of collecting duct into the interstitial fluid to raise latter's density for further uptake of water into collecting duct and bottom of Henle's loop
(d) Bicarbonate ions (HCOG) are also reabsorbed in DCT.
3)Tubular Secretion.: -
It occurs as under-
(a) Creatinine, hippuric acid and foreign substances (pigments, drugs including penicillin) are actively secreted into the filtrate in the PCT from the interstitial fluid. Hydrogen ions and ammonia (NH3) are also secreted into the PCT.
(b) Potassium, hydrogen, NH* and HCO; ions are secreted by ctive transport, into the filtrate in the DCT.
(c) Urea enters the filtrate by diffusion in the thin region of the ascending limb of Henle's loop. Removal of H* and NH* from the blood in the PCT and DCT helps to maintain the pH of the blood between 6 to 8. Any variation from this range is dangerous.
Tubular secretion probably plays only a minor role in the function of human kidneys, but in animals, such as marine fish and desert amphibians which lack glomeruli and Bowman's capsules, tubular secretion is the only mode of excretion. When the blood pressure, and consequently the filtration pressure, drop below a certain level, filtration stops, and urine is formed by tubular secretion only
Modified Glomerular Filtrate: -
The glomerular filtrate reaching the end of the collecting duct, after being modified by reabsorption of certain substances and addition of others, is called final urine. The volume of urine is far less than the violone of glomerular filtrate, and its composition is quite different from that of the glomerular filtrate due to loss and gain of many substances in the nephron. The composition of urine does not change beyond the collecting ducts, except that it may acquire some mucus and epithelial cells in the ureters, bladder and urethra.
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