Essay on the Physiology of Excretion (1300 Words)!
Urea Cycle/Ornithine Cycle:
i Process of urea synthesis is a repetitive cyclic sequence of a few reactions, which occur continuously in the liver cells.
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ii. Also referred to as Kreb-Henseleit cycle (1932) after the name of its discoverer.
iii. Formation and transformation of three amino acids, namely ornithine, citrulline and arginine one after the other, constitute the major steps in this cycle.
Ultrafiltration is filtration under pressure. It occurs in Bowman’s capsule. The efferent arteriole is narrower as compared to the afferent arteriole hence a hydrostatic pressure develops in glomerulus which forces a large amount of water, entire glucose, all amino acids, urea and salts of sodium and potassium out of the glomerulus into the Bowman’s capsule.
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Pressure increases the permeability of capillaries and Bowman’s capsule to some 100 times the permeability of common capillaries. Filteration takes place through tiny spaces amongst the cells of capillary walls and filteration slits of podocytes in the Bowman’s capsule.
Effective Filteration Pressure (EFP):
Is determined by 3 pressure –
1. GHP (Glomerular Hydrostatic Pressure) – It is blood pressure in glomerular capillaries due to narrower efferent arteriole and is the chief determinant of EFR. Its value is 75 mmHg.
2. BCOP (Blood Colloid Osmotic Pressure) – It is osmotic pressure created in the blood of glomerular capillaries due to plasma proteins. Its value is 30 mmHg.
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3. CHP (Capsular Hydrostatic Pressure) – It is caused by fluid occupying the Bowman’s capsule and resists filteration. Its value is 20 mmHg.
Hence, EFP = GHP – (BCOP + CHP)
= 75 – (30 + 20) = 25 mm Hg.
Glomerular filtration rate (GFR) is the filtered blood entering into the Bowman’s capsule. In a normal adult it is about 120 ml per minute and about 180 liters per day.
About 1250 ml of blood circulates per minute through the two kidneys, and out of it 650-700 ml is plasma. This is called renal plasma flow (RPF).
Filtration Fraction (FF) is the fraction of the plasma passing through kidneys which is filtered at the glomerulus, i.e., the ratio of GFR to renal plasma flow (RPF)
(b) Reabsorption is the process due to which the useful constituents of the glomerular filtrate are returned into the blood stream. It occurs in convoluted tubules as well as loop of Henle. Reabsorption involves both active and passive transport. Active absorption occurs in case of glucose, amino acids, vitamins C, Cr2+, K+(25%), Na+ (70%) and passive absorption occurs in case of water, CF and some other anions (sulphates, nitrates).
Proximal convoluted tubule (PCT):
i. It involves reabsorption by active transport, which is mainly due to large surface area (because of microvilli); numerous mitochondria and closeness of blood capillaries.
ii. Almost entire glucose, amino acid, 75% of water, Na+, K+, uric acid, chlorides by diffusion, are reabsorbed.
iii. Sulphates and creatinine are not reabsorbed.
Loop of Henle consists of two segments ascending and descending
Ascending limb:
i. It is impermeable to water so no water is reabsorbed with solutes. Due to which nephric filtrate become hypotonic.
ii. It actively reabsorbs the remaining 25% of the filtered K+ and some amounts of Cl–. Some Na+ is also reabsorbed by passive diffusion due to the electrostatic attraction of reabsorbed Cl–.
Descending limb:
i. It is not permeable to Na+ and other solutes. Only 5% of water in the filtrate is reabsorbed by osmosis.
ii. The filtrate becomes hypertonic to blood plasma. It involves the passive diffusion of Na+ into the tubule. About 80% of water is reabsorbed in PCT and loop of Henle. This is called obligatory water reabsorption.
Counter current mechanism (the process due to which the urine is made hypertonic) is regular exchange of Na+ ions between the descending and ascending limbs of loop of Henle. This mechanism involves passage of salts from ascending limb to descending limb via tissue fluid and from descending to ascending limb inside the lumen of loop.
Bottom of loop occurs in salty brew. As a result of which water diffuses out from bottom and descending limb of the loop. Some Na+ passes into descending limb and from there to ascending limb. In ascending limb, it comes out. The process is repeated forming a counter current multiplier system.
In the descending limb the water passes out of the collecting tubules due to high osmotic pressure created by NaCl, into the interstitial fluid and eventually to blood of vasa rectae. While Na+, Cl– and urea are absorbed this makes the filtrate (urine) hypertonic.
Hence Henle’s loop and vasa rectae play an important role of counter current multipliers to produce and maintain a high salt concentration in the medullary interstitial tissue.
Distal convoluted tubules (DCT):
DCT along with collecting ducts are involved in the active transport of Na+ under the influence of aldosterone iso-osmotic absorption of water and in exchange excretes some potassium in the urine. Water reabsorption in DCT mediated by ADH is called facultative water reabsorption.
Vasopressin (or ADH) hormone from the posterior pituitary play s an important role in regulating the amount of urine passed out by affecting the permeability of the DCT. If fluids are consumed in large amounts the permeability of DCT increases and the urine is passed out in more amounts.
Absence of ADH reduces the reabsorption of water in DCT causing dilute and increased urine, a pathological condition called diabetes insipidus.
In DCT, sodium reabsorption varies with concentration of the extracellular fluid.
Renin-angiotensin mechanism (auto regulation of GFR by Juxtaglomerular apparatus) – Juxtaglomerular apparatus consists of macula densa cells of DCT, which sense the amount of NaCl in DCT and regulate the sodium concentration and H2O concentration isomotically. When the blood pressure falls the hormone renin secreted by juxtaglomerular cells of kidney plays an important role in reabsorption of sodium.
Renin on reaching the blood changes angiotensinogen (inactive compound) released from liver) of plasma into angiotensin II via angiotensin I. Angiotensin I is converted into angiotensin II by another proteolytic enzyme called angiotensin converting enzyme (located in lungs).
The latter activates adrenal cortex (zona glomerulosa) to release aldosterone which in turn accelerates reabsorption of sodium and the excretion of potassium glomerular filterate. Angiotensin II is the active agent in blood pressure regulation and function.
(c) Tubular secretion:
i. DCT and collecting tubules constitute the area of tubular secretion.
ii. It is the active secretion or excretion of waste products from blood capillaries and interstitial fluid into the lumen of nephron.
iii. It is the opposite of tubular reabsorption
iv. The cells of tubules secrete H+ ions and K+ ions to maintain the pH of blood.
v. Most of the K+ eliminated in the mammalian urine is secreted by DCT and collecting ducts in exchange of absorbed Na+.
vi. In mammals the secreted substances are uric acid, K+, H+, ammonia, creatine, drugs – penicillin and para aminohippuric acid etc.
Micturition is a reflex of voiding urine. If the urine content of urinary bladder reaches more than 300 ml, micturition reflex starts.
Urine:
i. It is transparent, amber coloured, hypertonic fluid with a slightly acidic pH. The yellow colour of the urine is caused by the pigment urochrome, which is a breakdown product of haemoglobin from worn out RBCs. Volume of urine depends upon intake of fluids, external temperature and physical activities (1 to 2 litres in 24 hours but varies to a considerable extent)
ii. pH ranges from 5.0 to 7.8. Average pH = 6.0 (slightly acidic)
iii. The urine on standing gives a pungent smell. It is due to conversion of urea into ammonia by bacteria (hence alkaline)
iv. Least concentration of urea is found in renal vein because urea is excreted through urine formed in kidney.
v. Highest concentration of urea is found in hepatic vein because urea is synthesized in liver.
